Integrating modern seafloor and outcrop data in the analysis of slope channel architecture and fill

Abstract

Submarine slope channels can form important hydrocarbon reservoirs, but reconstruction of their geometry is often problematic, reflecting the complex interplay of spatially and temporally varying erosional and aggradational processes. The morphology and character of slope channel fills are consequently studied via a number of different approaches that, relying on different potential resolutions, yield general conclusions but fail to capture the complexity of slope channel make up. The integration of observations from modern and ancient systems can allow inferences about a large range of slope channel attributes from the 3D distribution and the hierarchical significance of their constituent geomorphic elements, to their stratigraphy and their lithology. In this paper such an integration has been carried out using modern data from the southeastern Tyrrhenian Sea and outcrop examples from the Rosario Formation in Baja California. Both data-sets give the opportunity to study relatively straight slope channels in coarse-grained continental margin settings. The modern data consistently show a hierarchy of elements. Slope channels are composed of a channel belt and external levee wedge. Channel elements, internal levees, inter-channel bars and side-attached bars compose channel belts. Thalwegs, thalweg margin areas and interthalweg- and side attached bars compose channel elements. The degree of relief associated with the different hierarchical elements provides varying scales of flow confinement contributing to the stratigraphic architecture of the slope channels. In particular, the relief of internal levees, in the order of 10-20 m, prevents the lower, coarser-grained parts of flows from spreading over the entire channel belt.Four types of channel element combinations are recognized. Type I has a sinuous thalweg associated with small scale laterally accreting deposits of coarse grain sediments; finer grained sediments are deposited in the overbank area marginal to the thalweg, in addition to coarse-grained scours fills. Type II has a straight thalweg with a complex scoured floor, flanked by side-attached bars. Type III is characterised by a straight thalweg and a fill mainly consisting of laterally accreted deposits, which are laterally and longitudinally extensive and show two scales of relief depending on the presence or absence of a thalweg margin area within the channel element. Type IV is relatively featureless and is characterised by sub-horizontal laterally continuous deposits that thin and coarsen in coincidence with an axial discontinuous low-relief thalweg or a wider depressed area.