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Abstract

Wilcox deposition from late Paleocene to early Eocene in age (60–52 Ma) recorded one of the largest and most laterally extensive periods of sand deposition within the Gulf of Mexico. The receiving basin extends seaward over 300 miles from its equivalent shelf margin and along strike approximately 450 miles, attaining thicknesses greater than 3000 feet. Based on previous research by Blum and others utilizing detrital zircons to reconstruct Cretaceous to Paleogene drainage systems, a rerouting of sediment flux occurred in going from Cretaceous to the Paleocene Wilcox, during which time the sediment drainage systems incorporate nearly the entire continental United States ranging from the Appalachians to the east and the Sierra Nevada’s to the west. Massive amounts of sediment were transported through large river systems seaward into coastal fluvial-deltaic systems. These extensive deltaic systems provided the staging area for deposition into the “deeper-water” environments through a sediment pathway system differing in gradient and paleotopography than the younger Miocene to Plio-Pleistocene systems that industry commonly uses for comparison with respect to sediment distribution and architecture. Fluctuations in climate during the mid-Paleocene–early Eocene resulted in erratic high rates of erosion experienced in hinterland sediment sources and contributed to an unusually high sediment flux into the Gulf of Mexico Basin. Unique to the Wilcox submarine fan deposition is that it occurred during an ice-cap free world when greenhouse conditions dominated and was not the characteristic “low stand fan” we associate within our sequence stratigraphic concepts.

Within the deep-water Gulf of Mexico, the Wilcox interval is commonly greater than 3000 feet in stratigraphic thickness. The chronostratigraphic framework is typically subdivided into four depositional sequences referred to as Wilcox 1–4. The Wilcox 4 is the oldest and serves as a proxy for early paleotopography of the basin as the architecture observed during the onset of Wilcox 4 deposition appears to be influenced by a resetting of the paleotopography in response to the Cretaceous Chicxulub meteorite impact on the Yucatan Peninsula. The deep-water Wilcox chronostratigraphic model integrates foraminifera, calcareous nannofossils, chemostratigraphy, palynology, and siliceous microfossils, specifically radiolarians, to aid in age determination as well as duration of sequences.

Wilcox 2–4 sequences span approximately 60–54 Ma and are underlain by the Midway Shale to Cretaceous age sediments and continue upward through the end of the Paleocene to Eocene Thermal Maximum (PETM). The PETM separates the Lower Wilcox (Wilcox 2–4) from Upper Wilcox (Wilcox 1) and is a major global warming event that occurred around 55 Ma and lasted approximately 200,000 years, with observed changes in sediment style and architecture across this boundary, and is considered by many to be a proxy for today’s climate change. Lower Wilcox intervals (2–4) commonly exhibit high net to gross and laterally extensive, weakly confined channelized distributive lobe architecture. Upper Wilcox 1 unit tends to be more variable across the basin possibly due to (1) sporadic sediment flux from the river systems and associated ephemeral nature of these deposits in response to the effects of high CO2 levels and extremely warm climate, (2) basin gradients approaching regional equilibrium establishing more bypass of sediments further out into the basin, and (3) sediment flux variability due to changing drainage basin inputs from the updip large riverine systems.

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