Sedimentology and sedimentary geochemistry of the Eagle Ford Group, Texas

The Eagle Ford Group is an organic-rich succession of rhythmically interbedded packstones and wackestones, deposited in the southern Western Interior Seaway during the Cenomanian-Turonian of the Upper Cretaceous. Recent investigations have suggested that either eustatic sea level change, astronomical forcings, change in primary productivity and dilution, or changes in local and global benthic oxygenation may be responsible for the Eagle Ford Groups lithological and geochemical variations. However, no consensus has yet been reached. Chapter 3 examines cm-scale geochemical, mineralogical and sedimentological changes to investigate the temporal and spatial variabilities of the Eagle Ford Group. This study suggests the stratigraphical cyclicity of the Eagle Ford Group is a function of alternating periods of high and low primary productivity and accumulation rates possibly modulated by the fertilizing effect of ash falls. Lateral variabilities are determined by the reworking of pelagic sediment by bottom currents, below storm wave base. Chapter 5 investigates changes in water-mass exchange, climate, water-column anoxia and hydrodynamics during the deposition of the Eagle Ford Group. The results of this study show four distinct stages in the deposition. The lowermost stratigraphic third of the Lower Eagle Ford was deposited under a humid climate with high continental runoff, anoxic/euxinic benthic conditions and exchange with a nutrient-rich water mass. The upper stratigraphic two thirds of the Lower Eagle Ford was deposited during a more arid climate, with lower continental runoff, a highly restricted anoxic benthic zone with some periods of euxinia, and little siliciclastic input. The lowermost stratigraphic half of the Upper Eagle Ford was deposited during a mostly humid climate with high continental runoff and the presence of a suboxic benthic zone that experienced multiple periods of oxygenation related to a more open water mass exchange. The upper stratigraphic half of the Upper Eagle Ford exhibits deposition under a more arid climate with lower continental runoff and reduced benthic oxygen content. Global controls, such as perturbations of the global carbon cycle, exert a fundamental role in the large-scale deposition of organic carbon. As such it is important to understand the effect and causes of global events when studying sedimentary systems. The Eagle Ford Group contains one such event Oceanic Anoxic Event 2 (OAE 2). However, the fundamental cause of OAE 2 is still poorly constrained. As such in Chapter 4 mercury concentrations are examined in sedimentary successions spanning the Mid-Cenomanian to Early Turonian interval. The data presented here are consistent with an initial magmatic pulse at the time of the MCE, with a second and greater, pulse at the onset of OAE 2, possibly related to the emplacement of LIPs in the Pacific Ocean and/or the High Arctic.