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Mixed Processes From Regressive and Transgressive Cycles: Examples From the Wilcox Group, Gulf of Mexico

Abstract

Two continuous cores in the Lower Wilcox Group provide a rare opportunity to document sedimentary facies, process regime, and the evolution of Late Paleocene paleo-Colorado deltaic systems. The 2,240-ft Pure Oil Vogelsang core in Colorado County records a stratigraphic succession within an area of major sediment influx. In contrast, the 1730-ft Letco TOH-2A core in Leon County represents off-axis, primarily interdeltaic deposits. The employment of a new semi-quantitative approach to interpret >4000 ft Lower Wilcox core shows that the type of shoreline (deltaic versus estuarine) and the dominant processes (fluvial, wave, tide) vary much more frequently than previous documentations. Even though Lower Wiclox deltaic systems in the area of Colorado and Leon Counties are commonly interpreted to be wave-dominated to fluvial-dominated, detailed documentation of the sediment structure and ichnology in these core suggest a complex interaction of fluvial, wave, and tidal processes. For example, the fair-weather deposits with wave ripples and intense bioturbation are commonly interrupted by the normal or inverse graded beds generated by hyperpycnal flows. The deltaic succession shows higher fluvial influences in the on-axis core and higher wave influences in the off-axis core in Leon County. The upward-coarsening deltaic deposits are commonly truncated by fluvial deposits with upward-coarsening and blocky vertical grain-size trends. These fluvial-channel deposits are commonly 2-5 meters thick and occur in amalgamated succession up into >10 m thick units. The on-axis Vogelsang core has relatively larger channels than the off-axis TOH-2A core. The fluvial deposits grade upward into tidally influences estuarine deposits. These estuarine deposits are associated with abundant tidal indicators with bi-directional ripples, rhythmites, and double mud drapes. Wave ripples also occur in estuarine successions, suggesting an outer estuarine setting. The delta-fluvial-estuary succession ranges from 80-250 ft thick in both cores, representing rapid (100s of thousand year time scale) shoreline regression and transgression. The frequent variations in the types of shoreline and dominant processes strongly impact the reservoir heterogeneities. Therefore it is necessary to factor these processes into reservoir characterization and modelling.