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Abstract

Understanding the origin and geometry of largescale erosional surfaces in fluvial and channelized submarine depositional settings is critical for interpreting reservoir architecture and connectivity, as these surfaces strongly influence reservoir heterogeneity. We use simple and fast-running forward stratigraphic models to investigate the geometry and the relative age of complex erosional surfaces that form in both the subaerial and submarine domain.

Because low-sinuosity systems tend to have relatively simple incisional and aggradational geometries, we focus on high-sinuosity systems. Fluvial deposits are commonly preserved on terraces that form during incision, and the basal erosional surface is highly time transgressive. Terraces can form without any external influence as a result of cessation of incision at channel cutoff locations. Similar processes and geometries can be observed in systems containing incising submarine channels. However, extensive deposition of fine-grained sediment in the overbank area of submarine channels tends to result in draping and long-term preservation of terrace geometries.

This is in contrast with fluvial systems, as the incisional terrace morphology can be quickly buried after valley filling initiates. Once incision ceases and aggradation begins, erosional surfaces become less continuous and form an intricate network inside the larger and longitudinally more continuous valley surface. Depending on the rate of aggradation and local rate of lateral migration, the internal erosional surfaces can be similar in vertical extent to a single channel depth, or to multiple channel depths and one channel bend in plan view. Phases of low aggradation cause these scallop-shaped surfaces to connect in the downslope direction and form an extensive erosional surface, without any significant re-incision. As relatively fine-grained deposits (e.g., shale drapes, slides, and debris-flow deposits) are primarily distributed along geomorphic surfaces, differentiating time-transgressive erosional surfaces from geomorphic ones results in a better prediction of reservoir compartmentalization and fluid flow. Understanding the origin and geometry of valleys and their deposits informs the controls on the sequence stratigraphy of basin margins. That is, most erosional surfaces are time transgressive and some of them reflect the autogenic dynamics of valley formation, rather than external forcing.

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