TAPHONOMY OF SAND-SIZED CARBONATE GRAINS IN CONTEMPORARY, TIDALLY-INFLUENCED ENVIRONMENTS OF PIGEON CREEK, SAN SALVADOR, BAHAMAS

Taphonomic processes such as encrustation, microboring, micritization, fragmentation and abrasion affecting skeletal grains are commonly regarded as negative factors that obscure genetic information conveyed by initial sediment grain composition. Yet, while making it more difficult to identify individual grains, they can be also a source of information on depositional environments. The objective of this study was to quantify taphonomic signatures of carbonate grains belonging to sand size fraction across a range of contemporary carbonate tidal channel and tidal lagoon environments. Using bulk samples of sediment collected from Pigeon Creek (San Salvador, Bahamas), taphonomic alteration of grains examined under binocular microscope has been recorded using five binary (presence/absence) variables. The results indicate that intensity of taphonomic processes varies among different fractions, with fragmentation, encrustation and boring more abundant in very coarse and coarse sand fractions and abrasion and biofilm-related calcification dominating in medium to fine sand fractions. Taphonomic profiles of studied sites result from different susceptibility of various skeletal grain types to these processes and are therefore correlated with grain composition in each site. However, type and intensity of alteration of grains belonging to particular group are controlled by the environment of deposition, e.g. algae and foraminifers are more abraded in tidal inlet sites than in calm tidal and lagoonal sites. Similar taphonomic signatures can originate from different processes, e.g. intensive fragmentation in lagoonal environment results mainly from biotic, pre-mortem factors, but in the main tidal channel it is predominantly mechanical in origin. Multivariate analyses show that taken together, the measured taphonomic variables allow for a more resolved and consistent discrimination of tidal channel-lagoon environments than grain composition data alone (i.e. data routinely included in sedimentologic studies).

Adaptability of these results to typical microfacies thin-section studies is estimated by comparison of the results obtained by examination of loose sediment grains with point-counting and recording of grains’ taphonomic traits in respective resin-embedded sediment samples.