Architecture of unusually thick bottomsets and multidirectional sediment sourcing in deep lake clinothems, Dacian Basin, Romania
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The models for basin floor fans formed at lowstand or highstand condition of sea level are now well known. In this project an additional complexity is added to deep water fan architecture where sediment is fed to an inland-lake basin floor from widely dispersed,multiple directions. Late Miocene clinoforms some 300-450 m high feed down onto the basin floor and are easily recognized in 3-D seismic of the western Dacian Basin, a para-Tethys basin in Romania. The basin was a closed lake basin during most of its existence. The basin margin clinoforms can be difficult to recognize even across closely spaced well logs but in seismic data the three clinoform segments are well imaged, namely a 100-150 m thick, lower interval with coarse sandy deposits, an overlying 150-200 m thick muddy succession and and an upper 50-100 m thick succession of sandy deposits. The 3-D seismic data combined with seven well cross-sections are the main tools used here to investigate the architecture of the basin-floor fans. The submarine fans are surprisingly aggradational, reaching thicknesses 100-150 m in places, and the fans can be followed laterally on the seismic data for hundreds of kilometers. Spontaneous potential (SP) and resistivity logs of some 200 closely spaced (~300 m) wells have been correlated over an area of thick basin-floor fans. Shelf, shelf edge, slope and basin floor depositional systems were mapped on seismic data and also recognized in well logs. The well logs show that the fans are composed of vertically repeated 10-30 m thick sandstone units, interpreted here as fan lobes. The lobes have variable sandstone distribution with a range of coarsening or thinning upwards, blocky or ratty log patterns suggesting variable facies and depositional settings. Individual lobes extend from the lower slope and onto the basin floor. In the ‘distal’ area of one of the clinoforms the fans have thicker sandstone intervals,possibly representing lobes formed by sediment shed from a different directions Into this location on the basin floor. We are able to document deepwater fan-lobe sandstones with different orientations and variable thicknesses through time, as these are linked with variable rates and directions of basin margin progradation. As a consequence, the complexity of fan lobes and feeding directions makes it likely that sediment was supplied from multiple catchment areas. The unusual thickness of the Dachian Basin submarine fans relates both to the interfingering of differently oriented fans and to the tendency for a greater frequency of river-derived sediment-gravity flows reaching the basin floor of relatively fresh-water lakes. This high partitioning of sandy sediment into the clinothem bottomsets is likely to be similar in other enclosed lake basins.