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The geological significance of the Dolyhir and Nash Scar limestone (Silurian) of the Welsh Borderland
Introduction and History
Ther regions of Old Radnor and Presteigne (Fig. 1), found on the outskirts of Hertfordshire and Powys, are home to two small inliers containing a mammoth unit of limestones of the early Silurian age, thick and distinctive in their form. In the Nash Inlier in the north-east, an eminent ridged is formed by the limestones which was once host to the quarry of Nash Scar, but is now abandoned. The Old Radnor Inlier in the south-west however, is still a part of a large network of working quarries at Dolyhir. It was in the first half of the nineteenth century that these limestones were first geologically described by Sir Roderick Murchison, who visited the area of Nash Scar as part of his investigations that led to his magnum opus, the Silurian System (Murchison 1893). Sir Murchison stratified these deposits to be Wenlockian and concluded them to be equivalent to Woolhope limestone (Murchison 1854), although he first labelled them as ‘identical in position and organic remains’ to the newer limestones of Wenlock Edge ((Murchison 1839, p. 313). Davis (1850) in a short paper established the link between the limestone and the underlying deposits at Nash Scar and added a faunal investigation to the study of the area. These studies laid the groundwork, but unfortunately, the Silurian of the area received little attention for almost a century except for Calloway’s brief commentary on the ‘Woolhope Limestone’ of Old Randor during his investigative study on the underlying Precambrian. The credit of the most detailed description on the limestones and the Silurian geology in both areas still goes to Garwood and Goodyear (1919) with their geological map of the Old Radnor Inlier. Also, they presented a view of limestone opposite Murchison’s and claimed it to be equivalent to the limestone at Woolhope, though thermally metamorphosed. They authors were of the view that the limestone deposits were instead a representation of an algae reef.
The region continues to be of interest and many works have been published based on the Silurian faunas in Nash and Dolyhir, the consequential biostratigraphic study of limestones themselves, and the overlying and underlying deposits to decipher the exact age age (Kirk 1951, Ziegler et al. 1968, Bassett 1974a, 1974b, Hurst 1975, Hurst et al. 1978, Aldridge et al. 1981), although no major study has been carried out following Garwood and Goodyear (1919). Woodcock (1988) addressed the impact of tectonic activity on the limestone in the Old Radnor Inlier and provided a map of the region. Moreover, other works on the topic include a brief revision of the description on the individual localities (Woodcock 1993, Siveter et al. 1989, Siveter 2000), and a detailed review of the local stratigraphy (Cocks et al. 1971, 1992). It was Bassett (1977) who coined a formal stratigraphic term for the Nash Scar Limestone Formation, Dolyhir and limestones.
Global and Regional Palaeogeography
Earlier during the Wenlock times, the region of Old Radnor Presteigne was buried in a mid-outer position below the warm and shallow continental sea on the eastern border of the Welsh Basin (Fig 2a). The Midland Platform, a topographic high, lies to the east. Although mostly submerged at this time, the platform probably covers much of present day Lincolnshire, Nottinghamshire and Yorkshire with a large low-lying island. On the south-west lay an even larger landmass, Pretannia, whose northern coastline falls in the present day region of South Wales. The north-eastern border falls in the region of the Mendip Hills, and was home to active volcanoes. The north-western part of the Welsh Basin connected with the Iapetus Ocean, the western side of the Welsh Basin was met by the Irsish Sea Landmass, another topographical high, while towards the north, it was open. This composition has remained unchanged for more than 100 million years.
The present day England and Wales formed a part of the microcontinent of Eatsern Avalonia, had drifted away towards the north since Ordovician times as the Iapetus Ocean narrowed and closed REASON (see Pickering et al. 1988, Pickering & Smith 1995). Towards the Wenlock times, a series of drastic changes began that over the course of a few million years altered the geography of the region beyond any resemblance to the initial structure. Firstly, the ocean was almost entirely replaced, Eastern Avalonia which had already collided with Baltica during the late Ordovician period (Cocks & Fortey 1998), began colliding with Laurentia, the continent which had on the north-western side of the Ipateus, had divided the equator. Recorded as the Caledonian Orogeny in the British geological record, these movements welded together the Eastern Avalonia and Baltica to the border of Laurentia. In the following Wenlock period however, the Welsh Basin maintained its marine form as well as its connections with presumably the remnants of the Iapetus Ocean in the north and Rheic Ocean that had emerged in the south. It is agreed that the Welsh Basin possessed a tropical or subtropical position in the Southern Hemisphere during this time, however, there is contradiction over its exact latitude when the Nash Scar and Dolyhir reefs began forming (estimates vary from around 20 to 40 degrees south, see e.g. Scotese & McKerrow 1990, Torsvik et al. 1990).
Sedimentology and Fauna
Currently, the outcrop of the Nash Scar Limestone formation can only be witnessed at the Nash Scar quarry and around Dolyhir where the workings are extensively active. Dolyhir consists of the ‘Dolyhir Quarry’ (Garwood & Goodyear 1919) and ‘Strinds Quarry’ as well as their subsidiary quarries C and D and the huge area of Yat Wood and its subsidiary quarries E, F and G (Garwood & Goodyear, 1919) in the north. The older southern region and Yat Wood area are now popular as the ‘Strinds Quarry’ and ‘Dolyhir Quarry’ respectively (Fig. 3b).
The limestone formation of the Nash Scar and Dolyhir can be described as a pure carbonate formation with over 99% of CaCO3 according to Garwood & Goodyear (1919), highly crystalline, with its colour ranging between bluish-grey and white. The limestone is poorly to massively bedded. The thickness is difficult to assess accurately as a result of extensive faulting but it is agreed that it has a minimum thickness of 24m and a thickness of up to 60m at Nash Scar (Bassett 1974a). In some places at the Dolyhir and Strind quarries, the basal part of the limestone consists of a basal rudite which is up to 2m thick The rudite contains apart from the rounded quartz pebbles, angular mudstone and standstone clasts. These casts are as big as 10 cm in diameter are derived from the local Precambrian basement, mixed with in-situ colonies of the tabulate coral Favosites. In spite of the difficulties that arise in collecting the fossils as a result of the crystalline formation and massiveness of the limestone, an abundant and diverse fauna has been observed. Calcareous algae in the form of oncolites dominate the fauna (Johnson 1966). These have a diameter of up to 20cm and include Girvanella problematica, Girvanella pusilla, Rothpletzella gotlandica, and Solenopora gracilis. These porcellaneous algal structures appear in a striking contrast with the coarse and crystalline limestone. The faunal list of seventy species by Garwood and Goodyear (1919), also names bivalves, brachiopods, bryozoans, cephalopods, corals, crinoids, trilobites and gastropods as being present in the limestone formation. The brachiopods present (Bassett 1974a, 1974b, 1977) include Antirhynchonella linguifera, Megastrophia (Protomegastrophia) quetra, Leptaena oligistis, Streptis grayii, and species of Whitfieldella and Plectatrypa. The trilobites (Thomas 1981) include Cornuproetus peraticus, Dudleyaspis portlockii, Planiscutellum kitharos and Radnoria syrphetodes. The conodonts which are observed to be in abundance include Dapsilodus obliquicostatus, Ozarkodina sagitta rhenana, Decoriconus fragilis and Ozarkodina excavate. An acritarch assemblage also thrives in the limestone formation although in low diversity with Micrhystridium Veryhachium, and Diexallophasis(thin-walled forms) dominating the species. The formation’s lower part has within itself, a tectonized and hence discontinuous shale horizon. The shale is abundant in crinoid debris and consists of greenish carbonate concretions nucleated around crinoid stems and brachiopods (Garwood & Goodyear 1919) which gives it its nodular form. The fauna here differs from the limestone somewhat in its detail. The species of Trilobites present in the lower part consist of Cyphoproetus depressus and species of Kosovopeltis and Scotoharpes, together with Tapinocalymene volsoriforma (Thomas 1978, Siveter 1980).
In the north-east side of the Nash Scar Quarry, the hardground encrusted surface of the top of the limestone, with crinoids holdfasts is exposed.
Geological Setting and Stratigraphy
Fig. 4. summarizes the stratigraphy of the Nash Scar and Dolyhir Limestone Formation, and the deposits vertically adjacent to them. A combination of the fine clastic Yat Wood and the dominantly sandy Strinds Formation shapes the Precambrian basement in the Old Radnor Inlier, where the formation lies uncomformably. In the Precambrian Longmyndian rocks of the Church Stretton area (Calloway 1900, Woodcock 1988, Woodcock & Pauley 1989), analogues of these lithologies can be found. However, at Nash Scar, the Folly Sandstone Formation (dated from late Aeronian to early Telychian), is overlaid by the limestone disconformably (Ziegler et al. 1968). Although the base of this sandstone cannot be seen, a 30m thickness is suggested by Cocks et al. (1992). Shales of the Coalbrookdale Formation overlay the reefal limestones of both localities (Siveter et al. 1989). Normally the contact is faulted or concealed but at Nash Scar it can be seen above the hardground. The graptolite faunas present at the site suggest basal shales lie within the C. lundgreni Biozone, of earliest Homerian age (Hurst et al. 1978). The outcrops of the Coalbrookdale Formation with a faulted contact with limestone also gives rise to the view that some of the shales may be of the Sheinwoodian age (Bassett 1974a, Kirk 1951, see also Siveter et al. 1989, fig. 5). Above the stratigraphic contact, the shales present are marked by an encrusted hardground at the top of the limestone which is perhaps derived from the crinoid debris present in the shales in abundance (Hurst et al. 1978). This also suggests that it is the original post-hardground sediment that has developed into lundgreni Biozone shales. It can therefore be concluded to some degree of effectiveness, that the Coalbrookdale Formation in the inliers has a diachronous base, and dating from the upper Sheinwoodian age.
Contemporaneous The study of trilobites, brachiopods (Bassett 1974a) and conodonts (Aldridge & Schönlaub 1989, Jeppsson et al. 1995) and the palaeontological dating of the limestones based on their study suggests that the limestones belong to the early Sheinwoodian age, occurring at the same time as Woolhope Limestone Formation, and spanning the centrifugus, murchisoni and riccartonensisgraptolite biozones (Fig. 4). It has also been suggested by Hurst et al. (1978) that at Nash Scar, the hardground found at the top of the limestone might be representative of the rigidus to ellesaebiozones of the upper Sheinwoodian.
According to Kirk (1951), at either or both localities the base of the limestone might extend below the Wenlock however, this supposition is not based on any palaeontological evidence (although see Bassett 1974a, p. 759). In the uppermost Llandovery, any evidence for a depositional gap is supplied by the weak angular unconformity with the underlying Folly Sandstone Formation found at the Nash Inleir (Ziegler et al. 1968). It can thus be concluded that the limestone may have been deposited 428 to 426 million years earlier (Fig. 4).
Described by both Davis (1850) and Garwood and Goodyear (1919), the limestone facies and their lateral extent is uncertain and is hampered greatly by outcrops at ‘the Sandbanks’, the region between Presteigne and Nash Scar. Within the shales, a band of non-reefal and stratified limestone, as thin as 2-3m, was exposed only ‘a mile … from the typical reef-development at Nash Scar’ (Garwood & Goodyear 1919, p. 21).
The Welsh Borderland Fault System is straddled at its easternmost element, the Church Stretton Fault Zone, by the Nash inlier and the Old Radnor inlier. The early Palaeozoic is marked by a transition between the high midland Platform and low Welsh Basin regions by this system which has a long geographical history (see Woodcock & Gibbons 1988, Woodcock 1988). The process of folding and faulting has tectonically disturbed the limestone formations at Nash Scar and Dolyhir as a result of the closeness of this major line of structural weakness. Towards the north and north-east, major faults dissect the Old Radnor Inlier, while towards the west-north-west, it dips steeply and is marked by many minor faults. A sinistral strike-slip movement marked the structures in post-Wenlock times. The action is most seemingly related to the late Early to early Mid-Devonian Acadian event (Woodcock 1988), the most indispensable phase in the buildup of the Avalonia with the Laurentian continent during the Caledonian Orogeny.