Depositional architecture and processes of sediment gravity flows: a 3D seismic case study from offshore Angola

Abstract

Deepwater environments are characterised by the deposits of mass flows (e.g. debrites, slumps, slides), sediment density flows (turbidites), and background hemipelagic and pelagic suspension fallout. ‘Mass transport’ is a general term used for the failure and downslope movement of sediment under the influence of gravity in both subaerial and subaqueous environments, the products of which are called Mass Transport Deposits (MTDs). The aims of this study are: (i) to investigate the temporal and spatial development of sediment gravity flows, particularly the coupling of MTDs, turbidites and hemipelagites (ii) to characterise and interpret the seismic facies present within a well-imaged deep-water succession, offshore Angola, with particular focus on the external morphology and distribution of MTDs and associated sediment gravity flows; (iii) to document the geometry, scale, distribution and kinematic importance of structures within the MTDs and relate these to the emplacement of these deposits, in order to understand the impact of seafloor bathymetry on the distribution of the deep-water deposits; and (iv) to characterise and classify the development of pockmarks that are formed contemporaneously or successive to the emplacement of the MTDs. Key results of this study are that: (1) the grade of the submarine slope is a primary intra-basinal control on the emplacement and depositional patterns of MTDs and associated sediment gravity flows, (2) the emplacement of MTDs in upper slope environments promotes the capturing and trapping of sediments, (3) strain indicators genetically linked to MTDs record kinematic information related to the initiation, translation and ultimate deposition of the MTDs, (4) pockmarks may develop from the dewatering of MTDs due to their rapid transportation and consequent entrapment of fluid, and (5) the distribution of pockmarks in slope sediments may be haphazard, but pockmarks may also form preferentially along fault traces and along the evacuation planes of MTDs.