A Lower Cretaceous clastic slope succession, Livingston Island, Antarctica: sand‐body characteristics, depositional processes and implications for slope apron depositional models

Abstract

The early Cretaceous fill of the forearc/intra‐arc Byers Basin includes a 600‐ to 900‐m‐thick interval of marine slope apron deposits, the President Beaches Formation. This is a predominantly argillaceous succession within which coarser‐grained deposits are largely confined to lenticular packages of low width–thickness ratios. The entire formation was deposited in mid‐ to late‐Berriasian times, coincident with a pulse of regional arc expansion, at minimum mean accumulation rates of 120–225 mm 1000 years–1. The mudstones are finely laminated, with a restricted benthic macrofauna and minimal bioturbation, indicating relatively inhospitable sea‐floor environments. Sand‐rich packages occur as 7‐ to 30‐m‐thick channel‐fill units composed chiefly of classical medium‐grained turbidites, in some cases associated with complex high‐concentration turbidity current deposits and minor mud‐rich debrites. These sand‐bodies are apparently elongate along (normal to) the NW‐facing palaeoslope implied by slump‐fold axes (and the strike of the volcanic arc). Similarly, palaeocurrent indices show a consistent arc‐parallel, NE‐directed trend, suggesting that transport processes were strongly influenced by the structural ‘grain’ of the irregular slope morphology. Slope instability is recorded by widespread slump and soft‐sediment collapse features promoted by a combination of steep sea‐floor gradients and relatively high rates of sedimentation. A lack of systematic vertical facies trends indicates that this was not a progradational or well‐organized system, despite high rates of sediment supply. However, the strong systematic relationship between palaeocurrents and palaeoslope suggests a promising basis for evaluating organization in otherwise poorly ordered slope apron depositional systems.