Early Ordovician: Skiddaw and Manx groups, introduction and stratigraphical framework, 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.
The Skiddaw (English Lake District) and Manx (Isle of Man) groups include the oldest rocks now seen in northern England. Their strata were deposited contemporaneously during the early to mid Ordovician, in relatively close proximity, on the northern margin of Avalonia (P916033). The thickness, duration and geographical extent of the two groups suggest that they were deposited in a large basin complex with a long history of subsidence. Although they are composed of broadly similar, mud-rich turbidite lithologies, the two groups differ considerably in detail and so deposition by localised sedimentary systems seems likely. For the geologist the Skiddaw and Manx groups’ strata are challenging. Lithological uniformity, complex structure and a lack of distinctive stratigraphical ‘marker’ beds combine to keep substantial parts of their outcrops enigmatic, with much detail currently unresolved.
During the early Ordovician, Avalonia formed the southern margin of the Iapetus Ocean and lay at about latitude 60°S within the circum-polar ‘Atlantic’ faunal province. This was characterised by an abundant, but low diversity, graptolite fauna and a sparse trilobite population. Examples of the graptolite fauna are widespread in the Skiddaw Group (from which a few trilobites have also been recovered) but macrofossils are rare in the Manx Group. Both groups contain acritarch floras that are the key to their biostratigraphical correlation (P916038). A selection of Skiddaw Group fossils is shown in (P711116), though the generally small and unsubstantial — yet biostratigraphically crucial — graptolites are under-represented.
The Skiddaw Group comprises a thick (about 5000 m) succession consisting largely of turbiditic sandstone and mudstone. The principal outcrop is in the main Skiddaw inlier of the northern Lake District, with the smaller Ullswater and Bampton inliers nearby to the south-east, and the Black Combe and Furness inliers farther away to the south (P916039). To the east of the Lake District, the Skiddaw Group appears in inliers of Lower Palaeozoic rock at Cross Fell and Teesdale, and has been proved by boreholes to underlie Carboniferous strata across much of the Alston Block. From its graptolite fauna and acritarch flora can be established an Ordovician biostratigraphy spanning the Tremadoc, Arenig and Llanvirn epochs (P916038). The only evidence for a pre-Ordovician component is provided by probable Cambrian acritarchs from a single locality close to Eycott Hill, in the north-east of the main Skiddaw inlier.
In the largest of the inliers (480 km2 including the eponymous mountain of Skiddaw), two distinct stratigraphical belts are present — the Northern Fells Belt and the Central Fells Belt (P916040) — separated by a major wrench and thrust fault system. This structure, the Causey Pike Fault, can be traced across the Lake District (P005045) and eastwards to Cross Fell (P916039) where it separates Skiddaw Group sequences that are stratigraphically equivalent to those forming the two belts of the main, Skiddaw inlier. The Causey Pike Fault coincides with a major geophysical lineament that is interpreted as a basement shear zone into which a concealed, approximately 400 Ma, elongate granitic body has been intruded.
To the north of the Causey Pike Fault, in the Northern Fells Belt, are preserved some 5000 m of mainly mudstone turbidites that were deposited between the Tremadoc and the early Llanvirn, although the acritarch flora from the Eycott Hill area suggests that sedimentation may have commenced in the Cambrian. Wacke-type sandstone beds occur sporadically throughout the succession but become dominant at two levels where they make up the Watch Hill and Loweswater formations (P916041). Their provenance was largely within an old, inactive, continental volcanic arc lying to the south-east. South of the Causey Pike Fault, in the Central Fells Belt, the Buttermere Formation is a major olistostrome that was emplaced from the south during the late Arenig. The olistostrome is overlain by 1000 m, perhaps more, of upper Arenig to Llanvirn siltstone and mudstone with sporadic volcaniclastic interbeds. These strata make up the Tarn Moor Formation, which also crops out farther south-east as inliers around Ullswater and Bampton.
It has been traditional to correlate the Skiddaw Group with the Ingleton Group, a superficially similar turbiditic lithofacies that crops out still farther south in the Craven inliers (P916039), and for which a description is given in the British Regional Geology guide for The Pennines and adjacent areas. However, despite some common features, there are significant compositional and structural differences between the two groups and it is possible that they are not closely related.
The Manx Group is dominated by laminated mudstone and siltstone (P018620) but also includes much thin- to medium-bedded, typically fine-grained sandstone, and intrabasinal debris flows of pebbly mudstone. Small amounts of volcanic rock occur locally and are apparently interbedded with the sedimentary strata. The succession is essentially steeply dipping to vertical and strikes north-north-east to east-north-east, so that the different formations have lenticular outcrops parallel to the long axis of the island (P916042). First described as the ‘Manx Slate’ the rocks were long-regarded as being of Cambrian age then, more recently, thought to be entirely of Ordovician (Arenig) age. In the latest development, newly discovered graptolites have established that part of the succession is Silurian, and most probably Wenlock, in age. The Silurian strata have now been separated as the Niarbyl Formation of the Dalby Group and can be informally associated with the Windermere Supergroup (see Chapter 4).
The stratigraphy and order of succession of the Manx Group have long been a matter of debate. From recent work, the rocks are considered to crop out in north-west younging succession across most of the island (P916042), although they are folded and disrupted by strike-parallel faults which locally repeat parts of the succession. The oldest and youngest parts of the sequence are structurally repeated in the north of the outcrop. It is difficult to estimate the thickness of the individual formations, but most are likely to comprise from several hundreds of metres up to about 1000 m of strata. The Lonan Formation is thicker, probably in excess of 2000 m. The total thickness of the exposed succession is likely to exceed 5000 m, but seems unlikely to have accumulated anywhere as a single entity.
Biostratigraphical control on the Manx Group is principally provided by acritarchs supported by a very sparse graptolite fauna. A Tremadoc to Arenig age range has been established (P916038), although the presence of some long-ranging forms leaves open the possibility that the base of the sequence may extend down into the Upper Cambrian. In general, and within a background of mudstone and siltstone, wacke-type sandstones are common in the older parts of the Manx Group (e.g. Lonan and Glen Dhoo formations), whereas quartz-arenite sandstones become locally important around the middle (e.g. Mull Hill and Creg Agneash formations); mudstone–siltstone sequences with little interbedded sandstone (e.g. Maughold and Creggan Moar formations) form the younger parts of the group. The depositional style is turbiditic and invites a general comparison with the Northern Fells Belt of the Skiddaw Group. Mass-flow deposits of pebbly mudstone in the Upper Arenig Lady Port Formation are possible equivalents of the Buttermere Formation olistostrome in the Skiddaw Group’s Central Fells Belt.
Both the Skiddaw Group and the Manx Group display a complex pattern of very low-grade metamorphism, elevated locally by contact effects around large granitic intrusions. Late diagenetic grades are found in the relatively soft mudstone that makes up, for example, the Tarn Moor and Bitter Beck formations of the Skiddaw Group. These rocks probably preserve syndepositional burial metamorphism augmented (at least in the Skiddaw Group) by the effects of burial beneath the Borrowdale Volcanic Group. Slightly higher grades of regional metamorphism were probably imparted by a combination of thermal effects during late Ordovician magmatism (i.e. associated with eruption of the Borrowdale Volcanic Group) and tectonic effects during Early Devonian (Acadian) deformation. Acadian contact metamorphism is illustrated by the hornfelsed aureoles around the Skiddaw Granite in the Lake District, and the Foxdale Granite in the Isle of Man. A more detailed account of the metamorphic effects is given in Chapter 5.
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