Submarine Lobes Confined Behind Depositional Relief at the Toe-Wall of a Seismic-Scale Mass Transport Complex, Neuquén Basin, Argentina

Hodgson, David; Duller, Robert A.; Jackson, Christopher A.; Spychala, Yvonne

Mass transport deposits can entrain large pieces of substrate, and generate significant depositional relief that influences subsequent dispersal patterns of sediment gravity flows. Highly-deformed Bathonian-to-Lower Callovian strata at Chacay Melehue, NW Neuquén Basin, Argentina, offer a valuable insight into the emplacement processes and influence on subsequent flow processes of a seismic-scale (>7 km-long and up to 70 m-thick), mass transport complex (MTC). The correlation of marker horizons between measured sections (200-500 m spacing) permits the structural and depositional architecture of the MTC to be constrained. The areal extent and relatively persistent thickness of basal marker beds suggests that the MTC was deposited in a relatively low gradient setting. The overlying MTC has a highly sheared basal unit that displays ~30 m of relief, before stepping up-stratigraphy to be concordant with the underlying stratigraphy. This surface is interpreted as the basal décollement to the overlying MTC; the downslope, inclined surface is interpreted as a steep, up-dip-facing toe-wall. The MTC is therefore frontally-emergent.

The MTC comprises two main sedimentary facies. The lower unit comprises tightly-folded, thin-bedded turbidites that are dissected by thrust faults, which is interpreted as a slide. This is overlain by a more extensive, poorly-sorted facies that contains well-rounded, extra-basinal clasts and sub-angular, laminated, intrabasinal clasts, which are supported by an argillaceous, poorly-sorted, sandy siltstone matrix that continues beyond the toe-wall. The upper unit also contains clasts of reworked shallow-marine facies that are 100's of metres in length and 10's of metres in thickness. Based on its texture the upper unit is interpreted as a debrite. In up-dip areas only, the MTC is overlain by a >20m-thick tabular sandstone-rich successions, with normally-graded sandstones that are commonly capped by clast-rich muddy sandstone. This succession is interpreted as a lobe complex that was confined by the depositional relief generate by emplacement of the MTC.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013