Early post-rift facies and architectural changes within a coarse-grained deepwater system deposited above a Mass Transport Complex (MTC)

Abstract

Local gradient changes and variable accommodation characterize the upper surface of mass transport complexes (MTCs). 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. A 60 m thick succession of very coarse-grained and poorly-sorted matrix-rich sandstones deposited above a seismic-scale mass transport complex (MTC) is investigated in the Jurassic stratigraphy of the Los Molles Formation, Neuquén Basin. The facies types and architectural changes documented within the deep-water succession provide insights into the interactions between coarse-grained sediment gravity flows and uneven seabed inherited from the top of a MTC. The succession was deposited in the Chacay Melehue hemi-graben outcrops along a 6.5 km long oblique downdip longitudinal profile which permitted correlation of individual beds between 16 stratigraphic sections (1:25 scale). Two units are defined: Unit 1 comprises heterolithic facies grading upwards into poorly sorted, very coarse- to fine-grained, thin- to medium-bedded sandstones with abundant pebble-sized clasts; Unit 2 comprises three thick conglomeratic event beds of very poorly-sorted, granular to medium-grained mud-rich sandstone matrix, supporting polygenic gravels ranging from pebble to boulder and large clasts.The sandstone-rich division of the deep-water succession studied is interpreted as a lobe complex emplaced by an out-of-equilibrium sand-rich system, which recorded 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. 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.