The Skiddaw Group (English Lake District) reviewed: early Palaeozoic sedimentation and tectonism at the northern margin of Avalonia

Abstract

Abstract The Skiddaw Group is a 5 km thick sequence of Tremadoc-Llanvirn turbiditic mudstone and sandstone, including a major olistostrome, which occupies the northern part of the Lake District Lower Palaeozoic Inlier. Sporadic outcrop and borehole records indicate that similar strata extend beneath other parts of northern England. To the west, the Manx Group of the Isle of Man is a regional correlative. The Skiddaw Group was deposited on the Avalonian margin of the Iapetus Ocean, with constituent sediment derived largely from an earlier, possibly Precambrian, continental margin volcanic arc. Nd isotope data confirm the absence of juvenile detritus. Olistostrome emplacement in the late Arenig preceded subduction-related uplift of the deep-marine Skiddaw Group to form the subaerial basement to the mainly Caradoc, Borrowdale and Eycott volcanic groups. The scale of the unconformity beneath the volcanic rocks requires considerable pre-volcanic disruption and erosion of the Skiddaw Group prior to structural disturbance by volcanotectonic faulting. Volcanism ended in the late Caradoc when thermal reequilibration, coupled with possible further extension, allowed marine transgression through the early Silurian. Ultimately, convergence of Avalonia with Laurentia initiated thrust imbrication of the Skiddaw Group as the Southern Uplands thrust belt extended across the sutured Iapetus Ocean. Thrust-related hydration caused widespread resetting of Rb-Sr isotope systems during the 430–420 Ma interval. A penetrative slaty cleavage with a broadly Caledonian trend was imposed during the Early Devonian, Acadian Orogeny and cuts an earlier, bedding-parallel (compaction) fabric. Later phases of Acadian compression probably involved reactivation of thrusts within the Skiddaw Group with associated strain partitioning resulting in domainal crenulation cleavage. Granite intrusion at c. 400 Ma coincided with the final cleavage episode.