Stockdale Group succession, Windemere Supergroup, late Ordovician to Silurian, Northern England
|From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England.
Fifth edition. Keyworth, Nottingham: British Geological Survey.
Above the Dent Group there is a marked lithological and faunal change with the abrupt appearance of black, graptolitic shale. This is the characteristic feature of the Stockdale Group which, whilst only attaining a maximum thickness of about 120 m, stratigraphically spans the whole of the Llandovery Epoch (P916053) and (P916056). There are two component formations, a lower Skellgill Formation overlain by the Browgill Formation. These can be recognised throughout the entire breadth of the outcrop, from Furness to Cross Fell, despite the thinness of the succession.
The Skelgill Formation is a condensed sequence consisting mostly of graptolitic black mudstone but with subordinate thin beds of calcareous siltstone. It is never more than 40 m thick but still spans 10 or more graptolite biozones, representing the very top of the Ashgill Hirnantian Stage, and the entire Rhuddanian and Aeronian stages of the Llandovery. The basal mudstone beds, of Hirnantian age, are commonly pyritous but a little paler in colour than the rest of the formation, and widely include thin developments of nodular limestone. This basal sequence is separated as the Spengill Member and contains a mixed, graptolitic and shelly fauna. Thin interbeds of calcareous siltstone increase in abundance towards the top of the formation and in places contain a sparse and dwarfed shelly fauna, but black graptolitic mudstone dominates throughout. Sporadic thin layers of bentonitic claystone attest to contemporaneous falls of volcanic ash.
The Browgill Formation consists largely of pale green, oxic siltstones with only a few thin interbeds of black graptolitic mudstone and intermittent horizons of pale grey, bentonitic claystone derived from volcanic ash. It ranges up to about 90 m in thickness and occupies the whole of the late Llandovery, Telychian Stage. Towards the top of the formation a number of local variations occur. Red-brown mudstones are widely seen right across the Lake District and Cross Fell outcrops, and some are calcareous and contain a sparse, shelly marine fauna. They are best developed in the Howgill Fells where up to 20 m are present and contain thin interbeds of black graptolitic mudstone. There, the red mudstones have been designated the Hebblethwaite Member. Above this member in the Howgill Fells, and more generally at the top of the Browgill Formation whether or not there are red beds present, is about 8 m of pale grey mudstone devoid of any black, graptolitic interbeds. Across the Lake District this unit has been recognised as the Far House Member, but it does not appear to be present at Cross Fell where the topmost beds of the Browgill Formation are red mudstones with black graptolitic laminae.
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