A Comparison of Depositional Styles: Late Cretaceous Eagle Ford and Late Devonian Woodford Formations, U.S.A.
Malicse, Ariel1 and Garcia, Judit
The Eagle Ford and Woodford formations are two prolific resource plays and source rocks, one in South Texas and the latter in Oklahoma and North Texas. The primary objective of this study is to compare and contrast the depositional processes of the Late Cretaceous Eagle Ford and the Late Devonian-Early Mississippian Woodford formations. The Eagle Ford was deposited at a time when coccoliths and planktonic foraminifera proliferated at the shallow to moderate deep water shelves of the North America Seaway.
The Eagle Ford can be divided stratigraphically into Lower and Upper units. Deposition of the Lower Eagle Ford coincided with the Late Cretaceous OAE2 event, characterized by deposition in a prolonged suboxic water column. Facies deposited during these episodes are dominated by dark-gray to black, lime-mudstones. Other minor facies are low TOC, calcareous turbidites and local storm deposits. The Cenomanian-Turonian boundary in the Eagle Ford marks a drop in sea level and a regional increase in clay and silt content. The Turonian, Upper Eagle Ford consists of alternating organic-rich lime mudstones and "clean", bioturbated lime mudstones. The interbedded facies is interpreted to be the result of alternating periods of suboxic to oxygenated conditions.
Deposition during the Late Devonian to early Mississippian in the proto North America was dominated by upwelling of silica & nutrient rich water near the continental slope break and the super estuarine-like circulation of this water into the adjacent epicontinental seas. The transgression of silica super-saturated water in the slope-shelf regions resulted in a prolonged stratified water column, which in turn favored the accumulation and deposition of organic, clay to silica rich mudstones known as the Woodford. Locally interbedded with the organic-rich mudstones are low TOC, chert beds. The bedded cherts appear to be the results of complex interaction of upwelling, migration, and winnowing underwater currents along paleo channels.
Both Eagle Ford and Woodford organic-rich sediment accumulation occurred during marine transgressions. The Eagle Ford formation is characterized by preservation of organic matter in anoxic and low energy water, with no evidence for oceanic upwelling. By contrast, the Woodford black shale accumulation is driven largely by upwelling. This upwelling, when coupled with a high rate of evaporation in the epeiric sea, led to a low oxygen concentrated bottom water.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013