A unique, far-travelled graptolite-bearing erratic pebble from the Lowestoft Till (Quaternary: Anglian Stage) of North Lopham, Norfolk

Abstract

An erratic chert pebble discovered on an exposure of the Lowestoft Till (Anglian Stage, Pleistocene) of North Lopham, Norfolk, UK, contains graptolites preserved three-dimensionally in silica in a mode not known in the UK. The graptolites are Monograptus parapriodon, Monograptus priodon, Monoclimacis linnarssoni and an undetermined retiolitid that indicate an Oktavites spiralis Biozone (mid-late Telychian, Late Llandovery, Silurian) age. The graptolites are associated with organic-walled microfossils, some containing internal bodies. The combination of lithology, preservation and low thermal maturity seem to exclude a British origin. The closest comparison is with Silurian chert pebbles in Miocene and Pleistocene gravels in central Germany, thought to be derived from bedrock in the Frankenwald region of Thuringia. A conjectured natural transport vector for this pebble involves drainage along the proto-Rhine system flowing into early/mid Pleistocene North Sea deltaic/marine deposits, with subsequent glacial transport to Norfolk. The possibility of an anthropogenic vector is also considered, but dismissed.