The Skiddaw Group of Cumbria : early Ordovician Turbidite sedimentation and provenance on an evolving microcontinental margin
During the Early Palaeozoic, the southern British Isles were part of a microcontinent Eastern Avalonia. In the early Ordovician Eastern Avalonia was close to the continent of Gondwana. in high southern latitudes. Northward drift of Eastern Avalonia during the Ordovician brought it towards Baltica and Laurentia, resulting in continental collision in the Silurian. Three major tectonic events have been postulated in the earlier history of the microcontinent rifting from Gondwana, onset of subduction and subduction of the mid-ocean ridge. Evidence for such events may be sought in the contemporaneous sedimentary basins: their type and geometry and the depositional environment and composition of their sediment fill. The Skiddaw Group (Tremadoc to Llanvirn) of Cumbria. comprises turbidites. debrites and a major olistostrome. Sediment was sourced from an orogenic terrain with noncoeval continental volcanic arcs, unroofed plutons, metamorphic basement and sedimentary cover. An additional source of quartzose sediment (e.g. a sandy shelf or delta) was present Two periods of submarine fan development display the following facies associations: depositional lobe, lobe fringe and interlobe, distributary channel, and proximal channel-levee. The first spans the Tremadoc-Arenig boundary and channels distributed sediment to both east and west Axial flow along a trough orientated approximately east-west is inferred. Contemporaneous depositional lobe fades association in the Isle of Man could represent a more distal portion of this system. The Manx Group displays sediment of similar facies and compositions to the Skiddaw Group. Turbidity current flow on a submarine fan of Arenig age was strongly influenced by sea floor topography. A palaeocurrent interpretation is presented which uses the theory of oblique reflection of turbidity currents (Kneller, Edwards, McCaffrey and Moore, 1991). The angular relationship between sole mark current directions and ripple current directions is used to infer the orientation of intra-basinal topographical features. With additional evidence from thickness variations of sandstone bodies and the orientation of pre-mid Ordovician faults, a tectonic model of syn-depositional extensional faults trending northnorthwest, with fault blocks tilted to the northeast is proposed. The controls of fan development are discussed. Gravitational deformation in the Llanvirn is expressed as slump folds in partially lithified sediment, debrites and an olistostrome with sandstone rafts. Slump fold orientations are used to demonstrate the development of a regional westerly palaeoslope which preceded uplift of the depositional basin prior to subaerial volcanism of the Eycott and Borrowdale volcanic groups. The undated Ingleton Group is described and contrasted with the Skiddaw and Manx Groups. A lithological correlation with Arenig sediments recovered from the Beckermonds Scar borehole is supported, but a continental margin volcanic arc provenance and deposition as a coarse grained lobe of a submarine fan suggests the dispersal system was separate from that of the Skiddaw Group. Lithological comparison is made with contemporaneous strata of southeast Ireland. With consideration of early Ordovician geology across the Southern British Isles, a tectonic model is proposed which incorporates transtensional and transpressional phases of oblique sinistral slip in the overriding plate above a subduction zone of general southerly dip. The present day active margin of Chile is used as an analogue. The long and complex history of Eastern Avalonia's northern margin is analysed to identify the three tectonic events mentioned above.