Late Quaternary co-seismic sedimentation in the Sea of Marmara's deep basins

The deep, northern, part of the Sea of Marmara (northwestern Turkey) is composed of several aligned, actively subsiding, basins, which are the direct structural and morphological expression of the North-Anatolian Fault's northern branch. The last 20 kyr of their sedimentary fill (non-marine before 12 kyr BP) have been investigated through giant piston coring onboard R/V MARION-DUFRESNE (MARMACORE Cruise, 2001) and by chirp sub-bottom profiler onboard R/V ATALANTE during MARMARASCARPS Cruise (2002). Especially during the lacustrine stage, the infilling of the deep basins (Tekirdaǧ, Central, Kumburgaz, and Çinarcic Basins; up to 1250-m depth) was dominated by turbidites (with coarse mixed siliciclastic and bioclastic basal parts), intercalated in "hemipelagic-type" fine-grained calcareous and slightly siliceous clays. Often the turbidites show strong segregation and a sharp boundary between a coarse lower part and a suspended-load upper part. In the Central Basin, 8 m of a unique sedimentary event include a 5 to 8-m thick "homogenite" well imaged on seismic profiles. The latter is interpreted as related to a major - possibly earthquake-triggered - tsunami effect, as described in the Eastern Mediterranean by Kastens and Cita [Kastens K. and Cita M.B., 1981. Tsunami-induced sediment transport in the abyssal Mediterranean Sea. Geological Society of America Bulletin, 92:845-857]. In the marine (Holocene) upper part of the sedimentary fill, repeated to-and-from structures, affecting silt or fine sand, are evidencing seiche-like effects and, thus, earthquake triggering. Detailed correlations between two deep coring sites (at 1250 m and 1200 m) indicate more than 100% over-thickening in the deepest one; this implies specific processes of distribution of terrigenous input by dense currents (high kinetic energy, seiche effects, complex reflections on steep slopes). The peculiar sedimentary fill of the Sea of Marmara's Central Basin is interpreted as a direct consequence of the strong seismic activity, (and, by extrapolation, to the whole set of deep basins). This imprint is more significant below the base of the Holocene, as environmental conditions favoured marginal accumulation (especially on the southern shelf) of large amounts of erosion products available for mass wasting.