Geostrophic versus friction-dominated storm flow: palaeocurrent evidence from the Late Permian Brotherton Formation, England

Abstract

The Late Permian Brotherton Formation of northeastern England is a 0–75 m thick carbonate sequence which exhibits abundant, erosively amalgamated, hummocky cross-stratification, gutter casts and shallow scour-fills, suggesting deposition in a storm-dominated, platformal shelf setting. The outcrop (ca. 15 m thick) is divisible into three facies associations: a basal transgressive interval, 4–5 m thick (facies association A), which passes upwards into a thinly bedded, argillaceous, condensed section < 1 m thick (facies association B), which in turn rapidly grades upwards into a 8–10 m thick, regressive, highstand succession of hummocky cross-stratified, amalgamated scour-fills (facies association C). The dominance of hummocky cross-stratification in all facies associations would suggest that post-storm deposition (and fairweather reworking) was largely under oscillatory currents. However, the erosional palaeocurrent data indicate that facies associations A and B were deposited following combined oscillatory and geostrophically balanced, shore-parallel flows. In contrast, facies association C was deposited by infilling the scours of friction-dominated, offshore-directed and downwelling, jet-like flows combined with oscillatory wave currents. The dominance of geostrophic or friction-dominated flows in the study area is interpreted to be controlled by relative sea level changes. During transgressive conditions increasing accommodation permitted rotation of the downwelling pressure-driven currents. In the regressive phase, particularly in the accommodation-limited late highstand, bottom friction was important in inhibiting flow rotation. It is suggested that the paucity of evidence for geostrophic flows in the ancient record may in part be attributed to the palaeocurrent indicators in sand-grade substrates being restricted to thin transgressive intervals (or surfaces), whilst in the volumetrically more important regressive deposits, palaeocurrent data are largely contained within facies from the friction-dominated zone.