AN IRIDIUM ANOMALY IN THE LUDLOW BONE BED FROM THE UPPER SILURIAN, ENGLAND

The Ludlow Bone Bed in the Upper Silurian of the Welsh Borderland shows an anomalously high concentration of iridium (0.49 ppb) compared with background (0.040 ppb Ir). Considering the overall major and trace element pattern and the mineralogy of the bone bed, it appears that the bulk of the Ir has precipitated from sea water and is not primarily related to an asteroid impact event. A secondary relation of the Ir to such an event, however, cannot be excluded. The profound sedimentological similarity (skeletal sands and hummocky cross-stratification) between the Ir-carrying 'storm deposit' at the Cretaceous-Tertiary boundary at Brazos River, Texas, and the LBB and overlying sediments may indicate such a relation.