Upper Old Red Sandstone, Bristol and Gloucester region

Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)

Unconformities at Woodhill Bay, near Portishead, Avon — between the Upper and Lower Old Red Sandstone (Devonian) with associated cornerstone calcrete. (P006754)

Stratigraphy

Upper old red sandstone

The Upper Old Red Sandstone has not received the same detailed study as the older rocks. In the north of the region the succession is divided into the basal Quartz Conglomerate and the overlying Tintern Sandstone, composed predominantly of interbedded sandstone, conglomerate and mudstone. Around Bristol and the Mendips, the much thicker succession is designated the Portishead Beds. Throughout the region the uppermost part includes interdigitations of fossiliferous marine shales and limestones indicating a coastal depositional environment. The fossils show that these passage beds are Carboniferous in age. The provenance of the terrigenous sediment appears to be similar to that of the Lower Old Red Sandstone.

West of River Severn

The Downton Castle Sandstone does not outcrop in this part of the region because of faulting, though it is present immediately to the east and west in the May Hill and Usk inliers respectively. The Raglan Mudstone gives rise to gently undulating ground, commonly with topographical features formed by the cornstone and sandstone beds. The best sections within the district occur in the coastal area between Blakeney and Lydney. Strongly indurated tuffaceous mudstone, estimated to lie about 100 m below the top of the formation, has been mapped in the area between Raglan and Monmouth where it caps steep ridges of mudstone. Allen and Williams (1981) have correlated it with the Townsend Tuff of Pembrokeshire and other occurrences over a wide area from Pembroke to the Clee Hills. The deposit, which is about 3 m thick, comprises three separate but closely spaced, graded, air-fall tuff beds, each with distinctive characteristics. The rocks are porcellanous and vary from green to white and purple. The Psammosteus Limestone is best developed in the area south-west of Monmouth, where the capping calcrete may attain 3 m or more in thickness and gives rise to prominent scarp features. Subsidiary calcrete horizons occur above and below this one. The total thickness of Raglan Mudstone in the M50 section is about 385 m, but it thickens southwards and, adjacent to the River Severn, appears to be at least 600 m.

The St Maughans Formation gives rise to hillier and higher ground than the Raglan Mudstone. The rocks are, in general, not well exposed, though there are numerous old quarries in sandstone and cornstone. The M50 section proved a total thickness of 695 m of strata, but southwards the formation has thinned to between 380 m and about 500 m.

The Brownstones form a striking scarp in the western and central parts of the area. On the east side of the Forest of Dean they dip steeply westwards and form a series of prominent north-south ridges from Mitcheldean southwards past Blakeney to Lydney. Fossils are sparse, apart from trace fossils in the finer-grained facies, and the age of the beds is uncertain. In the adjacent area to the west of the district the lowermost beds have yielded Breconian plants and Siegenian spores, whereas beds higher in the sequence near Mitcheldean have yielded late Dittonian fish remains. This apparent contradiction remains to be solved satisfactorily. A study (Allen, 1974) of far-travelled pebbles from the upper part of the Brownstones at Ross-on-Wye has shown that a wide variety of rock types is represented. They include acidic lava and tuffs, some of the latter with Ordovician graptolites, greywacke and various sandstones, some preserving Silurian, shelf-facies shelly faunas and others of ‘Grey Downtonian’ aspect. Metamorphic rocks, including mylonite, jasper and quartzite, invite comparison with the Precambrian rocks of Anglesey. Allen concluded that the assemblage originated in the Welsh region and provided the first evidence of uplift, denudation and the partial removal of early Devonian and Silurian rocks in that region. The Brownstones have been much quarried and one of the best sections is in the Wilderness Quarry east of Mitcheldean, which has yielded an abundant fish fauna, including the late Dittonian ostracoderm zonal fossil Althaspis leachi. The formation is thickest in the north-eastern part of the Forest of Dean where it attains about 1100 m. Elsewhere it is diminished by intra-Devonian or intra-Carboniferous overstep.

The Quartz Conglomerate, at the base of the Upper Old Red Sandstone, commonly makes a well-marked, in places craggy feature. The formation, from 2 to 30 m thick, comprises one to several thick conglomerate beds interbedded with sandstone. Individual conglomerate beds may reach 6 m in thickness, such as at Symonds Yat where the formation is at its thickest. Pebbles are largely vein quartz with some quartzite, jasper and decomposed igneous rocks. Together with the overlying Tintern Sandstone, the Quartz Conglomerate forms high upland tracts of poor sandy soil, usually devoted to forestry. The Tintern Sandstone, up to 100 m thick, consists of interbedded grey-green, red and purplish brown sandstone with subordinate quartz-pebble conglomerate and red and green mudstone. At the top, passage beds of alternating sandstone, shale and limestone grade up into the Carboniferous Lower Limestone Shale. These passage beds are 14 m thick at Drybrook.

Sharpness–Thornbury

The Downton Castle Sandstone is exposed on the foreshore at Tites Point beside the River Severn. The beds, which comprise 1.7 m of grey-brown to reddish brown, mostly soft, thin-bedded sandstone, rest non-sequentially on Ludlow mudstone and siltstone. The sandstone fills a shallow incised channel in the Ludlow rocks, and there is a representative of the Ludlow Bone Bed in the centre of the channel. The sandstone is sharply overlain by the red-brown silty mudstone of the Thornbury Beds. The best section through this formation, however, is in the Brookend Borehole. The name Thornbury Beds has been coined because the Psammosteus Limestone, which marks the top of the Raglan Mudstone, has not been found on this side of the River Severn and the possibility cannot be ruled out that part of the St Maughans Formation is represented in the upper part of the Thornbury Beds. The latter are present in discontinuous sections for some 6 km down the coast from Tites Point. Inland the beds give rise to a subdued outcrop, marked by discontinuous low ridges formed by the sandstone beds. The only firm evidence of higher formations of the Lower Old Red Sandstone is in the Severnside Borehole, about 1 km south of Pilning, in which 450 m of strata, divided between the Brownstones and the St Maughans Formation, were proved.

The Upper Old Red Sandstone succession is similar to that to the west of the River Severn. The most complete section is in a road cutting at Buckover, north-east of Thornbury, at the southern end of the Tortworth Inlier. The discovery, here, of a fish-bed with Bothriolepis, only 1.2 m above the base of the Quartz Conglomerate, provides evidence that the Upper Old Red Sandstone lies entirely within the Farlovian Stage. Middle Devonian earth movements reach their maximum known expression north of Bristol in the Tortworth Inlier, where a combination of gentle folding, horst development and erosion resulted in the Quartz Conglomerate resting directly on early Wenlock strata in the middle of the inlier. Small exposures of Thornbury Beds occur at Wickwar on the eastern side of the horst.

Bristol

In the Bristol area, the Lower and Upper Old Red Sandstone occur in the core of the Westbury-on-Trym Anticline and in the Clevedon-Portishead ridge. The type area for the Black Nore Sandstone and the Portishead Beds is on the north-west side of the latter ridge, where the rocks are well exposed along the coast. Lithologically, the Black Nore Sandstone is comparable with the Brownstones of the main outcrop west of the River Severn; in the absence of fossils, however, the correlation cannot be proved.

The Upper Old Red Sandstone has not here been subdivided into two formations and the whole sequence of red and reddish purple sandstone with subordinate, interbedded red and green mudstone, siltstone and conglomerate is given the name Portishead Beds; all the beds are lenticular. The formation varies from about 240 to 275 m thick in the Bristol–Portishead area. Apart from the Avon Gorge, the best exposures are seen in old stone quarries, where some of the beds were worked for building stone.

The lowest member of the Portishead Beds is a coarse, unsorted conglomerate, 3 to 5 m thick, known as the Woodhill Bay Conglomerate after its exposure in Woodhill Bay. The constituent pebbles are mostly augen-quartz and dark brown quartzite, but jasper, chert, quartz, mica schist and silicified igneous rocks have also been recorded, which suggests derivation from a Precambrian terrain (Wallis, 1927). The junction with the underlying Black Nore Sandstone is a sharp, irregular erosion surface (P006754). A calcrete extends into the Woodhill Bay conglomerate and downwards for about 1 to 2 m below the contact. Important fish faunas have been collected from the Woodhill Bay Fish Bed, a l0 m thick, mainly siltstone unit about 32 m above the base of the Portishead beds at Woodhill Bay, and from a conglomerate known as the Sneyd Park Fish Bed at the top of the sequence in the Avon Gorge to Westbury-on-Trym area. The overlying Shirehampton Beds are passage beds transitional to the Lower Limestone Shale, which have been traditionally included with the Carboniferous Limestone. Palaeocurrent data indicate derivation mainly from the north-west in both the Black Nore Sandstone and the Portishead Beds.

Bath

The Hamswell Borehole, some 5 km north-west of Bath, proved 293 m of Thornbury Beds without reaching their base. The lowest beds contain a Downtonian fish fauna. As elsewhere, it is thought that the Psammosteus Limestone may be absent and that Dittonian strata could be represented in the higher part of this formation. This occurrence may occupy a horst analagous to that of the Thornbury Beds near Wickwar to the north.

Mendip Hills

Old Red Sandstone strata occur in the cores of four periclines in the Mendips and in small inliers south-east of Cheddar and north of Frome. The rocks are poorly exposed, but appear to belong entirely to the Portishead Beds, of similar facies to that of the Bristol–Portishead area. The base is seen only in the Beacon Hill pericline. Here, the beds overlie Wenlock strata with an angular unconformity of 10° or more. This area is due south of Tortworth where the upper Old Red Sandstone is also unconformable on Wenlock strata. The succession is about 410 m thick, in the lowest 50 m of which are prominent conglomeratic and pebbly beds. The main body of the formation consists of red sandstone, but in the top 75 m grey sandstone and thick beds of red and green mudstone are abundant. A similar succession, about 500 m thick without the base being proved, appears to be present in the Blackdown Pericline. A fish- and plant-bearing conglomerate has been recorded at Burrington Combe 25 m below the top of the formation, possibly correlated with the Sneyd Park Fish Bed of Bristol, whilst spores in the uppermost beds suggest an early Tournaisian (Carboniferous) age. Unlike the areas to the north and south, there are no passage beds and the junction with the overlying Lower Limestone Shale (Carboniferous) is sharp.