The sedimentology, ichnology and invertebrate palaeontology of the late Silurian Downton Bone Bed

Abstract

The Downton Bone Bed is a Konzentrat-Lagerstätte deposit located in the Welsh Borderlands, United Kingdom. The Downton Bone Bed is late Silurian, considered to be Přídolí in age, and occurs within the Platyschisma Shale Member of the Downton Castle Sandstone Formation. The bone bed is exposed at Weir Quarry (Herefordshire), which this study proposes should become established as the type locality for this stratigraphic horizon, due to the destruction of other localities and lack of access to other sites. As Weir Quarry is one of the last remaining exposures of this unit, the objective of this study is to qualitatively describe the sedimentology, ichnology, and invertebrate palaeontology of the bone bed, to enhance the regional understanding of the palaeogeography, depositional environments and depositional processes of the Welsh Borderlands during the late Silurian. Parasequence thickness and frequency, and sedimentary structures such as hummocky cross stratification, observed within the Downton Castle Sandstone Formation, have traditionally been explained to have formed by sea-level oscillations. These are interpreted to have formed entirely by nearshore to shoreface, shallow marine autogenic sedimentary processes, such as storm events (tempestites) and tidal scour. It is interpreted that formation of the Downton Bone Bed occurred as a by-product of these autogenic sedimentary processes, through winnowing and erosion during storm-driven scour and reworking. The low diversity ichno- and invertebrate fauna observed within the Downton Bone Bed is consistent with a nearshore depositional environment and is indicative of a stressed ecosystem due to fluctuating salinity and oxygen levels.