Substrate Entrainment Processes and Depositional Architecture of an Exhumed Seismic-scale Mass Transport Deposit, Chacay Melehue, Neuquén Basin, Argentina

Hodgson, David¹; Jackson, Christopher A.; Duller, Robert; and Spychala, Yvonne
[email protected]

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 the influence depositional relief has on subsequent flow processes of a seismic-scale (>7 km-long and up to 70 m-thick), mass transport deposit (MTD). The correlation of marker horizons between measured sections (200-500 m spacing) permits the structural and depositional architecture of the MTD to be constrained. The areal extent and relatively persistent thickness of basal marker beds suggests that the MTD was deposited in a relatively low gradient setting. The overlying MTD 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 MTD; the downslope, inclined surface is interpreted as a steep, up-dip-facing toe-wall.

The MTD 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 MTD 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 MTD.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013