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AAPG Annual Convention and Exhibition

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Stratal Stacking Patterns of Continental Margin Successions in Low-Accommodation and Low-Sediment-Supply Settings

Abstract

Sequence stratigraphic models for continental margin successions emphasize the general case in which neither accommodation nor sediment supply are significant limiting factors on the formation and preservation of complete cycles. There are many circumstances, nonetheless, where these variables are limiting. The future evolution of sequence stratigraphic models must allow for a variety of limiting factors on stratal preservation in sedimentary basins. This presentation reviews the nature of, and controls on, depositional sequences formed under conditions where accommodation and/or sediment supply are limiting factors. Circumstances in which sediment accumulation is limited by accommodation and/or sediment supply include 1. Aggradation/degradation cycles on alluvial plains, both upstream and downstream of the backwater limit, 2. Basin-marginal areas, or slowly-subsiding basins such as cratonic basins, 3. Parts of basins where there is synsedimentary structural growth, such as in the forebulge region of foreland basins, and 4. Areas and periods of time characterized by large-Previous HitmagnitudeNext Hit relative sea-level fluctuations, such as during ice ages. The latter circumstances are known to give rise to distinctive stratal cycles termed “cyclothems”, wherein the thickness of preserved strata is considerably less than the Previous HitmagnitudeTop of the causal sea-level excursion, both proximal to, and distant from, the locus of glacial ice. Examples of all four situations reveal that stratal cycles formed under limited accommodation and/or limited sediment supply have several elements in common. Cycles are 1. thin (in many cases, < 10 m thick), 2. condensed, with subtle expression of systems tracts, 3. incomplete, in terms of development of systems tracts, and 4. ubiquitously top-truncated. Cyclicity can, nonetheless, be discerned in all cases by recognition of shallowing and deepening trends in facies successions. Cyclothems in particular may show evidence of sea-level drawdown and emergence abruptly superimposed upon offshore marine facies, thereby allowing first-order estimates of the magnitude of sea-level excursions during ice ages. In far-field locations, rates of eustatic sea-level change may exceed rates of tectonic accommodation by orders of magnitude. In ice-proximal regions, both rates of sea-level change and solid earth effects may be orders of magnitude more rapid than tectonic subsidence, further complicating stratal cycles.