| Computer Code: |
HYB |
Preferred Map Code: |
Hz |
| Status Code: |
Full |
| Age range: |
Oxfordian Age (JO)
— Oxfordian Age (JO) |
| Lithological Description: |
In north Dorset it is seen as coarsening upwards cycles typically from 8m to 10m thick, and up to 20m thick, of grey to brown sandy mudstone, muddy sandstone and yellow, brown and grey very fine- to medium-grained quartzose sand, with thin sandy siltstone beds in parts. Sand is locally cemented into lenticular beds and doggers of calcareous, 'blue-hearted' sandstone to sandy limestone. In Wiltshire, the sandy mudstone beds are subordinate. The beds are generally very shelly and bioclastic in Dorset to west Wiltshire, but poorly fossiliferous in north Wiltshire to Oxfordshire. Well-cemented fossiliferous sandstone beds occur in the upper part: the Seend Cleeve Sandstone Member in west Wiltshire (Wright and Cox, 2001), Lyon's Gate Bed in Dorset (Bristow et al. 1995), and a spiculitic sanstone bed, near Purton, Wiltshire (Ringsbury Spiculite Member; Arkell, 1941). Contains a fauna of bivalves and ammonites. |
| Definition of Lower Boundary: |
The base generally displays a change up from the grey mudstone of the Weymouth Member of the Oxford Clay Formation, into a sequence of sandy mudstone, muddy sandstone, sand and sandstone beds. In places, the basal bed is a fine-grained sand. |
| Definition of Upper Boundary: |
The upper boundary varies along the outcrop. In Dorset there is a sharp change from a sequence of sandy mudstone, sand and sandstone strata to a sand-free, or virtually sand-free, commonly ooidal, grey clay, of the Woodrow Clay Member and the Stour Formation. in East Somerset it is overlain unconformably by grey mudstone of the Gault Formation. In Wiltshire, the sandy strata are overlain non-sequentially by grey or brown mudstone of the Highworth Clay Member, or grey bioclastic ooidal and non-ooidal limestone of the Highworth Limestone Member, both part of the Kingston Formation, or else by a non-sequence, overlain by grey limestone of the Stanford Formation. In Oxfordshire, there is a passage up from poorly fossiliferous fine-grained sand, sandy siltstone and mudstone into moderately fossiliferous medium- to coarse-grained grey to brown sand of the Beckley Sand Member of the Kingston Formation. |
| Thickness: |
From 6m to 53m thick in north Dorset, and 6m to 18m thick in Wiltshire. |
| Geographical Limits: |
From north Dorset to southwest Oxfordshire, the unit passes east around Cumnor Hill (Oxford), into the more argillaceous West Walton Formation, and the Temple Cowley Member of the Kingston Formation. Between Farringdon and Cumnor the upper part may pass laterally east and vertically into the Beckley Sand Member of the Kingston Formation. |
| Parent Unit: |
Corallian Group (CR)
|
| Previous Name(s): |
Lower Calcareous Grit [Obsolete Name and Code: Use HYB, KTON, TECY, BYSA]
(-101)
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
East Stour Borehole, ST82SW/9, from 28.2m to 57.65m. The top is proved, but the unit continues below total depth, so the base is not seen. Core samples are held by BGS. |
| Partial Type Section |
Hazelbury Bryan Borehole, ST70NE/3, from 25.25m to 52.1m. The top is proved, although the formation continues to total depth, thus the base is not seen. Freshney, 1989. |
| Reference Section |
Cannings Court Borehole, ST70NW/2, from 19.9 m to 52.0m. The top is proved, but the formation continues below total depth, thus the base is not seen. Core samples are held by BGS at Keyworth. Freshney, 1989. |
| Reference Section |
Road cutttings on the A303 at Leigh Common (or Leigh Moor), which expose the middle part of the Formation. See Bristow et al. 1992; 1999. Some samples are held by BGS. |
| Reference(s): |
| Wright, J K and Cope, J C W. In press. Oxfordian - Kimmeridgian Correlation Chart. In: A correlation of Jurassic rocks in the British Isles: Middle and Upper Jurassic. Cope, J C W (editor). Special Report of the Geological Society of London. |
| Woodward, H B. 1895. the Jurassic Rocks of Britain. The Middle and Upper Oolitic rocks of England (Yorkshire excepted). Vol.5. Memoir or the Geological Survey of the United Kingdom. |
| Bristow, C R, Cox, B M, Woods, M A, Prudden, H C, Sole, D, Edmunds, M, and Callomon, J H. 1992. The geology of the A303 trunk road between Wincanton, Somerset and Mere, Wiltshire. Proceedings of the Dorset Natural History and Archaeological Society, Vol. 113, 139-143. |
| Bristow, C R. 1989. Geology of the East Stour - Shaftesbury district (Dorset) British Geological Survey Technical Report WA/89/58. |
| Bristow, C R. 1990. Geology of the West Stour - Cucklington district. British Geological Survey Technical Report WA/90/48. |
| Freshney, E C. 1989. Geology of Sheets ST70NW and ST70NE. (Hazelbury Bryan-Mappowder district, Dorset). British Geological Survey Technical Report WA/90/51. |
| Wright, J K, 1981. The Corallian rocks of north Dorset. Proceedings of the Geologists' Association, Vol.92, 17-32. |
| Wright, J K, and Cox, B M. 2001. British Upper Jurassic Stratigraphy (Oxfordian to Kimmeridgian). Geological Conservation Review Series. No. 21. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.) |
| Arkell, W J. 1939. Map of the Corallian Beds between Marcham and Faringdon, Berkshire. Proceedings of the Geologists' Association, Vol. 50, 487-509. |
| Arkell, W J. 1941. Map of the Corallian Beds around Highworth, Wiltshire. Proceedings of the Geologists' Association, Vol. 52, 79-109. |
| Arkell, W J. 1941. Map of the Corallian Beds around Purton, Wiltshire. Wiltshire Archaeological and Natural History Magazine, Vol. 49, 274-282. |
| Arkell, W J. 1951. The geology of the Corallian ridge between Wootton Bassett and Lyneham, Wiltshire. Wiltshire Archaeological and Natural History Magazine, Vol. 54, 1-18. |
| Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H C, and Taylor, R T. 1995. Geology of the country around Shaftesbury. Memoir of the British Geological Survey, Sheet 313 (England and Wales). |
| Bristow, C R. 1989. Geology of Sheets 71NE and 71SE. (Marnhull-Sturminster Newton, Dorset) Part of 1:10,000 Sheet 313 (Shaftesbury). British Geological Survey Technical Report WA/89/59. |
| Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, and Woods, M A. 1999. The Wincanton district - a concise account of the geology. Memoir of the British Geological Survey, Sheet 297 (England and Wales). |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E313
E297
E281
E253
E252
E266
|
