A Simplified Scheme to Classify the Surfaces and Geometries of Sequence Stratigraphy: the Accommodation Succession Method
This paper proposes a framework for the hierarchy of sedimentary geometries expressed by seismic data that is based entirely on the geometric relationship of the strata, independent of the duration of time taken for their deposition or any relationship to a sea level curve or inherited depositional profile. This framework of geometries is assumed to result from repeated successions of accommodation creation and sediment fill conditions. Recognition of succession is dependent on data resolution.
We trace much of the current controversy in sequence stratigraphy to the divergence in method that mixes interpretation of relative sea level with observational stratal terminations and stacking patterns, specifically the unfortunate association of “highstand” and “lowstand” designations that implies linkage to a position on a sea-level curve. Therefore, we propose a return to a physical stratigraphy, observation-based recognition of coastal accommodation-fill successions that follow a progradation-aggradation-retrogradation-aggradation-progradation-degradation stacking pattern, bounded above and below by sequence boundaries. Deposition responds to accommodation-fill successions across time scales of few thounsands to hundreds of million years that results in stacking patterns characteristic of the accommodation rate (δA)/sedimentation rate (δS) vector:
(1) δA/δS <1 and increasing = progradation to aggradation stacking (PA or lowstand);
(2) δA/δS <1 and decreasing = aggradation to progradation to degradation stacking (APD or highstand);
(3) δA/δS >1 = retrogradation stacking (R or transgressive).
Data resolution and coverage (lateral extent) determines if a unit can be identified as relatively conformable and correctly designated a depositional sequence. This stacking pattern repeats across a range of accommodation-fill succession durations and magnitudes that permits construction of a self-similar hierarchy of systems tracts, depositional sequence, sequence set, composite sequence, composite sequence set, and megasequence to describe a basin’s depositional fill. This framework has the advantages of being simple, objective observation-based, predictive, independent of time or sea level terminology, flexible for all scales of data and subsequent data resolution improvement, providing a guide to incorporate the new observations and make predictions of previously unrecognized complexity elsewhere.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009