Peculiar Deepwater Slope Morphology in the Semi-enclosed Mio-Pliocene Dacian Basin, Romania

Shelf margin clinoforms (400-500 m high) are the prominent building blocks of the Mio-Pliocene (8.5-4.1 Ma) Western Dacian Basin, a foreland of the Southern Carpathians of Romania. The Dacian Basin was a semi-enclosed elongate, para-Tethyan seaway that underwent several brackish to freshwater transitions while being connected to the Black Sea. 3-D seismic and well data show that the basin infill was dominated by coeval multidirectional clinoform progradation, dominantly from NE to SW and from SSE to NNW. The deepwater clinoform slopes were spectacularly irregular, with closely spaced canyons and channel-levee systems giving a ';sawtooth' appearance in both strike sections and horizontal slices. The canyons are approximately 1-2.5 km wide, 50-200 m deep and normally host slope channels. Slope channels range in width from 100-900 m and depth from 25 to 50 m. Most slope channels connected to an indented (valleyed or upper- slope scarred) shelf edge. The irregularity of the slope morphology differs from most open marine slopes which can be irregular downdip of the main shelf-edge fairway but becomes smoother laterally from the transport axis. It is likely that line-sourced hyperpycnal flows from multiple steep, Carpathian shelf-edge rivers were responsible for building and maintaining the unusual slope stratigraphy. The high frequency of river-flood underflows that would have been generated in the reduced salinity Dacian Basin also caused large volumes of river-derived sediments to be transported to the aggradational basin floor. Additional processes that might have contributed to the unusual slope stratigraphy are high-frequency base level changes and earthquake-generated slope failures that would have been common in the Dacian Basin during the studied interval.