The role of the sediment load in sequence stratigraphy: The influence of flexural isostasy and compaction

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Abstract
Simplified, physically based models are utilized in order to examine the effects of sea level, sedimentation, tectonic subsidence, isostatic compensation, and compaction on development of unconformities. Unconformities divide volumes of relatively conformable sediments defining the time and spatial extent of sedimentary sequences. The model parameters are varied both individually and in concert in order to isolate their contributions to sequence architecture. These models reveal the importance of sedimentation and subsidence rates, as well as sea level amplitude and rate, in determining the type and extent of sequence boundary formed. Sedimentation and subsidence rate can vary significantly within and between margins, producing different sequence boundaries given an identical eustatic sea level fall. It is shown that sea level amplitude plays an important role in determining which systems tracts are present and sequence boundary timing for type 1 sequences and that sea level rate controls the same attributes in type 2 sequences. Isostatic compensation and compaction, which have been described as secondary effects, are shown to have considerable influence on sequence architecture. These two processes, due to isostatic response to sediment loading and sediment self‐loading, initiate feedback by enhancing and partitioning accommodation space. These additional effects invalidate application of the equilibrium point as a guide to where and how all accommodation space is created. The manner in which flexure distributes accommodation space is a function of lithospheric rigidities: higher rigidities partition space laterally, producing wide shelves which favor type 1 sequence boundaries, whereas lower rigidities partition space vertically forming narrow shelves which favor type 2 sequence boundaries. Compaction generates a net landward shift of the shelf edge and favors type 2 sequence development Both isostatic compensation and compaction introduce time delays in sequence development. One consequence of compaction is to rotate sedimentary packages, generating growth and leakage of anticlinal structures as fluids are driven from the underlying section. This process has important implications for hydrocarbon migration in compaction‐dominated systems, such as the Gulf of Mexico margin.