CONTROLS ON BIOTURBATION AND SEDIMENT DISTRIBUTION WITHIN CARBONATE SHOREFACE DEPOSITS: INSIGHTS FROM HETEROGENEITY IN PLEISTOCENE AND RECENT STRATA

Abstract

Burrowing organisms alter sedimentary textures, influence cement distribution, and affect petrophysical characteristics of carbonate strata. Although many descriptions of carbonate successions reference bioturbation, quantitative data on spatial variability of trace fossils are rare, and fewer studies address trace-fossil influence on postdepositional modification of sedimentary deposits, which affects petrophysical properties. To address these unknowns and determine the controls on ichnology in carbonate shoreface successions, this study evaluates the distribution of sediment and bioturbation in recent, Holocene, and Pleistocene shoreface deposits on the leeward margin of Crooked-Acklins Platform (CAP), southern Bahamas. Results illustrate north–to-south along-strike variability on this margin. The extant north margin shelf is characterized by poorly to moderately sorted, very fine–fine, skeletal-peloid-ooid sand with an average of 16% mud (< 62.5 µm), and is moderately to intensely bioturbated (ii3–6). Trace assemblages are diverse, and include horizontal tracks and trails, abundant horizontal deposit-feeding and locomotion traces, as well as dwelling and resting burrows (e.g., Arenicolites, Conichnus, Thalassinoides) attributable to the proximal Cruziana Ichnofacies. South margin shelf deposits are well-sorted, medium ooid-peloid sand with < 1% mud, and display a range of bioturbation, from nonbioturbated to moderately intense bioturbation (ii1–4). Trace-fossil assemblages exhibit low ichnodiversity, and are dominated by vertical dwelling burrows with reinforced wall linings (e.g., Ophiomorpha, Skolithos), attributable to the Skolithos Ichnofacies. Earlier Holocene and Pleistocene strata show similar proximal-to-distal and along-strike variations in sediment attributes, ichnodiversity, and bioturbation intensity. These trends are interpreted to reflect a progressive, north–to-south increase in energy reflecting the change in margin orientation relative to the direction of dominant wave energy, analogous to the modern system. Results show relations among biologic processes (i.e., feeding, dwelling, resting, locomotion) of benthic organisms that produce a range of ichnofabrics, thereby altering depositional texture (i.e., primary porosity) as well as lateral and vertical facies patterns. This study provides an integrated conceptual model for sedimentologic-ichnologic processes and patterns of sediment accumulation on carbonate shorefaces that may be distinct from siliciclastic analogs, insights applicable to interpreting and modeling ancient reservoir analogs.