Castlechamber to Maw Wyke, North Yorkshire

[NZ 941 082]–[NZ 959 067]

K.N. Page

Introduction

The cliff and foreshore exposures along the North Yorkshire coast between Castle Chamber and Maw Wyke, near Hawsker Bottoms, provide one of the most complete Pliensbachian–Toarcian boundary sequences in Britain, and a continuous succession through the Staithes Sandstone, Cleveland Ironstone and Whitby Mudstone formations in the Pliensbachian and Toarcian of the North Yorkshire coast. The succession here includes the type sections of the Grey Shale and Mulgrave Shale members of the Whitby Mudstone Formation, while the facies developments in the Staithes Sandstone and Cleveland Ironstone formations below are critical to understanding the sedimentary history of the Cleveland Basin. The site is also the type locality for almost half of the known British species of amaltheid ammonites and has yielded indicator fossils for the basal Toarcian Paltus Subzone.

The strata dip gently west, unbroken by faults or folds, and represent a continuation from the succession exposed in the Normanby Stye Batts–Miller's Nab (Robin Hood's Bay) GCR site to the south (Figure 6.6) and (Figure 6.9). The base of the Upper Pliensbachian succession lies about 2 m above the floor of Castle Chamber and progressively younger strata come down to shore level to the north-west. Highest beds exposed in the site are at Maw Wyke where the upper part of the Bituminous Shales of the Mulgrave Shale Member is exposed on the shore. Still higher levels are exposed farther north but in the cliffs.

This section of coast was well known to early geologists and was described by Phillips (1875) and Tate and Blake (1876). The latter publication formed the basis for many subsequent accounts of the site. Further descriptions can be found in the [British] Geological Survey memoir for Whitby and Scarborough (Fox-Strangways and Barrow, 1882), although with some errors perpetuated in later memoirs (Fox-Strangways, 1892; Fox-Strangways and Barrow, 1915). Herries (1906a,b) provided an account of the succession, including a sketch section of the cliffs and an outcrop map of the shore. Further descriptions included those of Buckman (1915) and Arkell (1933). Tate and Blake's (1876) descriptions were not superseded until the Lias stratigraphy was revised in a series of papers by Howarth (1955, 1962a, 1973). Wilson et al. (1934) traced aspects of the variations in thickness of the Upper Pliensbachian sequence, consisting of the Staithes Sandstone Formation and Cleveland Ironstone Formation between Hawsker Bottoms and north-east of Staithes, and Whitehead et al. (1952) discussed the petrology of the ironstones. Howard (1985) revised the lithostratigraphy of the Upper Pliensbachian sequence, based in part on the framework of Powell (1984) and using Howarth's (1955) bed numbers, but did not include a bed-by-bed re-description. A graphic log of part of the Pliensbachian succession, published by Phelps (1985), extended up into the base of the succession here, while summary descriptions and logs of the full sequence have been published by Hesselbo and Jenkyns (1995) (Figure 6.10). Powell (1984) proposed that the section near Hawsker Bottoms should be designated as stratotypes for the Grey Shale Member and Jet Rock of the Mulgrave Shale Member, although clearly the latter should now be located at Port Mulgrave. He also proposed that the section from Hawsker Bottoms to Whitby Harbour be considered the stratotype for the Toarcian Whitby Mudstone Formation. The Toarcian sections at Hawsker Bottoms were referred to by Scrutton (1996), who noted that most of the Alum Shale Member is exposed in the cliff. Elements of the fauna from these higher levels are commonly found amongst fallen material on the beach. Reviews of the sedimentology and petrology of the Upper Pliensbachian succession are included in Hallam (1967a), Chowns (1968), Cattet al. (1971), Greensmith et al. (1980), Young et al., (1990a) and Parkinson (1996).

The site has long been a rich source of fossil material that was described by the early geologists, Including Simpson (1855) and Tate and Blake (1876), and in many other publications since, notably those of Doyle (1990–1992) on the belemnites and Howarth (1958, 1962a, 1973, 1992) on the ammonites. Almost half of the known species of amaltheid ammonite recorded in Britain were described originally from here and the site lends its name to the uppermost Pliensbachian ammonite Pleuroceras hawskerense. The section is also an important source of data contributing towards an understanding of patterns and processes of sedimentation in the Cleveland Basin. It has figured in discussion of the origin of the Cleveland Ironstone Formation (Young et al., 1990a).

Description

This section of coast exposes a complete sequence through the local Upper Pliensbachian and Toarcian succession, consisting of all or part of the Staithes Sandstone, Cleveland Ironstone and Whitby Mudstone formations. The following section is based on information from Howarth (1955, 1962a, 1973) including faunal records from the Hawsker Bottoms area extracted from his more general records. Belemnite records from Doyle (1990–1992) are incorporated for the Grey Shale Member. Bed thicknesses and faunal details for the higher beds are those for the Whitby to Saltwick GCR site section, although Howarth (1962a) states that there is very little thickness variation between these sites. The graphic log in (Figure 6.10), although compiled independently by Hesselbo and Jenkyns (1995), does not differ substantially from the descriptive log below and the same bed numbers, of Howarth (1955, 1962a, 1973) and Phelps (1985), are used.

Thickness (m)

The lower part of the Staithes Sandstone Formation is exposed in and around Castle Chamber (Figure 6.9). The base of the formation is taken at the distinctive ferruginous Shelly sandstone of the Oyster Bed. The formation here is about 25.8 m thick and dominated by siltstones with some mudstone units near the base and fine sandstone around the middle. Small-scale coarsening- and fining-upward cycles are evident. Most units are bioturbated, but some preserve hummocky cross-beds, wave ripples or gutter casts. Ironstone nodules are common at certain levels in the lower part of the formation and were noted by Parkinson (1996) as having anomalously high thorium–potassium ratios. Calcareous nodules occur at several levels towards the top of the formation.

The position of some of the lithostratigraphical boundaries has been the subject of some discussion. Cope et al. (1980a) placed the base of the Cleveland Ironstone Formation at the base of Bed 14, but Powell (1984) placed it at the base of Bed 12, and Howard (1985) placed it at the base of Bed 17 (Figure 6.10), a shale with scattered day nodules, which is the position adopted in this account. The formation is 25.4 m thick and dominated by metre-scale coarsening-upward cycles of light to dark mudstones and siltstones with ironstone nodules common at some levels and with several thin (< 0.3 m thick) but continuous ironstone bands at the tops of the cycles. Unlike the ironstone bands at the top of the cycles at Staithes (Myers, 1989), only the Pecten Seam shows a significant increase in thorium–potassium ratios (Parkinson, 1996). Some of the nodules and ironstone beds are richly fossiliferous and this locality has yielded more species types of amaltheid ammonite than any other site. These include Amaltheus bifurcus from Bed 13; possibly A. striatus from Bed 19; Amaltheus laevigatus, Pleuroceras solare and P. solare var. solitarium from Bed 25; Amaltheus reticularis and Pleuroceras paucicostatum from Bed 33; Amouroceras ferrugineum, Pleuroceras birdi and possibly Amouroceras lenticulare from Bed 33 or Bed 37; Pleuroceras hawskerense transient elaboratum from Bed 40; and P. hawskerense from Bed 42 (Howarth, 1958). The silty mudstones of Bed 21 at Far Jetticks yielded the holotype of the crinoid Balanocrinus solenotis (Simms, 1989). Morgans (1999) examined calcitized and pyritized driftwood from the Penny Nab Member at this site, finding evidence for a seasonally wet and dry climate.

The base of the Whitby Mudstone Formation was placed at the base of Bed 45 by Powell (1984), and around the middle of Bed 43 by Cope et al. (1980a) and by Hesselbo and Jenkyns (1995); the latter is the postion adopted in this account. Towards the middle of Bed 43 (Howarth, 1955) is a finely laminated shale, taken to represent the Sulphur Band. The lowest 0.3 m of Bed 43 is a sandy mudstone (Howarth, 1955).

The lower unit of the Whitby Mudstone Formation, the Grey Shale Member, is 13.3 m thick and consists of silty, micaceous, pyritic, mudstone with bands of typically small calcareous concretions, which tend to be more sideritic and weather a red colour in the lower part of the sequence, passing into laminated shales above. Ripple-laminated siltstones are also present in the lower part of the sequence. Howarth (1973) recognized that the general sequence of beds within the member is remarkably constant, in terms of lithology and fauna, throughout the coastal sections from Ravenscar to Port Mulgrave. Consequently he presented only a single composite section for the unit, but identified the individual source localities of the cited faunas. The first definite records of Toarcian style ammonites, namely Protogrammoceras paltum and Dactylioceras sp., are from Bed 3 of the Grey Shale Member at Hawsker Bottoms (Howarth, 1973, 1992; = Bed 45 of Howarth, 1955) and mark the base of the Toarcian Stage in the Cleveland Basin. The type of Tiltoniceras antiquum was obtained from Bed 17 here or at Staithes. A single specimen of Meneghiniceras lariense recovered from Bed 31 represents the only known British occurrence of this exclusively Tethyan juraphyllitid genus and is also stratigraphically younger than any other juraphyllitid in Britain (Howarth, 1976; Howarth and Donovan, 1964).

The base of the overlying Mulgrave Shale Member is taken at a prominent bed, the Cannon Ball Doggers (Bed 33) (Powell, 1984). The Mulgrave Shale Member is remarkably consistent in thickness throughout its coastal exposure, with a figure of 32.3 m (105 ft) cited by Howarth (1962a). It is dominated by organic-rich laminated shales with several distinctive bands of often large calcareous nodules. The fauna is dominated by nektonic and planktonic taxa, particularly ammonites and belernnites. Rare remains of teuthids, recorded as Geoteuthis by Tate and Blake (1876), also probably are from the Mulgrave Shale Member here. Vertebrate remains have rarely been recorded from here. Benton and Taylor (1984) listed an example of the ichthyosaur Leptopterygius acutirostris from an unspecified horizon, although examples of this species from elsewhere have been recorded from the Mulgrave Shale and Alum Shale members.

Interpretation

The ammonite faunas of the Upper Pliensbachian part of the succession were described by Howarth (1955, 1958). They include proposed stratotypes, or parastratotypes, for the bases of the Stokesi, Subnodosus, Gibbosus, Apyrenum and Hawskerense subzones and hence the Margaritatus and Spinatum zones themselves. The lectotype of the subzonal index fossil Pleuroceras hawskerense (Young and Bird, 1828) came from Hawsker Bottoms (Buckman, 1923). Meister (1988) discussed further the evolution of the Amaltheidae and figured a good selection of specimens from Hawsker. Howarth (1955) proposed that the base of the Hawskerense Subzone should be taken at the base of Bed 38 of the Cleveland Ironstone Formation at Hawsker. However, older faunas with Pleuroceras elaboratum were considered by Dommergues et al. (1997) to characterize an Elaboratum Zonule in the lower part of this subzone. Hence the base of the Hawskerense Subzone should be drawn lower according to this latter scheme, probably at the base of Bed 33 according to records by Meister (1988). Ammonites characteristic of the Solare Zonule are known from the Pecten Seam at Hawsker Bottoms, while the bed immediately below it near Staithes yields Amaltheusgibbosus (see Staithes to Port Mulgrave GCR site report), hence the base of the Apyrenum Sybzone can be drawn at the base of the Pecten Seam, Bed 25. However, information from France and elsewhere indicates that there are earlier Pleuroceras faunas, corresponding to the Transiens and Salebrosum zonules of the Apyrenum Subzone (Dommergues et al., 1997), that are not present in Yorkshire. The absence of early Pleuroceras faunas in Yorkshire, including P. transiens which is known elsewhere in Britain on Raasay (Howarth, 1958), indicates a widespread non-sequence as already predicted on lithological and sedimentological grounds by Howard (1985).

The succession here can be correlated with equivalent sections elsewhere along the Yorkshire coast on the basis of marker beds and ammonite faunas. Although the Toarcian succession remains relatively constant in thickness and facies (Howarth 1962a, 1973), there are marked variations in the Upper Pliensbachian successions. At the Staithes to Port Mulgrave GCR site, the Staithes Sandstone and Cleveland Ironstone formations are coarser than at Hawsker Bottoms, and the ironstone bands, poorly developed at Hawsker, are significantly thicker (Figure 6.11). This north-westerly thickening continues across to the iron workings at Eston, where the ironstone beds in the Kettleness Member form a single bed several metres thick with thin shale partings (Young et al., 1990a). Hesselbo and Jenkyns (1995) suggested that the finer-grained sediments at Hawsker were indicative of a more distal position within the basin. However, in apparent contradiction to this, the non-sequence at the base of the Kettleness Member (see (Figure 6.11)) is more marked at Hawsker than elsewhere, perhaps reflecting opposing directions of sediment progradation during the Penny Nab and Kettleness members. The thin laminated mudstone of the Sulphur Band has been attributed to eustatic sea-level rise. The influence of this becomes even more pervasive in the Mulgrave Shale and Alum Shale members which, except within the Peak Trough, show a remarkable uniformity of thickness and facies development along the entire coast.

Conclusions

The coastline between the Castle Chamber and Hawsker Bottoms area exposes a continuous section following on from that on the north side of Robin Hood's Bay (Normanby Stye Batts–Miller's Nab GCR site), extending from the base of the Upper Pliensbachian Substage, through the Pliensbachian–Toarcian boundary, and well into the mid Toarcian, encompassing the Staithes Sandstone Formation, the Cleveland Ironstone Formation and the Whitby Mudstone Formation. Fades developments in the Staithes Sandstone and Cleveland Ironstone formations here are fundamental to understanding the history of sedimentation in the Cleveland Basin. The Toarcian section here complements that nearer Whitby, where the lower part of the Toarcian succession is not exposed. The site is the location for the type populations of almost half of the known species of amaltheid ammonite and hence is of international importance for Upper Pliensbachian biostratigraphy.

References