ORGANIC-RICH MUDSTONE DEPOSITED UNDER SHALLOW-WATER ANOXIC AND EUXINIC CONDITIONS ON CONTINENTAL SHELVES AND EPICONTINENTAL SEAWAYS: AN EXAMPLE FROM THE LATE CRETACEOUS EAGLE FORD FORMATION, WEST TEXAS, USA

The Late Cretaceous Lower Eagle Ford Formation in west Texas is an example of an organic-rich mudstone deposited on a continental shelf and shallow epicontinental seaway. Within the Lower Member of the Lower Eagle Ford Formation are foraminiferal grainstone beds with hummocky and swaley cross-stratification (HCS and SCS, respectively) that are interbedded with organic-rich mudstone containing geochemical evidence of anoxia or euxinia such as: uranium and molybdenum enrichment, Mo/U ratios > seawater, low pristane/phytane (Pr/Ph) ratios, and aryl isoprenoids ratios (AIR). Furthermore, bioturbation within this facies is rare, consisting of mm scale horizontal burrows (Chondrites) and limited to the tops of storm event beds. This combination of geochemical, biological, and sedimentological data suggests that strata within the Lower Eagle Ford Formation, containing this facies, were deposited in relatively shallow waters (10’s of meters depth) on a shallow shelf. In this setting, storms, which mixed the water column, deposited the wave-stratified deposits. Stagnation between major storms resulted in prolonged intervals of shallow water anoxic, and even euxinic deposition, and water stratification, effectively excluding nearly all benthic organisms during deposition of these rocks. It is suggested that muddy, anoxic, shallow-water depositional environments are probaby more widespread, and the sedimentary record less complete due to hiatuses, than commonly thought within shallow epicontinental seas.