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Manx Group Sedimentation |
1 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
2 University of Portsmouth, Department of Geology, Burnaby Road, Portsmouth PO1 3QL, UK
3 British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK
The outcrop of Lower Palaeozoic rocks on the Isle of Man is dominated by thin- to medium-bedded sand-rich turbidites over most of the southeast side of the island (Lamplugh’s Lonan and Agneash Grits, comprising greywacke and quartz arenite, respectively, interbedded with mudstone) and on the northwest coast south of Peel (the Niarbyl Flags). Recent work has shown that the latter, the Niarbyl Formation, is Silurian in age and thus distinct from the otherwise early Ordovician sequences. The composition of the Niarbyl sandstone is also distinct, with comparatively low silica (SiO2 60–66%) but elevated CaO, MgO and Cr, relative to the Ordovician sandstones.
The Ordovician sandstones fall into two compositional groups: a very mature, silica-rich (SiO2 78–95%) quartz arenite (Agneash type) and a greywacke (Lonan type) with lower silica (SiO2 65–78%). Most element contents vary with silica but there is a compositional hiatus. It is here inferred that the two sandstone groups represent material from separate source areas. The three tectonostratigraphical sequences distinguished in the southeast of the island all include sandstone of both compositional types in different proportions. These usually occur as units of one or other composition, but in one sequence the two are locally closely interbedded while remaining compositionally distinct. Sandstone in two possibly equivalent units may, however, vary gradationally between the two types, implying more intimate mixing. In the absence of biostratigraphical control, the geochemical data are used to constrain the various ways in which the tectonostratigraphical sequences might correlate.
The chemical signature of the Isle of Man sandstones also provides constraints on possible correlatives in adjacent areas. The Lonan type is very similar to mid-Arenig sandstone in the upper part of the Skiddaw Group of the English Lake District. The more siliceous Agneash type has no compositional comparative in the main Skiddaw Group outcrop, although sandstone comprising the enigmatic Redmain Formation is similar in composition. The Wenlock Niarbyl Formation is lithologically and chemically comparable with Wenlock turbidite sequences in the Southern Uplands terrane and in the Windermere Supergroup in the Lake District, and they may all be closely related. None provides a precise match but compositionally the Niarbyl sandstone closely resembles the sandstone of the same age in the Birk Riggs Formation of the Windermere Supergroup. Subject to a number of constraints, the chemical composition of sandstone can also provide information on the probable tectonic environment of the source rocks. On this basis, the Lonan and Niarbyl sandstones include substantial components of first- or second-cycle volcanic debris, whereas the Agneash type is dominated by mature debris reworked from older sedimentary rocks or derived from a granite/gneiss basement source.
