Tectonic environment of the Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones

TLDR

The Portscatho Formation represents a deep‑water, fore‑arc turbidite sequence in south Cornwall, deposited during Devonian subduction beneath a southern arc that later closed by northward stacking of flysch–ophiolite nappes. Petrographic and geochemical analysis of the 3.5‑km‑thick Portscatho Formation shows a largely coherent, acidic continental magmatic arc provenance with a metamorphic‑clast‑rich upper section, variable Cr, low Zr, and evidence of subduction‑related arc activity.

Abstract

The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of south Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. The presence of an arc to the south of Cornwall during the Devonian implies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking of flysch–ophiolite nappes.