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

Abstract

© 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists 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.