Northern Province Chalk nomenclature - Welton Chalk Formation

Name

The Welton Chalk Formation was proposed by Wood and Smith (1978).

Type section

Melton Bottoms (Welton Wold) Quarry [SE 970 282] near North Ferriby, Yorkshire (Gaunt et al., 1992; and Whitham, 1991).

Reference Section

Bempton and Buckton coastal cliffs between North Landing and Speeton [TA 239 721to TA 165 750] (because of inaccessibility and structural complications, the section has never been described in detail, but see for example Rowe, 1904; Neale, 1974; Rawson and Whitham, 1992).

Formal subdivision

Includes at its base the Plenus Marl Member and the Black Band Member both defined herein. There are many named marl and flint bands throughout the succession that are used to divide the formation. They are all of bed status.

Lithology

White, massive or thickly bedded chalk with common flint nodules (‘burrow-form flints’) but generally lacking tabular flint bands; sporadic marl seams including the Plenus Marls Member (‘Black Band’ sensu lato) at the base.

Definition of upper boundary

Marked change from massive, rubbly-weathering chalks below, to harder, thinly bedded or nodular chalk (Burnham Formation) above. This horizon is found just below the Ravendale Flint, a tabular or semi-tabular flint up to 0.25 m thick, which is the lowest such flint in the Chalk Group and base of the chalk unit in which such flint bands are common.

Definition of lower boundary

Base of Plenus Marls Member, a unit of buff to green and grey marls and marly chalks, typically 0.5m thick, but up to 1.4 m in Cleveland Basin. This rests on an uneven erosion surface that may be stained with iron minerals and glauconite, at the top of a succession of marly chalk (the Ferriby Formation). The marly basal beds generally form a topographical slack at outcrop, which facilitates the mapping of the base of the formation, and can also be recognised from their geophysical log signature in boreholes. '

Thickness

The formation is approximately 53 m thick in the Burnham-Melton Ross area in the central part of the region, and at the type locality of Melton Bottoms or Welton Wold Quarry [SE 970 282]. It is thinner in the south, the equivalent beds averaging about 33 m in north Norfolk (Peake and Hancock, 1970) [Fig.5c/2]. It also may thin slightly across the Market Weighton High, perhaps to as little as 40 m in places, but thickens again into the Cleveland Basin where the gamma-ray log of the Fordon No 2 borehole [0689 7360] suggests it is about 55 m thick. Whilst Rowe (1904; see also Neale, 1974) suggests it is about 68 m thick in the Bempton and Buckton cliff sections between Flamborough and Speeton, according to Mitchell (2000) it is only 55 m (cf. Fordon) at Speeton, its most northerly outcrop. Curiously, however, at Thornwick Bay, at the south-eastern end of this section, measurement of the upper and greater part of the formation (see Whitham, 1991, [Fig.6/7]; Mitchell 2000) shows that it is slightly expanded relative to Speeton, suggesting a total of at least 60 m there. This may indicate an eastward thickening towards the offshore extension of the Cleveland Basin, or an error in one or other of these sections, possibly related to structural complications. '

Distribution

Throughout Yorkshire and Lincolnshire.

Previous names

‘Chalk with Flints’ (lower part) of early Geological Survey maps.

Middle Chalk plus uppermost part (Plenus Marls) of Lower Chalk.'

Parent

Chalk Group.

Age and biostratigraphy

Upper Cretaceous, Cenomanian (Metoicoceras geslinianum Zone) to Turonian (Terebratulina lata Zone).

References

Wood and Smith (1978); Gaunt et al. (1992).