The Skiddaw Group of the English Lake District : memoir for parts of sheets 22 Maryport, 23 Cockermouth, 24 Penrith, 28 Whitehaven, 29 Keswick, 30 Appleby, 31 Brough and 48 Ulverston

The stratigraphical and structural framework of the Skiddaw Group has been a controversial issue for much of the past century. Its rocks have been generally regarded as among the most intractable in England but a recently completed resurvey programme led by the British Geological Survey has resolved many of the outstanding difficulties and provided a comprehensive geological understanding. Important advances in graptolite and acritarch biostratigraphy underpin the lithostratigraphical interpretation, which in turn allows a more informed analysis of basin geometry, structure and burial metamorphism. This memoir presents and discusses the principal scientific findings of the resurvey. The Skiddaw Group occupies the northern part of the Lower Palaeozoic inlier of the English Lake District; smaller inliers occur to the east at Ullswater, Bampton, Cross Fell and Teesdale, and to the south at Ravenglass, Black Combe and Furness. Sporadic records from outcrop and boreholes farther afield indicate that similar strata extend beneath much of northern England. To the west, the Manx Group of the Isle of Man is a regional correlative. The group consists of turbidite mudstone and sandstone, which range from Tremadoc (or possibly Cambrian) to Llanvirn in age. The strata were deposited in an extensional environment at the Avalonian margin of the Iapetus Ocean. A range of petrological and geochemical parameters defines the provenance of the group within an extinct continental margin volcanic arc, possibly of Precambrian age. Palaeocurrent analysis and deduced basin configuration suggest that this source area was situated farther south, although no specific location can be identified. New graptolite and acritarch biostratigraphical zonal schemes have been developed. They assist in the recognition of three distinct lithostratigraphical domains, separated by major structural lineaments. In the north, about 5000 m of predominantly silty turbidites accumulated; two major sandstone incursions, the Watch Hill and Loweswater formations, alternate with three mudstone-dominated successions, the Bitter Beck, Hope Beck and Kirk Stile formations. Farther south, across the Causey Pike Fault, within a separate younger succession that consists mainly of early Llanvirn mudstone (Tarn Moor Formation) there are sporadic interbedded volcaniclastic turbidites and bentonites, a precursor of the extensive volcanism of the late Llanvirn to Caradoc. The Llanvirn strata overlie a major olistostrome, the Buttermere Formation, which contains Tremadoc and Arenig components that were emplaced from the south in the late Arenig. The olistostrome may have resulted from continuing extensional tectonic instability, but may alternatively have been triggered by the initiation of subduction- related uplift. The inliers of the southern Lake District lie to the south of the Southern Borrowdale Lineament, a major structural and geophysical feature, and they each show unique and characteristic geological features. By late Llanvirn or early Caradoc times, the deep marine environment in which the Skiddaw Group was deposited had been transformed into a subaerial basement for the Borrowdale and Eycott volcanic groups. The scale of the unconformity between the Skiddaw Group and the two volcanic groups indicates considerable disorganisation and erosion of the former prior to volcanicity. Further, during that phase, volcanotectonic extensional faulting seems likely to have imposed further complication on Skiddaw Group structural architecture. Volcanism ended in the late Caradoc, but it seems unlikely that the final closure of Iapetus Ocean occurred before the mid-Silurian, when convergence of Avalonia with Laurentia initiated polyphase tectonic deformation of the Skiddaw Group. Thrust imbrication probably began around late Wenlock, as the Southern Uplands thrust belt extended across the sutured Iapetus Ocean. However, there is no evidence for penetrative deformation and imposition of cleavage until the early Devonian Acadian Orogeny. A penetrative cleavage with a broadly north-east Caledonide trend was imposed at that time. It affects all of the early Palaeozoic rocks, including the Skiddaw Group, where it locally cuts an earlier, beddingparallel fabric. Consequently, the character of the earliest tectonic fabric v ries from slaty cleavage to a crenulation cleavage. Later phases of Acadian compression probably involved re-activation of thrusts and faults within the Skiddaw Group. This resulted in marked strain partitioning and the resultant development of crenulation cleavages that are difficult to correlate between domains. Granite intrusion at about 400 Ma spanned the final cleavage-forming episode. In addition to detailed lithostratigraphical, biostratigraphy and structural interpretation, the memoir presents supporting data that include mineralisation localities, geochemical analysis and comprehensive faunal lists. A section indicating other sources of information on the geology of he region and a full reference list is also included.