Sequence Boundary Ambiguities in Shelf-Margin Deltas and the Shelf-Slope Transition: Illustrations from the Pleistocene of the Gulf of Mexico
Charles D. Winker, R. Craig Shipp, 2013. "Sequence Boundary Ambiguities in Shelf-Margin Deltas and the Shelf-Slope Transition: Illustrations from the Pleistocene of the Gulf of Mexico", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Pleistocene shelf-margin deltas (SMDs) of the northern Gulf of Mexico (Mobile, Mississippi, Brazos-Trinity, Colorado, Rio Grande) and the corresponding shelf-slope transition illustrate some of the conceptual and procedural issues pertaining to sequence stratigraphy of continental margins. As one approaches the shelf margin from the landward side, it is customary to pick the sequence boundary (SB) at the erosional base of channelized fluvial deposits, typically cut into marine clinoforms. Near the shelf margin, the sequence boundary could be picked at the erosional base of a submarine canyon cut into a SMD, or alternatively at the base of the SMD, especially if no submarine canyon is present. Thus, the SMD can be placed either below the SB (making the SMD part of the HST) or above the SB (making the SMD part of the LST). In practice, the latter is seldom done, because there is rarely a distinctive surface or break in stratal geometry to uniquely mark the change from shelf-phase delta to SMD. Therefore, SMDs are usually considered part of the HST.
In some cases the SMD is characterized by submarine landslide deposits within the clinoforms, resulting in hummocky or chaotic clinoforms, which can grade downdip into massively chaotic, sand-bearing deposits on the upper slope. Even in these cases, the change from shelf-phase delta to SMD is typically gradational, without a single distinctive surface to uniquely define the SB or to place the SMD in the LST. In general, there is no consistent rule as to where the SB should occur relative to a SMD, or where the SMD should fit into the systems tract classification.
As one approaches the shelf margin from the basinward side, where mini-basins are present on the upper slope, the SB is typically picked at a major onlap surface, which in late Pleistocene deposits can often be correlated across saddles between mini-basins with little ambiguity. In the Brazos-Trinity deposystem, the SB defined by this onlap surface is clearly different from (and stratigraphically below) the SB defined by the erosional base of fluvial deposits landward of the shelf margin. In the Mississippi deposystem, SBs defined by such onlap surfaces are also clearly different from SBs defined by erosional bases of submarine canyons. Onlap surfaces (and immediately underlying MFS shales) are useful for correlation along strike, especially on the upper slope. In contrast, submarine canyon surfaces are useful for long-distance correlation in the dip direction from shelf to basin plain, but are of very limited extent in the strike direction. The basin-floor fan (BFF) phase of slope deposition typically occurs just above the onlap surface, whereas the slope fan (SF) phase occurs at and above the submarine canyons. A composite framework of onlap surfaces and submarine canyons is useful for establishing temporal relationships within the Mississippi depositional province, although this framework does not fit readily within standard systems tract nomenclature.
In concept, sequence boundaries are isochronous surfaces which separate deposits that are less closely genetically related while grouping deposits that are more closely related. Two difficulties are recognized with this concept. First, sequence boundaries picked at erosional surfaces are subject to regional diachroneity, such that some fluvial deposits above the SB may be coeval with some marine deltaic deposits farther downdip below the same SB. Secondly, the SB typically groups slope deposits with immediately younger transgressive deposits while separating them from immediately older deltaic deposits. However, in map view, Pleistocene deposystems of the northern Gulf of Mexico consistently show a close paleogeographic relationship between slope systems and immediately older deltaic systems. Conversely, the paleogeographic relationship between slope systems and immediately younger transgressive and highstand systems is typically much more distant.
From studying a variety of Quaternary deposystems associated with Gulf Coast rivers, we recognize a composite succession, although not all phases will necessarily be present in any one deposystem:
Incised valleys are filled by a combination of estuarine transgression, fluvial aggradation, and deltaic progradation.
Once the incised valley is filled, the alluvial plain continues to aggrade without the lateral confinement of valley walls. This aggradation may or may not be accompanied by deltaic progradation.
The delta progrades across the shelf, probably punctuated by minor transgressions and lobe switching. The net progradation is typically forced by sea-level fall, but may also occur purely by sedimentary progradation, as in the Holocene Mississippi delta. Forced regression of the delta is typically accompanied by valley incision farther updip.
As the delta approaches the shelf margin, the deltaic depocenter becomes thicker and smaller in areal extent, while the prodelta becomes steeper and increasingly prone to slope failure. Slope failures may be manifested in a variety of ways, such as a single slide complex which is healed by subsequent clinoform progradation, or as repeated slides during progradation, resulting in chaotic clinoforms. Also during this phase, turbidity currents may be generated at or near river mouths, which generate sinuous slope channels without significant incision of the shelf margin. Alternatively, the SMD can remain gravitationally stable, with minimal generation of sediment gravity flows.
The SMD is incised by a submarine canyon, typically connected to an incised valley. After a phase of sediment bypass to the slope and basin plain, the canyon is typically filled or healed by clinoform progradation.
Regional transgression resets the paleogeography, and the next depositional succession is likely to be offset along strike from the previous, due to large-scale lobe switching.
Overall, this depositional succession is controlled by eustacy. However, sea level and transgressive-regressive cycles are not necessarily in phase, and these phase relationships may vary from one one river to the next, and from one cycle to the next. In addition, the stratigraphic expression of a given eustatic cycle can be present in one locality and absent or cryptic in another. Therefore, inference of eustacy from a local stratigraphic record, or from a single dip section or corridor through one or more SMDs is likely to yield a sea-level history that is incomplete or otherwise inaccurate.
From an operational standpoint, we prefer a descriptive classification of major surfaces. The relationships of various kinds of stratigraphic surfaces define the stratigraphic framework. For sequence boundaries we use those surfaces that are most robust for regional correlation.