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Deepwater Clastic Systems Sediments Introduction
to deepwater system sediments and their classification Dott and Bourgeois (1983)
dismayed at the explosion of fluvial models, argued that this proliferation
“defeats the whole purpose of the conceptual model by encouraging
excessive pigeonholing, which obscures rather than reveals whatever
unity may exist among the variants’’. The same could
be said of deepwater sediments. Though deepwater sedimentary classifications
are changing rapidly (Shanmugam, 2000) and proliferating, this web
site argues that a consensus is developing amid the various schemes.
The intent is that these models be used as a guide to the interpretation
and the prediction of the continuity of each unique set of deepwater
sedimentary rocks and sediments. The solution is to consider the
origins of the deepwater sediment you are examining and describe
them carefully and in detail. This should be from the perspective
of process driven models, with classifications being used as needed
and modified to meet the specifics of the situation! Remember to
always define your terms! Deepwater Sedimentary
end members of subaqueous density flows; namely non-cohesive density
flow sediments (including turbidites), mass transport
debrites (MTD) and pelagic drape and/or condensed sequences.
The common characteristic of early classifications of deepwater systems was tied to the occurrence of beds of sandstone in which the sediment grain size fines upward. Beds with this texture became known as deepwater “turbidites” (Kuenen, 1957). This was based on recognition that deposition from a waning turbidity current had a matching decrease in grain size. Later this vertical change in fabric became known as the “Bouma Sequence” (Shanmugam, 2000). It is now recognized that while not every deepwater sedimentary cycle has the fabric of the Bouma Sequence turbidite (Shanmugam, 2000), process driven interrelationships exist between most deepwater sediment facies and their associated geometries. Many excellent examples of these process driven interrelationships exist. One example that has caught the attention of stratigraphers is the Carboniferous Ross Sandstone of Co. Clare, Ireland. Pyles and Bouroullec (2006) recognize that this section exposes an ascending order of interrelated mudstone sheets and slumps, linked debrites and mudstone sheets, and non-cohesive density flow sandy turbidite lobes.
The above illustration by Peter Haughton demonstrates the evolving interrelationship between the various sub-aqueous density flows and their transportation of sediment. The diagram demonstrates how cohesive flows change to non cohesive flows while passing through transitional hybrid flows. The sediment from these different flows are co-genetic and slurry flows accumulates as a mix both cohesive and non-cohesive material. To conclude the sediment transportation evolves from cohesive flows to non cohesive high to low density turbidity currents. The above diagram simplifies the classification of Mulder & Alexander (2001) below.
The classification of Mulder & Alexander (2001), seen in the illustration above, captures the interrelationships of subaqueous density flows and subdivides them into cohesive flows and frictional flows. This is "based on the cohesivity of the particles, flow duration, sediment concentration and particle-support mechanism".
This classification scheme of sedimentary density flows assumes that any flow may change in type both down and across flow and with time at any one point. The strength and weakness of the classification is the spectrum of representative sediments involved, making it general, interpretative and qualitative.
Building from the concept
of process driven interrelationships the interpretive models considered
below recognize that there is a strong association between turbidites,
mass transport debrites (MTD) and pelagic drape and/or condensed
sequences. These models are used as a first pass guide to the character
of the observations made in the field or lab. These models are then
modified to meet the character of ones observations, rather than
vice versa. Turbidites &
other Frictional Flows (Mulder & Alexander, 2001)
Turbidites This web site uses the term turbidite as Kuenen (1957) first used it to represent a deposit from a turbidity current. This supports the published work of Kuenen’s student - Arnold Bouma; the geologist who detailed the “man’s land” of the Annot Sandstone outcrops of SE France and established the first vertical facies model of turbidites (Bouma,1962). This stratigraphic section has become the type model for the "Bouma Sequence”.
Since the Bouma paper of 1962 it has been demonstrated the turbidite deposit is a “member” of an evolutionary trend of vertically associated sediments that accumulate from debris flows, hyperconcentrated to concentrated density flows, and turbidity flows (Mulder & Alexander, 2001). As Parsons et al (2003) observe, “Gravity-driven motions produced from turbulent suspensions are an important sediment-transport process in the modern ocean and the deposits that result from them constitute a significant portion of the sedimentary record”. Turbidites are one of these deposits. For instance on the continental margin of the Southwest Orphan Basin in the Labrador Sea, sediment moves under gravity as cohesionless debris flows that are transformed basinward into fluids moving under gravity in the form of high-density turbidity currents (Tripsanas & Piper’s, 2006). In this latter case no hemipelagic deposits were found to separate the sand-gravel unit from the mud flow and the seismic data suggests that the mud-flow deposit was initiated by slope failure. Mass-transport
deposits (MTD) and debrites One of the objectives of this web site is to help the user develop techniques to characterize potential and actual hydrocarbon reservoirs within these sediment complexes. Martinsen, and Posamentier, (2006) suggest to do this one should establish the deepwater stratigraphic architecture, its timing and tie to well and seismic data. Their basic approach examines age relationships, and characterizes the geometry, and the lithology of these sediments. They note that the processes that are responsible for the character of deepwater systems grade into one another as continuous spectrum down the subaqueous slope. These include
Nearly all-subaqueous slopes are unstable and down slope sediment movement is commonly expressed as mass transport complexes that vary in scale from cubic meters to several thousand cubic kilometers. Relative drops in sea level have often initiated large-scale margin collapse. This is often followed by confined chanalization down slope and then the cycle ends with unconfined fill that probably represents flushed shallow shelf sediments. Submarine fans in the Pyrenees (Pickering, and Bayliss, 2006) provide an example of this with Eocene mass-transport complexes (MTCs) confined to channels across them as they are transported to the basin-floor. Similarly on the Nile Fan, each of the large MTDs has a headwall scarp at the shelf edge or on the upper slope that forms a wide sloping corridor down which the sediment moves. “Channels systems appear to occupy these evacuation corridors immediately after most of these distinct mass failure episodes” (Newton et al 2006). Here and at other locations extensive deep-water mass transport deposits currently tend collect on slopes and basin settings. They form sheets, mounds lobes, and channel fills often 150 m or more in thickness (Posamentier & Martinsen, 2006). Their internal character includes heterogeneous deposits of mudstones to matrix-supported conglomerates with the chaotic, and slumped fabrics (Nilsen, et al, 2006). Debris flows and leveed channels tend to build out from canyon mouths directly onto the basin plain. Currently in offshore West Africa, large Pleistocene aprons have formed with multiple leveed channel systems and associated debris flows. Avulsing channels and associated levees emerging from the canyon mouth occur and sidewall failures and rotational slips have produced debris flows in all stages of development (Sutton and Mitchum, 2006). Sheet deposits seaward and onlapping the apron deposits often show gravitational instability and a range of deformation processes. These form small-scale soft-sediment deformation structures (convolute lamination, dishes and pipes), intensely dewatered intervals and the development of low velocity slope ‘creep' bodies. These fabrics occur in the Early Permian deepwater deposits of the Tanqua area in South Africa (Wild et al, 2006). The stacked deposits of co-genetic turbidity currents indicate other evidence of slope instability and debris flows are common to many down slope fan systems. These occur particularly downdip in relatively distal and lateral fan fringe settings, bypassing sandier mid-fan sections (Haughton et al, 2006). This sediment association is a response to erosion on unstable, tectonically controlled slopes or intrafan erosion during incision of the inner fan. Posamentier and Martinsen (2006) indicate that mass transport units are commonly amalgamated. So much so that surfaces between successive mass transport deposits may be difficult to recognize. Never the less they identify two distinct mass transport end members: 1) those formed by coherent and commonly discrete blocks that moved along relatively smooth decollement surfaces, and, 2) those with a less coherent, disrupted fabric scoured into the lower surface with linear grooves that may diverge down-system. Blocks imbedded at the base of the flow mass and dragged across the underlying substrate form these groves. Posamentier and Martinsen (2006) intimate that the upper bounding surface of the latter often has irregular to hummocky relief, bounded laterally by steep to gentle flanks. In some instances the termini of the mass transport lobes exhibit extensive low-angle thrust faults associated with compression. This is supported by Butler et al’s, (2006) recognition that submarine mass transport complexes on the modern sea floor often display complex rugosity on their upper surfaces, defined by features interpreted to be rafted blocks, pressure ridges, detached faults and folds. Pelagic drape These condensed sections can be seen to vary in thickness and their upper boundary forms broad surfaces that often mark the base of prograding downlapping clinoforms of highstand system tracts seen in outcrop, cores, and seismic. This makes condensed sections important stratigraphic markers that can often be correlated across most of a basin. Characteristically and conveniently these layers contain abundant and diverse microfaunas, glauconite, phosphorite and siderite (authigenic minerals), organic debris, and high concentrations of radioactive elements that include iridium. This latter is expressed as high gamma ray log signal values that reflect higher than normal concentrations of radioactive potassium, thorium and uranium sequestered by cyanobacteria. The use of intervening transgressive shaley interfan sediment sheets as stratigraphic markers was used to establish that the Tanqua Karoo Formation of the Permian of South Africa accumulated as series of prograding fans. Similarly the upper Miocene deltaic and deepwater elements of Gulf of Mexico sedimentary section are subdivided by similar pelagic drapes.
Unfortunately sedimentary drapes exist that have other origins than those associated with the maximum water depths of a sea level transgression and/or high stands. These other drapes are connected to autocyclic migration of the sand prone sections away from the axis of the deepwater fans, their channels, overbank fill and levees. For this reason the fine grained portions of homopycnal, hyperpycnal and hypopycnal sediment plumes (Bates, 1953); storm winnowed shelf sediment; glacially rafted material, pelagic fauna, and air born dust may dominate the depositional setting instead of the coarser potion of the fan driven deposition. From a stratigraphic perspective the fines that mark the passive phase of an autocyclic event look very similar in character to the fine-grained portion of the deepwater cycles and so be misinterpreted to be connected with the transgressive phase of a parasequence. However these autocyclic drapes are allostratigraphic in character and most have limited distribution within the internal variability of a fan or slump. Never the less, when they extend over a good portion of the fan complex, they can be used as stratigraphic markers. Even then, though the fine sediments match the pelagic drape described above, they often lack their condensed character and the associated microfaunas, glauconite, phosphorite and siderite, organic debris, and radioactive elements . Also because of their common lithologic character these may be mistakenly correlated with the other high frequency local events referred to above and without care their local character may be unrecognized. A mix of high frequency seismic tied to well data may help resolve this problem of miscorrelation.
Cited References Dott. R.H., JR., and
Bourgeois, J., 1983, "Hummocky stratification: significance
of its variable bedding sequences": reply to discussion
by R.G. Walker et al.: Geological Society of America, Bulletin,
v. 94, p. 1245–1251. |
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