The BGS Lexicon of Named Rock Units — Result Details

CROMHALL SANDSTONE FORMATION
Computer Code: CHSA  Status Code: INFORMAL, LOCAL 
Preferred Map Code:  
Age or Age Range: [ CJ ] ARUNDIAN  to [ CX ] BRIGANTIAN 
Lithological Description:
Includes up to three tongues. The lower tongue comprises brown and red fine- to coarse-grained quartzitic sandstone with subordinate mudstones and sparse thin limestones. Base is locally conglomeratic. The middle tongue is similar but contains units of dolomitised limestone. The upper tongue comprises grey and red coarse-grained quartzitic sandstones, sandy crinoidal and oolitic limestones, mudstones, siltstones and grey seatearths arranged in cyclic sequences. Units of crinoidal and oolitic limestone are locally developed. The formation was deposited mainly in a fluvial environment on a marine influenced coastal plain. 
Definition of Lower Boundary:
Lower boundary of the Cromhall Sandstone Formation is taken at the base of the lower tongue, which is taken at the base of the first sandstone overlying the calcite and dolomite mudstones and limestones of the underlying Clifton Down Mudstone Formation in the Bristol area and the Llanelly Formation in the Monmouth and Chepstow area. Base of middle tongue is taken at the base of the first sandstone in the upper part of the Clifton Down Limestone Formation. Base of the upper tongue is taken at the base of the first sandstone or mudstone above the skeletal and oolitic limestones of the Oxwich Head Limestone Formation. 
Definition of Upper Boundary:
Lower tongue: Top taken at the incoming of calcite mudstones and oolitic and skeletal limestones of the Clifton Down Limestone in the Bristol area and the Hunts Bay Oolite Subgroup in the Monmouth and Chepstow area above the sandstones and mudstones of the lower tongue. Middle tongue: Top taken at the base of the entrance of the skeletal limestones of the Oxwich Head Limestone Formation above the sandstones and mudstones of the middle tongue. Upper boundary of the Cromhall Sandstone Formation taken at top of upper tongue: Top taken at entrance of limestones of the overlying uppermost Oxwich Head Limestone Formation or cherts and cherty marine mudstones of the overlying undivided Tenby Group above the sandstones and mudstones of the upper tongue. 
Thickness:
Lower tongue: 6m at Chipping Sodbury to 43m at Cromhall. Middle tongue: 27m at Chipping Sodbury to 30m at Tytherington. Upper tongue: 18m on Broadfield Down thickening north to 240m at Cromhall. 
Geographical Limits:
Bristol, [ST 60 74] Monmouth and Chepstow area [ST 53 930]. Passes laterally southwards into formations of the Pembroke Limestone Group [Llanelly Formation, Clifton Down Mudstone, Clifton Down Limestone and Oxwich Head Limestone formations]. 
Parent Unit: Parent Unit Code:
PEMBROKE LIMESTONE GROUP  PEMB
Previous Name(s): Previous Code(s):
LOWER CROMHALL SANDSTONE FORMATION  LCS 
UPPER CROMHALL SANDSTONE FORMATION  UCS 
MIDDLE CROMHALL SANDSTONE FORMATION  MCS 
LOWER DRYBROOK SANDSTONE  LDS 
DRYBROOK SANDSTONE GROUP  DS 
UPPER DRYBROOK SANDSTONE  UDS 
Alternative Name(s):
none recorded or not applicable
Stratotypes:
Type Area  Cromwell area, north of Bristol. 
Reference(s):
Waters, C N, Barclay, W J, Davies, J R and Waters, R A. In press. Stratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
Trotter, F M, 1942. Geology of the Forest of Dean and Iron-ore Field. Memoir of the Geological Survey of Great Britain. 
Kellaway, G A and Welch, F B A, 1955. The Upper Old Red Sandstone and Lower Carboniferous rocks of the Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No.9, p.1-21. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
Map Code Sheet Name
none recorded or not applicable none recorded or not applicable

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