Facies and sedimentology of a carbonate delta drift

Abstract

The identification of sediment drifts typically relies on interpretation of reflection seismic data sets. Here, we sedimentologically analyze an example of a carbonate delta drift depositional system previously identified in seismics to provide a catalog of characteristic features at core and outcrop scale for allowing testing the occurrence of these poorly known type of deposit elsewhere. Cores and downhole logs recovered during IODP Expedition 359 to the Maldives in combination with seismic data were analyzed with this objective. The diagnostic criteria for the sedimentological recognition of a delta drift are: (1) the development of sigmoidal clinoforms that thin out towards proximal and distal settings, (2) proximal facies characterized by coarse-grained facies with abundant shallow-water components and distal facies dominated by fine-grained facies with rare to absent shallowwater components, (3) winnowing of the finer fraction in proximal facies, (4) extensive fragmentation of the bioclasts, (5) occurrence of large channels and bigradational intervals, and (6) the lobe to delta shaped outline of the sediment accumulation. The characteristic shallow-water fossil assemblage of the delta drift, which is Miocene in age, consists of large benthic foraminifera (Amphistegina, Cycloclypeus, Lepidocyclina, Operculina, Heterostegina), fragmented red algae and bryozoans, equinoid debris, and Halimeda plates. The deeper water part of the rift bodies consist of fine-grained planktonic foraminifera-rich wackestone. Condensed intervals may occur as result of enhanced bottom-current activity. In contrast to siliciclastic drift bodies, a carbonate delta drift has an important contribution by insitu shallow-water carbonate production, which is proposed as a major controlling factor as important as the pelagic settling or the shaping by density and bottom currents in siliciclastic drifts. In the absence of three-dimensional data and in two-dimensional views the carbonate delta drift sediment bodies resemble carbonate ramps, which indicates that there may be the need to re-evaluate various cases of such systems described from the geological record.