The upper part of the Carstone and the Hunstanton Red Chalk (Albian) of the Hunstanton Cliff, Norfolk

The upper part of the Carstone and the Hunstanton Red Chalk (Albian) of the Hunstanton Cliff, Norfolk H. G. Owen OWEN, H. G. 1995. The upper part of the Carstone and the Hunstanton Red Chalk (Albian) of the Hunstanton Cliff, Norfolk. Proceedings of the Geologists' Association, 106, 171-181. In northwest Norfolk, the Gault Formation passes from clays into sands at the top of the Carstone Formation and red and sandy, pebbly, limestones of the Hunstanton Red Chalk Formation, well exposed in the sea cliffs at Hunstanton. The Carstone at Hunstanton can be divided into two Members, separated by a marked erosion surface. The Lower Carstone is of early Aptian age at its base, but is otherwise unfossiliferous. The top of the Upper Carstone is of Middle Albian, Hoplites spathi Subzone age and passes lithologically into the basal beds (C) of the Hunstanton Red Chalk. Barely a metre in total thickness, the Hunstanton Red Chalk includes sediments of several Subzones of the Middle and Upper Albian within the three units comprising the Formation. The basal unit (C) is of Hoplites spathi Subzone age. The middle unit (B) has a complex depositional and erosional history. In the more northern part of the section, bedded pebbly limestone of late Anahoplites intermedius Subzone (Middle Albian) age rests on unit C. These are overlain non-sequentially in part of the section, by bedded concretionary limestones with interstitial marl of late Dipoloceras cristatum Subzone and early Hysteroceras orbignyi Subzone (Upper Albian) age. Strong post-formational current scour produced a boulder bed from these limestones which, in the southern part of the section, rests directly on unit C. The basal part of the upper unit (A) is of Hysteroceras varicosum Subzone age, followed in turn by sediments of Callihoplites auritus Subzone and Stoliczkaia dispar Zone age.

Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW75BD.

1. INTRODUCTION

Throughout much of East Anglia, the Gault Formation consists of clays with rare thin seams of marly limestone (Gallois & Morter, 1982). At outcrop in north Norfolk, the formation thins rapidly north of Marham and Gayton as its lithology passes into the reddish pebbly limestones of the Hunstanton Red Chalk Formation (the Hunstanton Red Rock of some authors). In the Dersingham area, these Middle and Upper Albian sediments are reduced in thickness to a little over 2 m and, at Hunstanton, the Red Chalk is reduced still further to an average of 1.25 m, to which must be added a small thickness of underlying Carstone. The ammonite and bivalve faunas contained in the individual beds at Hunstanton show that these sediments form a very incomplete representation of the Gault Formation to the south. Similar changes in facies and thickness are to be seen in borehole core sequences beneath the Chalk cover further east in north Norfolk (e.g. Gallois & Morter, 1982). The cliffs at Hunstanton provide one of the most colourful geological sections to be seen in England (e.g. Funnell, 1976). The Hunstanton Red Chalk Formation appears in the upper part of the sea-cliff at the northern end of North Promenade, New Hunstanton, as a brick-red seam surmounting the dun-brown-coloured Carstone and is overlain by creamy grey basal Chalk. The section extends for approximately 1300 m to the NNE until, in response to the Proceedings of the Geologists' Association, 106, 171-181.

dip of the beds, the outcrop and the cliff itself declines to near shore level (Fig. I). The Red Chalk sequence can be reached easily from beach level between Old Hunstanton and St Edmund's Point. Further south, the sections can be approached only by rope or by fortuitous and not infrequent collapses of the cliff face. Falls are frequent in the early part of the year, ranging from isolated blocks to substantial collapses of the cliff face and care should be taken when working the section. The Hunstanton section has been discussed in the geological literature since the early years of the nineteenth century (Smith, 1816 and see Eyles, 1969). The age equivalence of the bulk of the Hunstanton Red Chalk to the Gault was recognized at an early date by Sedgwick (1826, 342 footnote), Woodward (1833), Rose (1835), Wiltshire (1859a,b, 1869) and Seeley (1861, 1864a,b, 1866) among others. The stratigraphical account given by Wiltshire (1869) was the most accurate published to that date and for long after; it is still the framework used today. Although barely 1.25 m in thickness, the nomenclature, nature of the sediment and the age representation of the Hunstanton Red Chalk, its stratigraphical relationship both to the sediments above and below and to the slightly thicker Albian red and whitish chalks and marls of Lincolnshire, Humberside and North Yorkshire, has been the subject of considerable debate and dispute over the intervening 180 years; a situation which continues to the present day. The debate in the nineteenth century and in the twentieth century until 0016-7878/95 $07·00 © 1995 Geologists' Association

172

H.

G.

OWEN

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Earlier Lower Cretaceous

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Fig. 1. Sketch map of the coastal region of Hunstanton, Norfolk, with an inset of the outcrop of Albian sediments in Lincolnshire and Yorkshire.

1932 is documented by Rastall (1930, 1932). Despite all this work, the formation still has two names applied to it; the 'Red Chalk' and the 'Hunstanton Red Rock'. The stratigraphical rankings of the Carstone and the Red Chalk!Hunstanton Red Rock is not fully resolved, nor whether the latter should be grouped with the Chalk above or the Carstone below. The present paper describes in more detail than hitherto, the lithostratigraphy and biostratigraphy of the Hunstanton section. This is in order to stabilize the stratigraphical nomenclature, to establish more precisely the age of the uppermost Carstone and the individual beds of the Hunstanton Red Chalk and the relationship ofthese deposits to the lower part of the Carstone below and to the white Chalk above. Although zonally significant ammonites have been obtained from sections further south in Norfolk and across the Wash in Lincolnshire and Humberside, it is only at Hunstanton that the ammonite and bivalve molluscan

biostratigraphy of the Hunstanton Red Chalk has been elucidated in any detail.

2. REGIONAL DEPOSITIONAL SETTING (Fig. 1) The Hunstanton section is situated near to the southern margin of an outcrop of Carstone and Hunstanton Red Chalk, of approximately the same age range, which extends in the region of the East Midland Shelf northward across the wash into Lincolnshire and Humberside and eastward in the subcrop, extending into the North Sea. The Red Chalk extends northward into North Yorkshire, in the West Heslerton-Speeton area, where the age of the lower part is late Albian, representing only the topmost part of the Red Chalk further south. Here, in the marginal area of the East Midlands Shelf and the Cleveland Basin, it is underlain by

THE UPPER

PART OF THE CARSTONE

AND

the silty clays of Beds A3-Al of Middle and Upper Albian age (e.g. Ennis, 1937; Wright & Wright, 1942). Sedimentological and faunal studies indicate that the top of the Carstone and the Red Chalk were formed within a relatively shallow, current-swept sea-bottom environment, in which short periods of sediment accumulation in slack water alternated with long periods of non-deposition and current scour with the development of iron-oxide indurated hardgrounds. The extension of the southern margin of the Red Chalk facies eastward under the Chalk of northern Norfolk (Gallois & Morter, 1982), marks the southern boundary of this shallow sea. This shallow depositional environment was in marked contrast to that of the deeper and quieter waters to the south in which the contemporary muddy oozes of the Gault clay were deposited well below the effects of topographical or tidal-induced currents and storm-waves. The author has suggested (Owen, 1971: 140-141, fig. 51 and see Jeans, 1973, 1980) that this shoal area of the East Midland Shelf probably flanked the emergent Carboniferous massif of the Pennines and Peak District of Derbyshire, pebbles derived from which are to be found in the Middle Albian sediments. However, the derived Middle and Upper Albian fossils known from the Pleistocene boulder clays of the Midlands as far north as Chellaston, Derbyshire, including Gault-facies preserved Euhoplites cf. proboscideus (1. Sowerby) and species of Hysteroceras, suggest that a deeper water Gault clay facies separated these massifs from the shallows of eastern England and the adjacent North Sea.

3. PROBLEMS OF STRATIGRAPHIC NOMENCLATURE AND RELATIONSHIP (a) Red Chalk or Hunstanton Red Rock? Hancock (1972) provided a formal definition for the name Hunstanton Red Rock, which was first used by Seeley (I 864a, b) and by a number of authors to the present time (e.g. Rastall, 1930, 1932; Clarke, 1964; Owen, 1991). The name Red Chalk, however, has been widely used for contiguous deposits in Lincolnshire, Humberside and for the Hunstanton sequence by authors such as Wiltshire (1859a, b; 1869), Gunn (1878), Whitaker (1883), Harmer (1884), Spath (1943: 750-752 who uses both terms), Casey (1961), Gallois & Morter (1982), and Rawson (1992) and it is used for contiguous deposits in the North Sea (e.g. Crittenden (1982) who uses the term Red Chalk Formation and Cameron, Crosby, Balson, Jeffery, Lott, Bulat & Harrison (1992)). Wood & Smith (1978: 271) called the Red Chalk the Hunstanton Chalk Member and included it in the Ferriby Chalk Formation, which also includes the basal Chalk; a grouping promptly disputed by Jeans, Kent & Rawson (1978 in discussion of Wood & Smith). Rawson (in Kelly & Rawson, 1983), when considering the similar sequence in Lincolnshire, grouped the 'Hunstanton Member' with the Carstone Member beneath. Later, Rawson (1992) recognized separate Carstone and Hunstanton Formations. This practice is adopted here, albeit

HUNSTANTON

RED

CHALK

(ALBIAN)

173

that the name of the Hunstanton Formation is modified to reflect the widespread use of the descriptive lithological term Red Chalk which is used in a Formation sense in the North Sea (e.g. Crittenden, 1982). It is recommended, therefore, that the formal name for the sequence of red, pebbly, sandy marls and limestones intervening between the top of the Carstone and the base of the Chalk in north Norfolk, Lincolnshire and Humberside and which extends offshore in the North Sea, shall be the Hunstanton Red Chalk Formation. The best exposed and dated section is the sea-cliff at Hunstanton, Norfolk, described below. (b) Relationship to the Carstone Formation below and the Chalk above The relationship of the Hunstanton Red Chalk to the sediments above and below has been the subject of much debate and controversy. Workers such as Rose (1835), Seeley (1864a,b), Wiltshire (1869), Rastall (e.g. 1930, 1932) and Jackson (1911: 35) all recognized that the basal unit of the Hunstanton Red Chalk was a brick-red continuation of the sediments at the top of the Carstone at this locality. Indeed, Rose (1835) recorded a specimen of Hoplites dentatus (1. Sowerby) ('Ammonites interruptus') and so provided an early palaeontological link between the top of the Carstone and unit C of the Hunstanton Red Chalk. Ammonites found and recorded in the nineteenth century showed that units C and B of the Hunstanton Red Chalk contained Lower Gault fossils and unit A was of partial Upper Gault equivalence. Most of these authors recorded a seam of red marl separating unit A from the overlying Paradoxica Bed of the basal Chalk (Ferriby Formation). This evidence was ignored by Kitchin & Pringle (1922, 1932) who considered that only Upper Gault was represented in units C and B, that unit C rested non-sequentially on the uppermost Carstone of Aptian age and that unit A formed the base of the Chalk. Rastall (1930, 1932) argued strongly against Kitchin & Pringle's reading on detailed lithostratigraphical grounds and, while accepting a possible Cenomanian date for unit A in 1932, restated the fact that no sedimentary break existed between unit C and the top of the Carstone at Hunstanton. It is curious that Kitchin & Pringle should have been so emphatic in their views considering that the only Aptian ammonites which had been obtained from the Carstone had come from a nodule bed at its base (e.g. Jackson 1911; Casey, 1961: 570-1) and that Spath (1923-43) by 1932 had already described ammonites of Lower Gault equivalence from the Hunstanton Red Chalk. While not possessing exact stratigraphical information, the ammonites from the Hunstanton Red Chalk could be placed at the top of bed C and the lower part of B, using their distinctive preservation, by anyone with a detailed knowledge of the succession. Moreover, no attempt seems to have been made to check on the record of 'Ammonites interruptus' by Rose (1835), the original of which was found by this author housed in the collections of the Castle Museum, Norwich. It is a specimen of Hoplites

174

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dentatus (1. Sowerby) and complements another specimen in exact preservation obtained from phosphatic nodules near to the top of the Carstone by Mr Hamon Le Strange. Nor did they take into account the occurrences of Mortoniceras in bed A indicating an Upper Gault equivalence for that unit. However incomplete the sequence might be, the field data provided here confirm the close sedimentation and age link with the Upper Carstone below, the separate formational entity of the Hunstanton Red Rock, its total equivalence of the Gault Formation and the sedimentational and faunal break with the Paradoxica Bed (basal Chalk, Ferriby Formation) above. Morter & Wood (1983) have demonstrated that the topmost stone horizon of unit A is still Albian and, on the basis of the contained Aucellina, is of dispar Zone age.

4. STRATIGRAPHY (Figs 2-4) (a) The Carstone Formation Casey (1961) demonstrated that the pebble horizon in the basal part of the Carstone Formation at Hunstanton has yielded a condensed lower Aptian ammonite fauna. The age of the remainder of the lower part of the Carstone below a major erosion surface is unknown, as is much of the Upper Carstone above this surface . Comparison with sections of the Carstone seen further south in Norfolk, in which deposits of Lower Albian (Ley me riella regularis and early Douvilleiceras mammillatum Superzone) date are known, suggested to Casey (1961) and Casey & Gallois (1973) that the Carstone at Hunstanton is probably of Aptian and Lower Albian date. There is no palaeontological evidence yet to support this view, but the top beds of the Upper Carstone are definitely of Hoplites spathi Subzone (Middle Albian) age (Owen 1991: 291). Two distinct Members are recognized here: the Lower and Upper Carstone Members. Lower Carstone Member The Lower Carstone consists essentially of dark-brown, massive-bedded ferruginous pebbly grit, blocky above but cemented into large concretionary masses in the lower part. The base is marked by a phosphatic pebble bed containing ammonites of earlier Aptian age (Jackson, 1911; Casey, 1961) . The top of the Lower Carstone was deeply channelled by current scour prior to the deposition of the Upper Carstone. Its stratigraphy is not considered further here. Upper Carston e Member The Upper Carstone consists of warm-brown bedded pebbly sands. The north to south variations in thickness and lithology of the Upper Carstone and the Hunstanton Red Chalk along the cliff section are shown graphically in Figs 2-4. No fossils have been seen in Bed I of the Upper Carstone which has a remarkably uniform thickness along the complete cliff section, albeit that its lowest part is channelled into the top of the Lower Carstone. Variation in the total thickness of Bed 2 was noted; to a certain extent

OWEN

illustrated in Figs 2-4 and the sequence thickens southward. From the northern end of North Esplanade southward to the end of the exposed sequence, a wedge of current-bedded dark-brown sand, also thickening southward, intervenes between Beds I and 2. Towards the top of Bed 2, the detailed sequence, although variable, shows phosphatic nodules in sands with thin interbeds of reddish sand preceding a thin loamy sequence which passes rapidly into Bed C (i) of the Hunstanton Red Rock. These creamy-grey pebbly phosphatic nodules have a characteristic trace of reddish staining and contain , occasionally, small molluscs. Two ammonites have been obtained from these nodules, both specimens of Hoplites dentatus (1. Sowerby) : one is by Mr Hamon Le Strange in the cliff section near St Edmund' s Point (BMNH C 77572); the other is the specimen recorded as 'Ammonites interruptus' by Rose (1835) (Norwich Castle Museum Rose ColI. 10.1). Both arrunonites indicate that the top of the Carstone is of Middle Albian, Hoplites spathi Subzone age. Bed 2 of the Carstone is separated from the basal unit C of the Hunstanton Red Chalk by a thin residual earth. However, unit C is essentially the same lithology as the top of the Carstone , but with an upwardly increasing admixture of brick-red marl. This sedimentological feature led Casey (196 1: 571) and Gallois & Morter (1982 : 365) to speculate that the topmost Carstone here was of Albian age and this is now confirmed (and see Owen, 1991). The age of the underlying sediments of the Upper Carstone is uncertain . At West Dereham, 38.6 km to the south, the basal part of the Carstone is of Leym eriella regular is Subzone age (Lower Albian, latest Leym eriella tardefurcata Zone) (Casey, 1978: 6]) and the phosphatic nodule beds at the relatively abrupt Carstone-Gaultjunction are of Dou villeiceras mammillatum Superzone, Sonn eratia perinj7ata Subzone age (Owen, 1988: 2(0). Here, the Carstone is transgressive across sediments of the Dersingham Group (Casey & Galloi s, 1973). At Hunstanton, this transgression might be marked by the guttered erosion surface at the top of the Lower Carstone and the commencement of deposition of the Upper Carstone.

(b) Hunstanton Red Chalk Formation Wiltshire (1869) recognized three distinct lithological units of beds which he labelled A, Band C in downward succession. Albeit unconventional, these units are used here as the framework for the lithological sequence given in Figs 2-4. The ammonite fauna is sparse, but indicates the presence of sediments of both Middle and Upper Albian age, although the zonal and subzonal representation is very incomplete (Table 1). The principal collections were made by Seeley and Wiltshire in the nineteenth century and by Hamon Le Strange in this century (Le Strange, 1975). Despite Seeley's (I 864a, b) and Wiltshire's (1869) lists, many of the earlier collections are now devoid of precise stratigraphical data. However, subsequent collecting has permitted the exact matching of the lithology of specimens with individual discrete beds in the sequence. Spath (1923-43)

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THE UPPER

PART OF THE CARSTONE AND

HUNSTANTON

RED

CHALK

(ALBIAN)

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178

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Table 1. Ammonite zonal scheme of the Middle and Upper Albian in the hoplitinid faunal province; Subzones known to be present in the Hunstanton section are shown in bold Zones

Substage

Subzones

Stoliczkaia dispar {

Un-named Mortoniceras (Dumovarites) perinflatum Mortoniceras (Mortoniceras) rostratum

UPPER ALBIAN

Callihoplites auritus

{

(

Dipoloceras cristatum

{

Anahoplites daviesi Euhoplites nitidus

Euhoplites loricatus

(

Euhoplites meandrinus Mojsisovicsia subdelaruei Dimorphoplites niobe Anahoplites intermedius

Hoplites dentatus

{

Hoplites spathi Lyelliceras lyelli

Mortoniceras inflatum

Euhoplites lautus

MIDDLE ALBIAN

described the species known at that time, but the illustrations are unsatisfactory and the taxonomy and dating imperfect. In his stratigraphical review (1943: 750--2), he considered that the ammonites of Middle Albian age were confined to Bed C and that Beds B and A contained a fauna of Hysteroceras orbignyi Subzone age. Casey provided Larwood (1961, 1970) with a list of the ammonites described or recorded by Spath from this Formation, but no attempt was made to sort them into individual beds by lithological matching. The author had attempted to sort out the stratigraphical position of the ammonites, on the basis of lithological matching and on new material collected by Le Strange and himself (Owen, 1969: 158-65; 1979: 580; 1991: 291). However, records of ammonites of Euhoplites loricatus Zone age later than the Anahoplites intermedius Subzone are incorrect. The account of this section by Clarke (1964) is inaccurate and he reversed the measurements of units A and C. The three units of the Hunstanton Red Chalk can be further subdivided into beds. The lowest unit C, as mentioned above, is merely a continuation of the sedimentation of the uppermost part of the underlying Carstone, but with the irregular inclusion of brick-red marly sediment at the base and with a slightly calcareous binding. Towards the top, it becomes a brick-red, soft, pebbly sandy limestone and generally fossiliferous (Beds Cii & iii). Bed C (ii) contains scattered Moutonithyris dutempleana (d'Orbigny), Birostrina concentrica (Parkinson) among other bivalves, Neohibolites minimus (Miller) and Hoplites sp. juv. The specimens of Hoplites in the collections, including Hoplites canavarii Parana & Bonarelli (BMNH C 77573) and H. canavariiformis Spath (BMNH C 26232),

Hysteroceras varicosum Hysteroceras orbignyi

are preserved in the matrix of Bed C (iii) which also contains B. concentrica among other bivalves and N. minimus. A rock specimen of the top 5 em of unit C (i.e. Bed C iii) in the Sedgwick Museum, Cambridge (SM B 42267) shows well, the burrow-infill of chocolate-brown clay from Bed B (i) with a lautiform-ribbed Hoplites and an example of Moutonithyris dutempleana 3-4 em below the top of Bed C (iii). Like the top of the underlying Carstone, it is also of spathi Subzone age, but the relatively few ammonites indicate the later part of the Subzone. The thin, irregularly-bedded, fine-grained reddish brown earth - Bed B(i) - separating units C and B is probably an exsolution residue and is present throughout the whole length of the section resting upon an eroded and slightly burrowed top surface of Bed C (iii). Unit B has a complex history of formation. The bedded, pebbly, brick red limestones of Bed B (ii), nodular in places especially north of St Edmunds Point, were originally deposited throughout the section. These limestones contain an indigenous ammonite fauna of late Anahoplites intermedius Subzone age. Today, these limestones occur in situ from about 100 m north of the northern end of the North Esplanade at New Hunstanton, northward to the cliff end at Old Hunstanton; they reach a maximum development in the region 300 m north of North Esplanade. Further south they are represented by cobbles in the boulder beds. This remnant of bedded, pebbly-sandy limestone of intermedius Subzone age, was formerly included by this author in 'Bed' C, but in all the sections examined it occurs above the constant exsolution level capping that unit, marked by the chocolate-brown marl of Bed B(i). The ammonite fauna contained in Bed B (ii) is of considerable interest and includes Euhoplites microceras

THE

UPPER

PART OF THE

CARSTONE

AND

Spath (BMNH C 74989, BGS GSM 87950), E. loricatus Spath (BMNH C 71109) and E. cf. pricei Spath (SM B 42180 Holotype of Hoplites pretethydis Spath), Anahoplites intermedius Spath (BMNH C 76099, C 377la = 'Dimorphoplites altematus perelegans Spath'), A. praecox Spath (BMNH C 76098, 73955 and C 91203 and the coarsely-ribbed form 'Dimorphoplites altematus (Woodward)' C 2552 neotype) and forms with a tendency to lautifonn-ribbing common in the upper part of the intermedius Subzone seen in the Gault facies in southeast England. The ammonites are accompanied by Neohibolites minimus and Birostrina concentrica. The exsolution residual B (i) marks a non-sequence involving the whole of the lower part of the intermedius Subzone. In the contiguous Gault facies in England, intermedius Subzone sediments are relatively thick, show evidence of relatively rapid deposition and, in response to deepening water conditions, overstep more marginal mobile sediments of spathi Subzone age (Owen, 1971: 142; Gallois & Morter, 1982). Although at scattered localities and at individual horizons, uncrushed steinkerns, pyritized nuclei or phosphate-infilled body chambers occur, the bulk of the ammonite fauna is crushed flat. At no locality is a complete ammonite sequence known, although the sequence at Folkestone is the most representative (Owen, 1971). The ammonites of the early part of the intermedius Subzone, characterized by Anahoplites grimsdalei Owen and A. osmingtonensis Owen, are known from scattered localities throughout Europe; for example England (Owen, 1971), France (Owen, 1971; Destombes, 1979), Romania (Pauca & Patrulius, 1960) and in the Kopet Dag, Armenia (Glazunova, 1953). The principal part of the Subzone is well represented wherever sediments of this age are preserved (Owen, 1971). The later part of the Subzone, characterized by the range of the typical Euhoplites pricei Spath and Anahoplites of the intermedius group in which lautiform ribbing becomes more frequent, is much restricted in geographical occurrence. Although well developed at Folkestone in the upper part of Bed II (Bed II (iv) of Owen, 1971), the fauna is largely crushed. The importance of the ammonites from Bed B(ii) at Hunstanton, which are also of late intermedius Subzone age, is that they are preserved as 'solid' steinkerns. Throughout most of the section, these limestones of Bed B(ii) are overlain by a boulder bed derived from former nodular, bedded limestones (Bed B iii) formed during the later part of the Dipoloceras cristatum Subzone and still to be seen in the section (Fig. 3) situated 350 m north of North Esplanade. In this section, the soft interstitial brick-red marl of the nodular limestone seen in Bed B (iii) has been washed out of the upper part of the sequence only to leave a residue of boulders which have been recemented, subsequently, by an indurated brick-red marl. This gives the appearance of two distinct beds for this interval at this locality. Elsewhere, Beds B (iii-iv) consist entirely of reworked boulders resting directly upon Bed B (ii) in the northern part of the cliff section and, at the southernmost end close to New Hunstanton, directly upon unit C. At this

HUNSTANTON

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CHALK

(ALBIAN)

179

southern end of the cliff section, there are some cobbles derived from Bed B (ii) contained within the boulder bed. These boulders and cobbles of Bed B (iii-iv) were produced by a period of strong submarine current scour within the Hysteroceras orbignyi Subzone. The characteristic bivalve of Beds B ( iii-iv) is Birostrina sulcata (Parkinson) which ranges in age from the base of the Dipoloceras cristatum Subzone to the end of the Hysteroceras orbignyi Subzone. The ammonites include Beudanticeras sphaerotum (Seeley), Euhoplites ochetonotus (Seeley), E. solenotus (Seeley), E. sublautus Spath, E. trapezoidalis Spath, E. armatus (1. Sowerby), E. subcrenatus Spath, Epihoplites ('Metaclavites') spp. On balance, the ammonites indicate a late cristatum Subzone age, but there are a few examples of E. inomatus Spath known which indicate the presence of basal Hysteroceras orbignyi Subzone sediments. The top unit (A) of the Hunstanton Red Chalk consists of a number of individual limestone seams and hardgrounds, of variable thickness, separated from each other by thin indurated partings of red marl. The sedimentology of the unit as a whole and the fauna it contains, indicates intervals of shallow water limestone formation interrupted by periods of current winnow or exsolution. These periods of erosion and deposition may reflect the rhythmic sequence seen in the Gault facies further to the south in East Anglia (Gallois & Morter, 1982). Bed A (ii) contains specimens of Mortoniceras and very rare late forms of Semenovites. These are accompanied by the late, varicosum Subzone, form of Birostrina concentrica (Parkinson) which, together with the associated belemnite and bivalve fauna suggests a correlation with the upper part of the Hysteroceras varicosum Subzone developed in the Gault clay facies further south in East Anglia (Bed 14 of Gallois & Morter, 1982: 357, 361). Beds A (iii-iv) contains abundant fragments of large Inoceramus Lissa (Seeley), but no specimens of Birostrina concentrica (Parkinson) or ammonites have been found by the author; it is probably of Callihoplites auritus Subzone age and equivalent to Bed 15 of the clay facies to the south. Bed A (vi) contains specimens of Aucellina coquandiana (d'Orbigny) and Neohibolites praeultimus Spaeth suggesting a Stoliczkaia dispar Zone, possibly Mortoniceras rostratum Subzone, age (Morter & Wood, 1983). The surmounting brick-red marl-seam, formerly included in unit A by previous authors is separable into two distinct thin seams, the higher containing inclusions of nodular ferruginous limestone. The lower seam has yielded belemnites which suggest the possibility of a Cenomanian date and they are considered here to be the basal bed of the Paradoxica Bed Member of the Ferriby Chalk Formation.

5. SUMMARY A more precise picture than hitherto of the lithostratigraphy and ammonitelbivalve biostratigraphy of the Upper Carstone and Hunstanton Red Chalk at Hunstanton is provided in this paper. The records (Owen, 1991) of Hoplites spathi Subzone ammonites in the uppermost

180

H.

G.

Carstone and lowest unit (C) of the Hunstanton Red Chalk are substantiated. The sediments of Bed B (ii) contain rare uncrushed steinkem s of ammonites of late Anahoplites intermedius Subzone age. This fauna, although long known, has hitherto been assigned to a variety of Middle Albian dates. The fauna of Anahoplites of the intermedius group show the initial changes in the rib patterns which foreshadow species of Dimorphoplites. A marked non-sequence of sediments intervenes involving the remainder of the Middle Albian and early Upper Albian. Late Dipoloceras cristatum Subzone sediments were largely reworked within the Hysteroceras orbigny Subzone and rest unconformabl y on earlier sediments. Above the boulder beds, sediments of unit A, formerly thought to be of orbignyi Subzone age, are of Hysteroceras varicosum Subzone age in the lower part. Higher beds in A contain an auritus Subzone bivalve fauna with a possible rostratum Subzone fauna at the top.

OWEN

ACKNOWLEDGEMENTS It is a pleasure to acknowledge the work of Mr Hamon Le Strange, who has collected from the Hunstanton Red Rock since the 1920s and whose collection is housed in the Palaeontology Department of the Natural History Museum, London. I have greatly benefited by discussions with him in the past. My thanks are due also to Chris Wood, Adrian Mortar and Phil Palmer. The late Mr A. G. Brighton and the late Dr David Price of the Sedgwick Museum, Cambridge, gave me full access to the collections held there. Also to Mr P. W. Lamble y, formerly of the Norw ich Castle Museum, for similar facilities. I would also like to thank Mr M. R. Hum for generously pro viding additional ammonites for study and Peter Rawson for his helpful criticism of the original draft of this paper.

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Received 25 February 1994; revised typescript accepted 1 September 1994.