The BGS Lexicon of Named Rock Units — Result Details

Sandrock Formation

Computer Code: SIOW Preferred Map Code: SRS
Status Code: Full
Age range: Aptian Age (KP) — Albian Age (KA)
Lithological Description: The Sandrock Formation comprises up to four upward coarsening sedimentary rhythms. Although rarely complete, a full rhythm comprises: 1. Well-sorted fine- to coarse-grained, commonly cross-bedded, sands the top of which is scoured. 2. Very dark grey mudstone and finely laminated, fine-grained sands and silts. Bioturbated. 3. Pebble beds overlying a scoured surface. The sands with small-scale cross-bedding are believed to have been deposited as migrating shoals in shallow water, estuarine conditions, whereas the large scale cross-bedded sands are considered to be subtidal channels (Wach and Ruffell, 1990; Insole, Daley and Gale, 1998; Ruffell and Wach, 1998).
Definition of Lower Boundary: The lower boundary is defined at the top of grey and green glauconitic sands of the uppermost member of the Ferruginous Sands Formation at the base of a conspicuous thick band of black muds that is the basal unit of the upward-coarsening rhythmical succession of pebble beds, dark glauconitic mudstone, fine to coarse-grained, generally cross-bedded, sandstone of the Sandrock Formation. The boundary is drawn below the basal mudstone, which forms the lowest unit of "Group XV", the "Upper Clays and Sandrock" of Fitton (1847) and "clay band of the Group XV" of Casey (1961).
Definition of Upper Boundary: The top of the formation is an eroded surface where there is a change from the rhythmical sequence of fine- to coarse-grained, generally cross-bedded sands, dark grey mudstones and pebble beds of the formation up into the gritty, reddish-brown sandstone with pebbles and phosphatic nodules of the the Monk's Bay Sandstone Formation (formerly known as the "Carstone Formation of the Isle of Wight"). This depositional break is called the "Mid tardefurcata Break" by Casey (1961).
Thickness: Up to 70m.
Geographical Limits: Isle of Wight and in the neighbourhood of Punfield Bay, Dorset. The unit is not mapped on the mainland, where the Lower Greensand Group is divided differently.
Parent Unit: Lower Greensand Group (LGS)
Previous Name(s): Sandrock [Isle Of Wight] (-2422)
Sandrock "Series" [Obsolete Name And Code: See SIOW] (SRS)
Alternative Name(s): none recorded or not applicable
Reference Section  Locality: Punfield Bay, south Dorset. Strahan, 1898; Arkell, 1947. 
Reference Section  Compton Bay. The formation is about 25m thick and comprises two upward-coarsening cycles. The basal conformable contact and the eroded top contact are both visible in low cliffs along the foreshore. Strahan, 1889; Osborne White, 1921. 
Reference Section  Red Cliff, Sandown Bay. The formation is only about 28m thick here and is apparently composed of only two upward-coarsening cycles. The whole, northward dipping, formation is generally well exposed along the foreshore with both basal and top contacts visible in vertical cliffs. Osborne White, 1921; Insole et al., 1998. 
Reference Section  Luccombe Bay and Luccombe Chine. The cliffs of the bay and within Luccombe Chine are entirely composed of the full succession of the formation (here comprising three upward-coarsening cycles) and is about 40m thick. The base of the formation is commonly obscurred. Rawson et al., 1978; Insole et al., 1998. 
Type Section  Chale Bay, Rocken End-Blackgang Chine. About 70m of the full succession, comprising four coarsening-upward cycles, is visible west of Rocken End but is disturbed by landslides in places (most commonly obscuring the basal surface). Fitton, 1847; Insole et al., 1998. 
Ruffell, A and Wach, G. 1998. Firmgrounds - key surfaces in the recognition of parasequences in the Aptian Lower Greensand Group, Isle of Wight [Southern England]. Sedimentology, Vol.45, 91-107. 
Rawson, P F, Curry, D, Dilley, F C, Hancock, J M, Kennedy, W J, Neale, J W, Wood, C J and Worrsam, B C. 1978. A correlation of Cretaceous rocks in the British Isles. Geological Society of London, Special Report No.9. 
Strahan, A. 1898. The Geology of the Isle of Purbeck and Weymouth. Memoir of the Geological Survey of England and Wales, 278pp. [HMSO.] 
Wach, G and Ruffell, A. 1990. Sedimentology and sequence stratigraphy of a Lower Cretaceous tide and storm dominated clastic succession, Isle of Wight and SE England. Field Guide No.4, 13th International Sedimentological Congress, Nottingham, 1990. 100pp. 
Osborne White, H J. 1921 [1994 reprint]. A short account of the geology of the Isle of Wight. Memoirs of the Geological Survey of Great Britain. 235pp. [HMSO.] 
Reid, C, and Strahan, A. 1889. Geology of the Isle of Wight. Mem. Geol. Surv. G.B., (2nd edition). (Geological Survey of England and Wales.) 
Casey, R, 1961. The stratigraphical palaeontology of the Lower Greensand. Palaeontology, Vol.3, 487-621. 
Insole, A, Daley, B, and Gale, A. 1998. The Isle of Wight. Geologists’ Association Guide. No. 60. (The Geologists’ Association.)  
Arkell, W J. 1947. The Geology of Oxford. 267pp. [Oxford: Clarendon Press.] 
Fitton, W H. 1847. A stratigraphical account of the section from Atherfield to Rocken-end on the south-west coast of the Isle of Wight. Quarterly Journal of the Geological Society of London, Vol.3, 289-327. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E330 E331 E344 E345