USC Sequence Stratigraphy, Towards Formalization of Sequence Stratigraphy, Sequence stratigraphy terminology debate

The purpose of this page is to provide access to a dialogue on the formalization of Sequence Stratigraphy in terms of its methodology, surfaces, units and terminology. Earth Scientists interested in standardizing these criteria for Sequence Stratigraphy are encouraged to contribute to this process by submitting constructive discourse to this site. Submissions should be sent to seqstrat@gmail.com as text or word files.

The initial focus of this debate are the preliminary reports posted below of two groups of earth scientists developing schemes to formalize the terminology of sequence stratigraphy. Click on the highlighted text for "on line lectures" on this topic and other topics related to sequence stratigraphy.

Both groups are reviewing the sequence stratigraphic literature and making recommendations regarding the methods and terminology of sequence stratigraphy. The ISSC Task Group plans to publish their results in a revised edition of the International Stratigraphic Guide (ISG). The North American Commission on Stratigraphic Nomenclature (NACSN) will evaluate feedback from both the IWGSS and the ISSC reports in preparation for an upgraded version of the North American Stratigraphic Code.

Zoran Kun�tek wrote on Monday the 13 August at 4:48 am (See Zoran Kun�tek's Image below the text box too!) Dear Colleagues, The constructive debate about sequence stratigraphy, towards its formalization is undoubtedly reasonable. It becomes very useful and practical method in geological interpretation, especially for subsurface. But, following the discussion between IWGSS and ISSC it becomes obvious that somebody ignored the most important statements of sequence stratigraphy: Posamentier et. al, 1985: ?Sequence stratigraphic concept should be applied as tool to approach rather than as template.? Anderton, 1985: ?Sequence stratigraphy should be considered as way of looking at and ordering geologic data, rather than end in itself.? For the first time we have a useful method - sequence stratigraphy - that includes several tools and types of data as seismic + well-logs + stratigraphy (and not seismic vs. well-logs vs. stratigraphy � from IWGSS ?Towards the Formalisation�?) All of these data are equally important, and their integration allows us to fill in the gaps and inadequacies of each tool individually. Sequence stratigraphy allows us to understand the interplay of sedimentation and erosion equally. Those are two opposite processes that act simultaneously. Base level changes (erosional base, Marjanac, 1996), on the other hand defines accommodation space, and their changes cause the shift of facies basinward or landward as well as changes in depositional trends. We have to accept that some parts of the land for certain time span ?never? come under water so erosion acts continuously, simultaneously some parts of the basin have continuum in sedimentation. For that reason subareal erosion poses greater time span landward than basinward (shown by Wheeler�s diagrams). Correlative conformity represents maximal basinward shift of base level at particular moment and it is connected with small portion of SU over the part of time that belongs to subaquateous erosion (RAV or RSmE), and sediments reworked and/or removed basinward belong to new portion of time. MRS (TS; CTS, FS, �) is first onlap on erosional surface representing change in depositional trend and shift of base level landward. So, C.C. should be placed immediately below first onlap visible on seismic section. MFS, on the other hand, represents maximal onlap of seismic reflector (strata) on certain erosional surface (unconformity), for that reason there should be no further onlap reflectors above it, reason being obvious � change in depositional trend caused by base level shift. I understand that sequence stratigraphy is defined as ?study of rocks relationship within chronostratigraphic framework (time table) of the repetitive, genetically related strata bounded by surfaces of erosion or non deposition, or their correlative conformity? (Van Wagoner et. al, 1988), but some could take it too seriously. We should admit that chronostratigraphic approach is rather rigid for such diachronous sequence stratigraphic surfaces, even if those surfaces have small diachronosity. Sequence and the unconformities that define it, as well as other important surfaces, posses certain time spans that need to be defined, hence geochronological approach (certain sediment package for amount of time) should be more appropriate for interpretation. Later, at the very end of interpretation we can put it in the rather rigid chronostratigraphic framework (certain time span for amount of sediments). At the end, Embry�s simple T-R sequence is more appropriate for analysis and interpretation, rather than others with three or even four system tracts. It gives the explorer more freedom for interpretation, for nature while simple still isn�t mathematics. Sincerely Yours Zoran Kun�tek <Zoran.Kunstek@ina.hr> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Ashton Embry wrote on 2nd August 2007 at 1.02PM Dear Zoran, Thank you very much for your comments. It is always useful to hear from a practitioner with no axe to grind. I have only a few comments to make in reply to your statements �I have some doubts about usage of chronostratigraphy for sequence stratigraphic interpretation. Wouldn�t it be more appropriate to use geochronology for interpretation?� I could not agree more. The identification of specific time surfaces is always very difficult and subjective and is rarely reproducible. Our experiences over the last 100 years in lithostratigraphy and biostratigraphy have clearly shown us that chaos is the end result when you mix chronostratigraphy with material-based stratigraphy. The authors of the stratigraphic codes and guides purposefully separated chronostratigraphic practices from the material-based stratigraphic disciplines for very good reasons. Basically, almost all the problems and confusion currently associated with sequence stratigraphy are rooted in the attempted use of chronostratigraphic methods, surfaces and units in sequence stratigraphy. Unfortunately some of the strongest advocates of such a hybrid stratigraphic discipline (chrono-sequence stratigraphy) have little knowledge or understanding of the developmental struggles in stratigraphy which took place over the last 100 years. Then, why not use term �retrogradational system tract� instead of transgressive system tract for time span when basin is starving of sediment. Also, �progradational system tract� instead of regressive system tract for time span when accommodation space gets smaller because of great sediment income as well as base level fall. There are two reasons for preferring Transgressive ST over Retrogradational ST and Regressive ST over Progradational ST. One is that the name �transgressive systems tract� is entrenched in the literature and I don�t think it would be reasonable to change the name of this unit especially given that it is a well defined, material-based unit. Notably Galloway called the TST a Retrogradational ST unit but the name never caught on. Once you accept the name transgressive systems tract then name regressive systems tract becomes unavoidable. The real problem lies in claims that the regressive systems tract can be subdivided into three systems tracts on the basis of the conjecture that two time surfaces can be recognized within the RST. Despite the fact no one has ever been able to recognize these time surfaces on cross sections or in outcrop or offer any reasonable guidance on how one might recognize them, the advocates of chrono-sequence stratigraphy still base their entire sequence stratigraphic classification system on these abstract entities. To me this is a classic case of a model rather than data controlling classification. I would also note that a final phase of progradation of the shelf margin can occur during the initial phase of transgression and the early development of the TST. This does not happen often and it is often localized to areas of high supply. However, because it does happen, the start of progradation and the start of transgression do not always coincide. � Further, Embry (2002) connects SU with MRS via RAV (SR-U / SR-N) instead of MRS-RSmE-SU connection, why, if we deal with maximum regression?� The main problem with an SU-RSME-MRS boundary is that the basinward end of the SU is not co-terminus with the landward end of the RSME. Thus these three surfaces do not form one continuous boundary, a key requirement of a sequence boundary. As discussed in the report, the SU-SR-U-MRS very often do form one continuous boundary and such a combination of surfaces represents one option for a depositional sequence boundary (defined as an SU plus correlative surfaces). Such a boundary basically separates strata deposited during regression below from overlying strata deposited during transgression and thus is a boundary between two different �genetic� deposits. "Embry, Ashton" <AEmbry@nrcan.gc.ca> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Kun�tek Zoran wrote on Jul 19, 2007 at 3:01 AM Dear Colleagues, I have some doubts about usage of chronostratigraphy for sequence stratigraphic interpretation. Wouldn�t it be more appropriate to use geochronology for interpretation, according to no isochronous sequence surfaces? Later some can put it in to relatively rigid chronostratigraphic time span (time table). Next question is connected to Embry (2002) descriptive term, as he mentioned in papers, �maximum regressive surface� (MRS) and two system tracts that Embry proposed TST and RST. As he mentioned MRS is appropriate descriptive term. Than, why not use term �retrogradational system tract� instead of transgressive system tract for time span when basin is starving of sediment. Also, �progradational system tract� instead of regressive system tract for time span when accommodation space gets smaller because of great sediment income as well as base level fall. Further, Embry (2002) connects SU with MRS via RAV (SR-U / SR-N) instead of MRS-RSmE-SU connection, why, if we deal with maximum regression? Sorry for my English, Sincerely Yours BSc Zoran Kun�tek Senior Geologist INA d.d. SD Naftaplin Croatia, 10 000 Zagreb �ubiceva 29 Tell. 00385 1 4592632, Mob. 00385 98 1602965 e-mail: zoran.kunstek@ina.hr ++++++++++++++++++++++++++++++++++++++++++++++ Embry, Ashton wrote Jun 20, 2007 5:44 PM In reply to Alex MacNeil's Comments of June 12 on �Shoehorning Essay� It is nice to see someone commenting on my comments. That is what debate is all about. And it especially helpful to have Alex debating the subtleties of recognizing a BSFR and a CC because that is a key topic in this debate. If we can all agree how one can recognize BSFRs and CCs in our sections just like we do MFSs and MRSs we can all go away happy. Of course, if reasonable criteria are not established for recognizing and correlating these time surfaces, I suspect we will remain at a stalemate with the co-existence of a material-based approach as advocated by ISSC and a mixed time-based/material-based approach as advocated by IWGSS. I am confident that common sense will eventually win the day. MacNeil: �Dr. Embry�s paper on �shoehorning� in sequence stratigraphy focussed on two papers (MacNaughton et al. 2000; MacNeil and Jones 2006) that he considers to be good examples of scientists forcing their research into sequence stratigraphic models.� Embry: I just want to emphasize that the two examples discussed in the Shoehorning Essay were not chosen by me but were offered by the authors as good examples of studies in which BSFRs and CCs were recognized in well exposed strata. I simply commented on the problems associated with such interpretations. I do not know if these studies are representative or are �good� examples of such analysis. MacNeil: �I find his paper to be more an example of why grey literature should be taken with a �grain of salt� Embry: I don�t consider that Shoehorning essay as part of the literature, grey or any other colour. It is simply part of this online debate and I assume no one will ever formally reference anything on this webpage. MacNeil: �I do not know how Embry has come to conclude that deposits above the BSFR automatically correlate to the instantaneous switch from normal to forced regression at the shoreline, but it is an incorrect assumption.� Embry: Alex and I agree that the BSFR is a time surface that separates strata deposited during base level rise below from strata deposited during base level fall above. It is well accepted that the BSFR is a conformity and Catuneanu often refers to it as a �correlative conformity�. This means there is either a very small or no time gap across the surface. If an unconformable surface separates strata deposited during base level rise below from strata deposited during base level fall above then such a surface is referred to as the type of unconformity it is (e.g. MFS unconformity) and not as a BSFR. I would hope everyone accepts these rather straightforward constraints on a BSFR. Given the above, then the BSFR does indeed represent the time surface at the start of base level fall with the proviso there may be an immeasurable short time lag (days?) between the start of fall and the first particle of sediment to hit the sea floor at a given locality. As Posamentier and Morris (2000, p.39) state �this bounding surface (they are referring to the BSFR) has chronostratigraphic significance insofar as it represents the paleogeography at a moment in time� There is no doubt as to what moment in time they are referring to because on the same page they say the BSFR �represents the surface that exists at the time of initiation of sea level fall�. Notably Catuneanu also refers to the BSFR as the �paleo sea floor� at the start of forced regression as mentioned by Alex. I hope Alex will give some careful thought to what a BSFR is and what it represents. It is quite clear that, by definition, it forms at the moment of start base level fall. MacNeil: �the BSFR marks the base of sediments deposited during forced regression.� Embry: Once it is realized that the BSFR is a conformable surface then such a statement logically leads to the conclusion that the BSFR marks the start of base level fall at the shoreline (forced regression). MacNeil �With regards to our second BSFR and the falling stage systems tract at the top of the reef system, Embry states that a reasonable case could be made for the sediments atop of the MFS (that we assigned to the FSST) having been deposited during sea-level rise. No, this is precluded by the stratigraphic relationships, facies associations, and diagenetic fabrics, as explained on pages 349-352 in the paper and shown in Fig. 18,� Embry: There are two problems here. The main one is the surface referred to as a BSFR on the outer shelf is an unconformable MFS and not a BSFR. Marginward, MacNeil and Jones have interpreted a BSFR and show it as a dashed line downlapping onto the MFS (see their fig 18). They have not presented any evidence that at that precise horizon there is a change in sedimentary composition such that one could reasonably interpret the strata immediately below such a surface were deposited during base level rise and the strata immediately above were deposited during fall. Almost all of the arguments on pages 349-352 deal with the deposits on the outer shelf which directly overlie the MFS. These may well be all deposited during fall but the surface at their base is an MFS not a BSFR. The only bona fide attempt at identifying a BSFR is that dashed line on Fig 18 which is underlain by �HST� and its placement is more of a guess than a well supported interpretation. MacNeil: �This paragraph of Embry is completely incorrect � we do show onlapping and base level rise in our figure. If one looks at our figure and uses the water-table (WT) line as a datum, our sea level position in part C is LOWER than in part D. It is a subtle difference in position� Embry: I got out my magnifying glass and my calipers and indeed Alex is right, M+J do show a very small rise in sea level (a metre or two at best) between stage C and stage D. It is not easy to see on their diagram that the marine carbonate sediments are onlapping the basinward end of the unconformity which must be a SR-U, given that marine strata overlie it. I will accept their interpretation that some marine carbonates onlap the very end of the unconformity although I�d feel better if this contact was described in detail and it was demonstrated beyond a reasonable doubt that the strata underlying the onlapping strata had been subaerially exposed. Regardless, this interpretation has no bearing on their interpretation that the base of the Biohermal interval is a CC; that is, the time surface formed at the start of base level rise. Such a thin interval of onlapping strata could easily be equivalent to only the uppermost portion of the shallowing biohermal strata and could even be reasonably interpreted as being correlative with the TST interval at the top of the biohermal interval. The important consideration here is MacNeil and Jones� interpretation that carbonate sedimentation greatly increased at the start of base level rise (started at the CC). This is not sedimentologically reasonable. At the start of sea level fall the outer shelf was rather deep and was not the site of much in situ carbonate deposition (thin biostromal unit). As sea level fell it eventually resulted in the outer shelf becoming shallow enough to become a carbonate factory and this is marked by the initiation of the biohermal unit. It is not reasonable to interpret, as M+J do, that sea level fell exactly to the critical water depth to initiate a carbonate factory on the outer shelf and then IMMEDIATELY started to rise. It is likely it kept falling during part or all of the biohermal growth and then eventually started to rise, probably very near the top of the biohermal unit. My experience with initial base level rise in carbonates is that it is often very fast and the start of base level rise coincides with transgression. Such a �standard� scenario is easily accommodated in the stratigraphic relationships illustrated by M+J by correlating the onlapping strata with the TST (see M+J fig 18 for how easily this can done). There is no justification for interpreting the base of the biostromal unit (or any other specific horizon within it) as start rise. A better (simpler) case can be made for start rise at start transgression but even this interpretation is somewhat speculative. The bottom line is it is impossible to identify the horizon that marks the base of base level rise with any semblance of objectivity and any attempt to do so is simply shoehorning. MacNeil: I maintain that MacNeil and Jones (2006) provides a case study of the BSFR in outcrop and core, significantly undermining arguments made in the ISSC paper. The �shoehorning� argued for by Embry is based on his own definitions, and as outlined above, misrepresentation of the material presented in MacNeil and Jones (2006). Embry: I would maintain that M+J provides a good example of shoehorning. Alex felt he had to identify a BSFR and CC (Alex was taught sequence stratigraphy by Catuneanu at U of A) and thus chose surfaces he could see and labeled them as a BSFR and a CC. Unfortunately, as discussed herein and in the Shoehorning essay, such surfaces are not reasonable representations of the time surfaces at start BL fall and start BL rise on the basis of either empirical or theoretical considerations. If he had not been instructed to find such surfaces in order to recognize a HST, FSST and LST, I doubt if he would have made such interpretations. Such interpretations are forced on the data by the interpreter thinking that he/she needs to follow the prevailing mixed time-based/material-based approach (trying to see the unseeable). If Alex had used the material-based approach, as is done in all other stratigraphic disciplines, he would not have felt compelled to label (shoehorn) various facies changes or unconformities as time surfaces (BSFRs and CC). "Embry, Ashton" <AEmbry@nrcan.gc.ca> +++++++++++++++++++++++++++++++++++++++++++++++++++ Embry, Ashton wrote Jun 20 2007 1:59 PM in reply to Mark Smith Dear Mark, I just read your comments on the debate website. I enjoyed your talk at the CSPG conference but did not get the chance to speak with you. I think we can profitably exchange views. I would be interested to hear more about the surface you have interpreted as a BSFR and a picture or two and even a diagrammatic X section would be helpful for a discussion. First of all you have decide if the recognized surface is an unconformity (significant time gap across it) or is conformable (little to no time gap across it). If the surface is conformable, you have to ask yourself "Is it reasonable to interpret this conformable facies change as a time surface which formed at the moment (and I stress moment) base level began to fall?". If the surface is an unconformity, you have to ask how such an unconformity was created and whether or not it is reasonable to interpret that all strata below the surface formed during base level rise and all strata above were deposited during base level fall. As long as you can defend your interpretations with data and solid theoretical reasoning then the sharks won't have a feeding frenzy on a BSFR interpretation . You are wise to try and get some clarification at this stage of research. There is a great deal of extraneous material and innumerable, very suspect interpretations in the sequence stratigraphic literature and it is not easy for students to know what is good and what is not good. Best regards, Ashton Embry "Embry, Ashton" <AEmbry@nrcan.gc.ca> ++++++++++++++++++++++++++++++++++++++++++++++++ On Jun 18, 2007 1:27 PM Mark D Smith wrote As a student who plans on publishing his work to the scientific community in the near future, I am finding the recent debate on sequence stratigraphy, specifically the basal surface of forced regression of great interest. Unfortunately, also being a student makes me a little hesitant to jump into the waters with the sharks and so I will keep this communication brief, and if people on either side are interested I can elaborate further. My research examines Neoproterozoic deep-water (slope and basin facies) sedimentary rocks (Old Fort Point Formation) in the southern Canadian Cordillera and I recently presented some of the material at the 2007 AAPG and CSPG. Although the MRS and MFS can be identified, they are difficult to pinpoint where the exact surface is in outcrop (more like zones rather than surfaces). However, the BSFR (as I have interpreted it) is a sharp contact, easily recognizable in the field, possesses different facies (members of the OFP) above and below the delineated surface and can be correlated across the Windermere basin (>35,000 km2). The surface empirically exists, whether a label is attached to it or not. For those in the Alberta, Canada region, this surface can be easily viewed at the Lake Louise Ski Hill or along the Yellowhead highway about 14 km west of Jasper, Alberta. I have numerous photos, sections and thoughts on how this surface is expressed in my research (not neccessarily sequence stratigraphy overall) and would be interested to know what people think prior to submitting my work for publication. Thank you for your time. Mark... Mark D. Smith Department of Earth Sciences Marion Hall, 140 Louis Pasteur University of Ottawa Ottawa, ON K1N 6N5 Ph: 613.562.5800 x6729 <msmit074@uottawa.ca> "Look at me, I'm a grad student. I'm 30 years old and made 600 dollars last year." - Bart Simpson ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On June 12, 2007 at 2:39 pm Alex MacNeil wrote Reply to �Shoehorning in Sequence Stratigraphy� Dr. Embry�s paper on �shoehorning� in sequence stratigraphy focussed on two papers (MacNaughton et al. 2000; MacNeil and Jones 2006) that he considers to be good examples of scientists forcing their research into sequence stratigraphic models. As the primary author of MacNeil and Jones (2006), I find his paper to be more an example of why grey literature should be taken with a �grain of salt�, and not generally referenced in primary research articles. I feel that he has misrepresented and ignored important parts of MacNeil and Jones (2006), and distorted definitions to create/justify his arguments. While I offer my perspectives, I encourage everyone concerned with these issues to read the references and draw their own conclusions. 1. In our paper, we identify two BSFR�s in a Late Devonian succession. All of Embry�s arguments challenging our recognition of the surface are based on his definition of a BSFR, which is not the definition found in the literature. The BSFR was defined by Hunt and Tucker (1992) as: �a chronostratigraphic surface separating older sediments of the preceding highstand systems tract, deposited during slowing rates of relative sea-level rise and stillstand, from younger sediments, deposited during the base-level fall� (pg. 5; my italics added). From additional discussion in Hunt and Tucker (1992) it is clear that recognition of a BSFR does not imply that overlying deposits were deposited the instant base level started to fall � the definition otherwise applied by Embry. Furthermore, Hunt and Tucker (1995) stated that the BSFR marks the �onset of falling sea-level as recorded in the rock record� (pg. 149). I do not know how Embry has come to conclude that deposits above the BSFR automatically correlate to the instantaneous switch from normal to forced regression at the shoreline, but it is an incorrect assumption. Catuneanu (2002) was referenced in our paper as providing the definition for the BSFR. Catuneanu (2002) states, �The basal surface of forced regression was introduced by Hunt and Tucker (1992) to define the base of all deposits that accumulate in the marine environment during the forced regression at the shoreline� (p. 16). He goes on to state that the surface �represents the paleo-sea floor at the onset of base level fall at the shoreline�. This statement does not imply or require that deposits above the BSFR correspond to the instant that base-level started to fall, and we do not accept this assumption being applied to our study. Indeed, on pg. 340 in our paper, we state that a lag period existed between exposure of Reef Complex #1 and initiation of deposits that form the FSST � a statement ignored by Embry. Simply stating the work of Hunt and Tucker and Catuneanu, the BSFR marks the base of sediments deposited during forced regression. Yes, the surface is diachronous, but its recognition allows any geologist to walk up to the outcrop and understand that rocks above the surface were deposited at some point during the base-level fall, and are temporally distinct from underlying rocks (of deeper water origin) that belong to the preceding highstand systems tract. This can have important implications for understanding controls on the carbonate sediments that accumulated and how the system evolved. 2. With regards to our second BSFR and the falling stage systems tract at the top of the reef system, Embry states that a reasonable case could be made for the sediments atop of the MFS (that we assigned to the FSST) having been deposited during sea-level rise. No, this is precluded by the stratigraphic relationships, facies associations, and diagenetic fabrics, as explained on pages 349-352 in the paper and shown in Fig. 18, which Embry fails to mention. This discussion includes reference to recent analogs in the Bahamas and Cayman Islands. 3. On page 7 of the �Shoehorning� paper, Embry states, �Notably there is NO indication of base level rise during the deposition of the biohermal deposits of the outer ramp. This is clearly indicated on the MacNeil and Jones diagram which shows NO onlap on the unconformity during the deposition of the biohermal sediments. Also note that their sea level position is at exactly the same level in the Phase D section as it is in Phase C section despite their claim of base level rise during this time. Thus, by way of their diagram, even MacNeil and Jones acknowledge that base level was not rising during the deposition of the biohermal sediment.� This paragraph of Embry is completely incorrect � we do show onlapping and base level rise in our figure. If one looks at our figure and uses the water-table (WT) line as a datum, our sea level position in part C is LOWER than in part D. It is a subtle difference in position only because 1) we placed its lower position using rather conservative field evidence, as discussed on pg. 335, 2) the large scale of the figure, and 3) the figure was originally drafted at a full tabloid size and scaled-down to letter size in the publication process. The yellow and light-yellow facies at the left hand side of Reef Complex #2 are onlapping reef-front and fore-slope deposits of Reef Complex #1. This was discussed in the paper and figured close-up in Figure 13, which was titled �Deposits of Reef Complex #2 onlapping margin of Reef Complex #1� � but this was not mentioned in Embry�s paper.... The reef-front of Reef Complex #1 is onlapped by restricted lagoon deposits of Reef Complex #2, resulting in a clear juxtaposition of shallow water facies against older, deep water facies. Even Esther Jamieson, in her original work at this locality for her PhD thesis in the 1960s, which was only based on weathered outcrop (not the nice roadcut that we had), noted (Fig. 9 in her thesis and page 30 in the text) that there are facies discrepancies at this locality that are not �normal�. Even though she failed to recognize the significance of the facies relationships, it was obvious that a significant stratigraphic feature is present. To state that we show �no onlap� is incorrect, and to not acknowledge the text discussions about the onlapping succession, or the corresponding, full-page figure that details the onlapping, blatantly misrepresents our work. I maintain that MacNeil and Jones (2006) provides a case study of the BSFR in outcrop and core, significantly undermining arguments made in the ISSC paper. The �shoehorning� argued for by Embry is based on his own definitions, and as outlined above, misrepresentation of the material presented in MacNeil and Jones (2006). Sincerely, Alex MacNeil <alex.j.macneil@esso.ca> +++++++++++++++++++++++++++++++++++++++++++++ On Jun 8, 2007 at 12.00 pm Ashton Embry wrote John, Thank you for your very thoughtful and clearly stated comments. Some of your points were emphasized in the ISSC report and I would say we are in agreement on most of the points you make. I think there is some misunderstanding on a few points and I thought it would be worthwhile if I clarified them so that everyone is clear on what was actually recommended in the ISSC Report. "Certainly insertion of definitions into the code that use time or genetic origin as an interpretive element in defining units or boundaries would indeed be a violation of the spirit of either the North American or International stratigraphic codes". In general this is correct with one caveat. Using time as the interpretive element is fine for chronostratigraphy. Chronostratigraphy was introduced specifically in the 1940s to take time concepts and time surfaces/units out of the material-based disciplines, which now would include sequence stratigraphy, and gather them up into one discipline. Chronostratigraphy, as you rightly note, depends on the integration of correlations of material-based horizons/boundaries/surfaces. Basically one evaluates the diachroneity of all available material-based boundaries from the five disciplines and makes the best guestimate of the location of time surfaces. You can't actually see a time surface (it has no physical characteristics), only infer its position. Notably, each of the five material-based disciplines includes surfaces with low diachroneity and surfaces with high diachroneity. The trick is to use only those with low diachroneity (and time barriers of sequence stratigraphy) for interpreting a local chronostratigraphic framework. The available biostrat boundaries allow one to correlate the local framework to the global time scale. "We are all well familiar with the idea that we are not really mapping time-lines but are actually mapping first and last appearance of fossils in the rock column." In this case one would be mapping biostratigraphic boundaries and not time surfaces. Such boundaries often have low diachroneity so are great for interpreting time relationships by means of compensating for the diachroneity. I think it is crucial to keep the concept of a time surface completely separate from a material-based surface which essentially is never a time surface no matter how much we would like it to be. "Similarly, we all know that maximum flooding surfaces, regressive-surfaces of erosion, flooding surfaces, etc. are not recording truly synchronous events, and thus do not record absolute time-lines. We do, however, know that each of these has some time-significance." You are right that various surfaces recognized in sequence stratigraphy are either (approximate) time barriers or have low diachroniety and, as emphasized in the ISSC Report, such surfaces are most valuable for correlation, framework building and unit definition "Likewise, it seems perfectly fair, and of broad practice across all fields of stratigraphy, to use time-based techniques to hone in on a physical bounding characteristics (e.g. using biostratigraphy to help trace a correlative conformity through units where the surface is otherwise difficult to find and distinguish physically from various surface candidates, choosing one potential unit over a similar unit for lithostratigraphic correlation because its age is clearly more consistent with correlation than the alternative unit, etc.) ". This is exactly what is done in sequence stratigraphy as was discussed in the ISSC Report. One MFS can look exactly like another and correlation lines from other material-based disciplines (bio, mag, chemo, litho) can be most useful in guiding the matching of specific sequence stratigraphic surfaces. I might point out here that it is not acceptable to extend a sequence stratigraphic surface (the boundary of a sequence stratigraphic unit) such as an MFS along a biostratigraphic or magnetostratigraphic boundary just as we can't extend a biostratigraphic boundary (e.g. a FAD) along an MFS. A specific MFS can only be extended by correlation to MFSs at other localities with boundaries from the other disciplines sometimes guiding such correlation. "What would likely be a stretch is to define boundaries by specific point in time, on the premise that this time records a specific event like maximum transgression, particularly in applying this where there is no physical criterion or discrete surface to map. This would not be against the rules either, but it would be chronostratigraphy rather than sequence stratigraphy, which is already codified. Why bother?" Exactly!! This is chronostratigraphy and is NOT sequence stratigraphy. Thus it would be against the rules in sequence stratigraphy but completely within the rules for chronostratigraphy. As the authors of the Code and Guide emphasized, it is most important to not mix the two very different stratigraphic methods/boundaries/units. This is the basic reason why the ISSC Report recommended NOT to use the BSFR and CC which are time surfaces, for which, in John's words, "there is no physical criterion or discrete surface to map.". We could not have expressed it better!! "Most all of us are actually mapping physical surfaces with physical criteria when doing sequence stratigraphy and deliberately choosing surfaces because we know they will have time and/or genetic significance. There isn�t a thing wrong with that approach." I agree there is absolutely nothing wrong with this approach and this is exactly what was recommended and emphasized in the ISSC Report. Mapping physical surface on the basis of their characteristic physical criteria is the empirical approach we have strenuously argued for. I would emphasize that John's critical phrase is "mapping physical surfaces with physical criteria ". The physical criteria that characterize a given surface allow it to be recognized at various localities and make it available for correlation. "It�s pretty clear that the masses do not particularly like the Ashton choices." I think everyone feels fine about the utility of the SU, SR, MRS, MFS and RSME which are the surfaces the ISSC TG has recognized as valid surfaces of sequence stratigraphy and recommend for usage in sequence stratigraphic unit building. What the masses don't seem to like is our "choice" to interpret that the BSFR and CC are NOT valid surfaces of sequence stratigraphy because no one has ever presented any physical criteria to allow them to be recognized in outcrop or core. Furthermore, given that they are defined as time surfaces which are independent of sedimentation (paleo-sea floors in the IWGSS report), such surfaces meet all the criteria for being chronostratigraphic surfaces. "Bottom line is most all the surfaces discussed so far are valid surfaces within the context of that which they are meant to define." Given that you have not discussed a BSFR and a CC, I would agree with this. However the debate/ disagreement between the ISSC TG and IWGSS boils down to whether or not the BSFR and CC are material-based surfaces of sequence stratigraphy or are time-based surfaces of chronostratigraphy. It simply comes down to, are there or are there not adequate physical criteria for recognizing such surfaces in well exposed strata in most geological settings. The five agreed upon surfaces meet this requirement. I have recognized and correlated thousands of such surfaces and published cross sections illustrating them. So has Octavian for that matter. MacNaughton et al and MacNeil and Jones (and thousands of other authors) have also had no problem with delineating and correlating acceptable MFSs, MRSs, SRs and SUs. However, as discussed in the Shoehorning essay, they could not delineate and correlate a single, acceptable (meets the definition of the surface) BSFR or CC . Notably no one else has ever accomplished such a feat including Octavian. Of course many have used high diachroniety facies surfaces as shoehorned compromises which lead to not-so-great, non-actualistic interpretations. The fact that no one has been able to recognize such surfaces in rocks underscores the lack of physical criteria required to have them accepted as bona fide, material-based surfaces of sequence stratigraphy as discussed in the ISSC Report and by John in his earlier comment. The best anyone has done is to "tag" a seismic reflector as such a surface. I would emphasize such seismic interpretations are highly equivocal and do not constitute proof of existence of a discrete, specific surface type. Such seismic tagging is simply another form of shoehorning and Octavian�s favourite seismic section, which is used repeatedly in his text book, is a classic example of this. "While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary." Actually you might do well to begrudge Henry such a sequence boundary simply because it has no physical characteristics which allow it to be recognized and correlated. Even Henry acknowledges such a lack of physical attributes. I think it is most important to understand that Henry Posamentier sees sequence stratigraphy as equivalent to chronostratigraphy. Here are a few quotes from Henry: �The essence of sequence stratigraphy involves the recognition of time synchronous surfaces�; �The correlative conformity represents a surface that exists at the time of initiation of sea-level fall�; �The correlative conformity may have no obvious diagnostic attributes�. Henry and Octavian represent the view that sequence stratigraphy includes chronostratigraphy and that is the basic differences between their mixed time-based/material-based approach and the material-based approach of the ISSC TG. Given that you appear to be on the material-based only side of the street, you might want to more closely examine what Henry proposes as a sequence boundary. "That said, the choice of sequence-bounding surface seem to be the most contentious. There are issues with choosing any of the three/four favorites in the larger debate over the best fundamental sequence-bounding surface. While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary, I would also say that forcing me to abide by this as the true sequence boundary would render sequence stratigraphy pretty much useless to me, owing to my work being mostly within continental-interior basins." Here is where I think there is some significant misunderstanding of the recommendations of the ISSC Report. The Report emphatically recommends that a sequence/ sequence boundary be completely generic and that workers are free (encouraged) to define as many specific types of sequence boundaries as their data allow. The generic definition offered is that a sequence boundary is a specific type of unconformity and its correlative (i.e. physically join together) surfaces. Doesn�t get any simpler or flexible than this! Two common types of sequence boundaries were discussed in the Report and the definitions of these two types were taken straight from the literature (Van Wagoner et al. 1988 and Galloway, 1989). One was a depositional sequence boundary defined as a subaerial unconformity and its correlative surfaces. The other was an R-T or genetic stratigraphic sequence boundary defined as an unconformable MFS and its correlative surfaces. It was further noted that other types of sequence boundaries could be defined on the basis of other specific types of unconformities and their correlative surfaces. Also, it is even possible to have different sub-types of a given type of sequence boundary. For example we noted that very often a MRS and SR-U are correlative (join together) with an SU and thus represent a one possible combination of surfaces for a bona fide depositional boundary. If other workers demonstrate that other types of material-based surfaces correlate to an SU, they could establish other combinations for a depositional sequence boundary. We believe it would be inflexible if only one specific combination of correlative surfaces were allowed for a given type of sequence boundary as advocated by Catuneanu (2006) and the IWGSS report. It is the specific unconformity type which defines a given type of sequence boundary NOT the correlative surfaces. We believe this is about as flexible as it gets and certainly matches/exceeds the flexibility of the IWGSS proposal. "Codifying any of these surfaces as the �true bounding surface� just means that most of the remaining sequence-stratigraphic community will simply ignore the code." No argument here and I hope you agree that it is worthwhile to try to reach consensus on specific names for specific types of boundaries and units to help communication. We suggested commonly used names for the two types of sequence boundaries most often employed in sequence stratigraphy. We leave it to others to name other types of sequence boundaries. "Likewise, the other forms of stratigraphy don�t demand such unique criteria for defining unit boundaries. There are at least seven types of fossil combinations that can be used to define biozones in the North American code." As was noted in the ISSC Report, we used the biozone concept as the guide to how a sequence can be effectively defined. In both cases the definitions are generic and modifiers allow specific types to be defined - specific types of fossil change (e.g. assemblage biozone) or unconformity types (depositional sequence). As mentioned earlier, this system allows new types of biozones/sequences to be defined as required. "The individual worker is given enormous latitude as to what can be lumped into a lithostratigraphic unit or with regard to which rock layer is to be picked as the last placed within the unit. Why are we choosing to be so picky with sequence stratigraphy?" I hope you can now see we are not being picky but are being extremely flexible. All we ask is that material-based surfaces be used as they are in every other stratigraphic discipline except chronostratigraphy. Unfortunately a few of our most vociferous detractors have completely misrepresented the ISSC recommendations in various posts so I can understand why John had a wrong and negative impression of our recommendations for defining sequence boundaries. "While it has difficulties too, a more inclusive approach like that favored by Octavian." I would note that Octavian's approach is less inclusive. It does not offer a generic definition for a sequence. It does not allow different sub-types for a given sequence boundary. The biggest problem of course is that Octavian encourages the use of featureless time surfaces for the definition of different types of sequence boundaries. This last problem of course leads to all the problems discussed in the Shoehorning essay and is the source of the �difficulties� that John alludes to. I would again reiterate it is hard to imagine how one could come up with a more flexible and pragmatic approach than that recommended in the ISSC Report. A worker can mix and match as he/she pleases with the only caveat being that all surfaces have to be material-based. " I can think of no processes that demands shoehorning more than codification." There is no doubt that guidance or a codification in an official publication of an international stratigraphic commission could potentially lead to major shoehorning IF the guidance/codification is based on a non-material entity. It is critical to note that main problem which leads to shoehorning is not where the recommendation is but the nature of the recommendation. Once you recommend a non-material surface for a boundary, people are going to have to use "compromise" material-based surfaces to compensate. Thus, even though neither NACSN or ISSC have not offered any recommendations on sequence stratigraphy, there is MAJOR shoehorning going on simply because a couple of abstract time surfaces have become entrenched in the literature and textbooks on sequence stratigraphy. Sedimentary geologists are pressured to identify these entrenched, abstract surfaces (peer pressure) and, as discussed in my recent essay, this has lead to a lot of most unfortunate shoehorning and non-actualistic interpretations of sedimentary processes and depositional histories. "Ulrich-ism" reigns supreme right now with many people interpreting diachronous facies contacts as time surfaces. "The moment we codify any form of sequence stratigraphy, we are going to be attempting to set the demand that everyone from that day forth shoehorn into the decided code." Not really. As long as we recommend, material-based surfaces to define units there will be no need to shoehorn. I would note that there is no noticeable shoehorning in the other material-based stratigraphic disciplines. When the time surfaces are eliminated, everyone will be able to recognize the accepted surfaces and, in most instances, there will be agreement on their placement. I am sure, if I took John, Octavian and Ron up one of my sections, we would all agree on the approximate placement of the MRS, MFS, SU and SR-U. We might haggle over a few tens of centimetres for the exact placements and such is interpretation. The same goes for a well log section although there may be some slightly larger variation in placement choices due to the lower resolution. However we could still have a good discussion as to which picked surface was the best one. Most importantly, none of us would be able to point to a BSFR or a CC in the best exposed section even though such time surfaces would be there in theory. Each of us might shoehorn such surfaces in at very different horizons and there would be no way to discuss which is a better shoehorned pick. And that is the basic difference between the empirical surfaces which need no shoehorning and the unrecognizable time surfaces which do. "By the way, I noticed Octavian�s e-mail has somehow fallen from the mailing list." Good catch. It would be nice if Octavian could supply a few examples of where he has identified and correlated BSFRs and CCs in rocks (outcrop, logs) so we get beyond the �do as I say, not as I do� problem. He certainly has published lots of cross sections with MFSs, MRSs. SRs and SUs on them. The fact that his book lacks a single stratigraphic cross section which illustrates the delineation and correlation of BSFRs and CCs is telling. Ashton Embry: AEmbry@nrcan.gc.ca +++++++++++++++++++++++++++++++++++++++++++++++++++ On Jun 8, 2007 at 1:07pm John M Holbrook wrote Chris, Please to feel free to post these comments and the Ashton response. My comment pertained mostly to the e-mail exchanges, but Ashton points out that other people are involved in the debate who are not on the e-mail listing. Please to shift these on over. Thanks, John Folks: I have been following the sequence-stratigraphic debate between Aston and what appears to be the sequence-stratigraphic world at large with great interest. I would like to humbly offer some comments regarding points that seem to have emerged from the debate. 1) Aston does have a valid point. The stratigraphic code is built on the core principle that stratigraphic units should be defined based solely on �objective� and �reproducible� descriptive criteria. Each form of stratigraphy is defined based on the criteria that are used to define boundaries/units. The main issue with codification of sequence stratigraphy from the beginning has been that we are accused of imposing interpretive criteria for defining key units and boundaries. Certainly insertion of definitions into the code that use time or genetic origin as an interpretive element in defining units or boundaries would indeed be a violation of the spirit of either the North American or International stratigraphic codes. 2) Time as the force behind defining criteria While it might be bad form to use a time-interpretation as criteria do define a boundary, it is perfectly legitimate to define a boundary based on observable criteria that has time significance. Case in point, biostratigraphy. We all recognize this as the most fundamental means of defining �time-rock� units, and commonly define time-lines in the rock record using this method. We are all well familiar with the idea that we are not really mapping time-lines but are actually mapping first and last appearance of fossils in the rock column. We also know enough about evolution to know that species dispersion and extinction are diachronus events. Yet, we have long felt comfortable basing our original concept of geologic time, and the fundamental process of mapping time-rock units, based on biostratigraphic criteria. Similarly, we all know that maximum flooding surfaces, regressive-surfaces of erosion, flooding surfaces, etc. are not recording truly synchronous events, and thus do not record absolute time-lines. We do, however, know that each of these has some time-significance. It is perfectly legitimate to deliberately choose to map these surfaces because of the expectation that they will yield time/event-significant information. This is indeed one of the more powerful aspects of sequence stratigraphy. Likewise, it seems perfectly fair, and of broad practice across all fields of stratigraphy, to use time-based techniques to hone in on a physical bounding characteristics (e.g. using biostratigraphy to help trace a correlative conformity through units where the surface is otherwise difficult to find and distinguish physically from various surface candidates, choosing one potential unit over a similar unit for lithostratigraphic correlation because its age is clearly more consistent with correlation than the alternative unit, etc.). What would likely be a stretch is to define boundaries by specific point in time, on the premise that this time records a specific event like maximum transgression, particularly in applying this where there is no physical criterian or discrete surface to map. This would not be against the rules either, but it would be chronostratigraphy rather than sequence stratigraphy, which is already codified. Why bother. In short, mixing time and physical criteria to define units is far from unprecedented; it is in fact the norm. What we are charged to do, however, is clearly define the physical criteria we use to define boundaries, and state the time significance of this choice separately. Though we commonly appear to �define� boundaries by such time/event criteria as first episode of relative sea-level drop, most all of us are actually defining boundaries by such physically mapable criteria as landwardmost surface of downstep in coastal onlap. While it isn�t so hard to find a person who violates from time to time, most all of us are actually mapping physical surfaces with physical criteria when doing sequence stratigraphy and deliberately choosing surfaces because we know they will have time and/or genetic significance. There isn�t a thing wrong with that approach. 3) Surfaces surfaces everywhere! Most of the disagreements seem to stem from the fact that we each prefer to map different surfaces from a wide selection of possible surfaces. This is usually either because these are the surfaces that are easiest to map given the data and settings in which we tend to work, or because these are the surfaces that yield the information we prefer to know. It�s pretty clear that the masses do not particularly like the Ashton choices. While I�m thinking I might find more support for my choices, I would hardly expect them to be universally accepted as the pure surfaces either. Bottom line is most all the surfaces discussed so far are valid surfaces within the context of that which they are meant to define. That�s why we like to use them. That said, the choice of sequence-bounding surface seem to be the most contentious. There are issues with choosing any of the three/four favorites in the larger debate over the best fundamental sequence-bounding surface. While I would not begrudge Henry his favored choice of the first down-step as the sequence boundary, I would also say that forcing me to abide by this as the true sequence boundary would render sequence stratigraphy pretty much useless to me, owing to my work being mostly within continental-interior basins. Codifying any of these surfaces as the �true bounding surface� just means that most of the remaining sequence-stratigraphic community will simply ignore the code. This is destine to generate a lot of confusion. Likewise, the other forms of stratigraphy don�t demand such unique criteria for defining unit boundaries. There are at least seven types of fossil combinations that can be used to define biozones in the North American code. The individual worker is given enormous latitude as to what can be lumped into a lithostratigrahic unit or with regard to which rock layer is to be picked as the last placed within the unit. Why are we choosing to be so picky with sequence stratigraphy? While it has difficulties too, a more inclusive approach like that favored by Octavian makes a certain since if we are to bother with codification at all. 4) Shoehorning I can think of no processes that demands shoehorning more than codification. The moment we codify any form of sequence stratigraphy, we are going to be attempting to set the demand that everyone from that day forth shoehorn into the decided code. How would science have been served by codifying Vail, et al 1977? By the way, I noticed Octavian�s e-mail has somehow fallen from the mailing list. I added it back in my listing. Cheers, John "Holbrook, John M" <holbrook@uta.edu> +++++++++++++++++++++++++++++++++++++++++ On June 6th 2007 William Helland-Hansen wrote Guidelines for Sequence Stratigraphy � Comment to the ongoing debate Dear Sequence Stratigraphers There are a few points that I feel is important to communicate after having read most of the ongoing dialogue on the formalization of Sequence Stratigraphy: Without rephrasing the definition of sequence stratigraphy, this discipline basically deals with unconformities (with an unconformity defined as �a surface of erosion or nondeposition that separates younger strata from older rocks and represents a significant hiatus�, Mitchum, 1977) and the sedimentary deposits between the unconformities � all put together in a genetic and chronostratigraphic framework. Unconformities include levels of erosion, non-deposition and very slow deposition that can mark facies-belt jumps - or major environmental shifts - across surfaces or condenced intervals. They are caused by base-level changes and brought about most commonly by allogenic forcing mechanisms, but also by large scale autogenic forcing mechanisms (e.g. delta-lobe shifting). Deposits between unconformities consist of sediment and minor breaks (diastems) (e.g. common channel erosion, base of storm bed, base of turbidite etc) not caused by base-level changes (Embrys WTFC). These between-unconformity successions can be analyzed in terms of Walther�s law and are deposited in a response to a particular trend of environmental change � a migration trajectory of the depositional system. The systems tract concept, as currently applied, should be avoided in formalization attempts because it anticipates a succession in depositional trends. For example: 1) Vertically expanded successions of alternating regressive and transgressive strata without intervening base-level falls and 2) laterally expanded successions of alternating normal and forced regressions, without intervening transgressions, are both common in the stratigraphic record. These situations are not included in the proposed schemes. Furthermore, neither of the drafts from the two groups propose how the stratigraphic problems of sequences delineation in continental and deep-marine successions should be tackled. Formalization, if any, should be on the �tool box� level (cf. earlier comment by Ole Martinsen) � without any pre-conceived succession of sedimentary trends and tracts: What kind of breaks do we have, and what kind of trends do we have between the breaks? If we can really define all these as �concrete� � without forcing successions of events or trends into one universal template � then we may be a step further. Meanwhile, the concept of allostratigraphy plus the original definition by Mitchum 1977 (�.. a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities�) seem to work just fine. May June 6th 2007 William Helland-Hansen, University of Bergen E-mail <William.Helland-Hansen@geo.uib.no> ++++++++++++++++++++++++++++++++++++++ On June 3rd 2007 Ashton Embry wrote: Reply to Alex MacNeil It was a very nice surprise to see that our debate attracted comments from a member of Gen Y, the generation that represents the future of our discipline. It was also very encouraging that Alex addressed the main issue of the debate in his incisive comments and I thank him for reminding me of another example of an attempted application of the seven-surface/four systems tract approach to a real world succession. Such real world examples are completely lacking in Catuneanu (2006) and the draft IWGSS report and they are undoubtedly the acid tests for the acceptance or rejection of such an approach. Below are replies to some of Alex�s specific comments. �In several parts of the ISSC draft paper (e.g., page 44), the authors assert (rather strongly) that no one has ever presented any defining characteristics of a BSFR or demonstrated such a surface in core or outcrop. Before making such strong assertions, the authors should have taken the time to familiarize themselves with the recent literature on sequence stratigraphy. The MacNeil and Jones (2006) paper in Sedimentology documented the BSFR in outcrop and core, and undermines some of the key arguments made in the ISSC paper.� Actually we make the point that no one has ever presented any suitable/acceptable defining characteristics for a BSFR or a CC. Many people have used various types of real stratigraphic surfaces that have observable characteristics as �compromises� for the two time surfaces. However, in every case we looked at, such �compromise surfaces� were not time surfaces (paleo sea floors) or even reasonable approximations. I may be slowing down in regards to remaining current with the sequence stratigraphic literature but I try to read as many papers as I can. I hope Alex will be happy to hear that I read his paper soon after it was published. I cut my stratigraphic teeth on Devonian reefal carbonates almost 40 years ago you never forget your first love. I downloaded the MacNeil and Jones sequence stratigraphy paper as soon as I became aware of it. I was flattered that they referenced my very old papers from the early 70s and even used those less than euphonious, lithologic terms (e.g. bindstone) I dreamed up while lying in a hospital bed in 1969. As is discussed in the �Shoehorning and its Consequences� post, I found that the surfaces MacNeil and Jones labeled as BSFR (2 surfaces) and CC (1 surface) do NOT fit the definition of a BSFR and a CC. The compromise surfaces labeled as BSFR and CC are an unacceptable attempt to recognize such surfaces. I will not discuss the details of these surfaces in this reply and the reader is referred to �Shoehorning and its Consequences� for a detailed discussion of the MacNeil and Jones� attempt to recognize a BSFR and a CC. �Likewise, the biotic composition of the biostrome is consistent with an intermediate water depth (ironically, I can reference some of Embry�s original work from the Arctic on suggested absolute water depths for Devonian carbonate facies to support the intermediate water depth) between deeper water, nearly-drowned carbonates of the TST and HST and shallow water, high-energy carbonates of the ensuing LST.� I am pleased my past work helped Alex in facies interpretation. There is no doubt MacNeil and Jones have identified a nice shallowing upward succession from the basal TST to the top of the high energy carbonates which, as interpreted, are capped by a well documented MRS. The question becomes how one can recognize the time surfaces at the start and end of base level fall in such a shallowing upward succession. MacNeil and Jones have used within trend facies boundaries to do so and, as discussed in the �Shoehorning� post, their rationalizations that such facies changes occur at the instant that base level starts to fall or starts to rise are not convincing. In fact they demand very unlikely and non-actualistic sedimentological processes and events. I can understand why they chose such facies contacts as their best guess as to the placement of the time surfaces because the facies contacts were the only recognizable surfaces present. In other words, these surfaces were the only �compromise surfaces� available to label as a BSFR and a CC. �In carbonate systems with ramp-type geometries, the Basal Surface of Forced Regression can be identified as the surface that separates depth-controlled biofacies, and biofacies controlled or influenced by trophic resources, if the change in base-level shifted nutrient regimes�. Note that sediment shed in the HST has a specific grain size range (for review see MacNeil and Jones 2006) and changes in grain size can be used to constrain the shift from normal to forced regression.� The basic, underlying assumption here is that such changes (e.g. in nutrient regimes, grain size, etc) happen over a large area at the instant when base level starts to fall. Thus it is assumed that such changes mark a paleo-seafloor (time surface) at the start of BL fall. This assumption of very significant changes occurring over the basin at the moment of start base level fall is not reasonable or actualistic. There is no doubt changes occur as base level falls but the preserved record of such changes (i.e. the recognized facies change) varies in time and space. Thus labeling such a facies/ material-based change as recognized at various localities as a time surface is not realistic or acceptable. �Delineation of the BSFR with these empirical criteria is consistent with the type of data that Embry et al. argue (e.g., last paragraph on page 40) can be used to define the MRS and MFS. As such, to argue that there is no empirical data that can be used to identify a BSFR invokes a double standard and is incorrect� This is fundamentally not so. The BSFR is defined as a time surface whereas the empirical surfaces such as the MRS and MRS are defined by observable characteristics. The difference between the two types of definitions is profound. MacNeil and Jones see a change in trend from a shallowing to a deepening in their shallow water carbonate rocks and, as described in the ISSC report, such characteristics are sufficient to delineate an MRS. This is all well and good and I doubt if anyone would dispute their placement of the MRS in this instance. The BSFR is the time surface that marks the beginning of base level fall. MacNeil and Jones have not demonstrated that the observable facies change they observed (at the base of the biostrome) formed at the instant that base level started to fall. As argued in �Shoehorning�, it is reasonably certain that the recognized facies change did NOT form at the moment of the start of base level fall but most likely occurred AFTER base level began to fall. Thus, in contrast to their MRS, their BSFR is highly disputable. The reason for this is there are no noticeable changes in sedimentation over most of the basin at the instant base level starts to fall and consequently there are no reasonable criteria for delineating such a time surface. The bottom line is that MacNeil and Jones have failed to provide �empirical data� that can be used to identify a �BSFR� (the paleo-seafloor at the instant when base level starts to fall). All they have done is to try to rationalize that the facies change they do see formed at the moment that base level started to fall. As noted earlier, such a rationalization �does not hold water�. �It is not very �real-world� to limit sequence stratigraphic interpretations and systems tract definition to whether or not a specific surface is identifiable across a study region � a criterion argued for throughout the ISSC paper.� If you want to delineate a given stratigraphic unit, say a TST in a given sequence, then one has to correlate the boundaries of that unit over the study area. If this cannot be done, such a unit has little to no utility for subsequent facies analysis and reconstructing depositional history. Of course it is recognized that at some localities the data may not be sufficient for the exact placement of a given boundary but this can only be determined if one knows what data are needed to recognize the boundary in the first place. In most cases the placement of the empirical surfaces of sequence stratigraphy can be reliably done at most localities at which strata are well exposed and the work of MacNeil and Jones clearly demonstrates this. Their MFSs and MRSs are reliably delineated in most sections. The same cannot be said for the BSFRs and CC. �Systems tracts are delineated by sequence stratigraphic surfaces, but they are defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations.� This is a common error in understanding. The systems tracts are defined by their boundaries. Their boundaries are �defined by integrated data sets that include surfaces, stratal geometries/stacking patterns, and detailed analysis of facies associations.� It is important to understand the difference between how surfaces are defined and how systems tracts are defined. Data sets yield surfaces and surfaces yield systems tracts. Data sets do not directly yield systems tracts. One needs to go though the surface identification step first. �So yes, if one chooses to limit themselves to sequence stratigraphic surfaces, they may find themselves largely limited to defining T-R sequences. If one chooses, however, to actually include study of the sedimentology and stratigraphy of the deposits between the surfaces, they can conclude with much more advanced interpretations � and therein lies the powerful nature of sequence stratigraphic concepts and methods with significant implications for how we understand strata and advance the science�. We are getting close here. If we stick with surfaces, we can reliably identify an R-T sequence and a depositional sequence as defined in the ISSC report. So Alex is almost right with his first sentence. In his second sentence Alex suggests we can make some more advanced interpretations by examining the strata within the empirical systems tracts, the TST and the RST. There is no argument with this. Thus in the RST we can sometimes say these specific strata were deposited under conditions of rising base level and these specific strata were deposited under conditions of falling base level (e.g. they have a RSME at the base and a SU at the top). The critical point that must be stressed is that we CANNOT draw the boundary between rising base level deposits and falling base level deposits with any semblance of objectivity and thus cannot have separate stratigraphic units based on this concept. There will always be a large interval of uncertainty, and any attempt to draw a line between rising and falling base level deposits superimposes a very speculative and unsupportable interpretation on the data. This is the last thing we want to do. �I can not imagine that its guidelines, in the current form, will be accepted by the greater research community. In the very least, they need to correct their assertions that the BSFR is purely theoretical and has never been demonstrated in core or outcrop, as this is incorrect.� I like to think most sedimentary geologists are rational, critical thinkers. To me, when one applies critical thinking to Catuneanu (2006) and the IWGSS draft report, the fruitlessness of trying to delineate abstract time surfaces and to use them for unit boundaries becomes readily apparent. It just seems few have ever thought about the problems of such an approach. I hope Alex will do such critical thinking and always ask himself �Can this sedimentological change I want to label as a BSFR be instantaneous over the basin and coincide with the moment of base level fall?�. Based on current data, including the MacNeil and Jones paper, we stand by our �assertions that the BSFR is purely theoretical and has never been realistically demonstrated in core or outcrop�. Summary � I again thank Alex for his very useful and �on topic� contribution to the debate. His work illustrates a critical problem with sequence stratigraphy �forced and inappropriate interpretations of a BSFR and a CC. The interpretations that facies contacts are time surfaces demand the past occurrence of non-actualistic sedimentological processes and events. Ashston Embry <AEmbry@nrcan.gc.ca> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ On June 3rd 2007 Ashton Embry wrote: Reply to Bob Dalrymple These are by far the most comprehensive and insightful comments received so far and they will be of great value for revising our report. Thank you very much Bob for taking the time to be so helpful. Many of Bob�s comments will require only wording or �tone� changes in the report and I will not bother to discuss these. I would add here that such clarifications are very important and I am not trivializing such comments and the changes necessitated by them. It is just that they do not have anything to do with the main aspects of the debate which centre on classifying sequence stratigraphic surfaces and units. Below are comments on some of Bob�s �bigger� points that I think are worth considering in this forum. Bob�s �process sedimentologist� perspective of sequence stratigraphy has been especially enlightening and has helped me understand how the current problems have arisen. I can assure Bob and everyone else on this forum that any well-supported, substantive comments will bring about substantive changes in the Report. ISSC is looking to provide the best recommendations possible. That is why we have circulated the Report as widely as possible. Specific Comments �as the word �science� appears to be used in an extremely loose manner in the ISSC report. As Ashton himself has said, the deductive approach is an acceptable scientific methodology. Furthermore, the proposals that have been made by the deductive approach are definitely testable, one of the hallmarks of science.� I expect Bob is right that we went overboard with the adjective �unscientific� and this will be easily remedied in the revised Report. I think we all agree that deduction is an important part of science. However we enter the area of non-science (junk-science) when one uses a deduced entity, for which no empirical support has been yet demonstrated, as a real entity upon which other entities are then based (classic house of cards). Thus one can deduce almost anything imaginable and examples include a neutralino, a powerful deity, a Martian, and a stratigraphic surface generated at a specific instant in time (as defined by a specific event at a specific geographic location). Science thrives on imaginative deduction. For a deduced entity such as a Martian to enter the world of science, it is crucial to accumulate empirical data which support the existence of such a deduced entity. Thus neutralinos, powerful deities and Martians are NOT scientific entities because we have no empirical evidence to support their existence. A recent headline in our local paper announced that �The Museum of Creation Science� has just opened (only in Alberta you say). Lots of deduced concepts but not much science. I would claim that stratigraphic surfaces formed at the start and end of base level fall are also �Not Yet Scientific Entities� simply because we have no empirical evidence for their existence. If we did, we would not be having this debate and it would be clear how one might recognize a BSFR as we now recognize a MRS or an MFS or any other widely accepted, empirical surface of sequence stratigraphy. Notably, every attempt to delineate and correlate such time surfaces has failed (i.e. the deduced entities are falsified so far). Given this, to use such entities as a basis for defining specific types of stratigraphic units (e.g. FSST), has no scientific support. I will deal with a few attempts to delineate such baseless entities in a separate post on �Shoehorning and its Consequences�. All I will say now is, calling a dog a Martian does not make Martians (as originally deduced) real entities. Thus I would disagree with Bob�s assertion that �Therefore, none of the work being criticized is unscientific�. Searching for empirical data to support the existence of a BSFR is scientific activity. Claiming a BSFR exists in spite of no supporting empirical data and then using it to define a stratigraphic unit is not scientific. � I�m sure that the authors of the ISSC report, despite their claims of being �data driven� empiricists, must make �informed decisions� (i.e., model-driven interpretations) every time they construct stratigraphic sections. What they are attempting to do in this report is to create a highly simplified scheme for erecting sequences within sedimentary basins, so that the amount of interpretation is minimized.� When it comes to correlation and defining stratigraphic units, it is most important to do these activities with �real� surfaces. Once we allow entities that have no empirical support into the mix, we have crossed a major line between science and non-science. We certainly want to create as simple a classification scheme as possible while at the same time honoring the existence of all real sequence stratigraphic surfaces. This debate concerns the realness of the BSFR and CC. If they can be demonstrated to be real, then a seven-surface/four systems tract classification scheme is simplest. If not, a five-surface/ two-systems tract scheme is simplest and best. �If I understand the ISSC Report, its authors have a very restricted purpose behind the proposals that they make, namely to provide a simple and straight-forward, pragmatic means of mapping sequences throughout a sedimentary basin, and, to a lesser degree, to assign an order to each of them.� I see this as the �process sedimentologist� perspective on sequence stratigraphy. To stratigraphers, sequence stratigraphy is fundamentally all about correlation. And correlation entails the recognition of specific types of stratigraphic surfaces and matching them to specific, equivalent surfaces at other localities. Specific types of units �fall out� of such a correlation framework, being bounded by specific types of surfaces. A very important part of the guidance in the International Stratigraphic Guide is the definitions of various surfaces and units which are part of a given discipline. The ISSC Report on Sequence Stratigraphy provides recommendations on what surfaces and units are best to use when doing a sequence stratigraphic analysis. This is the heart of sequence stratigraphy and once such a correlation framework is established then the interpretations of facies relationships and depositional history naturally follow. Such interpretations can be considered a part of sequence stratigraphy but they are completely dependent on the established sequence stratigraphic framework. Bad framework, bad interpretations. We are trying hard to get some logical, well-supported feedback on the recommendations or surfaces and units but as can be seen from many of the replies on this webpage, such science-based feedback is tough to extract. If people do not like the recommendation that a BSFR is not an acceptable sequence stratigraphic surface (it is actually a chronostratigraphic surface) or that a FSST is not an acceptable sequence stratigraphic unit (it is, in part, a chronostratigraphic unit [chronozone]), we would really love to receive a good scientific argument why they are. �The more provocative proposal to use the MRS as the basinward extension of the subaerial unconformity is, from a strictly mapping point of view, perhaps defendable. However, I believe that the authors downplay the diachroneity of this surface� There is little doubt that in many cases the MRS joins the flanking unconformity simply because the basinward portion of most flanking unconformities is an SR-U. In fact this is the situation in almost all carbonate depositional systems (i.e. marine strata overlie the unconformity). Thus, on both empirical and theoretical grounds, the MRS is most often (not always!) a correlative surface of a basin flank unconformity. The diachroneity of the MRS is not a big issue because it is less than that of a within trend facies contact and it varies greatly in absolute terms depending on the order of the given MRS. Miscorrelation of MRSs is also another big problem which can lead to a false impression of excessive diachroniety. There is no doubt that the interpreter has to appraise the diachroneity when interpreting depositional history. �If the RSL lowstand is of longer duration than is argued (on flimsy grounds) in the ISSC report, then the amount of diachroneity of the MRS may be large.� This question of the lag between the times of start base level rise and start transgression is a very important one and requires much more empirical data to resolve it. I have not found any empirical evidence to support an interpretation that such a lag time is long and have found considerable evidence that it is short to basically non-existent. Again for those who disagree with this assertion, we would appreciate receiving empirical data (or references to such) that support their position of a long lag time. As noted in my reply to the IWGSS report, all the references that were used to support the existence of a unit deposited after the start of base level rise but before start transgression, turned out not to include such evidence. �I would suggest that the correlative conformity (i.e., the surface/timeline that truly does extend from the tip of the subaerial unconformity; the MRS is erroneously shown as tying to this position in some of the ISSC figures!) is a better surface on which to base sequences. Embry et al. argue strenuously (both in the ISSC report and in subsequent rebuttals to comments) that this surface is �invisible� (i.e., it is not easily seen in outcrops and cores). While I have not worked a great deal in the relevant deposits, I do have some minor experience (see MacNaughton et al., 2000, Canadian Jour. Earth Sci., 37: 997-1020). In that paper, we were able to identify on objective grounds (grain-size changes as controlled by the trapping, or not, of coarse material further updip in the source-to-sink system; the degree of development of slump and other mass-movement deposits, as a proxy for sedimentation rate; etc.), the boundaries of the four systems tracts that have been proposed by recent sequence-stratigraphic schemes.� It is most encouraging to have a member of IWGSS finally address the main point that distinguishes the different approaches taken by ISSC and IWGSS. Bob has offered a reference which demonstrates how he and co-workers recognized BSFRs and CCs in a well-exposed rock succession and thus delineated the 4 systems tracts in the succession. I had read this paper when it came out and have just finished re-reading it. I will discuss in some detail in a separate post on �Shoehorning in Sequence Stratigraphy and its Consequences� how Bob and his colleagues delineated the BSFRs and CCs and the consequent enclosed systems tracts. Some very useful insights into the problems with delineating the BSFR and CC time surfaces and the consequences that result can be extracted from a look at how MacNaughton et al �shoehorned� their excellent data into the four systems tracts approach now advocated by IWGSS. �As to the other �invisible� surface� the base of forced regression� my tendency is to agree with Embry et al. that this is not a realistic surface for correlation, or on which to base the definition of a sequence. . Embry et al. are wrong, however (in my opinion), to assert that there is no change in stacking pattern at the time when this surface forms.� It is nice to hear Bob agreeing that the time surface at the start of base level fall �is not a realistic surface for correlation� or to use to define a sequence stratigraphic unit. I would still claim there is no recognizable change in stacking pattern (in conformable successions) at such a time surface because regression simply continues in the offshore area and there is no discernable difference between a small scale sequence (depositional or R-T) directly below such a time surface and that directly above. Only in an unrealistic, perfect setting where each stacked unit represented the same time interval, each filled exactly to sea level and there was no erosion during transgression could such a difference possibly be determined. If anyone has empirical data which shows a change in stacking pattern at such a theoretical time surface, we would be pleased to receive them. We have not been able to find such data despite a focused search for them. �Sequence stratigraphy, as it is now practiced by most people, is much more than a tool for mapping. Instead, it encompasses a very sophisticated and holistic conceptual framework for interpreting many things about a sedimentary succession, including but not limited to: the presence/absence and location of specific facies, the vertical and lateral distribution of grain sizes, etc., etc. For this purpose, people require the most detailed model possible, because the system being interpreted is enormously complex. On this basis, the scheme proposed in the ISSC report is simpler than most other schemes and hence is a �step backward� as has been noted by some commentators�. This again is the �process sedimentologist� perspective which fails to appreciate the overwhelming importance of correlation in sequence stratigraphy. There is no doubt that once sequence stratigraphic surfaces have been delineated and correlated that many useful interpretations as to facies relationships, depositional history and paleogeographic evolution can be made. This point was clearly made in the Report. However unless such surfaces are properly defined, delineated and correlated then there is no �sophisticated and holistic conceptual framework for interpreting many things�. And that is the problem with current sequence stratigraphic practice. The deductive approach demands the recognition of unrecognizable time surfaces and many practitioners such as MacNaughton et al are using inappropriate stratigraphic surfaces as �proxies� for such unrecognizable time surfaces. This results in unrealistic interpretations of sedimentation processes, facies relations and depositional history as is discussed in the �Shoehorning� post. It seems to me all process sedimentologists should be concerned about this. I agree we need the most �detailed model� possible but it must be scientifically defendable and applicable to real world situations. We have argued that the seven-surface/four systems tract model is not scientifically defendable and is not applicable to real world situations. The five surface/two systems tract approach appears to be as detailed as we can presently get while at the same time having a scientifically defendable approach which is applicable to real world situations. �the formal recognition and endorsement of the Embry et al. scheme by the ISSC may, for some workers, give it a greater degree of sway than it should receive. If people attempt to use the Embry et al. scheme for detailed interpretations (as opposed to mapping), then their work will be limited unnecessarily.� This is very doubtful. People tend to apply what works. A classic example is the ISSC recommendation for the use of a �synthem� in the 1994 edition of the ISG. I did not detect �a greater degree of sway than it should receive� for the synthem. I ask everyone reading this post to reflect on how much he/she was influenced by the 1994 International Stratigraphic Guide and the specific recommendation for applying synthems. �.I suspect that many of the more vitriolic attacks on the ISSC proposal stem from the belief that this document proposes to discourage the application of other schemes (e.g., a four systems-tract model). This is (hopefully) not intended and should be explicitly stated, with guidelines as to situations where the use of each may be more appropriate.� I agree with Bob that the somewhat mean-spirited attacks we have received stem from the recommendations that discourage the application of a mixed, material-based/time-based approach to sequence stratigraphy which they strongly advocate. A number of the members of IWGSS including the chair have their careers closely associated with such an approach through the books they have written and the courses they teach. It is not surprising that these people have reacted in a very emotional manner and I understand why they are so upset. I would emphasize that the draft ISSC report DOES recommend that the four systems tract approach NOT be used until such time that empirical data are available to demonstrate the existence of the deduced time surfaces labeled as BSFR and CC. To us, this is a scientifically responsible approach and, if it upsets some people, so be it. We did not find any situations where the use of such an approach would be appropriate because we could not find any evidence to support it. We found lots of attempts to use it (e.g. MacNaughton et al, 2000; MacNeil and Jones, 2006) but in every case inappropriate stratigraphic surfaces were used as proxies for the time surfaces with less than satisfying results. We will discuss the 4 systems tract approach more explicitly in the revised text and outline our serious concerns (with real world examples such as MacNaughton et al) with it. �I urge the ISSC to explore this �pragmatic� alternative to the contentious ideas put forward in the Embry et al. report� Actually Bob�s proposal for a pragmatic approach is quite similar to that proposed in the ISSC report. We both agree that only empirical surfaces be used. We define our two types of sequences on the two primary types of regional unconformities which have been described - the SU/SR-U combo and the unconformable portion of the MFS. The only difference between our approach and that advocated by Bob is that we follow Vail et al�s innovation that the sequence boundary is extended along correlative surfaces (i.e. surfaces which join the unconformity so as to form a single through going boundary). The advantages of this approach over Bob�s are described in the report and are well accepted by most. Extension of the unconformable MFS boundary along the conformable portion of the MFS is straightforward and is not contentious. I don�t think anyone has a problem with the definition and delineation of an R-T (genetic stratigraphic) sequence. I think we all agree that a depositional sequence has an SU as its defining unconformity following Van Wagoner et al (1988). The disagreements come with what are the correlative surfaces for the SU. I would hope that in many cases there is no doubt that the SU correlates with (i.e. joins with) an SR-U and an MRS. Thus this is one type of viable depositional sequence and there are innumerable published examples of such a depositional sequence. The ISSC Report takes a very flexible approach to sequence definition and other types of depositional sequences can be proposed. All we ask is that the correlative surfaces used to extend the depositional boundary be empirical surfaces which can be recognized on the basis of observable characteristics. This does not seem to be an onerous and unreasonable requirement given that every boundary, for every defined stratigraphic unit, in every material- based stratigraphic discipline has the same requirement. In summary Bob�s proposal is perhaps too simplistic and, as he himself states, we need as detailed approach as possible. I see the empirical, five surface/two systems tracts approach as representing the most detailed approach that our current technology and understanding will allow. Attempts at more detail (e.g. the seven surface/four systems tract approach) unfortunately are not practical or scientifically defendable at this time. �The Embry et al. scheme is not very flexible, given than there are no alternatives that are to be allowed. This will be a disappointing outcome if it is accepted. The more flexible approach that has been suggested (in preliminary draft form only) by Catuneanu allows more freedom of choice and, thus, may be more readily embraced by the scientific community� I would counter that the recommended approach is extremely flexible with the caveat that only surfaces with empirical support can be used. Including deductive, time-based surfaces, which have no empirical support as Catuneanu (2006) and now the IWGSS advocate, does not add flexibility. It is a recipe for stratigraphic anarchy because material-based observation does not guide the recognition of abstract entities such as the BSFR or CC. �I would note that the guidelines for the creation of �formations� (for example) to not specify the precise rules to be used in the establishment of formation boundaries.� Actually the rules for the formalization of a lithostratigraphic unit such as a formation are rather rigorous. I can assure Bob that formation boundaries have to be observation-based, empirical boundaries that represent a major change in lithology. If a �flexible� approach of using deductive surfaces which had no material-based characteristics was adopted for formation boundaries, I would hate to see the resulting geological maps, not to mention the tops picked for a given well. �Sequence stratigraphy is such an important branch of stratigraphy that the entrenchment of ideas that do not have wide acceptance will do more harm than good.� The main problem currently dogging sequence stratigraphy is not the entrenchment of ideas which do not have wide acceptance (this seems to be a contradiction) but is the unfortunate entrenchment of ideas that are not supported by solid, empirical science. �I�m unclear what is meant by �concrete�. I very much doubt that most previous work on sequence stratigraphy is �non-concrete� because it does not agree with the authors� ideas. I would also note that there are no rigid statements of where a formation boundary should lie within the Guide or Code, so I see no reason why rigorous statements should be made in the case of sequence stratigraphy.� As noted in past posts, I think we will use the term material-based (as NACSN does) for what was referred to as concrete in the Report. Past work, which advocated the use of non-material based surfaces (i.e. time surfaces) for sequence stratigraphic units and correlation was non-material-based (non-concrete). Bob might want to dig into the struggles that lithostratigraphy and biostratigraphy had in shedding their methods and units of non-material based entities. Hedberg wrote a number of excellent papers on this theme. I can assure Bob that all formation boundaries are material-based (time surfaces need not apply) and the observable characteristics which define the boundaries are described in the formal definition of a given formation. �All stratigraphic units are fundamentally defined by their boundaries� I agree completely and that is why we need material-based boundaries with agreed upon observable criteria for their recognition. �We will be concentrating on the second use of sequence stratigraphy, that is, the definition of stratigraphic surfaces and units within the discipline�. This is a narrow and incomplete view of sequence stratigraphy. Therefore, any formulation that is based on it will be limiting as opposed to enabling, which is what any formulation should be.� This is a fundamental part of any stratigraphic discipline and is ISSC�s main concern. If �the definition of stratigraphic surfaces and units within the discipline� is not done well, the entire discipline will be in complete disarray. Because this has not been done well in the past for sequence stratigraphy, the discipline is experiencing some substantial problems and we are having this debate. �Emphasis is placed on the relative objectivity and reproducibility of data in defining units in each category� (NACSN, 2005)�. This statement does not require absolute objectivity, nor does it specify rigorously which surfaces are used, which it what these authors are doing.� There is a fundamental difference between recognizing an empirically based entity which is defined by observable characteristics (e.g. an MFS) and recognizing a deductive based entity which has no observable characteristics (e.g a BSFR). I suspect the failure of some people to understand this fundamental difference lies at the heart of the problems which beset sequence stratigraphy. �Farther basinward, a surface, which marked the depositional change from coarsening upward to fining upward and which was interpreted to represent the change from regression to transgression, was employed as the correlative conformity portion of the boundary (Fig. 8b)�. Simple logic (based on an understanding of the factors controlling sediment transport and deposition) shows that the surface defined in this quote forms later than the seaward-most extension of the subaerial unconformity where there is a period of relatively slow base-level rise and onlap of the landward-most point of deposition on top of the subaerial unconformity (see Fig. 11). Therefore, correlating it with the subaerial unconformity is inappropriate in the general case.� As has been described in the report, the MRS connects with (correlates with) the SU by means of the SR-U. This occurs in the vast majority of situations but certainly not all. This will not occur if a theoretical LST develops and separates the MRS from the SU. Real world examples of such a theoretical LST are exceedingly rare. Our request for references which illustrate such a stratigraphic relationship has been met with a telling silence. Thus correlating the MRS to the SU (through the SR-U) is the general case and I am sure rare exceptions to this do occur even if people have not been able to find any appropriate references. When such exceptions occur, extending the unconformable boundary of the depositional sequence basinward becomes problematic and perhaps impossible. �The surface must have various defining properties (i.e. physical features) which allow it to be recognized with reasonable objectivity in core or well-exposed strata and over a reasonable geographic extent so as to allow correlation to other sections and the establishment of mappable units�. The lack of reference to well logs and seismic data sets is an inappropriate limitation of sequence stratigraphy.� The point being made is that a given surface, above all, must be recognizable in rock because rock provides the most detailed and high resolution data and allows a given type of surface to be defined in the first place. Once it is recognizable in rock and everyone understands what it is (e.g. a subaerial unconformity), it then can be interpreted on lower resolution well log and seismic data. Such interpretations are somewhat speculative if no rock data are available to calibrate the seismic or logs. All the five accepted empirical surfaces of sequence stratigraphy were first recognized in rocks and are now interpreted on log and seismic sections. It is not reasonable to claim a bona fide sequence stratigraphic surface is recognizable on only seismic data because seismic data do not provide suitable information to characterize a surface. Seismic can be used to help recognize a new surface type in rock (e.g. it can help to pinpoint the stratigraphic position to determine if a new type of stratigraphic surface really exists or not). �For old deposits, there is no independent way of knowing what the actual sea-level curve was. To use the shape of the sea-level curves from the Quaternary as a model for ancient sea-level changes is highly problematic.� I completely agree with this and such a statement is important. The seven-surface/four systems tract approach ignores this statement and a priori uses a sinusoidal shape for the base level curve. Such a shape dictates the theoretical existence of four systems tracts. To paraphrase Bob �To use a sinusoidal shape for the base level curve as a model for ancient base level changes is highly problematic.� The interpretation of the base level curve shape should come from the observed stratigraphic architecture and the stratigraphic architecture should not be shoehorned into the a priori sinusoidal shape for the base level curve as Catuneanu (2006) and the draft IWGSS report advocate. �To us, it is essential that a depositional sequence be defined such that its boundaries are valid, recognizable sequence stratigraphic surfaces which all join together to form a continuous, unbroken boundary over much or all of a basin�. This requirement was not demanded of an R-T sequence, so why is it demanded here? This is inconsistent and suggests (to a cynical observer) that the stage is being set to promote the authors� choice of the MRS as the surface that should be the basinward equivalent of the SU.� This requirement holds for the R-T sequence as well as any new type anyone wants to propose. For the R-T sequence it just happens the MFS extends over the entire basin and thus no other type of surface is need to extend its boundary. There is no stage setting in such a requirement. It is difficult to imagine such a requirement not being part of any means of defining the boundary of any specific type of sequence. If you have a boundary of a unit you want it to be continuous. Thus if you have more than one surface forming that boundary they have to join together. �p. 55, Figure 13�This figure conveniently omits all reference to the deposits that form during the turn-around from regression to transgression, and thus inappropriately ties the MRS with the very tip of the SU. This figure also inappropriately shows the CC as not tying to the very tip of SU.� First of all the figure does NOT tie the MRS to the �very tip of the SU�. It ties it to the very tip of the SR-U which is both theoretically reasonable (both start to form at the initiation of transgression) and empirically demonstrable. As noted previously, if any deposits are formed after base level rise but before transgression , they are rarely preserved on the SU. Thus this diagram represents the general (but not every) case. The CC does not tie to the tip of the SR-U (not SU) as shown on the diagram because the SR has eroded the former tip of the SU which indeed did join the CC. �The �magnitude� of a sequence boundary is stated in two very different and inconsistent ways here. Explicitly, it is stated to be the �magnitude of the base-level change� that defines the magnitude of the sequence boundary, with the given example being that a 500 m change being a large-magnitude sequence boundary. However, most of the criteria given as �objective� criteria for determining the magnitude of a sequence boundary relate to the duration of the time gap! � Actually none of the criteria relate to the duration of the gap. �p. 80��The first is the reality that sequence methodology and terminology is still a hotly debated topic and only time will determine if the methods and units advocated herein are widely accepted�, and �Secondly, because sequence boundaries are somewhat interpretive and sequence designation depends to an extent on the establishment of a hierarchy which also has elements of subjectivity�� Statements such as these fly in the face of the call for strict �objectivity� earlier in this paper. Taken at face value, such statements mean that the formalization of sequence stratigraphy should stop now. This is, in fact, advocated by the subsequent statement: �Thus it may be best if formalization is never instituted so as to allow the easy and possibly frequent reinterpretation of sequence boundaries and their enclosed sequences� What is meant by objectivity in the Report is that interpretations are based on material-based observations. All such interpretations have an element of subjectivity but, because the interpretations are material based, scientific discussion is possible. There is no doubt that sequence stratigraphic interpretations are more subjective than lithostratigraphic ones but still include lots of reasonably objective observations. Interpretations of deductive surfaces that have no material based characteristics (BSFR) have no objectivity whatsoever. We are not recommending the formalization of INDIVIDUAL sequence stratigraphic units for the reasons stated but we are recommending some standardization of types of sequence stratigraphic surfaces and units. There is an important difference between these two approaches to classification. I trust all IWGSS understand this difference Summary � Bob has made some very good points and many of his comments will help us improve the Report. I am pleased he addressed the fundamental topic of the debate � the recognition of and use of time surfaces in sequence stratigraphy. I am pleased he agrees that the BSFR is not a recognizable surface. It has also been helpful to draw my attention to the MacNaughton et al paper which allows me to discuss the problems and consequences of attempts to apply the theoretical four systems tract approach in the real world. We would appreciate receiving other examples of attempts to apply the four systems tracts to real world stratigraphic successions. These provide the acid test of whether or not such an approach can be used in a �value added way� or if successions are basically being forced into an entrenched but unworkable approach. Ashton Embry: AEmbry@nrcan.gc.ca ++++++++++++++++++++++++++++++++++++++++++++++ On 15th May 2007 John Anderson wrote: Dear All I may well be the last one to register my comments on the Sequence Stratigraphy Report, but I just this week returned from a six week cruise in Antarctica where I did not have access to the web, and hence the report. I must confess that I still have had little time to do much more the skim the report and read letters from colleagues, some of which were forward to me on the ship. I agree, with most of the comments that have already been sent. I do not understand the need for a strict deadline on an issue of this type and, based on what I have read, I do not particularly appreciate the tone of the report or the approach that is taken. Many papers on the subject have passed the peer review test and it would appear that proper treatment of the literature is lacking. As one who has spent his time testing sequence stratigraphic concepts using the Quaternary stratigraphic record, I was annoyed to see that so little of this type of information is considered. It is my opinion that some of the questions raised by the report concerning various surfaces have been addressed by examination of strata deposited at a time when we know well the history of sea level rise and fall. I am not sure how we should proceed, but it is clear that the process of scientific debate has been short circuited and a different approach is needed. I for one would welcome a research symposium where those of us who are interested in this debate could present our most compelling results and try to resolve some of the issues raised in the report. Kind Regards John Anderson <johna@rice.edu> Maurice Ewing Professor of Oceanography Department of Earth Sciences Rice University Houston, Texas 77251-1892 +++++++++++++++++++++++++++++++++++++++++++++++++++++ On 9th May 2007 Rune Mj�s wrote: COMMENTS TO SEQUENCE STRATIGRAPHY AS A �CONCRETE� STRATIGRAPHIC DICIPLINE I agree with much of the discussions related to the difficulty of finding good criteria for determining sequence stratigraphic surfaces defined and picked by the �Exxon school�. Also, in some cases, poor stratigraphic conceptual thinking is responsible for the �Hunt for Surfaces� practise. I think the regressive � transgressive cyclic approach is simple and very useful in coastal areas. However, the focus on surface definitions and surface extensiveness may be dangerous and too simplistic. This is a report which focuses a lot on surfaces; the sedimentary facies in between the surfaces are not much incorporated in the discussions. This is strange since the surfaces itself are picked and defined based on sedimentary facies contrasts from below to above the surfaces. Also, a change in depositional trend is mentioned to be important in order to pick and define a seq strat surface. The meaning of �change in depositional trend� is not really pointed out clearly. There are spatial changes in depositional trends, pointed out earliest by A. Gressly (Cross and Homewood, 1997,) and followed by J. Walther (G.V. Middleton, 1973). The authors of this report are probably thinking along other lines, but please, do not ignore basic conceptual thinking given by Gressly and Walthers. - In the report base level is synonymous with sea level, hence nearly all discussions are related to absolute sea level changes as the only reason for changes in stratigraphic pattern. The approach in this report is therefore similar to the Exxon school. The terminology �stratigraphic base level� (Wheeler, 1964, Cross and Homewood, 1997, Cross and Lessenger, 1998) is not discussed. This terminology honours that the stratigraphic pattern is a response to interaction of several process variables; hence, base level is put into an energy domain by T. Cross and others. - Relationships to mass balance is not emphasized: - An important aspect of a significant erosional surface is that it correlates down dip to a pile of sediments. - An important aspect of a significant marine condensed horizon is that it correlate up dip to a pile of sediments - Stacking pattern of facies tracts is not mentioned at all, although in the Spitsbergen example at the report end (Fig 25) the importance of �shoreline trajectory� stacking pattern is shown. - Sediment volume partitioning is not mentioned at all. Sediments accumulate at different places along a depositional profile during a base level cycle. - Correct sediment volume partitioning is not shown in seq strat sketch figures; only surface drawings without much thickness considerations. - Geometry and stratal patterns are not discussed as being an important part of the sequence stratigraphy. Toplap, onlap and downlap terminations are not incorporated as being an important part of the sequence strat. In fact, sketches like Fig 11 shows no indication of that down lapping occur, although it�s quite obvious that MFS is a downlap surface. The petroleum industry would expect that stratal pattern from seismic would be important to incorporate in a sequence stratigraphic analysis. - Discrimination between an SU and SR is not a trivial task. Not as straightforward as it sounds after many repetitive arguments in the report. Also, in a vertical section the MRS or RS would probably be picked at the top of a coarse grained sandstone wh