Facies and sedimentology of a carbonate delta drift (Miocene, Maldives)

Abstract

The identification of sediment drifts typically relies on interpretation of reflection seismic data sets. This study sedimentologically analyzed an example of a carbonate delta drift previously identified in seismics in order to provide a catalogue of characteristic features at core and seismic scale for allowing testing the occurrence of this poorly known type of deposit elsewhere. Cores and downhole logs recovered during International Ocean Discovery Program 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: (i) the development of sigmoidal clinoforms that thin out towards proximal and distal settings; (ii) a proximal part characterized by coarse-grained facies with abundant shallow-water components and distal areas dominated by fine-grained facies with rare to absent shallow-water components; (iii) winnowing of the finer fraction in proximal facies; (iv) extensive fragmentation of most of the bioclasts with minor intervals of well-preserved components; (v) bigradational intervals in the proximal part and large channels in proximal to distal settings; and (vi) the lobe to delta shaped outline of the sediment accumulation. The characteristic shallow-water fossil assemblage of the Mid to Upper Miocene delta drift consists of large benthic foraminifera (Amphistegina, Cycloclypeus, Lepidocyclina, Operculina and Heterostegina), fragmented red algae and bryozoans, equinoid debris, and Halimeda plates. The deeper-water part of the drift bodies consists of fine-grained planktonic foraminifera-rich wackestone. Condensed intervals may occur as result of enhanced bottomcurrent activity. In contrast to siliciclastic drift bodies, the carbonate delta drift has an important contribution by in situ shallow-water carbonate production reminiscent of a shoal. In situ carbonate production is proposed as a major controlling factor which is 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.

The ichnology of carbonate drifts

“…The preservation of the trace fossils, which is generally poor, indicates that the surfaces of most of these carbonate drifts were soupgrounds to softgrounds. Locally, firmgrounds with well‐defined traces may occur which are related to omission surfaces created by enhanced currents (Eberli et al ., ; Isern et al ., ; Rodríguez‐Tovar & Hernández‐Molina, ; Reolid et al ., ). Such omission surfaces are a distinctive feature of drift deposits elsewhere (Huneke & Stow, ; Rodríguez‐Tovar & Hernández‐Molina, ).…”

“…In shallow environments such as in the delta drifts of the Maldives not only the current exerts a control on the bioturbation. Changes in the base level by sea‐level fluctuations or enhanced in situ carbonate production determine the type and intensity of the bioturbation (Figs C and B; Reolid et al ., ). In the case of the carbonate drifts of the Santaren Channel and the Marion Plateau the changes in colour are related to the amount of clay in the sediment, that is determined by the energy of the currents as is the case with the organic matter in the Maldives (Eberli et al ., ; Kroon et al ., ; Isern et al ., ; John & Mutti, ).…”

“…Detached drifts form at the downcurrent flanks of the atolls (Betzler et al ., , ) and the currents controlling the drift deposition in the Maldives prevail during the Quaternary (Betzler et al ., ; Bunzel et al ., ; Reolid et al ., ). The water depth at which the drifts were deposited changed through time from ca 50 m at the time of the delta drift deposition (Reolid et al ., ) to ca 500 m in present day (Betzler et al ., ).…”

Characteristics of modern carbonate contourite drifts

“…The drift sediments are composed of skeletal grains and bioclasts from the adjacent atolls, and the getaway itself and skeletal particles from the surface water such as planktonic foraminifera and nannofossils. As the delta drift builds out, it becomes a habitat for intermediate and deep water fauna, in particular echinoderms and large benthic foraminifera that are a major source of the drift sediment at the apex of the drift (Betzler et al ., 2016; Reolid et al ., ). The grain size is predominantly sand but coarser intervals, mostly benthic foraminifera rudstones, are present at the apex of the delta drift.…”

Do drifts deposited adjacent to carbonate platforms record the signal of global carbon isotopic values?

“…() and Reloid et al . (). Sites U1466 and U1468 are located closer to the Miocene shallow‐water platform than Sites U1467 and U1471 and penetrate the drift deposits and the underlying tongues of the prograding, sea‐level controlled platform system.…”