Welsh Basin ‘contourites’ reinterpreted as fine-grained turbidites: the Grogal Sandstones

Abstract

Since 1976 the Lower Silurian Grogal Sandstones of the Welsh Basin have been interpreted as contourites. Here we show that they are in fact fine-grained turbidites, frequently with pale-coloured oxidized top layers which enable distinction of individual event deposits. As reported previously, palaeocurrent directions derived from ripple cross-lamination are typically at a high angle, often close to perpendicular, to those measured from sole structures. The favoured explanation is that ripples migrated under transverse internal waves generated at an intrabasinal slope to the east. Evidence for the lateral confinement of the overlying Aberystwyth Grits by such a slope provides support for this explanation.

A fault-controlled depositional model for the Aberystwyth Grits turbidite system

“…1; Anketell. & Lovell, 1976;Smith & Anketell, 1992). The latter characterize the Mynydd Bach Formation in the south, displaying a noticeable northward arcuation across the crop, the implication of which is discussed below.…”

“…1), which swing into parallelism with the Bronnant Fault, confirms the influence of this fault-controlled depression on contemporary turbidite flow-paths (cf. Smith & Anketell, 1992).…”

“…Earlier workers (Wood & Smith, 1959;Cave & Hains, 1986;Loydell, 1991) have also assumed that the crop of the Aberystwyth Grits defines a broad periclinal syncline, sequentially underlain by the Borth Mudstones and Devil's Bridge Formation in the north and east (Cave & Hains, 1986), and by the Grogal Sandstones Formation in the southwest (Anketell & Lovell, 1976;Smith & Anketell, 1992). The Aberystwyth Grits and these underlying formations all occupy the early Telychian turriculatus Biozone.…”