Skiddaw 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.
In the Northern Fells Belt the Skiddaw Group comprises five formations overlain by the Eycott Volcanic Group (P916041). In ascending order these formations are: Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile. Boundaries between the formations are transitional.
The Bitter Beck Formation comprises thinly laminated, dark grey mudstone, silty mudstone and siltstone with minor amounts of fine grained, wacke-type sandstone. The latter is concentrated in the lower part of the formation where it forms up to 20 per cent of the succession in thin to medium beds (3–30 cm). In places there is abundant slump folding with dislocations sub-parallel to bedding. The formation is at least 500 m thick and lies within the upper Tremadoc murrayi graptolite Biozone (P916038).
The Watch Hill Formation comprises interbedded lithic sandstones (locally coarsening to microconglomerate) and mudstones. The sandstone is typically fine grained with abundant siltstone and mudstone intraclasts; bed thickness ranges up to about 1 m. Sandstone to mudstone proportions change vertically and laterally throughout the succession, but are typically around 60–70 per cent sandstone and 30–40 per cent mudstone. The maximum thickness of the formation is somewhere between 550 m and 800 m but it thins both westwards, to about 100 m, and eastwards, to 40 m. This pattern might have originated through deposition in a system of diverging channels, but interpretation in these terms is precluded by evidence derived from basal flute casts that shows a consistent palaeocurrent flow from an easterly direction. The age of the formation is within the latest Tremadoc murrayi graptolite Biozone (P916038). Equivalent strata further east in the Cross Fell inlier there form the Catterpallot Formation (P916041).
The Hope Beck Formation comprises laminated and thinly bedded mudstone with sporadic, thin sandstone and pebbly mudstone beds. Bioturbation is common, with burrows mainly subhorizontal to bedding and sometimes filled with faecal pellets. The sandstone is quartz-rich lithic wacke, generally medium to coarse grained, but including numerous beds with granules and pebbles. The clast assemblage is dominated by monocrystalline plutonic quartz and polycrystalline metamorphic quartz. Pebbly mudstones are matrix supported (about 50 per cent matrix) and probably represent debris flows. The thickness of the formation is between 600 m and 800 m with a biostratigraphical range from latest Tremadoc to early Arenig (P916038), spanning the phyllograptoides (more-or-less equivalent to the traditional approximatus) graptolite Biozone and extending up into the varicosus graptolite Biozone.
The Loweswater Formation is dominantly formed of quartz-rich wacke-type sandstone in which the individual grains are predominantly of monocrystalline quartz, mostly strained but with some larger, unstrained grains. The sandstone is mostly fine to medium grained, occurring in both parallel-and cross-laminated beds (P006929). The bed thickness increases gradually upwards to a maximum of 1 m near the middle of the formation; concurrently the maximum grain size increases to very coarse grained, though most beds are still fine to medium grained. The upper part of the formation is a mirror image of the lower part, bed thickness and grain size decreasing gradually while the percentage of siltstone and mudstone increases. Overall, the sandstone beds are slightly lenticular. Most have planar bases, but channels, groove casts and flute casts occur locally. Palaeocurrent flow was mainly from the southern quadrant. Small scale slump folds, up to a few metres across are widespread. The Loweswater Formation has a maximum thickness of approximately 900 m, decreasing northwards to an estimated 450 m. Biostratigraphically it occupies a position in the middle part of the Arenig, spanning the upper varicosus and lower simulans graptolite biozones (P916038).
The Kirk Stile Formation is between 1500 m and 2500 m thick but some of this apparent variation may be due to the repetition of strata through the stacking of slumped masses. The principal lithology is thinly laminated mudstone deposited from low-density turbidity currents. Interbedded with the mudstone are local lenticular units dominated by lithic wacke; these sandstone-rich units are typically thinly bedded with parallel- and cross-lamination. Strained quartz is the most abundant grain type, feldspar is rare, but various lithic grains, mostly volcaniclastic siltstone and altered mafic volcanic rock, are relatively common. In the upper part of the formation, sporadic units of sedimentary breccia and slumped strata, from 2 to more than 40 m thick, include intrabasinal clasts up to about 0.5 m in diameter contained in a silty mudstone matrix. Biostratigraphically, the formation ranges from the mid Arenig to the early Llanvirn, spanning the simulans to artus graptolite biozones (P916038); coeval strata, though with subtle lithological differences, occupy the Furness and Black Combe inliers in the south of the Lake District.
Late Arenig and Llanvirn: Central Fells Belt
South of the Causey Pike Fault, the Skiddaw Group comprises two divisions: the massive and chaotically disrupted olistostrome of the Buttermere Formation, unconformably overlain by the mudstone-dominated Tarn Moor Formation. The latter contains interbeds of tuffaceous sandstone and bentonitic claystone, both indicative of contemporaneous volcanicity. The Tarn Moor Formation also crops out in the small inliers of Ullswater and Bampton, and farther east in the Cross Fell inlier has an equivalent in the Kirkland Formation (P916041), albeit that the volcaniclastic component appears to be greater at Cross Fell. The Buttermere Formation olistostrome has a likely counterpart at Cross Fell in the Murton Formation, although the level of sedimentary disruption in the latter is less than in the former.
The Buttermere Formation is at least 1500 m thick, comprising mudstone, siltstone and sandstone turbidite olistoliths set in an argillaceous matrix. The olistoliths range in size from granules to blocks up to a kilometre or more across, though most are in the 5–10 m range; the largest are formed by wacke-type sandstone. Both clasts and matrix are intensely deformed by minor folds and shears, many of which were generated during emplacement of the olistostrome. Fossils from the Buttermere Formation (rare trilobites as well as the more widespread graptolites and acritarchs) indicate a range of ages from early Tremadoc to late Arenig whilst the overlying Tarn Moor Formation contains graptolites of latest Arenig and Llanvirn age. Hence the age of olistostrome emplacement is inferred to be in the late Arenig, possibly at about the boundary between the gibberulus and cucullus graptolite biozones. From the geometry of the major slump folds it can be inferred that the olistostrome was emplaced by down-slope movement towards the north.
Evidence for large-scale soft-sediment movement in the Murton Formation, the stratigraphical equivalent of the Buttermere Formation in the Cross Fell inlier, largely derives from the many exposures that show complex polyphase folding and shearing characteristic of slumped deposits. The interpretation is supported by the range of biostratigraphical ages present, from early Arenig to early Llanvirn, and their apparently random distribution. In lithology, the Murton Formation is mainly thinly-bedded grey siltstone with subordinate thicker interbeds of sandstone. It is probable that the Murton Formation illustrates an only partially disrupted part of the succession that was completely disaggregated in the Buttermere Formation olistostrome.
The Tarn Moor Formation comprises late Arenig to early Llanvirn mudstone and siltstone, with minor volcaniclastic turbidite and bentonite beds. The formation rests unconformably on the highly disrupted Buttermere Formation, although the time-break across the unconformity would appear to be very small. The top of the Tarn Moor Formation is cut out by the unconformable base of the overlying Borrowdale Volcanic Group and the youngest Skiddaw Group strata remaining (from the murchisoni Biozone) are those encountered during construction of the Tarn Moor aqueduct tunnel. The thickness of the formation is probably around 1000–1500 m. In its lower part, laminated mudstone is similar in overall character to the Kirk Stile Formation of the Northern Fells Belt, with which it partly overlaps in age, spanning the cucullus graptolite Biozone of the upper Arenig and the artus Biozone of the lower Llanvirn. The upper part of the formation is characterised by mudstone with up to five per cent of bentonite and volcaniclastic interbeds. The bentonite ranges up from very thin laminae to discrete beds a few centimetres thick. Graptolites indicate the artus Biozone in the Skiddaw and Bampton inliers but range up into the murchisoni Biozone in the Ullswater Inlier.
In both its lithology and fossil content the Tarn Moor Formation is similar to the Kirkland Formation of the Cross Fell inlier. The thickness of the latter cannot be estimated accurately since neither its base nor its top are exposed, but at least 1000 m of strata may be present. A twofold lithological division is possible: a lower sequence of volcaniclastic turbidites interbedded with mudstone and siltstone, and an upper sequence dominantly comprising black graptolitic mudstone. From both of these putative divisions the Kirkland Formation has yielded abundant graptolites representing the artus Biozone.
Farther east, the restricted exposures of the Skiddaw Group in the Teesdale inlier show cleaved, dark grey mudstone that contains graptolites and acritarchs of probable early Llanvirn age. A broad correlation with the Kirkland and Tarn Moor formations seems likely.
Late Arenig to early Llanvirn: southern inliers
In the Black Combe inlier (approximately 50 km2, (P916039) the dark grey siltstones and mudstones seen in its south-eastern part become paler in colour towards the north-west as a result of secondary metasomatism and metamorphism. Within the unaltered part of the sequence, laminae and thin beds of sandstone occur sporadically and in one instance, low in the exposed sequence, thicken and coalesce into the 300 m of the Knott Hill Sandstone Formation. The altered part of the sequence in the north-west of the inlier is affected by a pervasive cleavage fabric and intruded by numerous granitic sheets. The boundary between the dark, unaltered and the pale, metasomatised mudstones is transitional over a few tens of metres in a zone trending north-east to south-west across the south-east slopes of Black Combe. It is inclined to the north-west at about 40o, sub-parallel to the cleavage, and is interpreted as a mainly tectonic contact.
The thickness of the Skiddaw Group in the Black Combe inlier is uncertain because bedding is generally ill defined and, where discernible, reveals that the silty mudstone is folded on a scale that varies from less than one to several tens of metres, with no consistency in the direction and amount of dip. It is likely that at least some of the deformation arose from slumping of the accumulated sediment prior to its consolidation. The structural complexity combined with the lack of stratigraphical control makes estimates of the thickness speculative but as much as 2000 m of strata might be preserved. Sparse assemblages of graptolites and acritarchs suggest a late Arenig age.
The western margin of the Black Combe inlier is formed by the Lake District Boundary Fault, a regionally important structure trending approximately north–south. To the north of Black Combe and close to Ravenglass, two small inliers of Skiddaw Group strata are contained within the fault zone. The tectonised slivers have also been thermally metamorphosed by the adjacent Eskdale granitic pluton and now comprise hornfelsed black mudstone containing metamorphic biotite and chlorite with sporadic tourmaline and chiastolite; some indurated and recrystallised sandstone is also present. Correlation with the rest of the Skiddaw Group is unclear.
In the south of the Lake District, the Skiddaw Group is also exposed, though poorly so, in a structurally complex inlier (or series of inliers) on the Furness peninsula (P916039) where it is unconformably overlain by the Dent Group, the lowest division of the Windermere Supergroup and of Ashgill age (see Chapter 4). There is no good indication of the sequence or thickness of the Skiddaw Group hereabouts since the inlier is extensively drift covered, with few natural exposures. Most information has been gathered from quarry sections (notably those of the Furness Brick Company), temporary exposures and boreholes.
The rocks present are dominantly dark grey mudstones with sporadic silt laminae up to 2 mm thick. When freshly exposed in quarry sections, the mudstone is almost coal-black, but pyritic inclusions give rise to deep ferruginous stains on weathering. A faint lamination is apparent in places but strong cleavage, intense crushing and minor contortions all combine to obscure sedimentary detail. Graptolites and acritarchs have been recovered locally and indicate an age within the late Arenig to early Llanvirn range, making the Furness strata a little younger than the Skiddaw Group in the Black Combe inlier, a little to the north (P916041).
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