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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research
Vol. 90 (2020), No. 12. (December), Pages 1706-1733
DOI: 10.2110/jsr.2020.012

Out-of-Previous HitphaseNext Hit cyclical sediment supply: A potential causal mechanism for generating stratigraphic asymmetry and explaining sequence stratigraphic spatial variability

R. Bruce Ainsworth, Adam J. Vonk, Paul Wellington, Victorien Paumard

Abstract

Although acknowledged to be a simplification, the rate of sediment supply is usually assumed to be constant in sequence stratigraphic interpretations of clastic shelf systems. The simplified assumption taken in this work is that sediment supply can be represented by sine curves linked to climate changes driven by Milankovitch cycles. Three orders of sediment supply sine curves (amplitude and frequency scaled to order) are convolved with three orders of Milankovitch-forced eustatic sea-level sine curves and a constant rate of subsidence to generate curves for the ratio of rate of accommodation development to rate of sediment supply (δ A S ). The relative-sea-level curve is then held constant whilst sediment supply is systematically changed from being constant to being cyclical across the three orders of Milankovitch frequencies and being in-Previous HitphaseNext Hit, and out-of-Previous HitphaseNext Hit with the eustatic cycles by 90°, 180°, and 270°. For each scenario, stratal architecture is then represented for sixty consecutive parasequences (fifth-order, regressive–transgressive shelf transit cycles) by converting the δ A S curves into pseudo thickness / sandstone fraction plots (TSF plots). Constant sediment supply, in-Previous HitphaseNext Hit sediment supply, and 180°-out-of-Previous HitphaseNext Hit sediment supply produce symmetrical stratal geometries with equal periods of progradation, aggradation, and retrogradation. When sediment-supply cycles are 90°-out-of-Previous HitphaseNext Hit (supply peak occurs later than sea-level peak), stratal geometries are asymmetrical with progradational architectures being dominant. When sediment-supply cycles are 270°-out-of-Previous HitphaseNext Hit (supply peak occurs earlier than sea-level peak), stratal geometries are also asymmetrical but retrogradational architectures are dominant. These patterns are reproduced at all three orders of stratigraphic hierarchy (parasequence, sequence, and composite sequence). Comparison of these synthetic stratal geometries to real-world stratal geometries from Triassic to Neogene rocks across both the fifth-order (parasequence) and fourth-order (sequence) of stratal hierarchies suggests a consistently occurring asymmetrical, progradation-dominant motif. This indicates that 90°-out-of-Previous HitphaseNext Hit sediment supply (supply peak occurs later than sea-level peak) may be a common occurrence through geological time. The work also corroborates the findings of earlier workers and suggests that sequence stratigraphic surfaces can change nature along depositional strike due to out-of-Previous HitphaseNext Hit sediment supply and can thus also be diachronous. This work conceptually illustrates that Milankovitch climate-change-induced sinusoidal-sediment-supply cycles, out-of-Previous HitphaseTop with sinusoidal eustatic-sea-level cycles, may produce commonly observed asymmetrical stratal architectures and should be considered when invoking causal mechanisms for stratal architectures on clastic shelves.


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