Depositional Models for Fine-Grained Sediment in Western Hellenic Trench, Eastern Mediterranean: ABSTRACT

Abstract

Sediments in tectonically active, topographically restricted End_Page 787------------------------------ settings of the western Hellenic arc, eastern Mediterranean, consist primarily of seven clayey silt and silty clay facies. These sediment types are closely related to specific environment, time, or both. We propose two depositional models for Hellenic arc fine-grained sediments. The first emphasizes downslope transformations from slump and debris flow, to turbidity current, to low density turbidity current or turbid layer mechanisms. The distal end-member deposits settling from low concentration flows are thick, rapidly emplaced, fine-grained uniform muds closely associated with faintly laminated muds, and were ponded in flat trench basin plains. Planktonic and terrigenous fractions in the turbiditic, finely laminated and uniform muds record mixing of materials of gravitative and suspension origin. This sequence prevails under conditions of minimal stratification of water masses, as characterized by the present Mediterranean. A second depositional model is developed for conditions of well-developed water mass stratification, which temporarily occurred over large parts of the basin. In this example, well-laminated rather than uniform mud prevails as the end-product of low concentration flows. These more slowly deposited, very finely laminated and graded units record particle-by-particle settling from detached turbid layers concentrated along density interfaces. Well-laminated layers thus include material from turbid layers complemented by the normal rain of pelagic material settling through the water column. Stratification barriers resulted in region-wide distribution of such deposits, in both slope and trench environments. The applicability of both depositional schemes to other regions of the Mediterrane n and to other small ocean basins can be tested. End_of_Article - Last_Page 788------------