Lower 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.)|
Lower old red sandstone
Downton Castle Sandstone
The base of the Old Red Sandstone is marked by the well-known Ludlow Bone Bed, a thin, littoral lag deposit at the base of the Downton Castle Sandstone; it marks a probably small hiatus following deposition of the underlying Ludlow rocks. The type area of the sandstone formation is around Ludlow, but comparable, though not necessarily completely synchronous sandstones are present throughout the Welsh Borderland and South Wales. They were laid down under marine littoral conditions in either a delta or coastal plain during the final stage in the shallowing of the Silurian shelf seas. These rocks contain lingering elements of the shelly Ludlow fauna, including thin beds with Lingula and, more rarely, ostracods, bivalves and gastropods, with an admixture of the fish faunas that characterise the overlying strata. The rocks have been termed ‘Grey Downtonian’ in contrast to the ‘Red Downtonian’, applied to the overlying Raglan Mudstone Formation.
Raglan Mudstone Formation and St Maughans Formation
The Downton Castle Sandstone is sharply succeeded by the Raglan Mudstone Formation west of the River Severn, and the Thornbury Beds east of the Severn. Unlike some areas to the north, the basal junction of these two formations is sharp. They comprise thick, blocky, faintly laminated, red-brown silty mudstone interbedded with subordinate green, brown and purple sandstone. The sandstone commonly includes mudstone clasts and may infill mudcracks in the underlying mudstone. Deposits of air-fall tuff occur in the upper part of the Raglan Mudstone. The overlying St Maughans Formation is generally similar, but the proportion of sandstone to mudstone, approximately 1:3, is three or four times as high as in the underlying formation.
In these formations, the rocks are typically arranged in upward-fining sequences of intraformational conglomerate, channel sandstone and, finally, mudstone. The depositional environment was a wide alluvial plain intersected by rivers of moderate to high sinuosity draining towards the south. The positions of the river and stream channels are marked by the arenaceous rocks. At the bases of the thicker channel deposits there are laterally extensive pebbly beds with well-marked erosive bases. The pebbles are of local rock types, including abundant red marl. Disarticulated fish debris may be locally abundant.
Perhaps the most distinctive sedimentological feature of the Lower Old Red Sandstone is the presence, mainly within mudstone, of beds containing abundant micritic, calcitic or dolomitic concretions. These take the form of rounded nodules and subvertical, branching pencil-like forms. Aggregation of the nodules led to the formation of rubbly limestone beds known as ‘cornstone’. There is every gradation between vaguely defined beds or zones of mudstone with concretions to limestone beds. These may form cappings to concretion-rich zones ranging from a metre to 5 m or more in thickness. After decades of argument it is now recognised that these carbonate sediments correspond to present-day caliche or calcrete deposits which are precipitated during soil-forming (pedogenic) processes under hot arid conditions with low seasonal rainfall. They testify to long periods when extensive surfaces of the exposed interfluves of the alluvial plains were stable and starved of sediment. Flash flooding and surface sludging, following sporadic, locally intense storms caused the breaking up of the surface layers to give rise to cornstone-breccias and conglomerates which are distinct from the intraformational conglomerates that formed in the beds of less ephemeral streams (Allen and Williams, 1979).
Calcrete horizons occur most abundantly in the upper parts of the Raglan Mudstone and the lower parts of the St Maughans Formation and reach their maximum development in the widespread Psarnmosteus Limestone at the top of the former. A less well-developed sequence of carbonate-bearing beds also occurs at the top of the St Maughans Formation.
The St Maughans Formation passes up into the Brownstones west of the River Severn, and the Black Nore Sandstone on the east, without any sedimentary break, by upward diminution of the content of interbedded mudstone. In the lower part of the formation, mudstone accounts for about one-third of the total thickness, but higher up the proportion of mudstone becomes negligible and the sandstone becomes coarser grained. The sandstone is red and brown in colour, well sorted, fine to coarse grained, with several types of cross-bedding, parallel lamination and cross-lamination. The sandstone occurs in upward-fining cycles of from a few to many metres in thickness, commonly with conglomerate at the base and underlain by an extensive erosion surface. The sandstone was deposited in a low-sinuosity (braided) river system draining towards the south. The deposits represent the medial parts of the Old Red Sandstone drainage system, in contrast to the underlying formations which represent the distal parts. The conglomerates in the higher part of the formation may contain abundant far-travelled pebbles derived from the erosion of Palaeozoic and Precambrian terrains.
- Allen, J R L, and Williams, B P J. 1979. Interfluvial drainage on Siluro-Devonian alluvial plains in Wales and the Welsh Borders. Quarterly Journal of the Geological Society of London, Vol.136, 361–366.