The BGS Lexicon of Named Rock Units — Result Details

Gully Oolite Formation

Computer Code: GUO Preferred Map Code: notEntered
Status Code: Full
Age range: Chadian Substage (CI) — Chadian Substage (CI)
Lithological Description: Medium- to thick-bedded, pale grey, oolitic grainstone with subordinate beds of fine-grained skeletal packstones. Cross-laminated and cross-bedded, some burrowing. Locally dolomitized, especially on the southeast and east crop of the South Wales Coalfield and in the Chepstow area. A palaeokarstic surface overlain by a green/red mudstone/clay palaeosol caps the Formation. Locally in the north Bristol to Tytherington area 6-21m of grey crinoidal limestones, the Sub-Oolite Bed, occurs at the base of the Formation.
Definition of Lower Boundary: The base is taken at the gradational to sharp contact between the dolomitised sparsely crinoidal packstones of the Friars Point Limestone Formation or its parent the Black Rock Limestone Subgroup and the overlying oolitic grainstones or dolomitised oolite of the Formation. In the south the junction appears conformable but in the north a thin red clay overlying a palaeokarstic surface is locally present; in such cases the top is taken at the top of the red clay. The base of the formation in the Bristol area is locally taken at the junction of the sparsely crinoidal dolomites of the Black Rock Limestone Subgroup and the crinoidal limestones of the overlying Sub-Oolite Bed.
Definition of Upper Boundary: The top of the Formation is taken at the sharp erosional contact between the mudstone/clay palaeosol at the top of the Formation and the overlying thin-bedded calcite mudstones and mudstones of the Caswell Bay Mudstone Formation [South Wales] or the Clifton Down Mudstone Formation [Bristol and Broadfield Down area]. A conglomeratic lag is locally present at the base of the Caswell Bay Mudstone Formation. In the southern part of the outcrop, the Formation is overlain by the High Tor Limestone Formation, the capping palaeosol being absent. Here the top is taken at the irregular erosional contact between the oolitic grainstones of the Formation and the overlying coarse skeletal packstone/grainstones of the High Tor Limestone Formation.
Thickness: 83m in the southern Vale of Glamorgan, thinning to 19m on the southeast crop of the South Wales Coalfield.
Geographical Limits: Gower [SS 50 90], Vale of Glamorgan [ST 00 74], southeast crop of the South Wales Coalfield [ST 17 85], Monmouth/Chepstow [ST 53 93], Bristol [ST 60 74], Weston-super-Mare [ST 30 60] and Broadfield Down [ST 50 65] areas. Present but not mapped in the Tenby [SN 13 01] and Pembroke [SM 97 03] areas. Passes east in the Mendip Hills [ST 50 53] into the Burrington Oolite Subgroup.
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Crease Limestone [Obsolete Name And Code: Use GUO] (CRL)
Caninia Oolite [Obsolete Name And Code: Use GUO] (CAO)
Crease Limestone Formation (-4076)
Caswell Bay Oolite Formation [Obsolete Name And Code: Use GUO] (CBO)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Gully Quarry and adjacent road section on the eastern side of Avon Gorge, Bristol. Incomplete section [26m thick] in ooidal limestones, dolomitised in lowermost part. Lower contact with the dolomites of the Black Rock Limestone Subgroup seen, as is upper contact with the Clifton Down Mudstone (in adjacent road section). No Sub-Oolite Bed present at base of unit. 
Reference(s):
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. 
Kellaway, G A and Welch, F B A, 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. 
Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
Waters, R A and Lawrence, D J D. 1987. Geology of the South Wales Coalfield - Part III: the country around Cardiff. Memoir of the British Geological Survey, Sheet 263. (England and Wales). 
Institute of Geological Sciences. 1973. Swansea, England and Wales sheet 247. Solid Geology, 1:63,360. [Southampton: Ordnance Survey]. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E262 E279 E250 E251 E264 E265 E246