Clints Quarry, North Yorkshire

[SD 967 575]

Introduction

The Clints Quarry GCR site is a disused quarry located close to the Grassington-Skipton railway line a kilometre south of Rylstone [SD 967 575]. This site offers a stratigraphically important and fossiliferous Holkerian–Brigantian section that extends from the top of the Worston Shales (= the Hodder Mudstone Formation of Riley, 1990a; and see (Figure 6.2)) to a prominent limestone conglomerate, variously assigned to the Pendleside Limestone Formation by Arthurton et al. (1988) and to the Lower Bowland Shales by Riley (1990a). This conglomerate was formed by erosion at the northern margin of the Craven Basin during a significant period of late Asbian–early Brigantian earth movements. Important site descriptions are provided by Wilmore (1910) and Booker and Hudson (1926).

Description

Clints Quarry, also known as Itylstone Railway Quarry' (Wilmore, 1910; Hudson and Mitchell, 1937) and 'Clint Rock Quarry' (Arthurton et al., 1988), lies on south-eastern limb of the NE–SW-trending Eshton–Hetton Anticline, approximately 1 km south of the Winterburn Fault (an easterly trending splinter of the South Craven Fault System to the west; see (Figure 6.1)). The succession dips gently to the south-east (Figure 6.22).

At its base, the Worston Shales comprise approximately 4 m of calcareous mudstone with thin argillaceous limestone interbeds. These beds, containing Pustula cf. pyxidiformis (Arthurton et al., 1988) and Merocanites (Rose et al., 1973), were referred to as part of the 'upper mudstone-rich subdivision' of the Worston Shales by Arthurton et al. (1988), and as part of the Rylstone Limestones by Booker and Hudson (1926). A sharp but planar unconformity surface separates this unit from the overlying beds which Arthurton et al. (1988) regarded as part of the Pendleside Limestone Formation (= Skelterton Limestone of Booker and Hudson, 1926) and which Riley (1990a) considered as part of the Lower Bowland Shales (see Aitkenhead et al., 1992). Missing from the succession at this unconformity is the Hodderense Limestone Formation and, if Riley is correct, the Pendleside Limestone Formation (Riley, 1990a).

Above the Worston Shales an 8.4 m succession, ascribed to the Pendleside Limestone Formation by Arthurton et al. (1988) and to the Lower Bowland Shales by Riley (1990a) crops out, which can be subdivided into two unequal parts. The lower part (= the 'well bedded limestone subdivision' of Arthurton et al. (1988) and the 'Zaphrentid Bed' of Booker and Hudson, 1926) comprises well-bedded grey crinoidal packstones (c. 3 m) with possible chlorite mottlings near the base and limestone lithoclasts at its top. A rich fauna dominated by zaphrentid corals and brachiopods is reported from these beds including Amplexizaphrentis enniskilleni, cf. Fasciculophyllum densum, F. cf. junctoseptatum, Rotiphyllum rushianum, Zaphrentites parallela, Rylstonia benecompacta, R. cf. dentata, Dictyoclostus multispiniferus, Krotovia spinulosa, and 'an indeterminate goniatite' (Arthurton et al., 1988). The higher part (= the 'limestone conglomerate subdivision' of Arthurton et al. (1988) and the 'Lithostrotion arachnoideum Beds' of Booker and Hudson, 1926) is a massive (3.4 m) conglomerate (= 'Tiddeman's Breccia' — see Hudson and Mitchell, 1937; and Arthurton et al., 1988) with granule- to pebble-sized limestone lithoclasts up to 15 cm in diameter set in a bioclastic limestone matrix (Figure 6.22). Lithoclasts in the conglomerate are subrounded to subangular (Arthurton et al., 1988) and include pebbles of the Hodderense Limestone Formation (Riley, 1990a). Arthurton et al. (1988) recorded a mixed coral–brachiopod assemblage of Holkerian or Asbian age from this unit including Axophyllum vaughani, Siphonodendron martini, S. sociale, Avonia youngiana and Plicatifera plicatilis. Above this, the succession is represented by well-bedded to lenticular packstones (c. 2 m) containing a coral–brachiopod assemblage of early Brigantian age including Diphyphyllum furcatum, D. lateseptatum and Striatifera striata (Arthurton et al., 1988), Sutherlandia 'Emmonsia' parasitica and Michelinia tenuisepta (Booker and Hudson, 1926) but which also includes the late Asbian (P_1a) goniatite index Goniatites crenistria (Rose et al., 1973).

Interpretation

The Worston Shales (Hodder Mudstone Formation) deposits at this site represent hemipelagic muds (Riley, 1990a) and thin storm-generated limestone interbeds which formed on the upper part of a carbonate slope (Gawthorpe, 1986, 1987a) towards the northern margin of the Craven Basin during Holkerian times ((Figure 6.4)b).

By contrast, the massive limestone conglomerate, with its mixed and partially derived Holkerian–Asbian faunas near the base of the overlying unit (Lower Bowland Shales), most probably formed as a debris-flow deposit during a period of late Asbian-early Brigantian crustal instability; this causing uplift and widespread erosion of the basin margin and triggering gravity flows down the basin flanks. The unconformity below the conglomerate also developed at this time. Erosion on this unconformity surface evidently removed the Hodderense Limestone Formation (and, possibly, the Pendleside Limestone Formation — if deposited), from the Lower Carboniferous successions of the Eshton-Hetton area, its erosional remnants appearing as pebbles in the overlying limestone debris bed (Riley, 1990a). The associated packstones both above and below the conglomerate (but still within the Lower Bowland Shale sequence) most probably represent either similar but smaller-scale debris deposits or storm-generated beds.

The development of these features (debris bed and unconformity) together mark a significant late Asbian–early Brigantian phase in the evolution of the Craven Basin as early Carboniferous rifting gave way to thermally driven subsidence and regional basin sag (Gawthorpe, 1987a; Fraser and Gawthorpe, 1990).

Conclusions

This site provides one of the finest sections across the Worston Shales–Bowland Shales boundary in the Craven District. The sedimentological and palaeontological characteristics of the sequence (reworked faunas, derived pebbles and sediment gravity flows) provide evidence for the existence of a significant unconformity at the base of the Lower Bowland Shales which developed during a late Asbian–early Brigantian period of tectonic instability that is widely recognized within the Craven Basin.

References