Discovery of Coarse-Grained Carbonate Drifts in the Maldives – Implications for Ancient Deposits

Abstract

One of the discoveries of IODP Expedition 359 to the Maldives was that the current deposits in the Kardiva Channel are a drift fan. Although the drift package in the Kardiva Channel was identified based on the geometries seen on the seismic data, the cores revealed a facies evolution that has far-reaching implications for interpretations of neritic carbonates. No such system has been reported for carbonates before. The drift deposit started to form 12.9 Ma ago as the strengthening of the Indian monsoon winds triggered a current system in the Indian Ocean. As the current crossed the Maldives archipelago through the narrow Kardiva channel it increased its speed but in the entrance to the Inner Sea the current slowed and deposited a fine to coarse prograding drift deposit with a thickness of 450 m. The onset of the drift is indicated by wavy high-amplitude reflections. The waves have wavelengths of 250–450 m and a height of approximately 5 m. The sediments above the wavy bottom of the drift are partially lithified bioclastic wackestone to packstone followed by a series of unlithified to partially lithified coarsening-upward packages of large benthic foraminiferal grainstone to rudstone. The sedimentation rate in this drift is up to 16 cm/thousand years. The relatively fast infill of the basin resulted in the creation of an environment suitable to produce large amounts of large benthic foraminifers in its upper part. Today the top of the drift is in 520 m water depth and ornamented with large carbonate sand waves indicating that currents continuously remobilize the sediments. One of the interesting characteristics of these coarse-grained drift deposits is the unusual high porosity to great burial depth. Taking the seismic geometries together with the lithologies from the cores this drift is best classified as a drift fan. The comprehensive documentation of this carbonate drift fan will prompt the re-interpretation of deposits with these characteristics. For example, cores at Site U1468 from the apex of the drift mound contained large benthic foraminifera floatstones that resemble benthic foraminifera shoals. Likewise the prograding lobes of this drift fan are very reminiscent of what might be called a tidal delta front.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017