BIOTURBATION-INFLUENCED FLUID PATHWAYS WITHIN A CARBONATE PLATFORM SYSTEM: THE APTIAN-ALBIAN GLEN ROSE LIMESTONE

This study uses an integrated ichnological and sedimentological approach to record changes in ichnofabric index (ii) as a proxy for bioturbation within the Aptian–Albian Glen Rose Limestone (GRL) in central Texas and relates these changes to subsurface fluid flow. The GRL is an argillaceous shallow marine carbonate system deposited on the Central Texas Platform and forms the upper Trinity aquifer and the upper part of the middle Trinity aquifer. The GRL is divided into Upper and Lower GRL members, which have been subdivided into hydrostratigraphic units (HSUs). Fluid pathways within HSUs are controlled by the interaction of faults and fractures, karst development, and large-scale biogenic porosity and permeability. The effect of biogenic porosity (i.e., burrows, borings, and nodular preservation of bioturbation) is the least studied of these factors. Postdepositional solution enhancement of trace fossils is also common and has increased lateral and vertical fluid connectivity in some HSUs. Most GRL strata in central Texas are assigned to the Cruziana ichnofacies and are dominated by Thalassinoides networks, but also contain Cruziana, Ophiomorpha, Planolites, and Serpulid worm tubes. Rhizoliths associated with subaerial exposure are present due to sea-level fluctuations. A previous study showed permeability associated with Thalassinoides-dominated ichnofabrics controls horizontal fluid flow within most of the overlying Edwards aquifer, where unfilled ichnofossils form interconnected fluid conduits. The mudstone and marl within the GRL, however, complicates such ichnofabric analysis, as many GRL Thalassinoides are filled with grainstone to mudstone acting either as conduits or barriers. Burrow fill is commonly similar to overlying beds and is directly related to changes in depositional energy and rate of siliciclastic input. Transmissive HSUs have ii3–4 and burrows are commonly open or have permeable fill, whereas confining HSUs have either higher or lower ii than permeable units. Understanding these effects is vital for understanding subsurface water pathways of the Trinity aquifer. This study is also relevant to hydrocarbon exploration as changes in depositional environment seen at the surface and near surface in central Texas may correlate to changes within the deep GRL of south Texas.