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Stratigraphic Analysis of the Eagle Ford Formation, South Texas, USA: Application of Geochemistry and Quantitative Facies Analysis

Abstract

Geochemical and quantitative facies (Markov Chain) analyses show a link between coastal processes and pelagic-sedimentation. Using the Eagle Ford Formation as an example, the proper stratigraphic analysis required an understanding of the long- and short-term cycles of three interrelated parameters: 1) redox condition, 2) hydrodynamics, and 3) sediment input. Proxies for redox conditions are the abundances of Mo and V+Cr combined with the bioturbation index. Hydrodynamic energy at the depositional site is reflected by sedimentary structures, sole marks, and textures. Siliclastic input is determined using ratios of Si/Al, Ti/Al, and Zr/Al. The study shows that the Lower Eagle Ford (Late Cenomanian) was deposited during the earliest transgression of proto Gulf of Mexico (GOM) into the North America (NA) seaway. The position of oxygen minimum zone (OMZ) is shallow, allowing the preservation of organic-rich sediments. Facies deposited during fair-weather condition consists of dark, thinly laminated, calcareous mudrocks (Facies 1A). The interbedded facies will depend on the relative position to the storm wave base (SWB) and activity of bottom water currents. Sediments deposited above SWB are composed of Facies 1A interbedded with storm deposits (Facies 3). Sediments deposited below SWB are predominantly composed of Facies 1A with interbedded ripple to laminated rhythmites (Facies 2B). The Lower Eagle Ford is also characterized by high amounts of total organic carbon (TOC), uranium, iron, other metals, and high ratios of Si/Al and Ti/Al. The Upper Eagle Ford (Turonian) and lower portion of the Austin Chalk were deposited during the middle to late transgressive systems tract. Bottom water conditions during the Upper Eagle Ford deposition fluctuated from dysoxic to oxic. This resulted in interbedded dark gray, bioturbated lime mudstones (Facies 1B) and light-colored, bioturbated lime mudstones (Facies 1C). Storm deposits (Facies 3) were locally preserved but homogenized by bioturbation. The gradual deepening of the shelf resulted in the higher proportion of ripple to laminated rhythmites (Facies 2B) interbedded with Facies 1B and 1C. The Upper Eagle Ford is characterized by high Si, Al, and Ti decreasing in abundance up section.