XXX10.1144/pygs2015-344P. Stanway et al.Depositional environment of Ashfell Sandstone Fm., Cumbria 2015
Research article
PYGS
Proceedings of the Yorkshire Geological Society
Published online March 31, 2015 doi:10.1144/pygs2015-344 | Vol. 60 | 2015 | pp. 145–152
The depositional environment of the Ashfell Sandstone Formation (Arundian, Mississippian), Ash Fell Edge, Cumbria, NW England P. Stanway, J. Nudds* & F. Broadhurst† School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK † Deceased * Correspondence:
[email protected]
Abstract: The depositional environment of the upper strata of the Ashfell Sandstone Formation in east Cumbria is investigated. By recording the sedimentary sequence, we have determined a series of facies and facies associations from which we have interpreted the conditions of sedimentation during late Arundian (Visean, Mississippian) times. The most likely interpretation is that sedimentation occurred in a shallow equatorial sea, close to a shoreline, and possibly in a lagoon environment that was subject to periodic monsoonal storms. Received 28 November 2013; accepted 20 October 2014
Ash Fell is a flat-topped moorland at a maximum height of 385 m, lying between the market town of Kirkby Stephen and the village of Ravenstonedale in east Cumbria, UK [NY 736 047] (Fig. 1), and bordered by Scandal Beck on the west and south and by the River Eden on the east. Cutting across this moorland in a NW–SE orientation, Ash Fell Edge is a prominent escarpment formed by the resistant Ashfell Limestone Formation of Holkerian (Mississippian) age. Immediately underlying this limestone, the Ashfell Sandstone Formation of Arundian age is a predominantly clastic unit of alternating sandstones and mudstones with intercalated thin beds of muddy/sandy limestone. In the early 1970s, improvements on the A685 road from Tebay to Brough resulted in a new cutting through Ash Fell Edge that provided excellent exposures of these beds. Surprisingly, however, a log of the full sequence has never been published. Higgins & Varker (1982) and Barraclough (1983) both produced partial logs of the sequence, which were restricted to the upper c. 10 m or so of the Ashfell Sandstone that outcrops in the road cutting [NY 7361 0476]. For this paper, we have logged a total of c. 22 m of Ashfell Sandstone, including c. 7 m that outcrop in a series of small, disused quarries to the north of the road [NY 7352 0478 and NY 7346 0483] (Fig. 1). Several of the limestone beds were sampled and sectioned for microscopy to determine their composition. Macrofossils were collected and identified at least to generic level. Here we present our palaeoecological and sedimentological interpretation of these logs (Fig. 2). Previous Work The Ashfell Sandstone Formation forms part of the Great Scar Limestone Group (Waters & Davies 2006) (formerly part of the Orton Group) of Mississippian (Early Carboniferous) age. The formation is up to 160 m thick in
the vicinity of Ash Fell (i.e. in Scandal Beck and Ravenstonedale), thinning northwards (Dean et al. 2011). The British Geological Survey England and Wales Sheet 40 (Kirkby Stephen; British Geological Survey 1997) shows the Ashfell Sandstone lying above the Breakyneck Scar Limestone and below the Ashfell Limestone (Fig. 1). Garwood (1913), in a major study of the Mississippian in northwest England, included the Ashfell Sandstone in the upper portion of the Gastropod Beds subzone, the lower part of Vaughan’s C Zone (Vaughan 1905). Ash Fell Edge lies towards the western margin of the Ravenstonedale Gulf, which extends into the ENE-trending Stainmore Trough (Johnson 1967; Johnson & Marshall 1971; Cossey & Adams 2004) between the Alston Block to the north and the Askrigg Block to the south (Fig. 1). The boundary between the Ashfell Sandstone and the Ashfell Limestone is marked by a sharp transition from a series of sandstone, limestone and mudstone beds, to one of extensive and continuous limestone deposition. Dunham & Wilson (1985), describing patterns of Carboniferous sedimentation, considered that as the end of the Arundian approached, the Ashfell Sandstone was deposited in a moderate energy environment, indicated by current ripples and cross bedding, interrupted by periods of limestone deposition. Ramsbottom (1973) included the Ashfell Sandstone in the regressive phase of his 3rd major cycle. He suggested that a northern shoreline was indicated by the presence in Ravenstonedale of what might be “distal tongues of the upper beds of the Fell Sandstone” of the Northumberland Basin, identified as the Ashfell Sandstone. Leeder (1988, 1992) also considered that the Ashfell Sandstone had a northerly provenance. He described the Ashfell Sandstone as a complex combination of clastic beds, which had travelled south as a series of delta lobes originating from the Northumberland Gulf, interspersed with marine limestones.
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Fig. 1. Locality map to show position of Ash Fell road cutting and the disused quarries to the north of the road referred to in text. Based on OS landplan map 1:10 000. Geological boundaries from BGS 1:50 000 Provisional Sheet 40, Kirkby Stephen (British Geological Survey 1997). Bre, Breakyneck Scar Limestone; AfS, Ashfell Sandstone; AfL, Ashfell Limestone; SU, Southern Uplands; SB, Solway Basin; NT, Northumberland Trough; LDH, Lake District High; Al, Alston Block; Ask, Askrigg Block; St, Stainmore Trough; KS, Kirkby Stephen; R, Ravenstonedale. Asterisks mark position of logged sections. EA15/014, by permission of the British Geological Survey © NERC 2015.
Leeder (1988) suggested that the interdigitation of limestones with the clastic deposits in the Ashfell Sandstone was due to autocyclic lobe abandonment and compactioninduced marine transgression at a time of more or less constant sea level. Waters & Davies (2006) also regarded the Ashfell Sandstone as a distal extension of the Fell Sandstone. In contrast, George (1958), on the evidence that the mineral content had been derived from the southeast, argued that the Ashfell Sandstone was not to be regarded as a tongue of the Fell Sandstone. Also, Johnson & Marshall (1971) considered the marine transgression that initiated the deposition of the Mississippian succession at Ravenstonedale to have come from the east through the Stainmore Trough. Additionally, Armstrong & Purnell (1987), using conodont evidence, were of the opinion that the Ashfell Sandstone in Ravenstonedale is in part older than the Fell Sandstone of Northumberland. Collier (1991), using seismic data, suggested that the deposition of the 6 km thick Lower–Middle Mississippian (i.e. Dinantian of older terminology) succession in the Stainmore Trough was due to extensional subsidence. The Ashfell Sandstone at Ash Fell Edge has also been studied by Turner (1950, 1959) as part of wider studies of the Carboniferous in this area. The Ash Fell Edge area is noted in particular for its fossil content in upper Arundian and lower Holkerian strata. Early work on the fauna in this area was carried out by Garwood (1916). Johnson & Nudds (1975) and Nudds & Day (1997) reported on the exceptionally well-preserved coral Siphonodendron martini, surviving in muddy conditions. Bancroft (1986) recorded the rare presence of the bryozoan Eridopora macrostoma, encrusting corallites together with Fistulipora incrustans. Higgins & Varker (1982) studied conodont faunas at this locality and Nudds & Taylor (1978) reported on the unusual preservation of fossil plants.
Results The succession under study is bounded at both top and bottom by thick, massive-bedded, medium grained, cross-bedded sandstones. The base of the sequence is a sandstone that outcrops in the small, disused quarry [NY 7352 0478] (see Fig. 2, beds 1–7), while the top of the sequence is a 3.2 m thick sandstone (Fig. 2, bed 104) that marks the top of the Ashfell Sandstone and is overlain in the road cutting by the Ashfell Limestone. These two sandstones are separated by approximately 17 m of alternating mudstones and limestones that chronicle the development of a shallow offshore basin, or small lagoon, which for much of its existence was receiving sediment from the land surface. First we will define the major facies present in the sequence in order to determine the processes involved, then we will define any facies associations to see if these suggest a plausible environmental reconstruction. Major facies are defined on the basis of lithological and, where appropriate, palaeontological criteria (‘biolithofacies’). Major facies Biolithofacies A Biolithofacies A is massive-bedded, medium grained, crossbedded sandstone, only seen at the base and the top of the sequence studied here. The sandstones at the base of the succession (Fig. 2, beds 1–7) have dunes on their upper bedding surface (wavelength 94 cm, height 10 cm) and smaller ripples on the surface of the dunes (wavelength 40 mm, height 5 mm, Ripple Index (RI) = 8). The dunes, observed on the bedding planes, are asymmetrical and the foresets, observed in the vertical section, are uni-directional with a height of c. 20 cm. The sandstones at the top of the sequence (Fig. 2, bed 104) have load casts and worm casts at the base
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Fig. 2. A composite grain-size profile log of the Ashfell Sandstone compiled from the disused quarries [NY 7352 0478 and NY 7346 0483] and the road cutting at [NY 7361 0476].
and rootlets at the top. The beds vary in colour from white and buff to deep red/brown or deep red/purple, and may exhibit thin shale partings. Interpretation and discussion. The asymmetry and unidirectional nature suggests that the dunes in the basal sandstones were current generated and not wave generated. Mean dune height is c. 2.9 times the mean foreset thickness (Leclair & Bridge 2001) and was therefore c. 60 cm. Dune height is directly proportional to water depth (approximately 20–30%), which was therefore c. 2–3 m (Merren Jones, pers. comm. 2013). We interpret the basal sandstones as having been deposited in a large river channel in a fluvial regime, or as a migrating, submarine offshore bar, while the upper beds seem to indicate subaerial conditions. Biolithofacies B Biolithofacies B is a soft, reddish-purple, red-brown or blue-grey shaley mudstone (for example Fig. 2, beds 8, 9, 13, 19, 22, 25, 27, 28, etc.). Bed 59 (the uppermost bed
exposed in the small quarries, which we believe correlates with the lowermost bed exposed in the road cutting), and also beds 90 and 92 have plentiful individual corallites of Siphonodendron martini, weathering out and lying on the surface. Fresh exposures of the latter two beds, observed in the original 1970s road cutting by one of us (JN), revealed several large colonies of this coral in growth position, now reposited in the collections of Manchester University Museum (LL. 10880 et seq.). Beds 59 and 85 also have colonies of S. martini in growth position, while bed 83 has abundant erect trepostome bryozoans at its base identified as Anisotrypa sp. (Caroline Buttler, pers. comm. 2013). Interpretation and discussion. The corals and bryozoans in growth position indicate that fully marine conditions were established during deposition of these beds, even though the basin was continually receiving a large amount of muddy sediment supplied from the land. It is unusual for colonial rugose corals to survive in such muddy conditions, and Nudds & Day (1997) have shown that these
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Fig. 3. Gastropods from Bed 17 with pound coin for scale (21 mm diameter).
corals were indeed adversely affected by this stressful environment in a number of ways. These include an accelerated vertical growth rate, a failure to reach normal adult dimensions, a lack of asexual budding, and the common occurrence of rejuvenesence. We interpret the continuous deposition of this mud to represent the ‘normal’, quiet conditions of a muddy marine basin, which we term the ‘background mud beds’. Biolithofacies C Biolithofacies C consists of thin, impersistent beds of nodular, friable, sandy and muddy limestone layers (for example Fig. 2, beds 21, 24, 26, 29, 32, 35). The beds are red/brown in colour and usually contain a fauna of fragmented corallites of S. martini and numerous crinoid ossicles of varying sizes. Bed 70 also contains fragments of colonies of the bryozoan Fenestella and large productid brachiopods, while bed 84 contains fragmented colonies of the tabulate coral Aulopora. Interpretation and discussion. We interpret these impersistent beds as tempestites, in agreement with Barraclough (1983) who described them as storm layers. These beds were laid down after violent storms that brought coarser grained clastic material from the land, and which would have reduced the salinity of the basin by flooding with fresh water. Coral thickets, crinoid meadows and bryozoan tufts would have been overcome and broken up by the current action, and deposited haphazardly within the sediment. We term these layers the ‘storm event sandy limestone beds’. Note that these sandy tempestites always contain a high percentage of calcareous mud presumably from the disturbed lime mud forming the bed of the basin or lagoon. Biolithofacies D Biolithofacies D comprises dark, muddy, bioturbated limestones. These consist of packstones and wackestones, always with a high proportion of land-derived mud so that
recognition of a ‘muddy limestone’ from a ‘calcareous mud’ is not always easy. The lowermost limestone (Fig. 2, beds 14–18) is characterized by a rich fauna of gastropods (Fig. 3). The succeeding five limestones (Fig. 2, beds 23, 38–39, 45, 58, 61–62) are mostly notable for their strongly bioturbated upper bedding surfaces, each being characterized by a particular trace fossil ichnogenus. Bed 23 has large diameter burrows of Thalassinoides (Fig. 4), along with spines and spines bases of Archaeocidaris and productid brachiopods. Bed 39 shows widespread development of Zoophycos (Fig. 5), while the upper surface of bed 45 is covered by small diameter burrows of Chondrites (Fig. 6). Finally, bed 62 has well-developed trails of Cruziana (Fig. 7) on its upper surface, along with large and small diameter horizontal burrows. Interpretation and discussion. In contrast to the short-lived storm deposits (tempestites), we interpret these limestones as lower energy episodes of longer duration caused by further apparent pulses of the marine transgression. The bioturbated bedding planes of many of these limestones suggest long periods of quieter conditions, while the high proportion of mud suggests that the current activity was never excessive, otherwise the mud would have been washed out. The gastropods in the lowermost limestone indicate shallow water (shoreline or lagoonal according to Tucker 2003) suggesting that water depth did not increase dramatically during the transgressions. Perhaps these apparent transgressions were merely caused by a switching off (or at least a reduction) of the clastic sediment supply? Biolithofacies E Biolithofacies E is a dark, muddy, non-bioturbated limestone (packstones and wackestones), but is highly fossiliferous with common examples of Siphonodendron martini in growth position (for example Fig. 2, beds 68, 74–82, 86–89, 91). Beds 74–82 represent a thicker limestone with a colony of S. martini in growth position near the base of bed 78. Bed
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Fig. 4. The trace fossil Thalassinoides from Bed 23 with pound coin for scale (21 mm diameter).
Fig. 5. The trace fossil Zoophycos from Bed 39 with pound coin for scale (21 mm diameter).
89 includes Siphonodendron, Fenestella, erect bryozoans and spriferid brachiopods. Bed 91 includes brachiopods and the rugose coral S. sociale. Interpretation and discussion. Similar to biolithofacies D, these limestones represent low energy episodes of longer duration caused by further apparent pulses of the marine transgression. The established growth of coral thickets with associated bryozoans and brachiopods suggests long periods of quieter conditions. The absence of an ichnofauna is difficult to explain. Biolithofacies F Biolithofacies F is a silty micrite lacking the red colouration of the event beds (biolithofacies C) and either lacking any fauna (Fig. 2, bed 97), or with a sparse fauna of echinoid
plates, calcareous algae and occasional bryozoans (Fig. 2, beds 97, 100–101). Interpretation and discussion. These are interpreted as marine limestones deposited as the basin neared the end of its life and became clogged with silty sediment. The paucity of fauna prevents any further interpretation. Facies associations Four facies associations have been defined, with each occurring in a distinct part of the sequence at Ash Fell Edge. The associations have been defined by the repetition or occurrence of a combination of facies within the sequence. Where there is a marked change in the facies pattern, a new facies association has been defined.
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Fig. 6. The trace fossil Chondrites from Bed 45 with cm/mm scale bar.
Fig. 7. The trace fossil Cruziana from Bed 62 with pound coin for scale (21 mm diameter).
Facies association 1 (FA1): A–B This association occurs only in the first 1.8 m of the log (Fig. 2, beds 1–13). The basal sandstone (beds 1–7) is followed by blue-purple and reddish-purple mudstones (beds 8, 9), then by a further buff sandstone (beds 10–12), and finally another red-purple mudstone (bed 13). We assume that after the deposition of the basal sandstone in a river channel or as a submarine offshore bar (section Biolithofacies A above), the area was affected by a marine transgression, the result of either a eustatic rise in sea level or a relative rise in sea level caused by basin subsidence. The basin was receiving a small amount of red-purple coloured mud supplied from the adjacent land surface. A temporary shallowing of the basin led to the deposition of the subsequent sandstone, before marine conditions became firmly established.
Facies Association 2 (FA2): B-C-D This association occurs from 1.8 to 7.2 m (Fig. 2, beds 14– 62) and is marked by the disappearance of facies A, and the appearance of facies C and D. Facies B occurs 16 times in this sequence, facies C occurs 12 times, while facies D occurs 6 times. (Note that Bed 59 forms the top of the logged section in the 4 small quarries to the north of the road. We estimate by triangulation that this bed correlates with that at the base of the road cutting section; whether there is a small unexposed gap, or a small overlap we cannot be certain.) This association represents the time when the offshore marine basin was becoming established. Most of the time the basin was receiving a small amount of mud supplied from the adjacent land surface (facies B). A marine fauna of gastropods, brachiopods, crinoids, echinoids, bryozoans, and even rugose and tabulate corals, was able to flourish in
Depositional environment of Ashfell Sandstone Fm., Cumbria these somewhat muddy conditions, although the rugose corals, at least, were adversely affected by it (Nudds & Day 1997). The slow deposition of this mud represents the ‘normal’ conditions for this basin, which we have termed the ‘background mud beds’ (e.g. Fig. 2, beds 9, 19, 59). Such conditions were periodically interrupted by one of two quite different situations. Violent storms repeatedly brought coarser grained clastic material from the land and reduced salinity of the basin by flooding with fresh water. Coral thickets and crinoid meadows were overcome and broken up by the current action, and were deposited haphazardly within the tempestites (facies C). In contrast to these short-lived storm deposits, there were a number of lower energy episodes of longer duration caused by further episodes of marine transgression. These happened in one of three ways, either by further eustatic rises in sea level, or by further basin subsidence causing a relative rise in sea level, or by a turning off or slowing down of the sediment supply causing an apparent transgressive series of facies changes. These transgressive events are evidenced by a number of bioturbated limestone beds (facies D) that punctuate the succession, especially in the lower part. Water depth remained shallow for the entire existence of this basin, possibly not much more than 10 m. Evidence for this comes from the trace fossil assemblage of Cruziana, Chondrites and Thalassinoides, all of which belong to the Cruziana ichnofacies, characteristic of the sublittoral zone, below normal wave base between 10 and 100 m depth (Tucker 2003). The only suggestion of deeper water comes from the presence of Zoophycos, which is normally characteristic of the bathyal zone, 100–2000 m depth. However, both the Cruziana and Zoophycos ichnofacies also occur in lagoons, and perhaps this is the most compatible model for Ash Fell Edge. Facies association 3 (FA3): B-C-E This association occurs from 7.2 to 13.8 m (Fig. 2, beds 63–91) and is marked by the disappearance of facies D, and the appearance of facies E. Facies B occurs 9 times in this sequence, facies C occurs 5 times, while facies E occurs 4 times. This association is not dissimilar to FA2, but the facies proportions are different. Because facies C beds (the tempestites) are less frequent, facies B beds (the background muds) tend to be thicker. Moreover, the limestones (facies E) are generally thicker in this part of the sequence than those of facies D in FA2. This all suggests that water depth in FA3 was greater than at any other time in the evolution of the basin. The transgression was at its maximum, true marine conditions became established for longer periods, while the increased depth and distance from the shoreline reduced the effect of the storms, and only the very severe storms deposited tempestites. Facies association 4 (FA4): B-C-E-F This association occurs from 13.8 to 18.4 m (Fig. 2, beds 92–103) and is marked by the appearance of facies F. Facies B occurs 5 times in this sequence, facies C and F both occur twice, while facies E occurs only once. This association occurs only at the top of the sequence. Bed 92 (facies B) is rich in corallites of S. martini, so conditions were clearly still marine at this point, but FA4 is marked by the reappearance of the thin, impersistent
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tempestites of facies C (beds 93, 95), previously seen near the base of the sequence in FA2. This suggests that the water was shallowing once more and the basin had by now entered a regressive phase. Near the top of the section, facies F makes its only appearance in the succession (beds 97, 100–101). These friable, silty micrites, with a sparse fauna of echinoid plates and occasional bryozoans, are interpreted as marine limestones, but were deposited as the basin neared the end of its life and became clogged with silty sediment. Only a single typical limestone (facies E) occurs in this association (bed 102), representing one last minor transgression before the basin was filled. Above this, limestone bed 103 (facies B) revealed a large bivalve near its base, but unfortunately it was not possibly to determine whether this was marine or non-marine. The uppermost sandstone (bed 104) represents the final filling of the basin and return to fluvial conditions. It has load casts and worm casts on its under surface, long rootlets on its upper surface and shows evidence of soft sediment deformation, probably by dewatering of the underlying muds. Conclusions The Ashfell Sandstone is noted for its reddish colouration due to a relatively high iron content. We consider this to indicate that the area of deposition lay close to an arid, if not desert, shoreline. During the Mississippian, this part of the British Isles was thought to lie in equatorial latitudes with a semi-arid climate (Wright 1990). In such a position and with such a climate, a desert-like environment would not be unusual and, if close to shore, could account for the ironrich content in the rocks at Ash Fell. An equatorial climate could also provide hot and humid conditions with perhaps monsoon or storm events and seasonal rainfall, changing a low energy environment, when lower levels of siliciclastic material were deposited, to a higher energy environment with rivers in spate and increased levels of mud and sand being transported into the area of deposition. Inundations of terrigenous material overwhelming benthic fauna suggest that periods of rapid sedimentation were not unusual in areas subjected to monsoon climatic conditions (Broadhurst et al. 1980). Acknowledgements and Funding The authors express their gratitude to Mr Martin Dent at Ashfell Farm for permission to excavate on his land. Dr Merren Jones (University of Manchester) advised on the implications of the dune bedding, Dr Caroline Buttler (National Museum of Wales) identified the bryozoans and Dr James Jepson (Humboldt Museum, Berlin) kindly drafted Figs 1 and 2. We also thank Dr Tony Adams (University of Manchester) who read and commented on a previous version of the manuscript, and Mrs Lisa Jepson (University of Manchester) for fruitful discussions on the definition of facies and facies associations. Finally we are grateful to Dr Stewart Molyneux, Dr Patrick Cossey and an anonymous reviewer for their helpful improvements to the manuscript.
Scientific editing by Stewart Molyneux
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