The stratigraphy and tectonics of the Manx Slate Series, Isle of Man

Abstract

The geosynclinal sequence of the Manx Slate Series has been subdivided into eleven distinct formations with a thickness of about 25 000 ft. This thick sedimentary pile was deposited in the Caledonian geosyncline and is most probably all of Cambrian age. The stratigraphical positions of two thick sheets of slump-breccia have been established. The tectonics of the Manx Slate Series are more complex than previously envisaged. The island forms part of the polyphase Caledonian orogenic belt. Three phases of tectonic movement ( F 1 , F 2 , F 3 ) each formed large-scale folds and a suite of minor structures. Later structures deform those that originated earlier. The fundamental structure that affects the whole island is the acute Caledonoid Isle of Man syncline of F 1 age. During the second movement-phase ( F 2 ) this pre-existing fold was flexed into a major synform and two complementary antiforms. These more open flexures have Caledonoid hinges and their axial planes dip gently north-westwards. During the last phase of folding ( F 3 ), the complexity of the pile was further increased by several large, open cross-flexures round axial planes dipping steeply east-north-east. The final event in the Caledonian deformation was a brittle fracturing of the complex block by a sinistral system of large and small transcurrent faults with a north-westerly strike. Most of the small normal faults that also intersect the Manx Slates are probably post-Carboniferous in age.

The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District

“…Group has so far proved to be only sparsely fossiliferous, the biostratigraphical evidence does not support the order of superposition in Simpson's (1963) scheme. This evidence suggests that formations towards the top of Simpson's succession are among the oldest exposed on the island, whereas the lowest formation in Simpson's scheme should be placed higher in the succession.…”

“…15). It comprises a thick pile predominantly of mudstone, siltstone and sandstone turbidites with sedimentary breccias (Gillott 1956;Simpson 1963), and has its closest similarities with the Skiddaw Group succession in the Northern Fells Belt.…”

“…The Glen Dhoo Flags, overly the Cronk Sumark Slates in Simpson's (1963) scheme, and have yielded an acritarch microflora that Molyneux (1979) considered to be of probable latest…”

“…A second microflora, from poorly exposed Niarbyl Flags interbedded with supposed volcanic rocks near Peel (Simpson 1963), was previously thought to be of late Arenig age (Molyneux 1979), but can now be reinterpreted as the uppermost sub-assemblage (5) of the messaoudiitrifidum assemblage. The microfloras suggest that the Niarbyl Flags are partly temporal equivalents to the topmost Watch Hill and basal Hope Beck formations in the Lake District.…”

“…The `Peel volcanic rocks' were recorded as andesitic lava, tuff and agglomerate with sedimentary intercalations (Simpson 1963). Hughes & Kokelaar (1993) Group (Molyneux 1979) Roberts et al (1990).…”