Are large-scale submarine erosive features created by erosive debris flows or turbidity currents?: Field evidence from the Middle Eocene Ainsa Basin, Spanish Pyrenees

The Mid-Eocene Ainsa basin, Spanish Pyrenees, preserves exceptional exposures of ancient deep-marine sedimentary deposits that include a wide range of mass transport deposits (MTDs), interpreted as cohesive debris flows and sediment slides. With growing observational and theoretical evidence for erosive processes associated with subaerial debris flows, we present outcrop data showing cross-sectional profiles of erosive debris flow processes in Eocene subaqueous MTDs, some of which have channel-like cross-sectional geometry (Dakin et al. 2013). This study suggests that individual debris flows may have been more erosive than turbidity currents, an observation that is opposed to many previous studies from the Ainsa Basin and other ancient deep-water clastic systems. Turbidity currents are not interpreted as the main erosive mechanism observed in outcrop as data shows debrites immediately above the erosion surface directly incorporating sandstone blocks from underlying beds. The blocks are commonly plucked, sheared, disaggregated, brecciated, and partially liquefied as they were incorporated into the base of the pebbly mudstone. Erosive basal surfaces of debris flows also show flow-parallel grooves gouging into the substrate, pebble scours and isolated pebbles embedded in to the immediately underlying sandstone beds. Thin-section analysis shows that erosive debris flows are deposited from wet-type flows, primarily composed of a homogenous, well-mixed texture and containing high sand contents. The most erosive subaqueous debris-flows appear to have created megascours removing up to 35 m thickness of underlying sandy submarine-fan deposits in a lower-slope to base-of-slope setting. Under appropriate conditions, large-volume cohesive debris flows can therefore be at least as erosive as turbidity currents in producing submarine megascours (probably chutes that, in cross section, superficially resemble submarine channels). These observations have important implications for hydrocarbon reservoir continuity and heterogeneity, including the origin and recognition of mudstone-filled chutes or channels identified in seismic profiles. Reference Dakin, N., Pickering, K.T., Mohrig, D. & Bayliss, N.J. 2013. Channel-like features created by erosive submarine debris flows: Field evidence from the Middle Eocene Ainsa Basin, Spanish Pyrenees. Mar. Petrol. Geol., 41, 62-71.