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Ichnofacies and Ichnoporosity-Ichnopermeability in Carbonate Strata: Comparison of Recent, Holocene, and Pleistocene Successions of Crooked-Acklins Platform, Bahamas

Abstract

Bioturbation in carbonate strata result in ichnofacies and ichnofabrics that often modify depositional porosity and permeability. These processes are less understood than those in siliciclastic environments in the recent and deep time. To improve understanding of the nature of bioturbation in carbonate strata, this study examines the distribution of modern and Pleistocene trace-making organisms on the Crooked-Acklins Platform, Bahamas, focusing on organism-media (=substrate) interactions in a shoreface and a tidal flat system. Across the oolitic shoreface, one 2-km-long transect that extended from the shoreline to the lower shoreface complimented previous data collection. In aquatic settings, a 50-cm2-grid was used in a cruciform pattern (5 replicates) to photograph bottom sedimentary structures and bioturbation patterns. Data collection from the shoreline inland to the beach berm and lake settings where sampled across 25-m-long (parallel to beach) x 4 to 15-m-wide (shoreline to inland) transects. Cast of burrows made with dental plaster in shallow water and subaerial settings to record the architecture and depth of biogenic structures, as well as their orientation to the shoreline. Traces in the 2-km-long transect include polychaete worms and acorn worm burrows (cf. Skolithos, Arenicolites, Rosselia), lined and unlined shrimp burrows (cf. Ophiomorpha, Thalassinoides), stingray feeding pits and fish nests (cf. Piscichnus), echinoid surface and shallow subsurface trails (cf. Taphrhelminthopsis, Scolicia), and snail self-crossing, looping trails (cf. Gordia, Mermia) in this offshore deepening and fining region. Preservation of physical sedimentary structures was greater where water depth was shallower and water energy was greater. Burrow depths appear to deepen as water depth increased and energy decreased. Preservation of sedimentary structures was greatest in the beach and backshore area but decreased dramatically landward. A comparable sampling strategy in subtidal, intertidal, and supratidal deposits in Turtle Sound revealed a reduction in burrow densities and depth from subtidal to supratidal settings, with an increase in bioturbation and depth in terrestrial settings with no marine influence. Comparison of traces and facies in Holocene beachrock and Pleistocene outcrop revealed similar bioturbation patterns to that observed in the shoreface and tidal flat systems.