Sedimentology and stratigraphy of the earliest deltaic shorelines of the Paleocene Lower Wilcox Group in the Gulf of Mexico

Abstract

Large-scale growth of shelf-to-slope margins is well documented in regional 2-D seismic data, but it is only rarely linked to sedimentological processes and detailed stratigraphic architecture at smaller scales. This study documents the building of a shelf-to-slope margin topset through successive shoreline regressions and transgressions by using data from 824 wells, 80 m of core from 4 wells, and 3750 km3 of seismic data with a focus on the complexities of shoreline behavior during repeated cross-shelf transits. Our combined data set allows understanding of the variability of depositional processes on the shelf at the scale of individual regressive–transgressive cycles (less than 105 years), and shows that the paleogeography of the Wilcox Group in the northern Gulf of Mexico is more diverse than commonly depicted. At a fourth-order scale dominant processes (river, wave, and tide) control shoreline morphology and as a result are critical to understanding sandstone body distribution on the shelf and sediment delivery to the shelf margin and deep water. The earliest deltas of the Lower Wilcox prograded southward under conditions of rising relative sea level across a 50-km-wide shelf, but the sandstone rich deltas remained 10–20 km updip from the shelf edge, and the shelf margin grew through accretion of mud-rich clinothems. The plan-view morphology and internal architecture reflect depositional systems with wave-influenced and tide-modified deltas on the inner and mid shelf, and river flood and wave-reworked hyperpycnite deposits on the outer shelf. An abundance of normal and inverse graded beds, alternation of massive to flat to undulating lamination, and low intensities of bioturbation observed in cores indicate that the shelf sandstone bodies were deposited from hyperpycnal flows. Overall the study shows that the deltaic depositional systems of the Lower Wilcox Group in Texas have a greater architectural complexity and process variability than previously interpreted. Wave-modified hyperpycnites were identified for the first time in the Wilcox and are considered to form a significant component of the muddy shelf succession. The hyperpycnite lobes are located laterally from the deltaic supply fairway in more distal positions than previous studies identified on the shelf. This along-shore redistribution of sediment suggests that it takes more time for deltas to reach the shelf edge than is inferred for many shelf-to-slope margins, and there is likely to be correspondingly less delivery of sediment to deep water.