Conodont biostratigraphy of the attenuated Dent Group (upper Ordovician) at Hartley Ground, Broughton in Furness, Cumbria, UK

Abstract

SUMMARY At Hartley Ground, Broughton in Furness, Cumbria, the Dent Group, a mixed clastic-carbonate succession at the base of the Windermere Supergroup, is attenuated (only 20 m thick) and rests unconformably on the Borrowdale Volcanic Group. This succession includes a debris-flow deposit (Lumholm Member), the age of which was thought previously to be either Ashgill or Llandovery. Five samples from the Hartley Ground succession have yielded an Amorphognathus ordovicicus Biozone conodont fauna, considered to be of mid- to late Rawtheyan (Ashgill) age. The conodont fauna occurs in strata below and within the matrix of the Lumholm Member, although reworking within the latter cannot be completely discounted. Conodont colour alteration index (CAI) values of 5 for all the samples suggest these strata have been heated to temperatures of 300–400°C. At normal continental geothermal gradients this would indicate a minimum depth of burial of approximately 10km. New multi-element reconstructions are proposed for Amorphognathus ordovicicus Branson & Mehl and Birksfeldia circumplicata Orchard; Hamarodus europaeus (Serpagli) is considered to be a member of the family Periodontidae.