Geological Bounding Surfaces or Boundaries

Bounding Surfaces, Boundaries and their Hierachies

Sedimentary sections are subdivided by a variety of surfaces that envelope & enclose discrete geometric bodies of sediment. The partings with the greatest frequency are those in shales but bedding planes are the commonest of these surfaces to catch the geologists eye and are used by sequence stratigraphers to interpret the origin of the sedimentary section.

Brookfield (1977) was one of the first to recognize the importance of hierarchical order to the bounding surfaces seen in sedimentary rocks. He applied this concept to the Stokes (1968) surface boundaries that occur in aeolian sediments. He classified these as:

First order boundaries that cut across underlying aeolian sediments when the migration of “draa” dunes occured
Second order that were related to migration of transverse dunes
Third order boundaries that enclose groups of laminae interpreted to be the products of local events within the depositional cycle

Allen (1983) went on to establish, using fluviatile sediments as an example, that there at least four kinds of bounding sufaces:

Concordant non-erosional (normal bedding)
Disconcordant non-erosional
Concordant erosional
Disconcordant erosional contacts.

This led to his concept of "architectural elements" from which Miall (1985) developed a hierarchy of scales for the depositional units of fluvial depositional systems to better classify communicate something of their origins.

Pickering et al (1998) working with deeper water sedimentary bodies recognized that these were subdivided by a hierarchy of enveloping boundaries that define genetically related discrete stratigraphic “architectural elements", "bodies", or "units" or “groups”. They defined the architectural elements enclosed by these boundaries as:

An interpretive characterization of a sedimentary feature distinguished on the basis of its geometry, scale and facies.

The top end of this hierarchy includes the boundaries of basins, and/or the larger stacked channel complexes, or larger reef complexes while the low end of this hierarchy includes the boundaries to laminae or beds found within groups of discrete stratigraphic"elements", "bodies", or "units".

Sequence stratigraphers use a framework of certain of these surfaces or boundaries to define "sequences" and the "systems tracts" they contain. The commonest boundaries and surfaces used in this characterization are:

The enveloping sequence boundary (SB) of a sequence, or its down dip Correlative Conformity, lies between the Highstand Systems Tract (HST) and the Falling Stage Systems Tract (FSST)
Basal surface of Falling Stage Systems Tract (FSST) (Plint and Nummedal, 2000) (also known as the Early Lowstand Systems Tract (ELST) (Posamentier and Allen, 1999)
Transgressive surface (TS) that lies over the Lowstand System Tract (LST) and the Transgressive Systems Tract (TST)
Maximum Flooding surface (mfs)

Sequence stratigraphic interpretation involves the identification of the subdividing "surfaces" that envelope and enclose discrete geometric bodies of sediment. It tracks these in order from oldest to youngest. They are mentally disassembled (backstripped) and then reassembled in order in which they formed. This reassembly considers the subdividing surfaces, geometry, lithofacies and fauna and their evolving character in terms of the depositional setting. ‘‘Each stratal unit is defined and identified only by physical relationships of the strata, including lateral continuity and geometry of the surfaces bounding the units, vertical stacking patterns, and lateral geometry of the strata within the units." (Van Wagoner et al., 1990).

In the process of interpreting the origins of these surfaces, the depositional setting and gross sedimentary geometry of the rocks that form that sequence are established. Underlying these interpretations is the use of a simplified version of Walther's Law that states:

Sedimentary facies with adjacent depositional settings in plan view will succeed one another in a vertical progression of facies within a system tract or a sequence.

Paradoxically the surfaces used to subdivide stratigraphic sections are diachronous (transgress time), rendering Walther's law invalid. However, if in the process of interpretation, the diachronous character of the surfaces is ignored, an oversimplified version of Walther's Law can be applied to vertically adjacent sediments subdivided by the surfaces within the system tract or a sequence. The result is a powerful interpretation of depositional setting and a prediction of gross sedimentary geometry, both of which combine data from these bounding surfaces and the parasequence stacking patterns exhibited by the Systems Tracts (Van Wagoner et al., 1988).

The terminology of a number of the surfaces used in sequence stratigraphy are listed in the table below (modified from Catineanu, 2006). Highlighted in red are the more commonly used terminology for boundaries and surfaces used to characterize sequences and their systems tracts. The less commonly used terms and surfaces associated with shallow shelf clastics and the immediate shoreline are not highlighted.

Surfaces of Sequence Stratigraphy

Base Level Fall	Base Level Rise
Sequence boundary (SB) (and its correlative conformity)	Maximum regressive surface
Basal surface of forced regression	Transgressive surface (TS or Ravinement)
Regressive surface of marine erosion	Maximum flooding surface (mfs)

To conclude fundamental keys to sequence stratigraphic interpretation are the surfaces that subdivide the sedimentary section. Unfortunately the nomenclature of each of these sequence stratigraphic surfaces is constantly changing as our understanding of sedimentary systems and their interpretation improves. Though the changes in nomenclature are well intentioned they often add to the confusion to a scientific methodology that is already weighed down with complex multi-word and multi-syllable terminology. A surface can be given a name that has been used before for a different surface. The innocent reader, even the seasoned stratigrapher, not knowing the terminology has been changed and lacking the understanding of the reason for the change, may feel that they are going stark raving mad as they try to make sense of what they read.

References
Allen, J. R. L. 1983, Studies in fluviatile sedimentation: bars, bar complexes and sandstone sheets (low sinuosity braided streams) in the Brownstonews (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293.
Catuneanu,O., 2002, Sequence stratigraphy of clastic systems: concepts, merits, and pitfalls Journal of African Earth Sciences, Volume 35, Issue 1, Pages 1-43.
Miall 1985, Architectural elements and bounding surfaces: A new method of facies analysis applied to fluvial deposits: Earth-Science Reviews, v, 22, p. 261-308.
Pickering, K.T., Hiscott, R., and Hein, F.J., 1989. Deep-marine Environments: Clastic Sedimentation and Tectonics: London (Unwin Hyman).
Plint, A.G., Nummedal, D., 2000, The falling stage systems tract: recognition and importance in sequence stratigraphic analysis. In: Hunt, D., Gawthorpe, R.L. (Eds.), Sedimentary Response to Forced Regression, vol. 172. Geol. Soc. London Special Publ, pp. 1–17.
Posamentier, H.W., Allen, G.P., 1999, Siliciclastic sequence stratigraphy: concepts and applications. SEPM Concepts in Sedimentology and Paleontology no. 7, 209 p
Stokes, W.L. 1968, Multiple parallel-truncation bedding planes: a feature of wind-deposited sandstone formations. Journal of Sedimentary Petrology, 38(2):510-515.
Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., Hardenbol, J., 1988, An overview of sequence stratigraphy and key definitions. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes––An Integrated Approach, vol. 42. SEPM Special Publication, pp. 39–45.
Van Wagoner, J.C., Mitchum Jr., R.M., Campion, K.M., Rahmanian, V.D., 1990, Siliciclastic sequence stratigraphy in well logs, core, and outcrops: concepts for high-resolution correlation of time and facies. American Association of Petroleum Geologists Methods in Exploration Series 7, 55 pp.