The role of structural growth in controlling the facies and distribution of mass transport deposits in a deep-water salt minibasin

Abstract

We distinguish two types of mass-transport deposits (MTDs) within a salt-related slope minibasin. The locally-derived MTDs are sourced from growing structures bounding the minibasin, and the more extensive, regional, MTDs are sourced from the shelf of the continental margin, at least 100 km east of the study area. The two types of MTDs have different internal characteristics, distribution within the basin and depositional mechanisms. Using a high-resolution 3D seismic dataset we characterise the internal structures and dimensions of the MTDs, and also quantify the deformation history of the salt structures. Through these analyses we demonstrate that the ‘local’ and ‘regional’ MTDs react in different ways to growth on the structures within, and bounding, the slope minibasin. The regionally derived MTDs are debris flows, with individual deposits up to 300 m thick and covering an area of up to 1300 km2 within the minibasin. Seismic facies analysis suggests they are mostly composed of chaotic material but in some places contain discrete blocks of sediment ranging from 50 m to 250 m in length. In contrast, the more locally derived MTDs are submarine slides consisting of translated material from the flanks of salt structures. Each slide has an up-dip extensional domain with normal faults and a down-dip contractional domain with thrusts and folds. They cover areas between 11 and 28 km2 with frontal and lateral ramps up to 200 m high. Relict topography on the top surface of the MTDs is infilled with younger sediment. Shortening analysis of the intra-slope salt structures within and bounding the minibasin is used as a proxy for the growth rate of the structures. This novel approach of combining shortening with the detailed observations of the local and regional MTDs has allowed us to draw new conclusions on the relationship between structural growth and MTD initiation. Local MTDs either form during periods of high structural growth (10–30 m/ma) or in a period of quiescence (