A day at the beach: a portrait of British geology through sand

Unconsolidated sediment can retain a fingerprint of geological history, stored in refractory detrital grains that endure multi-cycle reworking. Isotopic analyses of these grains can help decipher source-to-sink pathways, crystalline provenance and regional tectonomagmatic evolution. Pleistocene glaciations denuded and transported the eroded product of crystalline basement into the regolith of Britain. Here, we present in situ U–Pb and Lu–Hf isotope data for detrital zircon grains from beach sands at Talacharn and Cwm-Yr-Eglwys in SW Wales. These grains are predominantly of magmatic origin, with zircon ages spanning c. 2934–54 Ma, providing insight into grain provenance of both distal and proximal sources and tectonic processes since the Archean. Eocene zircon demonstrate a source from Britain's North Atlantic Igneous Province, whereas Archean grains represent multi-cycle detritus that transited across the Gondwanan craton to form Old Red Sandstone. Indeed, detrital zircon dates for Anglo–Welsh Basin Old Red Sandstone are statistically indistinguishable from the beach sand samples, demonstrating a recycled East Avalonian source for the sand. ɛHf signatures help refine provenance interpretations and reflect episodic crustal reworking with peaks at c. 1900, 1000 and 600 Ma, consistent with the assembly of Nuna, Laurentia and Gondwana. Thus, isotopic analyses of detrital zircon from modern-day Welsh beach sands reveal patterns of Quaternary denudation and subsequent sediment dispersal across the post-glacial landscape of southern Britain. Ultimately, this detrital archive retains a comprehensive record of regional magmatic history through time.