The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences

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stratigraphy of the Kimmeridm2 Clay of the Dotset type area and is%:correlation with some other Kimmeridgian sequences

INSTITUTE OF GEOLOGICAL SCIENCES Natural Environment Research Council

Report 80/4

The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences

B. M. Cox and R. W. Gallois

O Crown copyright 1981

lSBN 0 1l 884182 3

London Her Majesty's Stationery Office

1981

CONTENTS Summary 1 The Kimmeridge Clay in Dorset 1 Introduction 1 Details of the sections 4 Wyke Regis 4 Black Head and Osmington Mills 4 Ringstead B v 9 Kimmeridge area 10 Lithologies 10 Fauna, ammonite distribution and zonation Ammonite distribution and zonation 13 Pictonia baylei Zone 13 Rasenia cymodoce Zone 14 Aulacostephanus mutabilis Zone 14 Aulacostephanus eudoxus Zone 14 Aulacostephanus autissiodorenris Zone 15 Pectinatites zones 16 Pavlovia and jittoni zones 16

13

Correlation with other English sections 18

I

1

l

Introduction 18 Pictonia 18 Rasenia 18 Aulacostephanus and Xenostephanus Amoeboceras 19 Aspidoceras 19 Sutneria 22 Pectinatites 22 Gravesia 23 Pavlovia and Virgatopavlovia 23 Other macrofauna 23

18

Correlation with other European sequences 24 Correlation based on ammonites, notably Gravesia 24 The Kimmeridgian Stage 25 Acknowledgements 28 References 29 Appendix 1 32 Black Head 33 Osmington Mills 34 Ringstead Bay 35 Brandy Bay 36 Hobarrow Bay to Chapman's Pool 39

6 Correlation between the basal beds of the Kimmeridge Clay exposed at Wyke Regis, Black Head, Osmington Mills and Ringstead Bay 9 7 Sketch map of the solid geology of the Kimmeridgc area 11 8 Geological sketch sections of the Kifnmeridge Clay exposed in the cliffs between Brandy Bay and Chapman's Pool 12 9 Generalised Kimmeridge Clay rhythms 13 10 Ammonite ranges and zonal schemes applied to thc beds adjacent to the Lower-Upper Kimmeridgian boundary 17 11 Sketch map of the Kimmeridge Clay outcrop and subcrop in England 19 12 Correlation of the Lower Kimmeridge Clay sequence of the Dorset coast with those of the Warlingham Borehole and the Wash area 20 13 Correlation of the Upper Kimmeridge Clay sequence of the Dorset coast with those of the Warlingham Borehole and the Wash area 21 14 Ammonite ranges and lithologies adjacent to the eudoxus-autissiodorem's zonal boundary at Kimmeridge Bay and in the Warlingham Borehole h 15 Palaeogeography in Lower Kimmeridgian times showing the positions of outcrops referred to in the text 27

TABLES

1 Zonal schemes : past and present 3 2 Correlation between Dorset and south-west Germany sequences 25 3 Stage names : past and present 26

FIGURES

1 Kimmeridge Clay outcrops in the Dorset type area 2 2 Generalised vertical section of the Lower Kimmeridge Clay in Dorset 5 3 Generalised vertical section of the Upper Kimmeridge Clay in Dorset 6 4 Sketch map of the solid geology of the Black ~ead-Ringstead Bay areas 7 5 Correlation between the Kimmeridge Clay sequences exposed at Black Head-Osmington Mills, Ringstead Bay and Kimmeridge Bay 8

Bibliographical reference Cox, B. M. and GALLOIS,R. W. 1981. The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences. Rep. Znrt. Geol. Sci., No. 8014. Authors B. M. Cox, B.Sc. and R. W. Gallois, B.Sc., F.I.M.M.. Institute of Geological Sciences, Exhibition Road, London SM.7 2DE.

The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences B. M. Cox and R. W. GALLOI~ SUMMARY The Kimmeridge Clay of the type area crops out in cliff sections at Kimmeridge, Osmington Mills, Ringstead Bay and Wyke Regis in south Dorset. At Kimmeridge, the sections, although continuous, expose only the upper part ofthe formation. At Osmington Mills and Ringstead Bay, a section through the lower part ofthe Kimmeridge Clay and through part of the remainder of the formation can be determined, althoLigh only in isolated exposures which are separated from one another by landslips and complicated by tectonic structures. At Wyke Regis, only the basal beds are exposed. Descriptions and graphic sections are given for the exposed sequences. Comparison of the Dorset succession with those elsewhere in England has shown that the lithological and faunal sequences are similar throughout the country. A number of distinctive thin marker-bands have been recognised in Dorset, and these are correlated with similar bands recorded from the Warlingham Borehole, Surrey, and from boreholes in East Anglia. The ammonite faunas are discussed and the zonal scheme in current use is reviewed. It is suggested that the abundance of Gravesia has been underestimated in the past and that consideration should be given to the introduction ofa new Zone of Gravesia S@. The base ofsuch a zone would mark the Lower-Upper Kimmeridgian boundary. It would also form an important link with Tithonian and Volgian sequences elsewhere in Europe. Relationships among the Kimmeridgian, Portlandian, Tithonian and Volgian stages are discussed. It is recommended that most of the Kimmeridge Clay of the type area should be included in the Kimmeridgian Stage, and that the use of the term Kimmeridgian sensu gallico (which places only the Lower Kimmeridge Clay in the Kimmeridgian Stage) should be abandoned. THE KIMMERIDGE CLAY IN DORSET INTRODUCTION The Kimmeridge Clay was first recognised as a discrete division by William Smith on his map of 1815, where it was named in the table of strata as the Oaktree Soil. Subsequently (1817), he used the name Oaktree Clay for the clays lying between the 'Coral Rag and Pisolite' and the 'Portland Rock'. Webster (1816) provided the first descriptive details of the formation and changed the name to Kimmeridge Clay after Kimmeridge Bay in Dorset. The Kimmeridge Clay has an outcrop and subcrop in England which extends from Dorset to North Yorkshire. I t is poorly exposed inland and almost all our understanding of the stratigraphy of the formation has been made through study of the coastal sections between Weymouth and Swanage in Dorset (Figure 1) and borehole sequences elsewhere. Fitton (1836) described the general faunal and lithological characters of the formation in the Dorset cliff sections and further details

were added by Waagen (1865), Blake (1875), Woodward (1895) and Strahan (1898). As with most Jurassic strata1 terms, it can be argued that the original definition of the Kimmeridge Clay was that of a chronostratigraphical unit, in that it embraced a lithological and faunal assemblage bounded by what were believed to be isochronous surfaces. D'Orbigny (1842-1851, p. 610) introduced the term 'l'etage kimmeridgien' to cover the interval of time during which the Kimmeridge Clay was deposited and in recent years, most British workers have treated the terms Kimmeridge Clay and Kimmeridgian as if synonymous. The following notes are provided to clarify the position of these terms and their derivatives (Lower Kimmeridge Clay, Lower Kimmeridgian, and others) within the current system of stratigraphical nomenclature (Hedberg 1976; Holland and others, 1978). The Kimmeridge Clay (Formation) is an argillaceous lithostratigraphical unit which, in south Dorset, ranges from about 300 m to more than 500 m in thickness. The lithology of the formation is described on p. 10. The position of its base has remained unchanged since Waagen (1865) defined it at the base of a clay with fossils including 'Rhynchonella' inconstans, resting on a clay with iron concretions, in cliff sections near Wyke Regis (Figure 1). The base of the formation is also exposed at Ringstead Bay, 9 km north-east of Wyke Regis, where clays with 'R.' inconstans overlie a thin, ironshot, muddy limestone with corals (the Ringstead Coral Bed of Arkell (1929, p. 10) ), (see also Pictonia baylei Zone, p. 13.) Here, and in the adjacent sections at Osmington Mills and Black Head, the boundary lies at a lithological change from the mixed lithologies of the Corallian Beds to the more uniform, soft mudstones of the Kimmeridge Clay. This lithological change is accompanied by a general change in the macrofaunal assemblages. The Ringstead Coral Bed and its equivalents contain the last of the typical 'Corallian' fauna, characterised by corals, echinoids, gastropods, and large, commonly coarsely ornamented, bivalves. The abruptness of the faunal and lithological change is due to a minor, but widespread, erosion surface at this level. The base of the formation is coincident with the base of the Kimmeridgian Stage (see p. 3 for details). Brookfield (1978) has recently revived a suggestion of Blake (1875) that the basal part of the Kimmeridge Clay should be grouped with the top part of the Corallian Beds to form a 'Passage Beds Formation'. Arkell (1933, p.385) dismissed Blake's suggestion as impractical and retained Waagen's (1865) definition of the base of the formation. Although Brookfield cited faunal, lithological and mineralogical evidence to support the case for the new formation, his main contention was that the Kimmeridge Clay (mutabilis Zone and above in his definition) is composed of uniform black shales with an impoverished macrofauna and microfauna, whereas the 'Passage Beds' are dominantly arenaceous and contain a typical Corallian fauna. These observations are not in

~ l

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Corfs Castle

Fault. crossmark on downthrow s~de

r(z General dip an degrees

- 70

PORTLAND

60

Figure 1 Kimmeridge Clay outcrops in the Dorset type area

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I

accord with those of the present authors who regard the lithologies and faunas of the Kimmeridge Clay part of the 'Passage Beds' as typical of the Kimmeridge Clay. Throughout the baylei to eudoxus zones, the Kimmeridge Clay is made up of rhythmic alternations of silty mudstone, dark grey mudstone and pale grey, very calcareous mudstone (see p.10 for details) with a fauna that is similar in character to the remainder of the Kimmeridge Clay. The position of the upper limit of the Kimmeridge Clav in Dorset is more controversial. Arkell (1933, pp.443-6) summarised the reasons given by authors, from Fitton (1836) to Cox (1929), for their particular choices of a boundary within what is clearly a lithologically transitional sequence from clay to silt and sand. Arkell (1933; 1947) placed the boundary at the base of the Massive Bed, a prominent bed of sandstone exposed in Houns-tout cliff [SY 9505 77201, and most subsequent authors have followed this. Townson (1975) lowered the boundary to the base of the Rhynchonella Marls but Cope (1978) and Wimbledon and Cope (1978) continued to use the Massive Bed. This part of the sequence is poorly exposed and has not been remeasured in the present work; for the purpose of comparison with earlier literature, the top of the Kimmeridge Clay has been taken at the base of the Massive Bed. It seems likely however that Townson's (1975) position for the boundary will become generally accepted in the future because it is based on detailed sedimentolo~icalwork. At outcrop outside Dorset, theuhighest beds of the Kimmeridge Clay have been removed and the base of the overlying Portland Beds (or their equivalents) rests on an erosion surface. Blake (1875, p.197) was the first to divide the formation into Lower Kimmeridge Clay and Upper Kimmeridge Clay. I t has proved useful for descriptive purposes to have these terms available because of the substantial thickness of the Kimmeridge Clay in its type area; they have been widely used by subsequent authors such as Arkell (1947) and Cope (1967). Both divisions consist of relatively uniform soft mudstones, but they are easily distinguishable on the basis of their ammonite

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2

3 m

4

i 80

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Minor faults and folds omitted

5km l

Vert~callimb of Purbeck monocl~ne

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s 90

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7

faunas. The Lower Kimmeridge Clay is characterised br species of Pictonia, Rasenia and Aulacostephanus and &c Upper Kimmeridge Clay by species of Pectinatitri. Pavlovia and related forms. Ammonites are abundant at the level of the Lower-Upper Kimmeridge Cla? boundary and the distinction between the two assemb lages can be made even from very small shell fragments (see p.16 for discussion). Elsewhere in southern England, this boundary has proved readily identifiable in boreholes and its outcrop position can be determined even in areas of poor exposure. Arkell (1947) introduced terms for nearly all tbc subdivisions of the Kimmeridge Clay at and adjacent to Kimmeridge Bay (for example, Cattle Ledge Shales, Hen Cliff Shales, Maple Ledge Shales, Gaulters Gap Shales, Washing Ledge Shales), defining each as tbc stratum between particularly prominent, and usually named, thin limestone bands. These names have m usefulness outside the type section, and it is suggested that they should be abandoned. D'Orbigny's original definition of the Kimmeridgiam Stage was subsequently refined by Salfeld (1913) to encompass a number of fossil zones. Arkell ( 1933; 1945: 1947; 1956), Callomon (in Torrens and Callomon, 1968; in Callomon and Cope, 1971), Casey (1967), Cope (1967; 1978), Neaverson (1925), Spath (1935; 1936) and Ziegla ( 1962a; 1964) have modified Salfeld's zonation to produce the present zonal scheme, based on ammonite (Table 1). The boundaries of most of the lower zones have not been precisely specified in the sections because tbt published descriptions of the Lower Kimmeridge Clay h Dorset are mostly of generally poorly correlated short sections, which give only an approximate stratigraptpI n modern terminology, these zones are range or assemblage biozones (biostratigraphical units), because they have been defined in terms of the ammonite fauna and are unrelated to the lithological sequence. The?cannot be used alone therefore to define the Kimmeridgian Stage because 'stages' are now considered to be units of chronostratigraphy (Hedberg, 1976). However. in fossiliferous marine clays such as the Kimmeridge

Table 1 Zonal schemes for the Kimmeridgian Salfeld 1913'

Neaverson 1925

Arkell 1933

Spath 1935; 1936

Arkell 1945; 1956

Perisphinctes pallasianw

Holcosphinctes pallasioides

Zonally unclassified

Paulouia pallasioides

Pavlouia pdlasioidcs

Ziegler , Casey 1967' Callomon 1962a; 1964 Cope 19673 ' 1968; 1971

Pavlovia rotunda

Virgatopaalovit jttoni

Paulouia pallasioides

Pavlovia rotunda

Pallasiceras rotundurn

Paulouia rotunda

Paulouia rotunda

Pavlovia rotunda

Pavlovia pallasioides

Paulouia pallasioides

Pectinatites pectinatus

Pectinatites pectinatus

Pectinatites pectinatus

~cctinatites pectinatus

Pectinatites pectinatus

Pectinatites pectinatus

Virgatosphinctoides whcatleymcis

Pectinatites hudlestoni

Subplanites ~ p p .

Pectinatites wheatleymcis

Virgatosphinctoides Subplanifcs nodifcm wheatlcymris

Virgatites rniatschkouiemis

-----

Vir atosphinctoides Virgatosphinctoides Subplanites nh+ms wheatlcyensis grandis

-----

Virgatosphinctoides Subplanites ~ p p . Subplanites wheatlcymris uimincus? Grauesia irius

Gravesia irius

'Gravesiazones'

Gravesia gigas

Pcctinatites scitulus Pectinatites elegans

Grauesia gigas

Grauesia grauesiana

Grauesia grauesiana

Aulacostephanus pseudomutabilis

Aulacostcphanus pseudomutabilis

Aulacostephanus pseudomutabilis B A . undorae

Aulacostephanw yo

Aulacostephanus YO

Aulacostephanusyo B Aspidoceras longispinurn

Rasenia mutabilis

Pararasenia mutabilis

Pararasmia mutabilis

Rasmia mutabilis

Aulacostc&nus mutabilis

Aulacostephanus mutabilis

Rasenia cymodoce

Rasmia cymodoce

Rmia uralensis B Amoebites kitchini

Ramia cymodoce

Rarmia uralensis

Rasenia cymodou

Pictonia baylei

Pictonia baylei

Pictonia baylei G3 Rmia cymadoce

Pictonia baylei

Pictonia baylci

Pictonia baylei

O

X

F

O

Cope 1978

R

D

I

A

Grauesia graucsiana Aulacostephanus pscudomutabilis

Aulacostcphanus autissiodorensis

Aulacostephanus autissiodorensis Aulacostephanus cudoxus

N

'

This sequence represented the whole of Salfeld's Kimmeridgian Stage and the lower part of his Portlandian Stage (see Figure 10). Pavlouia zones only. Pectinatites zones only. Note:

Horizontal lines do not imply precise correlation between zonal boundaries.

study in a single, gently dipping section in which the zonal boundaries have been more accurately determined (Cope, 1967; 1978). Although as defined by Cope these zones are biozones, some are probably also chronozones. Until the situation is clarified and the whole of the Kimmeridgian Stage can be divided into chronozones it is suggested, in the interests of consistency of usage, that all the zones retain their italicised fossil names. In recent years, the Kimmeridgian has been divided into two substages (Lowerand Upper). The base of the Upper Kimmeridgian was taken by Cope (1967, p.4) at the base of the Grauesia zones of Arkell (1956). Arkell (1947, p. 67) believed this division to be in accord with the most natural grouping of the ammonites; he had earlier and again subsequently (Arkell, 1945; 1956) used this level as the boundary between a Lower and Middle

Clay where correlations have been achieved in the past almost entirely by the use of fossils, the practical distinction between biostratigraphy and chronostratigraphy is slight. Recent work on the Lower Kimmeridge Clay of East Anglia (Gallois and Cox, 1976) has shown that minor erosion surfaces occur at the bases of the baylei, cymodoce, mutabilis and eudorrs zones. These same erosion surfaces, and the lithological and fauna1 marker beds that enable them to be recognised, have been recorded in Dorset in the Dresent work. The authors believe that these erosion surfaces can be regarded as isochronous for all practical purposes, and therefore enable these zones to be elevated to the status of chronozones. By contrast with the Lower Kimmeridge Clay, the whole of ihe--Upper Kimmeridge Clay is available for 3

substage in a threefold division of the stage. Cope (1967, p. 68)proposed that the use of Gravesia as a zonal index should be discontinued because of its rarity in Dorset, and placed the substage boundary at the junction of the autissiodorensis and elegans zones. In the type section, this boundary was believed to be coincident with a limestone rhat forms a convenient lithological marker (Bed 42 of Blake, 1875). The present work has reaffirmed the usefulness of Gravesia and has suggested that the zonal scheme and the substage boundary might require revision (see p.25 for details). The purpose of the present work has been to bring together new information on the faunal and lithological sequences observed on the Dorset coast and to compare this information with that from cored boreholes elsewhere in England. Graphic sections have been drawn (Appendix 1) in the hope that these will provide a framework fi5r future palaeontological, sedimentological and mineralogical research on the type sections.

DETAILS OF THE SECTIONS The Kimmeridge Clay crops out in two main coastal areas of Dorset. At Wyke Regis, Osmington Mills and Ringstead Bay (Figure l), a number of small exposures, disturbed by tectonic structure and landslip, expose most of the Kimmeridge Clay. In the Kimmeridge area, between Brandy Bay and Chapman's Pool, a broad anticline exposes beds ranging from the middle part of the tudoxus Zone to the top of the Kimmeridge Clay along about 8 km of cliffs in continuous sections that are largely unaffected by structural complication. Arkell (1947, pp. 80-85) described the broad stratigraphical features of the Osmington-Ringstead sections, and suggested that the total Kimmeridge Clay thickness in that area was about 800 ft [244 m], compared to his estimate of about 1650 ft [503 m] in the Kimmeridge Bay area. The Kimmeridge Clay is poorly exposed inland in Dorset. Between Osmington and the Ridgeway Fault at Portesham (Figure l), the soft mudstones of the formation give rise to a poorly drained clay vale lying between the escarpments of the Corallian Beds and the

Portland Beds. At the western end of this outcrop, near Abbotsbury, the lowest part of the formation is represented by a sandy limonite oolite (the Abbotsbury Iron Ore). The stratigraphical sequence of the Kimmeridge Clay in Dorset is summarised in Figures 2 and 3. Wyke Regis The lowest part of the Kimmeridge Clay (baylei and cymodocG zones) is exposed at Wyke Regis in low, slipped and weathered cliffs at East Fleet [SY 661 7661 and near Sandsfoot Castle [SY 671 7701 (Figure 1). Both sections are protected from marine erosion, the former by Chesil Beach and the latter by Portland Harbour breakwater, and are consequently degraded. Nevertheless, with some digging, almost continuous sections can be measured through the baylei and lower cymodoce zones at both localities. In the 19th century, the Sandsfoot section was the best available exposure of the Corallian Beds Kimmeridge Clay junction and for this reason it was used by Waagen (1865) for the first definition of this boundary. At present, the most prominent single bed in the Wykc Regis sections is a thin siltstone crowded with myid and other bivalves. Birkelund and others (1978, pp. 35-36) have recorded the sequence of raseniid faunas ( c y w t h z Zone) of the Fleet section and confirmed the earlier observations of Arkell (1947, p.88) and Morris (1968. p.8) that the base of the zone is marked by this siltstone which they named the Wyke Siltstone. A bed of similar lithology, occurring 1-2 m above the Wyke Siltstone, has been recorded in the same sections and at Black Head and Osmington Mills: for ease of stratigraphical reference, the higher bed is here named the Black Head Siltstone. B!ack Head and Osmington Mills The most complete section of Lower Kimmeridge Clay in Dorset is exposed in slip-faces on the east side of a prominent ridge running down from the highest point of Black Head [SY 7258 82001 (Figure 4, section 1) to the beach [SY 7259 81921. Viewed from Osmington Mills, Sca1e:l to 1000

Mudstone, undifferentiated; mostly dark grey

Mudstone, very calcareous (pale grey)

Silty mudstone and siltstone

N.B. Thicknesses for the Lower Kimmeridge Clay are based on Kimmeridge Bay (down to tha Hobarrow Bay Stone Bandland the Broad Bench No.1 Borehole with faunal datril transposed from Weymouth area sections.

Oil shale

- 1

Tabular bed Dolomitic limestone (cementstone

dD 0

Concretions

W

W

Bivalve - r i c h

db

Rhynchonellid brachiopods

W

L 0

Individual oil-shale seams and limestones not to scale Corresponding Bed Number in East Anglia (Wash area boreholel

VERl RANGE OF

Figure 2 Generalised vertical section of the Lower Kimmeridge Clay in Donet

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VERTICAL RANGE OF SECTIONS

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Virgatopavlovia fittoni Zone(prs1 . F -

silty mdstone and muddy siltstone

Pavlovia

Rotunda Nodules

Pectinatites pallasioides Zone

A

Freshwater Steps Stone Band

Middle White Stone Band A White Stone Band

Basalt Stone Band

*

E; m

S

n

Rope Lake Head Stone Band Blackstone

j

E r

Y 5 m Grey Ledge Stone Band

Cattle Ledge Stone Bend

Yellow Ledge Stone Bend

Blrkef Bed 42

V V V Zone (pars)

Figure 3 Generalised vertical section of the Upper Kimmeridge Clay in Dorset

crossmark on downthrow side

Fault.

_L-

m

Kimmeridge Clay

@

Measured section

2 kilometres

Figure 4 Sketch map of the solid geology of the Black Head - Ringstead Bay areas (based on Arkell, 1947, pl. XIX) showing the positions of sections referred to in the text

these slips appear to form an almost continuous black cliff from which the head takes its name. There are scattered exposures in smaller slip faces to the west of Black Head and eastwards to Osmington Mills village, and these provide additional details to the main section. Few prominent marker bands are visible from the beach but, in the middle and upper part of the ridge, a line of cementstone doggers in the mutabilis Zone, the Virgula Limestone and the Nannocardioceras Cementstone form useful datum planes (Figure 5). The eudoxus Zone above the Nannocardioceras Cementstone and the autissiodorensis Zone are poorly exposed here, but much of the sequence can be revealed by digging. Dips are generally steep throughout the section, being about 80" near the base of the Kimmeridge Clay and decreasing to 50" to 60"in the upper part of the section. Camber and landslip add to the difficulties of making accurate thickness measurements. Much of the lower part of the section is obscured by a thin crust of weathered clay, but in dry weather this can be readily cleared to expose clean sections of essentially unweathered material, often with beautifully preservecl calcareous fossils. On the western side of Black Head for a distance of about 300 m [SY 7259 8192 to 7229 81981, there are small exposures, mostly in the baylei, cynrodoce and mutabilis zones, which can be correlated with the main section. These sections, although varying from year to year as new cliff falls occur, often show continuous

exposures through the baylei Zone and much of the cynodoce Zone. There is little overlap between the Lower Kimmeridge Clay exposed at Black Head and that in Kimmeridge Bay; the beds in the range of overlap are considerably thinner at Black Head and, for the most part, poorly exposed (Figure 5). To the east of Black Head, almost as far as Osmington Mills village, there is a large landslip. Along its eastern edge [SY 7336 81861, a number d adjacent exposures, although complicated by faulting and steep dips, provide an almost continuous section from the base of the Kimmeridge Clay to a level within the upper part of the eudoxus Z.one. At the present time, the baylei and cymodoce zones are well displayed here and in a fault-bounded mass of Kimmeridge Clay at beach level [SY 7342 81741 (Figure 4, section 2). Dips in this latter exposure are steep (70" to 80") and, along the northern margin of the outcrop, clays low in the mutabilis Zone are faulted against Corallian Beds. The Corallian Beds-Kimmeridge Clay junction is well exposed with Torquirhynchia inc01utans U.Sowerby) and Pictonia common in the lowest beds of the Kimmeridge Clay. This part of the sequence can be matched in detail with that at Ringstead Bay, although at Black Head and Osmington Mills, the Ringstead Coral Bed is replaced by a shelly oolitic ironstone (Figure 6).

Kimmeridge Bay end adjacent c l i f f s west of Clavell's Herd Grey Ledge S.B.

I Cattle Ledge S.B

Black Head and Osmington Mills

/ /

Ringstead Bav

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1

1

/

Blake's Bed 41 Blake's Bed 42

Freshwater Steps S. B.

I

Washing Ledge S.B.

Nannocardioceras Cementstone -----

L- -

===% Virgula Limestone

Hobarrow Bay S. B.

I I I l

I I

I

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I I

I

I I I not exposed:l I I

3 S

]

AI-1

Supracorallina Bed

E

thickness not

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I

precisely known

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I

""

lnconstans and Nana beds Ringstead Coral Bed

!I I

t

I

I I

KEY

r 30 metres 25

I

I

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1

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I

Pictcnnia ...baylei ~ ~ n eV t , , , ,

I

Beds below Hobarrow Bav Stone Band proved in Broad Bench No.1 Borehole

Lithologies as Fig.2

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!

S-S-S-s Saccocoma -rich band

1

lnconstans and Nana beds

Ringstead Waxy Clay (Corallian Beds)

Figure 5 Correlation between the Kirnrneridge Clay sequences exposed at Black Head - Osrnington Mills, Ringstead Bay and Kimmeridge Bay

WYKE REGlS (~ANO~FDOT SY67067700

KEY Siltstone

m m

Oolitic (limonitel mudstone Sandy mudstone

Oark grey , .m du

0

1,

Limestone Erosion surface w i t h phosphatic pebbles and burrowing Scale

I' \

'.

MILLS

not exposed

(Y

D C

N

..

Pale grey mudstones with thin tabular clay ironstone