Deep sea turbidites in brine-filled anoxic basins of the Mediterranean Ridge. Depositional models based on sedimentological and geochemical characterization

Abstract

The detailed sedimentological and geochemical study of four long piston cores collected from the Bannock Libeccio, Bannock Scirocco, Tyro and Urania basins (eastern Mediterranean) lead to the distinction of two basic types of muddy turbidites, one of grey color, the other of green. Individual turbidites are several meters thick, up to 13.30 meters for the mid Holocene Homogenite related to the Santorini tsunami. A conceptual model was presented soon after the first discovery of the deep-seated brines related to submarine dissolution of Messinian evaporites, some twenty years ago: the turbid flow triggered by seismic events and/or by seismically induced tsunami, when reaching the pycnocline that separates the high density anoxic brines from normal sea-water, was split in two parts, one running above the interface, the other breaking it and descending along the basin slope. The model was tested with flume experiment in a tank, and applies to both types of turbidites. The green turbidites are richer in organic C, rare and trace elements and in siliceous plankton, and prevail in the lower portion of the cores, pre-dating the Homogenite event. The grey turbidites are characteristic of the Homogenite and of the lower part of the cores: they record a strong mixing of fine sediments whose source area was above the brine top layer and the bacterial pellicles that characterize the interface. An interplay ofa) the original composition of the source sediment,b) the density and chemical composition of the fluid, andc) of the basin size and configuration controls the depositional models in this complex part of the Mediterranean Ridge. The core collected from the deepest part of the Urania Basin (pit) is strongly different. Early diagenesis, encrustations of fossil shells, and sulphur nodules characterize this core. The anomaly is accounted for by the expulsion of fluids of deep origin along a fault plane. The very high temperatures measured in the Urania pit support this interpretation.