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).
4.3.1 Plenus Marls Member
Name
The Plenus Marl Member as defined in Sumbler (1999) included beds A to H of Dodsworth (1996) and thus contained the Black Band. It is herein proposed to redefine the term to include only beds A and B of Dodsworth with the higher beds C to H forming the newly proposed formal term Black Band Member (see below). In so doing the Plenus Marl Member as defined in the Northern Province is the direct equivalent of the member as described for the Southern Province.
Type section
Melton Ross Quarry, Lincolnshire [TA 082 112].
Reference Section
South Ferriby Quarry, Lincolnshire [SE 9915 2045].
Melton Bottom upper quarry [SE 970 280] the stratotypes for the whole Welton Chalk Formation.
Formal subdivision
None
Lithology
Thinly bedded dark-coloured calcareous mudstone (marl) overlain by rubbly limestone and calcareous silty beds followed by a higher calcareous mudstone siltstone (marl).
Definition of upper boundary
Conformable at the base of the Black Band ss (the ‘Main Black Band’ of Wood et al., 1997), the lowest unit of the Black Band Member.
Definition of lower boundary
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). This member and the newly proposed Black Band Member above, generally form a topographical slack at outcrop, which facilitates the mapping of the base of the Welton Formation, and can also be recognised from its higher gamma-ray geophysical log signature in boreholes.'
Thickness
About 1.15 at type-site but to as little as 0.25 m elsewhere.
Distribution
Not known in detail but it is suspected that this member forms part of the succession formerly identified as the Black Band in boreholes and seismic sections.
Previous names
Equivalent as defined herein to the Plenus Marls (Member) of Jefferies (1963) and the Plenus Marls Member as defined herein in the Southern Province.
Equivalent to Beds A and B of Dodsworth (1996).
Lower part of the Flixton Member of Jeans (1980).
Lower part of the Plenus Marls Member of Witham (1991).
Lower part of the Basal Beds including the Black Band in Gaunt, Fletcher and Wood, (1992).
Lower part of the Variegated Beds of Wood and Mortimore (1995) also referred to as the ‘expanded standard Black Band Succession’ by Wood et al. (1997).
Parent
Welton Chalk Formation.
Age and biostratigraphy
Upper Cretaceous, Cenomanian (Metoicoceras geslinianum Zone).
References
Dodsworth (1996); Sumbler (1999); Wood and Mortimore (1995); Wood, Batten, Mortimore and Wray (1997).
4.3.2 Black Band Member
Name
The Black Band Member is formally proposed herein. It is equivalent to beds C to H of Dodsworth (1996) and together the Black Band (bed) itself and superposed Variegated Beds of Wood and Mortimore (1995).
Type section
South Ferriby Quarry, Lincolnshire [SE 9915 2045].
Reference Section
Melton Ross Quarry, Lincolnshire [TA 082 112].
Melton Bottom upper quarry [SE 970 280] the stratotype for the whole Welton Chalk Formation.
Formal subdivision
None
Lithology
Interbedded black or very dark carbonaceous calcareous mudstone (the Black Band) and variegated calcareous mudstone (marl) succession terminating in the ‘sticky green clay’ (Wood and Mortimore, 1995).
Definition of upper boundary
Conformable at the junction between a sticky green clay (Bed 6 i.e. the top of the ‘Variegated beds, in Figure 4, Wood and Mortimore, 1995) and an extremely condensed succession of shell-detrital limestones with some thin very dark ‘marl’ seams.
Definition of lower boundary
Conformable at the basal surface of the Main Black Band as defined in Wood and Mortimore (1995) and Wood et al. (1997) with the underlying Plenus Marls Member defined herein.
Thickness
Generally about 0.5 to 0.6 m but up to 1.4 m in the Cleveland Basin.
Distribution
A well-known marker throughout the Northern Province and in the offshore areas.
Previous names
Equivalent, as defined herein, to the Melbourn Rock Member (Meads Marls) in the Southern Province
Equivalent to Beds C and H of Dodsworth (1996).
Upper part of the Flixton Member of Jeans (1980).
Upper part of the Plenus Marls Member of Witham (1991).
Upper part of the Basal Beds including the Black Band in Gaunt, Fletcher and Wood, (1992).
Upper part of the Variegated Beds of Wood and Mortimore (1995) also referred to as the ‘expanded standard Black Band Succession’ by Wood et al. (1997).
Parent
Welton Chalk Formation.
Age and biostratigraphy
Upper Cretaceous, Cenomanian (Neocardioceras juddi Zone) to Turonian (Mytiloides spp. Zone).
References
Dodsworth (1996); Sumbler (1999); Wood and Mortimore (1995); Wood, Batten, Mortimore and Wray (1997).