Bioturbation-influenced fluid pathways within a carbonate platform system: The Lower Cretaceous (Aptian-Albian) Glen Rose Limestone

The Aptian-Albian Glen Rose Limestone (GRL) is an argillaceous shallow-marine carbonate deposit on the Central Texas Platform and contains 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 further subdivided into hydrostratigraphic units (HSUs). This study uses an integrated ichnological and sedimentological approach to record changes in ichnofabric index (ii) as a proxy for bioturbation within the GRL and relates these changes to fluid flow. Fluid pathways within HSUs are controlled by the complex interaction of faults and fractures, karst development, and large-scale bioturbation-influenced porosity and permeability. The effect of bioturbation-influenced porosity as an aquifer characteristic is the least studied of these factors. Post-depositional solution enhancement of ichnofossils is also common and has increased lateral and vertical fluid connectivity in some HSUs. Most GRL strata are dominated by Thalassinoides networks, but also contain Palaeophycus, Planolites, Ophiomorpha, Serpulid worm tubes, rhizoliths, and Cruziana. Thalassinoides are commonly filled with coarser sediment than the surrounding matrix and act as fluid conduits within an otherwise low permeability matrix. Beds with ii3-4 and burrows with permeable fill transmit water readily. Beds with ii5-6 are commonly muddy and heavily homogenized, restricting fluid flow. Grainstone beds commonly have ii1-2 and are well cemented, restricting fluid flow to low intergranular flow. Pore systems dominated by Thalassinoides ichnofabrics, such as the GRL, are difficult to characterize on a large scale using many laboratory methods because they create heterogeneous flow paths depending on difference in permeability between the matrix and burrow fill. Understanding the effects of bioturbation-influenced porosity and permeability on subsurface fluid pathways is vital for creating a geologic and hydrostratigraphic framework for the Trinity aquifer. (C) 2016 Elsevier B.V. All rights reserved.