Cephalopod accumulations from the Late Ordovician of Cumbria: Mud mounds and stromatolites

Abstract

Accumulations of cephalopods in the Late Ordovician (Katian [Ka4]) Troutbeck Siltstone Member of the Ash Gill Mudstone Formation at Skelghyll Beck in the Lake District and in the Keisley Limestone Formation of the Cross Fell Inlier contribute toward an understanding of their respective depositional environments as well as the palaeogeography of the southeastern margins of the Lakesman Terrane. The accumulation present in the Troutbeck Siltstone Member is interpreted as having been deposited in a shallow, near-shore lagoon that supported stromatolite growth. The shore lay to the north of this site. Geopetals infilling cephalopods forming concentrations in the Keisley Limestone Formation indicate that the conchs came to rest on angles of 30–40° to horizontal. The lithologies of the matrix suggest that they were deposited in cavities or fissures from which fine-grained carbonate was either excluded or winnowed away. Both observations accord with hypotheses that the Keisley Limestone Formation represents the remnants of a carbonate mud mound, and facilitate comparison with the contemporaneous Boda mounds of Siljan, Sweden. Comparison with local late Katian – Hirnantian successions indicate that the Keisley mound would have formed a prominent feature on the sea floor, becoming largely buried by argillaceous sediments during the Hirnantian.