Sedimentation on Continental Margins, VI: A Regime Model for Depositional Sequences, their Component Systems Tracts, and Bounding Surfaces
- D. J. P. Swift3,
- G. F. Oertel3,
- R. W. Tillman4 and
- J. A. Thorne5
Published Online: 14 APR 2009
Copyright © 1991 The International Association of Sedimentologists
Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy
How to Cite
Thorne, J. A. and Swift, D. J. P. (1992) Sedimentation on Continental Margins, VI: A Regime Model for Depositional Sequences, their Component Systems Tracts, and Bounding Surfaces, in Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy (eds D. J. P. Swift, G. F. Oertel, R. W. Tillman and J. A. Thorne), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303933.ch6
Norfolk, Virginia, USA
Tulsa, Oklahoma, USA
Plano, Texas, USA
- Published Online: 14 APR 2009
- Published Print: 30 JAN 1992
Print ISBN: 9780632032372
Online ISBN: 9781444303933
- sedimentation on continental margins, VI - regime model for depositional sequences;
- sequence stratigraphic terminology;
- sequence versus cycle boundaries;
- Galloway model for genetic stratigraphic sequences;
- geometric systems tract terminology;
- topographic fill sedimentation terminology;
- regime sedimentation terminology;
- basin fan geometric systems tract - comparison with modern lowstand deposits;
- offlap wedge geometric systems tract - models and observations;
- regime model for sequence interpretation
This study evaluates the sequence architecture models of Vail (1987) and Galloway (1989) in terms of the regime concepts presented in the first five papers of this volume. Comparison of these models with modern and submodern shelf deposits, in which sediment dispersal systems are presently functioning, leads to the development of a new, regime-based model for their system tracts and bounding surfaces. To compare effectively the Vail, Galloway and regime-based models, four geometric systems tracts are defined that make no explicit reference to eustatic sea-level. Rough Vail or Galloway equivalents can be found for the (1) basin fan, (2) back-step wedge, and (3) offlap wedge geometric systems tracts. The fourth alluvial fan geometric systems tract has no equivalent. Geometric systems tracts are defined in terms of stratal geometry, primarily as observed through seismic records or well-log correlations. They are generally larger in scale and more inclusive in character than the facies-defined depositional systems tracts discussed by Swift et al. (this volume, pp. 89–152.)
Our survey of modern sedimentation, based mostly on high-resolution seismic profiles, largely confirms the descriptive aspects of the sequence architecture model of Vail. The model of Vail, however, may need to be modified to include several additional architectural elements that can be present under certain regime conditions. One modification concerns the transgressive surface. The transgressive surface is a surface whose time range encompasses the turnaround event, when paracycles of progradational sedimentation give way to paracycles of back-stepping sedimentation. A single transgressive surface may include portions of: (1) a ravinement surface formed by erosional shoreface retreat landward of the maximum basinward shoreline position; (2) a marine erosion surface that may form seaward of the maximum basinward shoreline position; and (3) a conformity between beds of the back-step wedge and beds of the preceding offlap wedge systems tract. Other elements recognized in this study are; (4) a time-transgressive coastal plain unconformity formed by fluvial entrenchment during offlap wedge progradation, which is not related in origin to the slope unconformity formed during type 1 sequence boundaries; (5) a wedge of back-barrier sediments between the transgressive surface and the ravinement surface; (6) a sedimentary wedge that onlaps the relict slope and downlaps the deep basin formed during transgression; and (6) the formation of extensive coastal-plain–slope unconformities that mimic sequence boundaries at times other than during falling sea-level. A few of these modifications have been suggested by Galloway.
The Galloway model, although giving a less detailed picture of architectural sequence elements, appears essentially realistic in its recognition that other regime variables besides relative sea-level change can largely control sequence architecture. The regime concepts of Galloway, however, are modified to include more explicitly the effects of regime variables, R, related to the rate of relative sea-level change, D, the rate of sediment transport, Q, the rate of sediment supply and, M, the sediment grain size.
Specifically we find that the product Q · M is a regime expression for effective sediment supply while the product R · D is a regime expression for effective accommodation potential. It is suggested that the ratio of accommodation to supply, defined as ¥ = R′ · D′/Q′ · M′, is fundamental to the architectural response of sedimentation to changing regime conditions. A semi-quantitative model for each systems tract, the subsystems within it, and its bounding surfaces is proposed based on the theoretical response of regime topographic profiles to changes in ¥, the accommodation/supply ratio. Hypothetical examples of sequence development in response to changes in: (1) sediment supply without changes in sea-level; and (2) sea-level without changes in sediment supply serve to illustrate the differences between the new regime model and the eustatic model of Vail.
The regime-based, ¥-dependent model is used to interpret observed sequence stratigraphic architecture of the Campanian–Maastrichtian deposits of the USA Western Interior. This analysis suggests that second-order (c.10 Ma) sequence cycles may be controlled by episodic subsidence caused by in-sequence thrusts, while third or higher order (<1 Ma) sequence cycles may be controlled by sediment supply changes caused by out-of-sequence thrusts.
A comparison of regime conditions for various time periods on the Northwestern Gulf of Mexico shelf and Middle Atlantic shelf sections indicates that the preservation potential of sequence architecture is dependent on the relative roles of sediment input (Q) and sea-level change (R). As sediment input and subsidence increase relative to the frequency and intensity of sea-level falls, the preservation potential of highstand deposits increases.
This study is the last of six papers in this volume that apply the concept of regime sedimentation to continental-margin deposition. These six papers have shown, at various spatial and temporal scales, that sedimentation systems are governed by interdependent variables, whose mutual adjustments tend towards a state of dynamic equilibrium. As a first approximation, semi-quantitative regime models can be developed in terms of equilibrium surfaces controlled by homeostatic responses of erosion or deposition.