Cossey, P.J., Adams, A.E., Purnell, M.A., Whiteley, M.J., Whyte, M.A. & Wright, V.P. 2004 British Lower Carboniferous Stratigraphy. Geological Conservation Review Series, No. 29, JNCC, Peterborough. The original source material for these web pages has been made available by the JNCC under the Open Government Licence 3.0. Full details in the JNCC Open Data Policy
Spara Bridge, Devon
Introduction
The Spara Bridge GCR site is a road cutting developed along the minor road (B3193) that runs through the Teign Valley near Chudleigh, south Devon. It extends for nearly one kilometre northwards from Spara Bridge
Description
The structure of the Teign Valley is dominated by an anticlinorium that plunges eastwards away from the Dartmoor granite and preserves conformable Lower Carboniferous sequences in several folds. This site provides a section through the southern limb of the Ashton Anticline from the road cutting at
The oldest rocks are exposed in the road cutting north of Ashton Mill Weir and are ascribed to the Hyner Shale. They consist of grey-green and blue-green silty shales with tough calcareous siltstone bands. Sanguinolites ellipticus is abundant and a horizon bearing Phacops granulatus indicates a Late Devonian age. Miospores from the same locality (Whiteley, 1983; Dean, 1992) suggest an age that approximates to the Devonian–Carboniferous boundary whereas the lowermost Carboniferous index ostracode Richterina latior has been identified at a slightly higher stratigraphical level (Gooday, 1983). Thus the Devonian–Carboniferous boundary occurs in the upper part of the Hyner Shale.
A conformable passage into the overlying Trusham Shale is marked by increasingly micaceous and calcareous shales. This unit is about 60 m thick and is characterized by olive-green and pale-grey shales with a well-developed conchoidal fracture. They are Interbedded with more micaceous shales that are finely banded and locally calcareous. The Trusham Shale is not richly fossiliferous, but Gooday (1983) records lowermost Carboniferous (latior-Zone) ostracodes from nearby localities, whilst the bivalve Sanguinolites? ellipticus, athyrid brachiopods and bryozoans are evident in the higher part of the succession. On this basis, the Trusham Shale is considered to be of Tournaisian age.
Roadside exposures and small quarries provide the type section for the succeeding Combe Shale, which comprises highly fissile blue-black or black shales with white silty laminations. The shales are about 150 m thick and they are intruded by two dolerite sills that are well exposed in quarries near Ashton Mill Weir
This site also serves as the stratotype for the Teign Chert, and Selwood et al. (1984) described the following measured succession from north to south, in ascending stratigraphical order:
cherts and siliceous shales containing a 6 m coarse crystal-tuff horizon | 45 m |
cherts and shales with thin bands of jasper and mineralized (manganese) veins | 27 m |
radiolarian cherts and siliceous shale with thin tuff bands towards the top | 109 m |
gap in exposure | 40 m |
interbedded tuffs, shales and cherts, locally folded and sheared | 42 m |
well-bedded cherts and siliceous mudstones | 55 m |
pale-grey calcareous cherts and mudstones with radiolarians | 13 m |
black shale and grey limestone (Posidonia Beds) | 4 m |
The numerous volcanic tuff bands contribute to this unusually thick succession, but apart from poorly preserved radiolarians (Hinde and Fox, 1895; Ussher, 1913), no fossils have been found below the level of the Posidonia Beds. Other localities in the immediate vicinity show that the Posidonia Beds are typically 30–45 m thick and contain both large and small forms of P. becheri and the anunonoids Neoglyphyioceras spirale and Sudeticeras aff. ordinatum.
South of Spara Bridge
Interpretation
The Spara Bridge succession in the Teign Valley is probably the thickest (c. 450 m) and certainly the most complete Lower Carboniferous succession in the Culm Trough. Although the exposure is not continuous, it does appear that the succession is conformable and wholly marine in origin. It is characterized by shales and cherts and In this respect resembles the stratigraphy in the northern outcrops around Barnstaple
The palaeontology of the Teign Valley strata is also interesting. Faunas in the Upper Devonian (lower part of the Hyner Shale) strata are dominated by a variety of benthos, notably trilobites and brachiopods, but some pelagic elements are evident. A comparatively shallow-water setting, free from strong current activity, is envisaged, with uninterrupted access to deeper water allowing pelagic forms to drift into the area. The overlying Dinantian rocks become increasingly fine-grained and siliceous, with a rapidly diminishing bcnthos suggesting euxinic conditions. Increased levels of ventilation in the water column during late Asbian and Brigantian times are indicated by the sudden appearance of abundant pelagic forms in the Posidonia Beds and the associated development of calcareous lithologies (Thomas, 1982). The succession as a whole is interpreted as the product of an extended transgressive phase of Dinantian sedimentation, with the cherts representing a condensed sequence in a very widespread, starved sedimentary basin.
The structural setting of the Teign Valley succession is considered to be largely in situ or autochthonous (Selwood et al., 1984). It thus provides evidence that the Culm Trough extended at least this far south, although overlying thrust sheets contain rocks of equivalent age that probably formed closer to the currently unidentified southern margin of the basin.
Conclusions
The Spara Bridge GCR site provides the most informative section through the Lower Carboniferous successions in the Culm Trough. Palaeontological data are sufficient to define the Devonian–Carboniferous boundary with some accuracy, and the presence of the Cravenoceratoides indicates an Arnsbergian age for the top of the succession. The site contains stratotypes for the Combe Shale and Teign Chert successions and abundant igneous rocks that contribute to an understanding of basin evolution.