The BGS Lexicon of Named Rock Units — Result Details

Thorncroft Sand Member

Computer Code: THS Preferred Map Code: ThS
Status Code: Full
Age range: Bajocian Age (JB) — Bathonian Age (JN)
Lithological Description: A grey to yellow, locally bioturbated massive parallel or cross-bedded fine-grained, patchily iron-cemented but otherwise friable sandstone with plant debris, rootlets and thin lignite lenses, with thinly bedded and laminated locally sphaerosideritic and pyritic sandstone, siltstone and mudstone, seatearths and thin coals, which may include a median unit of marine fossiliferous calcareous mudstone or siltstone.
Definition of Lower Boundary: Passage up from the ooidal limestone-dominated succession of the Lincolnshire Limestone Formation into a predominantly sand-dominated, generally non-calcareous succession, which in places comprises a dark sooty mudstone, accentuated in the weathered zone by the development of a secondary limonitic bed at the base, known as the 'Ironstone Junction Bed' which lies at the base of the Thorncroft Sand, although elsewhere, there may be up to 2 m of interdigitated ooidal limestone and sandstone strata, or indications of a non-sequence, such as limestone pebbles or oxidation of the top of the Lincolnshire Limestone.
Definition of Upper Boundary: A sharp often rootleted contact with the mudstone-dominated upper part of the Rutland Formation described as the Priestland Clay Member of Gaunt et al, 1992.
Thickness: Up 18 m or more thick.
Geographical Limits: North Lincolnshire and the East Riding of Yorkshire (Humberside).The northern limit is located around North Newbald near Market Weighton (Sheet 72), where the unit is overstepped by the Cretaceous.The southern limit is uncertain, although it is perhaps defined by the absence of the lower cycle, (see note above) either by passage into the Lincolnshire Limestone facies and/or by overstep through downcutting of the upper (Stamford) cycle into the Lincolnshire Limestone, which can be seen in the area between the Grantham district (Sheet 127) and the Brigg district (89), and probably also to the north of Lincoln. Provisionally the arbitrary boundary is taken at the southern edge of Sheet 89.
Parent Unit: Rutland Formation (RLD)
Previous Name(s): Thorncroft Sands Member (-4370)
Lower Sandy Beds (-2484)
Alternative Name(s): none recorded or not applicable
Type Area  North Lincolnshire and Humbeside Gaunt et al., 1992. 
Type Section  Worlaby East Borehole (SE91NE/45), between 113.49 m and 120.70 m. Richardson, 1979. 
Reference Section  Nettleton Bottom Borehole (TF19NW/54), at between about 338.09 m to 345.16 m, depths revised by MGS, Oct 2001. Bradshaw and Penny, 1982; Gaunt et al., 1992. 
Gaunt, G D, Fletcher, T P and Wood, C J. 1992. Geology of the country around Kingston upon Hull and Brigg. Memoir of the British Geological Survey, sheets 80 and 89 (England and Wales). 172pp. 
Bradshaw, M J. 1978. A facies analysis of the Bathonian of eastern England. (University of Oxford: Unpublished PhD thesis.) 
Bradshaw, M J and Penny, S R. 1982. A cored Jurassic sequence from North Lincolnshire England: stratigraphy, facies analysis and regional context. Geological Magazine Vol. 119, p. 113-134. 
Gaunt, G D, Ivimey-Cook, H C, Penn, I E and Cox, B M. 1980. Mesozoic rocks proved by the Institute of Geological Sciences boreholes in the Humber and Acklam areas. Report of the Institute of Geological Sciences, No. 79/13. 
Richardson, G 1979. The Mesozoic stratigraphy of two boreholes near Worlaby, Humberside. Bulletin of the Geological Survey of Great Britain. No. 58, p.1-24. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E080 E089 E072