Hebden Formation is part of the Millstone Grit Group
The new name Hebden Formation is proposed to identify all Millstone Grit Group strata of Kinderscoutian age. The term Kinderscout Grit Group of Bradford (Stephens et al., 1953) is unsuitable as there is a sandstone already called Kinderscout Grit, and the name has also been used for the Stage. The new name was chosen from Hebden Bridge (SD 990 270) and Hebden Water, which provide excellent sections representative of much of the Kinderscoutian succession. The basal marine band has, however, not been found within this area.
Fine- to very coarse-grained and pebbly, feldspathic sandstone interbedded with grey siltstone and mudstone, with subordinate marine black shales, thin coals and seatearths. The lower part of the formation is dominated by a turbiditic facies of thinly interbedded siltstone and fine-grained sandstone with laterally impersistent and locally thick, massive, coarse- to very coarse-grained sandstones. The upper part of the formation is dominated by sheet-like laterally persistent, cross-bedded sandstones, interbedded with siltstone and mudstone; Coal and seatearth are largely restricted to the upper part of the formation. A distinctive feature of the lower part of the formation within the Askrigg Block succession is the presence of brachiopod-bearing sandstones, the Cayton Gill Shell Bed and Ure Shell Bed (Wilson and Thompson, 1965; Wilson, 1977).
During the early Kinderscoutian there is a return to deposition in deep-water deltaic successions in the northern part of the basin. In Wharfedale this is dominated by 125.m of mostly delta slope siltstones with possible turbidite feeder channels, the Addlethorpe Grit. This marks the initiation of a major deltaic advance with the distributary channel sandstones of the Addingham Edge Grit, 15–55 m thick prograding over the delta slope deposits. The delta system appears to be thickest in the Pennines, thinning westwards into Lancashire. In the south of the basin, where hemipelagic shales had continued to be deposited until Kinderscoutian times, the greater accommodation space resulted in a marked southward thickening of the deltaic succession to about 600 m in north Derbyshire. Here, the onset of deltaic sedimentation is marked by the thin-bedded distal turbidites of the Mam Tor Sandstone (Allen, 1960). This passes up into the very thick-bedded, more erosional turbidites of the Shale Grit (Walker, 1966). The overlying Grindslow Shales represent the delta-slope deposits and the Lower Kinderscout Grit represents fluvial distributary channels (Collinson, 1969; McCabe, 1978; Hampson, 1997). The general southward progradation of deltas resulted in the youngest Kinderscoutian sandstone, the Upper Kinderscout Grit, extending furthest south. In the Goyt Trough the Longnor Sandstone, a turbiditic sandstone equivalent to the Upper Kinderscout Grit, marks the base of the Millstone Grit Group. Heavy mineral studies have demonstrated the northerly provenance of the Addingham Edge Grit (Cliff et al., 1991) and Kinderscout Grit (Chisholm and Hallsworth, 2005). Sporadic marine environments are evidenced by the presence of subordinate marine black mudstones, and in the Askrigg Block the presence of the brachiopod-bearing sandstones, the Cayton Gill Shell Bed and Ure Shell Bed.
Partial type sections include Crimsworth Dean, north of Hebden Bridge (SD 9899 3079 to 9926 3141), which shows the upper boundary with the Marsden Formation as a transition from the underlying turbiditic facies to the overlying fluvial sheet sandstone facies (Stephens et al., 1953; Davies and McLean, 1996); and Blackden Brook, Kinder Scout, Derbyshire (SK 1223 8842 to 1171 8833), which provides an excellent stream section 150 m thick of the Shale Grit, Grindslow Shales and the base of the Kinderscout Grit (Stevenson and Gaunt, 1971; Davies and McLean, 1996).
Lower and upper boundaries
The base of the formation is taken at the base of the dark grey, shaly mudstone Hodsonites magistrorum Marine Band with an eponymous fauna, where the formation overlies the mudstone-dominated Samlesbury Formation. Elsewhere, where this marine band has not been found (Figure 15, Column 5), the base of the formation equates with the base of the Cayton Gill Shale in the south of the Askrigg Block (Wilson, 1977) and the base of the Mousegill Marine Beds in the Stainmore Trough (Owens and Burgess, 1965). In the south of the Central Pennine Basin the base is taken at the base of the first thick quartz-feldspathic sandstone of Kinderscoutian age, present above the Bowland Shale Formation.
The top is taken at the sharp conformable base of the dark grey, shaly mudstone of the Bilinguites gracilis Marine Band with an eponymous fauna, typically overlain by mudstone of the Marsden Formation. The marine band is underlain by sandstone including the Upper Kinderscout Grit of the Hebden Formation. Elsewhere, where the marine band is absent, in the Askrigg Block the base of the formation is taken at the base of the Wandley Gill Shale, described by Wilson and Thompson (1965) as 3 m of shale with a Lingula-band at the base, overlying a seatearth at the top of the Upper Brimham Grit. In the Stainmore Trough the base of the Marsden Formation is taken at the base of a 6.m-thick shale with Productus carbonarius, which overlies a 0.15.m-thick shaly coal (Owens and Burgess, 1965).
Preston 335.m; Bradford 275.m; Derbyshire 600.m; southern part of the Askrigg Block about 100 m; Stainmore Trough 40 m.
Distribution and regional correlation
North Lancashire and north Yorkshire, between Lancaster (SD 47 61) and Harrogate (SE 30 55), extending southward to north Derbyshire (SK18) and on the Askrigg Block and Stainmore Trough. The formation passes southward into basinal mudstones of the Bowland Shale Formation (Craven Group).
Age and biostratigraphical characterisation
Kinderscoutian (R1a). The base is taken at the base of the Hodsonites magistrorum Marine Band (R1a) and the top at the base of the Bilinguites gracilis Marine Band (R2a).
Marine shales of the R1a Zone (equivalents of the Mousegill Marine Beds of the Stainmore Trough) occur within the Cayton Gill Shale of the southern part of the Askrigg Block (see Owens and Burgess, 1965; Wilson, 1977). Thick sandstones present in the Askrigg Block area include the Libishaw Sandstone and the Lower and Upper Brimham Grits.
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