Scale and lateral variability of mass-transport deposits within submarine slope and basinfloor systems

Abstract

Mass-transport deposits (MTDs), and their larger-scale occurrence as mass-transport complexes (MTCs), are common in many deep-water sedimentary basins, forming a substantial component of the stratigraphic record in slope and basinfloor environments. They are the result of emplacement by gravitationally driven mass-transport processes that are influenced by allogenic and autogenic controls, which deposit beds at a range of thickness scales (m-dm to 100s m) with marked spatial and stratigraphic variability. 3D seismic reflection data has allowed the occurrence, structure, and morphometry of MTCs to be documented in a diverse range of basin settings. However, uncertainties remain regarding: (i) the quantification and distribution of strain within MTCs, (ii) the lateral variability and stratigraphic occurrence of MTCs along degradational basin margins, and (iii) the distribution and rock properties of sub-seismic MTDs, genetically related transitional flow deposits, and their effect on deep-water turbidite reservoir quality. These questions are considered in the context of three contrasting MTD/MTC settings: (i) the Neogene continental slope, offshore Uruguay; (ii) the Eocene shelf edge system, in the central Santos Basin, offshore Brazil; and (iii) the Upper Jurassic Magnus Sandstone Reservoir, in the Northern North Sea, UK continental shelf. This investigation utilises a variety of subsurface datasets, including 3D seismic-reflection surveys, petrophysical well logs, and core samples, which enables the deposits to be studied in 3D at passive margin-, system-, and bed-scale. The results establish that: (i) on a seismic-scale, the studied MTC did not balance the magnitude of updip extension with downdip contraction, and strain within the contractional domain was depth dependant, with deeper horizons accommodating more strain; (ii) degradational margins may follow a predictable stacking pattern of oversteepening and pre conditioning of the shelf-edge, followed by degradation and emplacement of the collapsed shelf-edge as MTCs, and demonstrates lateral (1-10 km-scale) variability in shelf-edge accretionary styles, and (iii) clay-rich, sub-seismic MTDs can act as competent seals to subsurface fluid flow, and transitional flow deposits may form baffles or barriers impacting sweep efficiency, compartmentalisation and hydrocarbon recovery from turbidite reservoirs. The range of scale of analysis presented here is essential to understand the scale, geometry, and internal characteristics of MTDs/MTCs, to reconstruct basin margin behaviour and to predict MTD rock property distribution.