The influence of topography on subaqueous gravity flows: a case study from the Jurassic Los Molles Formation, Neuquén Basin, Argentina

Abstract

ABSTRACT Subaqueous sediment gravity flows experience modifications when they interact with slope and basin-floor topography, impacting facies, geometries, and architectural patterns of the deposits. Understanding these processes is critical for reservoir-quality and trap predictions in deep-water stratigraphic plays. A comprehensive literature summary of the topographic influence on subaqueous gravity flows and an interpretation applied to the basin-floor turbidite system of the late synrift Los Molles Formation in the Neuquén Basin of Argentina are provided. In the study area, a high-resolution satellite image, drone imagery, and 30 logs (about 5000 m total thickness) were measured, focusing on facies analysis, paleocurrents, bed thicknesses, and large-scale thickness variations. Studies describing topographic confinement of sediment gravity flows have used five approaches: 1) Paleocurrent analysis: paleocurrents following structural trends and variations within the same bed (flow deflection and reflection). 2) Facies analysis: complex facies variations on short distances and increased number of hybrid beds and debrites near barriers (flow transformation), loading, convoluted structures, and bidirectional ripples (flow reflection), and sand–mud couplets or thick mudcaps (flow ponding). 3) Small-scale (decimeter to meter) thickness variations: beds thickening towards topography, beds thinning on top of topography (onlaps), and low thinning rates (loss of flow competence, flow containment). 4) Architectural analysis: aggradational stacking, lateral stacking away from topographic barriers, and fill–spill successions. 5) Large-scale thickness (tens of meters) variations: increased fan system thickness across topographic lows. An extended version of an existing classification scheme on degree of confinement is proposed: C0 (unconfined): no flow modification evidence present, and compensational stacking is a classic architectural pattern. C1 (weakly confined): variations in regional paleocurrent directions, minor thickness variations against small-relief barriers, large (basin scale) thickness variations. C2a (confined) to C2b (highly confined): onlaps against high-relief barriers, paleocurrent direction variations on the same bed, bed thickening against topography, facies evidencing flow reflection, increased number of hybrid beds, aggradational stacking, and high percentage of beds continuous over 500 m distances (tabular beds). C2a and C2b are considered end members of a spectrum where the amount of evidence listed can vary. C3 (ponded): sand mud couplets or thick mudcaps. Higher confinement categories might have indicators from the weaker confinement categories. The lower fan in the Los Molles Fm. turbidite system is classified as weakly confined. A complex sediment routing followed structural trends inherited from the rift phase of the basin. The basin-floor had irregular water depths that led to preferred sites of deposition. Hybrid beds are mostly found at the fringes of the lower part of the succession and might be associated with an above-grade slope profile on a footwall scarp. Lastly, the Los Molles Fm. basin-floor beds present high thinning rates (average 1.5 m/km) and the system stacks compensationally, pointing to null interaction against basin margins.