Seismic stratigraphy of the southeastern Black Sea shelf from high-resolution seismic records

Abstract

High-resolution seismic reflection profiles obtained from the shelves off the Samsun and Trabzon coasts (southeastern Black Sea) have revealed the existence of two distinct reflectors (R and T). Reflector R forms the boundary between the upper depositional sequence A and the lower depositional sequence B. Seismo-stratigraphically, upper sequence A is characterized by stratified (simple, complex and lenticular) seismic reflection configurations which imply changing depositional systems and/or conditions, changes of source and supply of sediments due to the relative change of sea level. Additionally, the observed onlapping pattern within this sequence denotes rising of sea level during the last post-Glacial transgression. The mounded seismic facies beyond the steep gradients on the shelf area of Trabzon imply slump masses. The lower depositional sequence B on the Samsun and Trabzon shelves is commonly characterized by chaotic reflection configurations which are interpreted either as strata deposited in a variable, relatively high energy setting, or as initially continous strata that have been deformed. Additionally, the presence of some parallel-subparallel reflections within this sequence denotes its stratified character. The upper boundary of sequence B (reflector R) reflects a pre-Holocene unconformity, probably caused by falling sea level at the time of the Last Glacial Maximum. Additionally, toplap termination at the upper boundary of sequence B indicates a stillstand of sea level at 91 m depth during the Last Glacial Maximum. Based on borehole data, sequence B is composed of consolidated sand and gravel admixtures in the Samsun area, and basalt-agglomerate-tuff series in the Trabzon area. Reflector T forms the boundary of a zone of anomalous reflections (acoustic turbidity, AT) that are interpreted to represent gas in the sediments of the upper depositional sequence A. The gas accumulations tend to occur in “blanket” and “curtain” forms depending on their appearances on seismic records.