Architectural changes in a coarse-grained deep-water system deposited above a Mass Transport Complex (MTC)

Abstract

Local gradient changes and variable accommodation characterize the upper surface of mass transport complexes (MTCs). The associated bathymetric variability can influence the longitudinal and vertical processes occurring within coarse-grained sediment gravity flows, which are challenging to interpret from the rock record alone. Here we document an exhumed deep-water succession from the Los Molles Formation, Neuquén Basin, Argentina to investigate the facies types occurring within and the architecture of, coarse-grained beds deposited above a seismic-scale MTC (20-60 m-thick). The study area is a 12 km-long oblique downdip section within which individual beds are correlated between 21 measured sections (1:25 scale). The stratigraphic succession comprises a 25-70 m thick, sandstone-rich division (69% sandstone) containing two units (Figure 1). Unit 1 comprises a lower heterolithic succession, grading upwards into a sandstone-prone succession that consists in poorly sorted, very coarse to fine-grained, thin to medium bedded sandstones with pebble-sized clasts. Sandstone beds in Unit 1 have a poor bed continuity, high amalgamation and abrupt lateral pinch-out, with evidence for erosion, sediment bypass and lateral facies changes occurring over short down-dip distances (<100s m). Unit 2 is characterized by three thick and laterally extensive, conglomeratic event beds intercalated with thin to medium bedded very coarse- to coarse-grained or fine-grained sandstones, siltstones and mudstones. Conglomeratic event beds have a very poorly sorted, granular to medium-grained, mud-rich sandstone matrix, supporting polygenic gravels (ranging from pebble to boulder size) and large clasts from the underlying strata (Figure 2). The base of the conglomeratic event beds can be amalgamated over long distance and show local thickness variations related to compensational stacking and enhanced basal erosion. The vertical stacking of component facies with significant lateral offset, together with the increase of lateral bed extension and thickness upwards, indicates a progressive decrease of confinement from Unit 1 to Unit 2. The MTD-related topography controlled stratigraphic trapping of considerable sand volumes in Unit 1. Once the MTC-related accommodation was filled, relief associated with individual supra-MTC sandbodies produced subtle changes in depositional processes in the Unit 2. The changes of depositional processes and associated spatial segregation of material transported conferred a significant bed heterogeneity downdip local topographic highs and controlled internal bed architecture. We infer that the sandstone-rich division corresponds to a lobe complex emplaced by an out-of-equilibrium sand-rich system. The sand-rich system was fed by unstable flows sensitive to seabed topography and close to an erosional state with intermediate turbulent-laminar behavior. Predictive stratigraphic outcrop-based models can provide insights into spatial distribution and internal architecture of heterogeneous sandbodies able to generate multiscale net/gross variations that make-up the internal complexity of subsurface reservoirs hosted in lobes above MTCs.