The BGS Lexicon of Named Rock Units — Result Details

Ferruginous Sands Formation

Computer Code: FRS Preferred Map Code: FrS
Status Code: Full
Age range: Aptian Age (KP) — Aptian Age (KP)
Lithological Description: The formation comprises a number of heavily bioturbated coarsening-upward units each comprising dark grey sandy muds or muddy sands passing up into fine- to medium-grained grey to green glauconitic sands. Weak cementation occurs at the top of many of the units producing discontinuous calcareous, phosphatic or pyritic concretions that are generally fossiliferous. The middle of the formation contains plant debris. Eleven units (IV to XV) described as "members" are informally identified within the succession. Full lithological descriptions for each of these informal members are given in Insole et al. (1998, pp. 65-68). The succession at Chale Bay is divided into eleven units considered to be of member status in Insole et al. (1998) and whose numbering follows on from the members as defined by Fitton (1847, "Groups" I, II and III) of the preceding Atherfield Clay Formation. The "members" of Insole et al. (1998) are difficult to recognise and trace laterally away from the type site of Chale Bay and must therefore be regarded as informal terms. The "members", in ascending order, are Member IV, Member V, Member VI, Whale Chine Member, Member VIII, Ladder Chine Member, Member X, Member XI, Old Walpen Chine Member, New Walpen Chine Member, Member XIV and Member XV.
Definition of Lower Boundary: The lower boundary is marked by a short transition from silty clays of the Upper Atherfield Clay Formation (the Upper Lobster Member) up into ferruginous sands and sandstones with concretions.
Definition of Upper Boundary: The upper boundary has been defined in a number of ways. Fitton (1847) and Casey (1961) placed the boundary at the top of their "Group XV" (equivalent to Member XV), where coarsening-upward estuarine silty mudstones pass up into interlaminated black clays and white sands (of their Ferruginous Sands) and are overlain by the white, quartzose sands of the Sandrock Formation. This boundary (Fitton, 1847; Casey, 1961) is a scoured erosion surface separating the estuarine mudstones from marine sandstones with a highly burrowed and indurated top. It marks a sequence boundary and is the current definition of the base of the succeeding unit (Sandrock Formation) (Insole et al., 1998; Ruffell and Wach, 1998). Wach and Ruffell (1990) placed the boundary within Member XV where the estuarine silty mudstone passes up into the interlaminated black clays and white sands. They regarded this unit as a transitional bed of estuarine mudstone passing up into the estuarine sands of the Sandrock Formation. The underlying ferruginous, glauconitic sands and sandstones are essentially marine.
Thickness: Up to about 161m.
Geographical Limits: Isle of Wight and southern Dorset.
Parent Unit: Lower Greensand Group (LGS)
Previous Name(s): Ferruginous Sand (-4051)
Ferruginous Sands (-2194)
Alternative Name(s): none recorded or not applicable
Reference Section  Coastal section at Punfield Cove [SZ 037 807] in Swanage Bay, Dorset. Strahan (1898). 
Type Section  Coastal section between Chale Bay [SZ 453 791], southeast of Atherfield Point to Rocken End [SZ 489 761], Isle of Wight. The whole succession is seen in the cliffs above the beach but the exposure is prone to landslip that can obscure parts of the succession at times. There are a number of accounts based on exposures at the time. See Fitton (1847), Casey (1961), Insole et al. (1998). 
Arkell, W J. 1947. The Geology of Oxford. 267pp. [Oxford: Clarendon Press.] 
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.) 
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. 
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. 
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. 
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.)  
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E330 E331 E334 E345