Comparative Ichnology of Pleistocene, Holocene and Modern Carbonate Shorefaces: A Predictive Ichnofacies Model
The sedimentologic aspects of carbonate facies are well documented; however the distribution and effects of bioturbation of carbonate strata are less well constrained. To address this issue, this study evaluates traces and ichnofabrics in Pleistocene, Holocene, and modern carbonate shoreface successions on Crooked-Acklins Platform, southern Bahamas. The goal of this project is to produce a conceptual ichnofacies model that relates the nature, control, and impact of bioturbation to specific carbonate sedimentary textures and facies. Such ichnological and sedimentological understanding can be used to reconstruct facies distributions within heterogeneous ancient carbonate systems. As part of a larger study, this project focuses on Pleistocene strata of Crooked Island and Long Cay, Bahamas. Pleistocene strata include coralline boundstone and rudstone deposits <0.5 km wide along strike, interpreted as patch reefs, and cross-stratified peloid-skeletal-ooid grainstone, interpreted as shoreface and backshore deposits. Pleistocene strata form elongate, margin-parallel topographic ridges characterized by an upward decrease in grain size and ichnofabric index (ii), and an increase in rhizolith content. Trace fossils within the topographic highs of ridge successions include vertical burrows (e.g., Conichnus, Skolithos) and boxwork burrows (e.g., Ophiomorpha) that have lined or reinforced walls, indicating a higher energy, shifting seafloor. These areas exhibit ii 2–3, and generally have less diverse trace fossil assemblages than the low-lying areas. Conversely, paleotopographic lows between ridges exhibit traces that are predominantly vertical to oblique to bedding planes (e.g., Cylindrichnus, Rosselia, escape burrows), and generally have ii 4–5. These highly bioturbated areas represent stabilized, subtidal sediments deposited in an upper shoreface environment. Petrographic analyses reveal that these shoreface facies of Crooked Island are dominantly fine-sand peloid-ooid grainstone with varying amounts of skeletal debris. In the Pleistocene samples, primary porosity is preserved with clear equant calcite cement and can exceed 25%. These results indicate how trace fossil associations, ichnofabric indices, and porosity and permeability trends are related to depositional energy, facies, and position within geomorphic bodies. Insights from this study provide a conceptual framework for stratigraphic architecture and heterogeneity in subsurface shoreface analogs.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015