Lithofacies, biostratigraphy, chemostratigraphy, and stratal architecture of the Boquillas Formation and Eagle Ford Group : comparison of outcrop and core data from Big Bend National Park to Maverick Basin, Southwest Texas, USA

Abstract

The late Cretaceous-aged Eagle Ford and Boquillas Formations were deposited on the Texas paleoshelf during a major transgressive sequence wherein organic-rich mudrocks accumulated across the paleoshelf. This study investigates lithofacies; biostratigraphy, chemostratigraphy, and stratal architecture of the Eagle Ford and Boquillas Formations to characterize the depositional environment present during deposition, as well as define and describe the Cenomanian-Turonian (C-T) stage boundary and the Oceanic Anoxic Event II (OAE2). Comprehensive, high-resolution data sets compare geologic and geochemical interpretations of subsurface Eagle Ford Group conventional core from the Maverick Basin and chrono-synchronous Boquillas Formation outcrop strata from Big Bend National Park. Results from core and outcrop show a dynamic depositional environment regularly influenced by bottom-currents, debris-flows, and deposition during anoxic bottom-water conditions. Elemental and biostratigraphic data show that the water-column was stratified -- surface-waters experienced high levels of primary productivity while deeper waters were anoxic to euxinic. The Eagle Ford strata are divided into a lower and upper group defined geochemically by the appearance of a titanium-rich chemofaceis correlative to massive argillaceous claystone. This change in deposition has been defined to occur concurrently with the C-T boundary (identified biostratigraphically) and OAE2 (identified chemostratigraphically and isotopically). The OAE2 as documented within both cores shows an 'oxygenated' anoxic event, wherein burrowing and low molybdenum are documented during the positive δ¹³C isotope excursion. Outcrop and core data comparison demonstrate a similar depositional system between Big Bend National Park and Maverick Basin; bottom-currents, debris-flows, and periodic anoxia are all documented within both sections, however, further investigations are needed to correlate the sections.