Middle Lias, Jurassic, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
The Middle Lias comprises two formations, a lower, thick silty and arenaceous group (the Dyrham Silts) and an upper, much thinner, ferruginous shallow-water limestone (the Marlstone Rock Bed). These represent the final parts of a regressive phase in the major sedimentary rhythm that commenced with the thick argillaceous Lower Lias succession.
Apart from the outliers of Dundry Hill and Bitton Hill, where thin Marlstone Rock Bed, with (at Dundry) or without the underlying silts, is present, both formations of the Middle Lias are apparently absent between Bitton and the south side of the Mendips. The silts are very thin between Upper Cheyney and Dyrham, but thicken northwards; the Marlstone Rock Bed is absent or unmappable some considerable distance northwards, to near Hawkesbury. The absence or attenuation of the Middle Lias in these areas was mainly due to intra-Bajocian erosion near the Mendips and intra-Liassic erosion farther north.
Dyrham Silts (formerly ‘Middle Lias Silts’)
The zonal age of these generally poorly fossiliferous beds in most places probably encompasses the margaritatus Zone and varying proportions of the underlying davoei Zone. The thickness variations broadly follow those of the underlying Lower Lias, though the changes are less well marked (P948996).
Around Chard and Ilminster, in the extreme south of the region, the lowest deposits consist of blue-grey, micaceous silty mudstone and silt up to 30 m in thickness. These are succeeded by micaceous silt and fine yellow sand, with occasional doggers, known as the Pennard Sands. In the Yeovil district and at Pennard Hill in central Somerset, there is a gradual passage from silty beds up into the Pennard Sands, here 10 to 25 m thick. The thickest recorded sequence of Dyrham Silts (86.8 m) is in the BGS borehole near Bruton.
At Dyrham, the Dyrham Silts are about 20 m thick, and are thought to belong mainly to the davoei Zone. Northwards from here, they steadily increase to about 75 m in the mid-Cotswolds. A complete section has been recorded from the now disused Tuffley Brickpit, Robins Wood Hill, south of Gloucester. Here the Marlstone Rock Bed sharply overlies about 58 m of mainly grey silty shales with scattered ironstone nodules. The upper 24 m are assigned to the margaritatus Zone and the remainder to the davoei Zone. The Lower Lias Clay, belonging to the ibex Zone, was represented in the pit bottom by grey clay with calcareous nodules. An almost identical succession was described at the former Stonehouse Brickpit near Stroud. In both pits three beds (0.3 to 1.3 m) of fossiliferous, ferruginous limestone, similar to the later Marlstone Rock Bed, occur within the top 16.5 m of the silts. The uppermost of these beds was also recognised in the Stowell Park Borehole, where it consisted of dark green, calcareous and sideritic, ‘chamosite’ oolite, about 2 m thick. Similar ferruginous and calcareous beds have been described from areas to the north of the region.
Marlstone Rock Bed
The Marlstone Rock Bed marks a widespread change in conditions of sedimentation. It is typically a shelly, ferruginous, locally oolitic limestone, which may pass into a calcareous ferruginous sandstone. Fossils are abundant and include several species of the ammonite Pleuroceras, and numerous belemnites and bivalves; brachiopods such as Tetrarhynchia tetrahedra and Lobothyris punctata commonly occur in large numbers.
The Marlstone Rock Bed is rarely more than 6 m thick and is only 0.3 m thick at Yeovil on the Yeovil ‘high’, over which stratal attenuation is known to have occurred at intervals up to and including late Bajocian and early Bathonian times. In south Somerset, the thin Marlstone Rock Bed is not mapped separately from the immediately overlying limestones known as the ‘Junction Bed’.
In the Ilminster district the top 0.2 m of the Marlstone Rock Bed yields ammonites that indicate the presence of the tenuicostatum Zone. Over the remaining part of Somerset, this zone has not been recognised and a nonsequence is therefore postulated at the top of the Marlstone Rock Bed.
Unlike the Marlstone of Oxfordshire, the Marlstone Rock Bed of Gloucestershire and Somerset is normally too thin and its iron content too low to warrant its use as an ironstone. Its pleasant, rusty-brown colour makes it a most attractive building stone, but it is rather soft and there is much waste in quarrying.
Owing to their relative hardness in comparison with the soft sands and clays above and below, the Marlstone Rock Bed and the Junction Bed give rise to characteristic platform topography, both in Somerset and in the Cotswolds. The Junction Bed platform is well-marked around Ilminster and Corton Denham in Somerset; the Marlstone Rock Bed platform is well seen in the Glastonbury outlier. In Gloucestershire, the Marlstone Rock Bed forms a conspicuous ledge below the Cotswold scarp, a feature which is well exhibited near Wotton-under-Edge and Stinchcombe (P948979). It also gives rise to the flat-topped hills of Diston and Dumbleton, and the ledges on Oxenton and Alderton Hills, between Cheltenham and Broadway.