The sedimentology, palaeoecology and stratigraphy of cretaceous rocks in N.W. Scotland
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Sediments of Cretaceous age in N.W. Scotland outcrop in small, often isolated exposures throughout the Inner Hebrides and Morvern, and have been dealt with cursorily in most previous work on the Cretaceous rocks of Britain. The aims of this study were (i)to propose a formal integrated stratigraphic scheme for the Cretaceous strata of N.W. Scotland and (ii)to model the development of the Inner Hebrides Basin (where these strata outcrop) during the Cretaceous Period. Detailed field observations, macro- and micropalaeontology (including palynology), sedimentology and structural data were integrated in an attempt to achieve these aims. Fieldwork was conducted in Morvern (Argy 11) and the Inner Hebridean islands' of lYlul], Eigg and Skye. In the proposed lithostratigraphy the Mo~ern Greensand becomes the M.orvern Greensand Formation, of which the former "Lochaline Glass Sand" or "Loch Aline White Sandstone" becomes the Lochaline White Sandstone Member. The overlying silicified chalk, outcropping in Morvern and Mull, becomes the Gribun Chalk Formation. Dark grey micritic limestone, previously undifferentiated from the silicified chalk, becomes the Strathaird Limestone Formation of which there are two clastic members: the basal Laig Gorge Sandstone IVIember and the Clach Alasdair Conglomerate lVlember. The "Upper Estua.rine Series" of Judd (1878), becomes the Beinn Iadain Mudstone Member of which there is a coarser clastic member: the Feorlin Sandstone ~ember. r]:lhese Formations comprise the Inner Hebrides Group. Biostratigraphic evidence (based primarily on dinoflagellate cysts and foraminifera) indicates a latest-Albian to IVlid-Cenomanian age for the Morvern Greensand Formation; a Late Cenomanian age for the Gribun Chalk Formation, and an Early - IVliddle Turonian age for the Str2thaird Limestone Formation. The most refined biostratigraphical range for the Beinn Iadain lVIudstone F'ormation was Albian to Palaeocene. The IVlid-Late Cretaceous development of the Inner Hebrides Basin includes two periods of major transgression, the first of which began in the latest Albian and continued through the Early Cenomanian with the deposition of the marginal clastic facies of the Morvern Greensand Formation. A minor period of regression preceded the onset of carbonate deposition in the Late Cenomanian, recorded in the Gribun Chalk Formation. A second major transgressive episode followed the silicification, uplift and erosion of the Gribun Chalk, and reflects rapid deepening of the basin during the Early to Middle Turonian, poorly sorted clastic sediments (the Laig Gorge Sandstone Member) being overlain by biomicritic limestones intercalated with debris flows (the Strathaird Limestone Formation). These two major transgressive episodes are marked by the deposition of similar lithofacies throughout N.W. Europe (although no in situ deposits of Turonian age are found in Northern Ireland). Some feature; of the Cretaceous sediments of N.W. Scotland are found elsewhere, for instance the high abundance of calcispheres and organic-rich nature of the Strathaird Limestone are typical features of the Early Turonian transgressive episode in the AngloParis Basin. However, in the Inner Hebrides Basin, these eustatic changes in sea level were imposed upon local tectonic movements as indicated Qy the weathering and erosion of the silicified Gribun Chalk prior to the deposition of the Strathaird Limestone Formation, and the debris flow events, apparently related to synsedimentary movements along the Camasunary Fault, recorded within that Formation.
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