Turbidite to storm transition in a migrating foreland basin: the Kendal Group (Upper Silurian), northwest England

Abstract

Abstract The uppermost Windermere Supergroup (Kendal Group) of northwest England records the passage from Wenlock and lower Ludlow deep water ‘flysch’ deposits to upper Ludlow and Přídolí shallower water ‘molasse’ deposits within an evolving foreland basin. An upwards progression is preserved from oxygen-poor basin-slope turbidite deposits through more oxygenated, bioturbated dilute density flow deposits, to storm and wave-influenced sediments. The storm-influenced sediments display hummocky cross-stratification, a Skolithos ichnofacies, shelly lags, and symmetrical wave ripple cross-lamination. Convolute lamination increases in magnitude and frequency in the upper part of the sequence, apparently nucleated above ripple crests. Tropical hurricanes probably controlled storm deposition, as suggested by late Silurian palaeogeographic reconstructions. Structures in the heterolithic muddy siltstones suggest deposition in a lower energy, wave-influenced setting. Mud-drapes, short wavelength symmetrical ripples and multi-directional ripple cross-lamination are common. The Kendal Group shows a regional palaeocurrent distribution consistent with an arcuate basin geometry, bounded to the northwest and northeast by topographic slopes. As well as a temporal trend, facies and faunal diachroneity imply a southwards migration of the foreland basin depocentre through Ludlow and Přídolí time, probably ahead of a rising mountain front to the north. The increase in pre-lithification sediment disturbance may reflect greater earthquake activity as this mountain front advanced and the basin began to invert.