A slippery slope: Submarine slope processes on tectonically active basin margins

Abstract

Siliciclastic submarine slopes (slopes) are relatively steep surfaces that connect low-angle shallow(er) water shelves, to flat basin floors of the World's oceans, and deeper seas and lakes. Advances in the understanding of subaqueous slopes have been driven by improvements in bathymetric imaging, the resolution of seismic data, and growing core inventory. Nevertheless, knowledge-gaps remain, particularly with reference to the initiation and evolution of, and the suite of (changing) sedimentary processes and depositional products on slopes in tectonically active basins. To address these uncertainties, a field-based study was undertaken on the exhumed lower Eocene Fosado Unit (Hecho Group) which represents the initiation, evolution and subsequent burial of a submarine slope system during active contractional tectonics on the eastern margin of the deep-water Ai­nsa Basin (Spanish Pyrenees). The slope was initiated by thrust propagation through a pre-existing mixed carbonate-siliciclastic ramp. Shallow water, shelfal deposition was maintained on hangingwall anticlinal crests, whilst deepened footwall synclines became conduits for sediment gravity flows. The interface between shelfal and slope deposition (the shelf-slope rollover) retrograded with time, as thrust footwall strata were steepened. Cessation of thrusting in the study area, reduced accommodation in footwalls, and the shelf-slope rollover was able to prograde. The retrogradational slope was characterised by submarine landsliding, which generated seafloor topography and trapped coarse-grained turbidity currents in ponds and channels. By contrast the progradational slope was characterised by unconfined mudstone deposition and sedimentation by shelf-edge delta-fed turbidity currents. Detailed analysis of submarine landslide deposits show that they were characterised by creep, and incorporated the deposits of turbidity currents into their creeping top-surface. Several aspects of slope processes identified in this study may be unique to, or characteristic of tectonically-active basin-margin settings: 1) the occurrence of retrogradational shelf-slope rollovers stacking patterns. 2) The formation of sediment conduits and traps through fault-driven slope deformation, rather than via erosional processes, which have distinct characteristics, in part because of; 3) a highly mobile seafloor on which turbidites are readily incorporated into, and can be difficult to distinguish from submarine landslide deposits. Additionally, the outcomes of this study overturn long-held sequence stratigraphic models that invoke erosional canyon-formation on the eastern margin of the Ainsa Basin.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor), Rhodri Jerrett (Supervisor) & Ian Kane (Supervisor)