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Relating Changes in Stratigraphic Architecture of Fluvial Deposits to Changes in Accommodation and Sediment Supply, Sobrarbe and Escanilla Formations, Spain

Abstract

Fluvial systems are important hydrocarbon reservoirs found around the world. One of the problems facing the development of fluvial reservoirs is our understanding of how varying rates of accommodation and sediment supply during deposition relate to spatial and temporal variations in stratigraphic architecture of fluvial deposits. The Ainsa Basin, Spain contains world-class outcrops of fluvial strata of the Escanilla Formation, which can be correlated to coevally deposited progradational deltaic deposits of the Sobrarbe Formation. This study uses outcrop data to document how stratigraphic architecture of fluvial floodplain and channel deposits vary in relation to temporal variations of the rate of accommodation versus the rate of sediment supply. Data from stratigraphic columns, interpreted photopanels, and a geologic map are used to document spatially varying characteristics including, architectural elements, lithofacies, grain size, stratal boundaries, sedimentary structures, and net-sand content of fluvial deposits of the Escanilla Formation. Shelf-edge trajectories are used as proxies for the ratios between accommodation and sediment supply during deposition. Examination of shelf edge trajectories of four consecutive shelf edges documents a spatial trend of decreasing shoreline trajectory, which we relate to a temporal (upward) increase in the rate of sediment supply in relation to the rate of accommodation. These data are used to construct a longitudinal cross section that documents the location of the paleoshoreline, regressive and transgressive intervals, cycle boundaries, and the distribution of fluvial floodplain and channel deposits. An upward change in the stratigraphic architecture of both crevasse-splays and fluvial channels is documented. In the lower part of the stratigraphic succession, crevasse-splay beds are relatively thin, although crevasse-splay stories are relatively thick, the overall net-sand content of floodplain deposits is relatively high, and fluvial channel belts are relatively thin. In contrast, the upper part of the stratigraphic succession contains relatively few but thick crevasse-splay beds, the overall net-sand content of floodplain deposits is relatively low, and fluvial channel deposits are relatively thick.