Abstract:
The origin of fine-grained deep-sea facies is often blurred because of interplay of diverse transport mechanisms: sediment gravity flow, traction related to fluid-driven circulation, and pelagic and hemipelagic "rain" mechanisms. Physical and chemical attributes of the Mediterranean amplify petrologic differences, thus facilitating distinction between mud types in this sea. Important attributes include small distances between sediment input and depositional site, generally low bottom current velocities in the deep basins, and shallow depths that permit preservation of carbonate components, an important criterion for mud facies definition. Particularly important in the Mediterranean are periodic development of intense water mass stratification and pycnoclines which act as sediment barriers, i.e., deviation of low concentration sediment gravity flows, and temporary retention of particles from turbid layer flows and hemipelagic settling. Release and differential settling of terrigenous silt and clay flocs and reworked benthic and planktonic (largely coccolith and foraminifera) components from well-marked density interfaces occur in a manner such that particles are segregated according to size and density. The resulting varve-like deposits display fine parallel laminae of alternating coccolith- and terrigenous-rich layers that show diverse fining-upward trends. Finely laminated sections of this type accumulate more rapidly than hemipelagites and are distributed over larger surfaces than mud turbidites. Analysis of bedform, texture-fabric, composition, geometry, and rates of sedimentation help distinguish (1) fine parallel laminated muds derived from coupled sediment gravity flow and hemipelagic settling from (2) laminated mud turbidites, (3) laminated hemipelagites, and (4) contourites as commonly defined. Study of mud lithofacies in small to moderate size seas, such as the Mediterranean, holds promise for better interpretation of deep-marine fine-grained deposits.
