Stratigraphy, facies and evolution of deep-water lobe complexes within a salt-controlled intra-slope mini-basin

Abstract

A succession of four deep-water lobe complexes deposited within a salt-controlled minibasin have been imaged in unprecedented detail on high resolution, high frequency 3D seismic reflection data. The ponded interval was deposited over approximately 2.7 m.y. and consists of four discrete sequences, each of which contains one lobe complex. There is a systematic change in the shape and orientation of the lobe complexes through time: the two older lobe complexes are oriented broadly north-south and are up to 10 km (6.2 mi) long by 5 km (3.1 mi) wide, whereas the youngest lobe complexes are oriented southeast-northwest and have a rounder shape (9 km (5.6 mi) long by 8 km (5 mi) wide). The north to south migration of the feeder channel entry point and the change in lobe complex orientation is attributed to growth of the basin-bounding salt structures. Each lobe complex is composed of a feeder channel, multiple individual lobes formed of a trunk channel and a diverging network of smaller distributary channels, commonly fringed by a high amplitude band. The lobes are on average 1.6 km (1 mi) long by 1.3 km (0.8 mi) wide and are fed by trunk channels that range from 60 m (197 ft) to 200 m (656 ft) wide, with thicknesses up to 15 m (49 ft). Variation in lobe shape and spatial location is driven by the response of the lobes to topographic growth along the edge of the basin and as well as inherited seabed relief generated by previous lobe growth. In areas where lobe development is constrained by structural growth along the edge of the basin the lobes become elongated and divert away from the growing topography. Lobe complexes of similar scales have been described in detail in outcrops and in unconfined settings on the seafloor but this is the first study to describe these systems in such detail in the subsurface, resolving the individual lobes and lobe elements within a ponded intra-slope basin The high resolution plan-view images help bridge the gap between the fine-scale sedimentological studies that have been carried out on lobe complexes and sheet sands in outcrop for the past twenty years with more recent research on less well-resolved seismically imaged systems. The sheet sands described in outcrop studies can be correlated with features seen in the plan-view amplitude extraction maps: we record densely channelized lobes passing laterally into more branched, thinner channels and lobe elements then terminating in a high amplitude fringe. We relate these seismic characteristics to outcrop facies of channnelised, amalgamated and layered sheets.