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Anatomy of Shelf-Margin Scale Lacustrine Clinoforms With Thin Topsets, Irregular Foresets and Thick Bottomsets: Miocene Dacian Basin, Romania


Late Miocene lacustrine clinoforms with 400 m thickness are mapped using a 1,500 sq km 3-D seismic dataset of the Para-tethyan Dacian Basin, Romania. The clinoforms, fed by sediment from the nearby South Carpathians, have thin topsets, greatly disrupted foresets and thick bottomsets. The clinoform topsets attained <100 m aggradation while progressing for 25 km into the basin and this led to overall flat shelf-edge trajectory. During the early stage, the 3.5° dipping slope was highly irregular and dominated by closely spaced 1.5–2 km wide and 50–100 m deep V-shaped submarine canyons that were slope confined and shelf-edge indenting. The canyons width increased to 2–4 km as the clinoforms prograded. They gradually disappear on smaller (200 m high) and younger clinoforms with gentler (2°) gradient. The individual clinothem isochore maps show thickest sediment accumulation on the slope (300 m) and basin floor (150 m). The bottomset deposits of are extremely thick (10x topset thickness). Base level of the semi-enclosed Dacian Basin was controlled by tectonic subsidence, water supply under influence of climate and the presence of a threshold sill with the Black Sea Basin to the east. Water level in the basin could not rise over the sill height or spilled to the Black Sea. The sill-bounded lake level and low tectonic subsidence contributed to a persistent normal to force regression which resulted in thin clinoform topsets. The prolonged regressions had another impact. They caused steady sediment supply at the shelf edges that, when combining with steep long-lived slope conduits, allowed large sediment volumes to be transported and accumulated on the basin floors. Decreasing clinoform amplitude due to declined accommodation relative to high sediment supply rates was concurrent with reduced slope conduits and smoother slope morphology.