Clinoform growth in a Miocene, Para-tethyan deep lake basin: thin topsets, irregular foresets and thick bottomsets

Late Miocene lacustrine clinoforms of up to 400m high are mapped using a 1700km(2) 3-D seismic data set in the Dacian foreland basin, Romania. Eight Meotian clinoforms, constructed by sediment from the South Carpathians, prograded around 25km towards southwest. The individual clinothems show thin (10-60m thick), if any, topsets, disrupted foresets and highly aggradational bottomsets. Basin-margin accretion occurred in three stages with changing of clinoform heights and foreset gradients. The deltaic system prograded into an early-stage deep depocenter and contributed to high gradient clinoforms whose foresets were dominated by closely (100-200m) spaced 1.5-2km wide V-shaped sub-lacustrine canyons. During intermediate-stage growth, 2-4km wide canyons were dominant on the clinoform foresets. From the early to intermediate stages, the lacustrine shelf edges were consistently indented. The late-stage outbuilding was characterised by smaller clinoforms with smoother foresets and less indentation along the shelf edge. Truncated and thin topsets persisted through all three stages of clinoform evolution. Nevertheless, the resulting long-term flat trajectory shows alternating segments of forced and low-amplitude normal regressions. The relatively flat trajectory implies a constant base level over time and was due to the presence of the Dacian-Black Sea barrier that limited water level rise by spilling to the Black Sea. Besides the characteristic shelf-edge incision of the thin clinoform topsets and the resultant sediment bypass at the shelf edge, the prolonged regressions of the shelf margin promoted steady sediment supply to the basin. The high sediment supply at the shelf edges generated long-lived slope sediment conduits that provided sustained sediment transport to the basin floor. Clinothem isochore maps show that large volumes of sediment were partitioned into the clinoform foresets, and especially the bottomsets. Sediment predominantly derived from frequent hyperpycnal flows contributed to very thick, ca. 300-400m in total, bottomsets. Decreasing subsidence over time from the foredeep resulted in diminishing accommodation and clinoform height, reduced slope channelization and smoother slope morphology.