Stratigraphical framework for the Ordovician and Silurian sedimentary strata of northern England and the Isle of Man

This report provides a comprehensive review of the lithostratigraphy of the Ordovician and Silurian sedimentary, excluding volcaniclastic, strata within the Lake District Lower Palaeozoic Inlier, the nearby northern England inliers of Cross Fell, Cautley and Dent, Craven and Teesdale, and the Isle of Man. It gives summary definitions of all the existing lithostratigraphical units, and attempts to resolve some of the inevitable anomalies resulting from the more than 20 years of recent research by members of the Lake District Regional Geological Survey team and academic collaborators. That research has led to publication of a new set of British Geological Survey (BGS) maps. This report complements the previously published review of the volcanic strata and intrusive igneous rocks of the same region (Millward, 2004, BGS Research Report RR/01/07). The Ordovician sedimentary rocks of Cumbria comprise the Skiddaw Group, whereas those of the Isle of Man form the Manx Group. These groups are correlatives and the stratigraphy is essentially that previously published by the BGS, but with definitions expanded where required. The main change is that the Tailbert Formation is now re-assigned to the Borrowdale Volcanic Group, in recognition of its dominantly volcaniclastic composition and its unconformable relationships with the underlying rocks of the Skiddaw Group; in this respect it resembles the Latterbarrow Sandstone Formation seen at the base of the Borrowdale Volcanic Group in the west of its outcrop. As its definition was omitted from Millward (2004), the Tailbert Sandstone Formation is included in Appendix 4. The Ingleton Group is considered here because of its long history of correlation with Lower Palaeozoic rocks in the Lake District. However, the absence of biostratigraphically significant fossils means that the group may equally be considered to be Precambrian in age. Though this conundrum remains unsolved, the petrological, structural and metamorphic characteristics of the Ingleton Group suggest that, on balance, these rocks should be regarded as Neoproterozoic in age. The uppermost Ordovician and Silurian rocks of the region are included within the Windermere Supergroup, wholly divided into groups that reflect the dominant packages of lithofacies present: in ascending order they are the Dent Group, of Ordovician age, succeeded by the Silurian Stockdale, Tranearth, Coniston and Kendal groups. The definition of the Windermere Supergroup is now widened to include the Silurian (Wenlock) Dalby Group in the Isle of Man to emphasise likely correlation. Only minor revisions have been made to the established constituent formations and their members, though definitions of many of these entries in the BGS Lexicon of named rock units are to be amplified from diverse literature sources. The most significant changes are summarised as follows: • Within the Dent Group, the Low Scales Sandstone Member, formerly at the base of the Kirkley Bank Limestone Formation in the Furness district, is transferred to the Stile End Formation, remaining at the same rank. In the same area, the Ireleth Member of the Kirkley Bank Limestone Formation is replaced by the Kentmere Limestone Member of the same formation. • In the Cross Fell Inlier, the term Swindale Shales is replaced by the Lake District Ash Gill Mudstone Formation. • In the Craven inliers, it is recommended that the Llandovery rocks are assigned to the historic Stockdale Group and its constituent Skelgill and Browgill mudstone formations, rendering the relatively recently named Crummack Formation and its component Hunterstye and Capple Bank members obsolete. In the Craven inliers, it is further recommended that the term Arcow Formation is replaced by Coldwell Siltstone Formation which is in use across the rest of the region. • Also in the Craven inliers, the Austwick Formation (Tranearth Group) is redefined to comprise only the sandstone-dominated succession, with its original lower part assigned to the Brathay Mudstone Formation. It is also recommended that the Horton Formation reverts to its earlier definition; consequently, its parent is the Tranearth Group. The Studfold Sandstone is elevated to formation rank within the Coniston Group. The siltstone succession (also previously part of the Horton Formation), overlying the Studfold Sandstone Formation and underlying the Neals Ing Sandstone Formation, is newly defined as the Sannat Hall Siltstone Formation. • Within the Kendal Group of the southern Lake District, the Underbarrow Flag and Scout Hill formations become redundant and the strata subsumed within the Kirkby Moor Sandstone Formation.