Manx Group succession, early Ordovician, 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.
Late Tremadoc to early Arenig turbidite facies
The oldest Manx Group rocks seen at outcrop comprise the Glen Dhoo Formation in the north of the Isle of Man, and the Lonan Formation, which crops out over much of the southern and eastern part of the island (P916042). Both of these formations contain late Tremadoc to early Arenig acritarch floras. They are dominated by mudstone with a variable proportion of siltstone and fine-grained sandstone forming thin beds that are commonly either planar- or cross-laminated. Locally, immature fine-grained sandstone, in beds ranging up to 2 m thick, is dominant within the Glen Dhoo Formation. This formation also includes, near Peel, a minor occurrence of andesitic volcanic breccia that contains an acritarch flora in its mudstone matrix. Sandstone-dominant members are more widely developed in the Lonan Formation, where they make up much of the south-east coast of the island (P018605). Two distinct sandstone types are present, one forming the Keristal Member, and the other forming the Santon and Ny Garvain members.
Although the Keristal Member is only a few metres thick and locally lenticular, it is laterally persistent over at least 15 km as beds of distinctive, pale grey and fine-grained quartz arenite. The beds are ungraded or weakly graded, commonly massive or with parallel-lamination, but in some cases with ripple cross-lamination in thinner beds. They may be organised in either thinning-up or thickening-up sequences. In thinning-up sequences, the basal bed is commonly strongly erosional into underlying thin-bedded turbidites. The Keristal Member represents a short-lived system of sand-rich gravity flows that tapped a source of relatively clean quartz sand.
The Santon Member (at least 600 m thick) and the along-strike but slightly younger and much thinner Ny Garvain Member, are characterised by well-bedded sequences of grey, fine-grained wacke intercalated with the more typical facies of the Lonan Formation. Sandstone beds grade upwards into the intervening mudstone, parallel- and ripple cross-lamination is common, and some bed bases preserve flute marks and horizontal burrows. Locally, beds of pale grey quartz arenite punctuate the less-mature wacke successions. The members are interpreted as the deposits of relatively high-concentration turbidity flows; the palaeocurrents, as determined from sole marks, were directed consistently towards the west.
The base of the Lonan Formation is not seen but at least 2500 m of its strata are exposed. The top is transitional into overlying formations and can be readily defined relative to the lithologically distinct Creg Agneash Formation in the north-east and Mull Hill Formation in the south (P916038) and (P916042). The transition is more subtle in the centre of the island where the Lonan Formation passes directly up into the lithologically similar Maughold Formation.
The relatively high-concentration quartz arenite turbidity flows represented only locally in the Lonan Formation become dominant in the overlying Creg Agneash and Mull Hill formations. Both are characterised by sequences of white or pale grey, quartz arenite interbedded with a variable amount of mudstone. The sandstone beds are massive or weakly graded from medium sand to silt. A few beds have coarse-grained sandstone bases with mudstone rip-up clasts. Internal structure is typically a weakly defined, upward thinning parallel-lamination, in some cases with a thin ripple cross-laminated division at the top. Slump folding on a scale of tens of centimetres to metres is commonly developed, with close to tight disharmonic folds of widely varying orientation. The top of the Mull Hill Formation is not seen due to faulting. The Creg Agneash Formation becomes more mudstone-dominated upwards and passes transitionally into the Maughold Formation as sandstone beds decrease in abundance. The age of the Lonan Formation beneath the Santon Member is constrained by acritarch floras from several localities that are indicative of the late Tremadoc to early Arenig interval.
Acritarchs from the Santon Member itself established correlation with the earliest Arenig phyllograptoides graptolite Biozone (P916038) but graptolites, though present, are poorly preserved and allow only a general Arenig age to be allocated.
Early to mid Arenig mudstone dominated facies
The upland spine of the Isle of Man is composed of mudstone-dominated formations (P916042). The Maughold Formation is mainly composed of dark grey, laminated mudstone in bedded or disrupted facies, but locally includes a significant proportion of either siltstone or quartz arenite interbeds. Bioturbation is common as spots or as discordant silt-filled or mud-filled burrows. In the south of the outcrop, pebbly mudstone locally forms a substantial part of the succession, though it is absent in the north-east. This lithology is matrix supported, with clasts generally up to a few centimetres in diameter but ranging up to a maximum of 30 cm. The clasts are mostly intraformational but include rare fragments of a fine-grained igneous rock.
The contact of the Maughold Formation with the Barrule Formation is faulted, but an originally conformable stratigraphical contact between the two formations seems likely. The outcrop of the Barrule Formation, duplicated by thrusting, consists of homogeneous or faintly laminated grey to black mudstone interpreted as hemipelagic sediment deposited in an anoxic sea-bed environment. The formation may be several hundred metres to 1 km thick, depending on internal structure, and passes gradationally into the Injebreck Formation with the appearance of distinctive silty laminae.
The Injebreck Formation is in many respects similar to the Maughold Formation. It is dominated by dark mudstone with pale grey silty laminae and a variable proportion of interbedded siltstone and fine-grained sandstone; the latter may be either wacke or quartz arenite. The sandstone beds typically have sharp tops and bases and are parallel- and ripple cross-laminated. Pebbly mudstone occurs locally in the south-west of the outcrop, but becomes dominant through much of the lower part of the formation in the north-east. Clasts range up to 10 cm across, occasionally to 35 cm, and are matrix supported. They consist mainly of intrabasinal material but include some fine-grained igneous rock types.
The Injebreck Formation appears to pass stratigraphically up into the Glen Rushen Formation, although the evidence is equivocal. The Glen Rushen Formation is composed predominantly of laminated, grey mudstone with a variable proportion of pale siltstone laminae that are generally less than 1 mm thick. Bioturbation is absent and the laminae are laterally persistent. Thin beds of quartzose sandstone or pebbly mudstone occur sporadically. The Glen Rushen Formation passes stratigraphically upwards into the Creggan Moar Formation with a gradual increase in the proportion of siltstone laminae and the appearance of very thin manganiferous ironstone beds.
The mudstone-dominated facies reflects deposition from hemipelagic fall-out and low-concentration turbidity flows into a periodically oxygenated marine basin. Sandstone-rich intervals record periods of medium- to high-concentration turbidity flow, with some of the introduced sediment being highly quartzose. Periodic basin instability resulted in disruption and resedimentation of the bedded facies as pebbly mudstone.
The age of the Maughold, Barrule and Injebreck formations is poorly constrained. Their interpreted stratigraphical position (P916038), above the Tremadoc to lower Arenig Lonan Formation and below the middle Arenig Creggan Moar Formation, implies an early to mid Arenig age. Samples from all of the formations have yielded sparse, low-diversity assemblages of acritarchs consistent with the lower Arenig (varicosus graptolite Biozone) in the Lake District. Most samples from the Glen Rushen Formation have yielded similar sparse assemblages but two samples close to its transition into the Creggan Moar Formation have yielded acritarchs diagnostic of the mid Arenig in the Lake District.
Mid to late Arenig manganiferous iron stone bearing facies
Two units that crop out in the north-west of the Isle of Man are biostratigraphically constrained - as the youngest parts of the Manx Group, ranging from mid to late Arenig in age (P916038). The Creggan Moar Formation crops out in sequence above the Glen Rushen Formation but is structurally truncated along its western margin. The Lady Port Formation forms a tectonically isolated sliver on the north-west coast of the Isle of Man (P916042).
The Creggan Moar Formation was formed mainly from low-concentration turbidity flows into oxygenated bottom waters, with deposition punctuated by intermittent episodes of chemical precipitation. The formation is dominated by thinly bedded, dark grey mudstone, siltstone and very fine-grained pale sandstone. The siltstone and sandstone beds are typically parallel-laminated, rarely cross-laminated, and grade through bioturbated tops into the mudstone. Very thin beds of manganiferous ironstone occur intermittently or in clusters and weather distinctively to shades of reddish brown or dark brown to black. Parallel-laminated siltstone is locally dominant, forming thick uniform successions with rare mudstone or manganiferous beds. Quartz arenite occurs sporadically as pale grey, faintly laminated, thin to medium beds.
The age of the Creggan Moar Formation was only recently constrained as mid to late Arenig (P916038). Acritarchs, sparse near the transition from the Glen Rushen Formation but becoming more abundant and more varied elsewhere, commonly include species indicative of the mid Arenig. However, one locality in the middle of the outcrop has yielded a variety of forms that suggest a late Arenig age. The top of the Creggan Moar Formation is not seen; in the north the Glen Dhoo Formation has been thrust above it and in the south it is faulted into juxtaposition with Silurian rocks of the Dalby Group.
The Lady Port Formation is heterogeneous and extensively disrupted, recording major basin instability. The dominant lithology is matrix-supported pebbly mudstone containing a varying proportion of sandstone and siltstone clasts, commonly up to 20 cm in size but also including rafts of bedded strata up to at least tens of metres in size. Fragments of intact lithostratigraphy include thinly-bedded wacke, and mudstone with siltstone laminae that resemble parts of the Glen Rushen Formation. Alternating with the pebbly mudstone are rare sections of interbedded millimetre-scale manganiferous ironstone bands, siltstone and fine-grained quartzose sandstone that form a lithofacies similar to that of the Creggan Moar Formation. The Lady Port Formation has yielded poorly preserved acritarchs considered to be of late Arenig age, but the likelihood that material has been reworked from underlying formations means that this can only be a maximum.
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