Subsurface and outcrop characteristics of fluvial‐dominated deep‐lacustrine clinoforms

Abstract

Abstract Deep‐lacustrine deposits provide records of palaeoclimate and tectonics, and often host major hydrocarbon reservoirs, but their facies description and long‐term stratigraphic architecture are not sufficiently reported. A combined outcrop and three‐dimensional seismic dataset in the western Dacian Basin of Romania is used to decipher depositional systems, basin‐fill architecture and clastic sediment dispersal across a narrow shelf. The lake clinoforms are about 400 m high in seismic images and all of their aggradational bottomsets total up to 350 m in thickness. Depositional elements and morphology of fluvial channels, delta lobe complexes, sublacustrine channel forms, sublacustrine canyons and deep‐lacustrine lobes are interpreted on the seismic attribute maps. Outcrops show that sharp‐based deltaic units contain thin delta‐front deposits. The slope succession is dominated by channel‐levée thin‐bedded turbidites with terrestrial debris. Thicker and coarser turbidites are found in sublacustrine channels. The channelized sandstones on the slope are 10 to 25 m thick and often overlie tens of metres thick mass‐transport deposits. Tabular turbidite beds, sandy‐conglomeratic debrites with shallow‐water fossils, mud‐rich mass‐transport deposits and hybrid event beds within fan lobes are found on the basin floor. The integrated seismic‐outcrop analysis suggests that low accommodation on the narrow (10 to 30 km) morphological shelf and high sediment supply resulted in the prograding lacustrine shelf‐margin clinoforms with fluvial‐dominated topsets and significant sediment bypass to the deep lacustrine. The late Miocene–Pliocene Dacian Basin provides a typical example of a supply (river)‐dominated basin margin and possible recognition criteria of deep‐lacustrine clinothems including: fluvial‐dominated topset deposits with abrupt vertical facies changes, bottomset‐dominated sediment partitioning and frequent sediment gravity flow activities denoted by closely spaced and aggradational channel‐levée systems, thick bottomsets and rare indications of sediment starvation in the deep‐lacustrine deposits.