Mechanisms of sediment transport and deposition: Sediment sequences and accumulation during the Holocene on the Thermaikos plateau, the continental slope, and basin (Sporadhes basin), northwestern Aegean Sea, Greece

Abstract

Detailed 3.5-KHz seismic reflection profiling carried out on the Thermaikos plateau, the slope and the basin (Sporadhes basin), northwestern Aegean, has shown that during the Holocene, the area received fine-grained sediments discharged mainly by the rivers Axios, Aliakmon and Pinios. A well-developed oblique sigmoid progradation pattern has been observed on the seismic profiles taken across the deltaic Holocene sequences of these rivers. Three seismic facies have been identified within the oblique-sigmoid pattern: (1) Dark homogeneous facies that represent fine sand deposits along the delta front (topsets), (2) stratified facies that represent alternating muddier or sandier horizons (foresets), and (3) transparent facies in which the muddy sediments predominate (bottomsets). The base of the Holocene (reflector C) has been found beneath the oblique-sigmoid pattern. The thin homogeneous and transparent seismic facies throughout the slope and the basin has been interpreted as the Holocene muddy sediment sequence in the area. Bottom samples from the outer Thermaikos plateau have shown that it is floored by relict terrigenous sand at depths of 90–110 m. The sediment and clay mineral distribution and the presence of relict sand confirm that during the Holocene period, the fine-grained sediment dispersion and deposition was effected by the water-mass circulation. The majority (80–90%) of the sediments dispersed from the rivers during the Holocene has been captured by the extensive shelf (Thermaikos plateau), having deposited from the near-bottom nepheloid layer. Minor amounts (10–20%) of very fine-grained material have escaped and deposited from suspension (hemipelagic settling) in the slope and basin of the studied area.