Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district

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Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district John C.W. Cope * Department of Geology, National Museum of Wales, Cardiff CF10 3NP, UK

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 March 2013 Received in revised form 16 July 2013 Accepted 19 July 2013 Available online xxx

An account is given of a Geologists’ Association meeting in the Isle of Purbeck held on 28th–30th September 2012 and the stratigraphy and structures of the rocks examined during the weekend are described. Uppermost Jurassic Stage nomenclature and recent changes to stratigraphical nomenclature in the uppermost part of the Kimmeridge Clay Formation are discussed and the conclusion reached that the long-established divisions (Members) of this Formation are both readily recognisable and have nomenclatorial priority. The recent change to the position of Pallasioides-Rotunda zonal boundary ignores the ammonite fauna and is inappropriate. For the Lulworth district the stratigraphy of the uppermost Jurassic (Portlandian) through Lower and Upper Cretaceous formations are described and their associated structures discussed. The coastal evolution of the Lulworth coast is briefly discussed. ß 2013 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Dorset coast Upper Kimmeridge Clay Upper Jurassic Cretaceous

1. Friday September 28 Most of the Members attending the weekend field meeting were present at the library in Wareham, Dorset for an introductory talk by the Director, who presented an outline of the stratigraphy and structure of the area and explained the occurrence of oil in the Isle of Purbeck. The fieldtrip was arranged to follow the publication of the new Geologists’ Association Guide to the Dorset Coast (Cope, 2012). This particular weekend had been chosen because of the ideal tides and it was not until after fixing this date that it was discovered that this was one of the few weekends when the Lulworth Ranges would be closed to the public; thus the Fossil Forest (Fig. 3) would not be accessible. 2. Saturday September 29 The following morning saw the group assembled in bright sunshine on the steep-sided Chalk ridge some 5 km SSW of Wareham. From here the view to the south took in the northern limb of the Purbeck Anticline. On the coast to the south cliffs of Kimmeridge Clay were visible around Kimmeridge and above the village the change of slope on Smedmore Hill where the Portland Sand succeeded the Kimmeridge Clay was clear. Quarries in the overlying Portland Stone were visible on that hill. To the southwest we could see the dip slope of the basal part of the Purbeck Group, and inland from Gad Cliff the scalloped edge of the dip slope

* Tel.: +44 029 2057 3164. E-mail address: [email protected]

could be made out by the cliff-line. In the valley below us the Purbeck Group is succeeded by Wealden Group and Lower Greensand and westwards the deserted village of Tyneham and Worbarrow Bay were clearly visible. The slope below our position is formed of Upper Greensand that, in this locality, overlies the Gault (the top of the latter marked by a line of rushes) and the Upper Greensand in turn is overlain by the Chalk (Table 1). The dip progressively steepens northwards on the northern limb of the Purbeck Anticline, the hog’s back of the Chalk (here vertical) being formed as a result of its outcrop overlying the trace of the pre-Albian Purbeck Fault. This Late Cimmerian fault throws down to the south, whereas on the footwall to the north most of the Upper Jurassic and all pre-Gault Cretaceous rocks were absent (largely through non-deposition). The missing rocks are thickly developed on the hanging wall to the south presenting clear evidence that the fault was active during the Upper Jurassic and Lower Cretaceous. Looking north, the view was over the Frome Syncline, a flat-lying structure in Palaeocene and Lower Eocene strata that is succeeded northwards by the low hills of the Chalk as it emerged from beneath the syncline on the southern margin of Salisbury Plain. The Frome Syncline is effectively the western end of the Hampshire Basin. So underneath the Tertiary succession is the Chalk, Upper Greensand and Gault, below which are much earlier rocks, most probably Oxford Clay, Kellaways and Cornbrash formations. The location of the Wytch Farm oilfield (carefully screened) was pointed out and beyond this the view took in Poole Harbour and the Poole-Bournemouth conurbation. The party then took the toll-road to the cliff-top car park at Kimmeridge (Fig. 1). The Director pointed out the nodding donkey at the site of the Kimmeridge oil-well on the crest of the Broad

0016-7878/$ – see front matter ß 2013 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pgeola.2013.07.004

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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Fig. 1. Outline map of the coast around Kimmeridge. Modified after Callomon and Cope (1993). Inset shows location of the Isle of Purbeck.

Bench Anticline on the west side of Kimmeridge Bay, the only productive site on the hanging wall of the Purbeck Fault. The oil reservoir here is in fractured Cornbrash, met at 550 m depth. Although the oil, which is sourced in the lower part of the Lias Group, initially flowed under its own pressure, for many years it has required pumping. Since 1960 the total yield has been some three million barrels of oil. Nowadays the yield is some 65 barrels per day which is taken to Wytch Farm by road. To the east of Kimmeridge Bay the Clavell Tower on Hen Cliff had been moved from the cliff edge in 2007–8 and rebuilt some 25 m inland. The party walked eastwards towards Hen Cliff and the Director explained that the quay at the base of the cliff was originally built for the export of the Kimmeridge ‘Coal’ — a rich oilshale — to France in the 19th century, where its gas was used for street lighting in Paris. Under Hen Cliff (Fig. 2) the party then stopped at a marked double cementstone band, Blake’s Bed 42 (Blake, 1875). This, the Director explained, marked the base of the Bolonian Secondary Standard Stage and contains fragments of the pectinatitid Table 1 The geological succession in the Isle of Purbeck. Thickness figures are the maximum figures obtaining within the area studied. Note that the basal few metres of the Purbeck Limestone Group are of Jurassic age. Upper Cretaceous

White Chalk Subgroup

Grey Chalk Subgroup Lower Cretaceous

Wealden Gp. Purbeck Lst. Group

Upper Jurassic (pars)

Portland Group

Newhaven Chalk Formation (seen to) 30 m Seaford Chalk Formation 70 m Lewes Nodular Chalk Fm 80 m New Pit Chalk Formation 80 m Holywell Nodular Chalk Fm 40 m Zig Zag Chalk Formation 70 m Cenomanian Basement Bed 1 m Upper Greensand Formation 25 m Gault Formation 15 m Lower Greensand Fm 0–15 cm Wealden Group 150 m Durlston Formation 20 m Lulworth Formation 25 m Portland Stone Formation 30 m Portland Sand Formation 35 m Kimmeridge Clay Fm (base not seen) 500 m

Fig. 2. The succession of the Kimmeridge Clay between Kimmeridge Bay and St Aldhelm’s Head. Modified after Callomon and Cope (1993).

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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Fig. 3. The western face of Hounstout Cliff showing the seepage lines defining the boundaries between the uppermost members of the Kimmeridge Clay Formation and the Massive Bed at the base of the Portland Sand Formation. Modified after Cope (2012).

ammonite Pectinatites (Arkellites), which are better preserved and complete in the succeeding shales. Because of extreme provincialism of the ammonite faunas above the Kimmeridgian Stage, the faunas of the overlying (Primary Standard) Tithonian Stage do not occur in Britain and direct correlation is impossible. When d’Orbigny set up the stages of the Jurassic System in the middle of the 19th Century he had proposed a Kimmeridgian Stage overlain by a Portlandian Stage (d’Orbigny, 1842–51). As he had equated the Kimmeridgian Stage with the Kimmeridge Clay of Britain and the Portlandian Stage with the Portland Sand and Stone of Britain, the interpretation of these stages seemed totally unambiguous to British geologists. However, in naming ammonites characteristic of the Portlandian Stage, d’Orbigny listed Ammonites gigas, irius and gravesianus. These ammonites were not known in Britain at that time, but J.F. Blake (1881), working on correlation of the Kimmeridgian and Portlandian rocks of Britain with those of northern France, realised that those ammonites occurred in France in older rocks than the British Portlandian, but on the basis of their contained ammonite species the French were calling them Portlandian. Thus arose a dual meaning of the Kimmeridgian and Portlandian Stages. The French recognised a short Kimmeridgian overlain by a long Portlandian, whereas in Britain we had a long Kimmeridgian overlain by a short Portlandian. The suffixes sensu anglico or sensu gallico were needed for the best part of a century to clarify the interpretation used. However, Blake had realised, as early as 1881, that action was needed to end this confusion and proposed the Bolonian Stage as equivalent to The Upper Kimmeridgian sensu anglico and the Lower Portlandian sensu gallico. Blake even (1881) referred to the Lower Bolonian as the zone of Ammonites gigas. In 1913 Salfeld discovered these species of ammonites in the Kimmeridge Clay and referred them to his genus Gravesia; the genus is now known to range in Dorset from the Autissiodorensis Zone to the Scitulus Zone. Meanwhile over much of the world, a thick limestone facies occurs at the top of the Jurassic, quite unlike the north-western European successions. To this limestone facies the name Tithonian Stage (Oppel, 1865) was widely applied. In some areas it contains species of Gravesia so that its base correlates rather approximately

with the base of Blake’s Bolonian Stage. Close correlation is, however, not possible at this level and above it the ammonite faunas are totally distinct, so that Tithonian fossils are unknown in northwest Europe. The Tithonian Stage was internationally ratified as the terminal Jurassic Stage in the early 1990s, with the Kimmeridgian Stage beneath it. The Kimmeridgian was thus internationally ratified in its shorter sense and its use in the longer (sensu anglico) sense can no longer be countenanced, although the British Geological Survey (e.g. Barton et al., 2011) has continued to do so. Recognising that correlation with the Tithonian was not yet possible Cope (1993) proposed that Blake’s Bolonian Stage and d’Orbigny’s Portlandian Stage be recognised as Secondary Standard Stages for northwest Europe, broadly equivalent to the Tithonian Primary Standard Stage, until such time as correlation may become possible. The base of the Bolonian Stage was to be drawn at the base of the Elegans Zone at the base of Blake’s Bed 42 at Hen Cliff; the base of the Portlandian had already been defined at the base of the Albani Zone at the base of the Massive Bed on Hounstout Cliff (Wimbledon and Cope, 1978). Following the ratification of the Tithonian Stage as the Primary Standard terminal Jurassic Stage, the Berriasian became the International Standard for the basal Cretaceous Stage. The Volgian Stage, originally proposed by Nikitin (1881) for rocks of in part equivalent age to the Tithonian in Russia and Poland, was modified by Gerasimov and Michailov (1986) to make its base approximately coincident with the base of the Tithonian. This was as a political move to advance the Volgian as a contender for the Primary Standard terminal Jurassic Stage. The use of Volgian was also advocated in Britain by Casey (1971, 1973) and a subsequent publication by Riley (1977) promoted the Volgian for use by exploration companies in the North Sea. This was particularly unfortunate since, as shown by Cope (2008) all the uppermost Jurassic ammonites of the North Sea are typical Bolonian and Portlandian species and there are no known Volgian forms. The only Volgian ammonites known from Britain are from eastern England and the North Sea; however, these are entirely Upper Volgian species, which are now known to be of earliest Cretaceous (Early Berriasian) age (Cope, 2008). The fact that the Volgian Stage

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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is now known to be in part Jurassic and in part Cretaceous in age disqualifies it as a valid unit, since a stage cannot belong to two systems. This was indeed recognised by Stratigraphic Commission of the Russian Federation who have withdrawn the Volgian Stage from general use (Zhamoida and Prozorovskaya, 1997). The Kimmeridge Clay of Hen Cliff, like much of the Kimmeridge Clay shows clear cyclicity, as first recognised by Downie (1955). An idealised cycle starts with a clay (A), frequently with a benthic fauna and with an organic content of up to 10%. Member B of the cycle is bituminous clay with 10–30% organic matter. C is an oil shale; with up to 70% organic matter and D is a coccolith limestone. Many cycles are incomplete and on Hen Cliff the cycles are principally of the ABAB type. The origin of the cycles has been attributed to Milankovitch cyclicity: both longer wavelength orbital obliquity cycles (41 ka) and shorter wavelength precession cycles (21 ka) (Weedon et al., 2004). These authors recognised 95 longer wavelength cycles for the Kimmeridgian and 103 for the Bolonian giving respective durations of 3.6 Ma and 3.9 Ma. There had been much debate about water depth and the anoxic conditions required for the accumulation of oil-shales in the Kimmeridge Clay. The consensus of opinion is that the rocks accumulated in a stratified water column which allowed periodic anoxia to develop on the sea-bed. Water depth was not great, as the margins of the Kimmeridge Clay deposition seem little different from those of the preceding Corallian Group; additionally, storm activity was able to produce rip-up clasts of the oil-shale at various horizons, showing that even in the basinal area of Kimmeridge the sea-bed was not below storm wave-base. The stone bands in the cliff (apart from the coccolith limestones) were largely secondary in origin. They had formed under burial in the methanogenic zone and were principally dolomites (Irwin et al., 1977; Scotchman, 1989); such stone bands were laterally impersistent, whereas the coccolith limestones were persistent laterally. In the ledges exposed at the low state of tide crushed ammonites of the Elegans Zone belonging to the genus Pectinatites were seen, but no specimens of Gravesia, which is a rare ammonite, were found. Continuing on eastwards the party stopped on the next stone band, the Yellow Ledge Stone Band, cut by a small fault with a fault breccia developed along it. This stone band is a secondary dolomitic band and marks the base of the succeeding Scitulus Zone. The Director was able to demonstrate from the abundant ammonites in the ledges of shale above Yellow Ledge that the ammonites were of two sizes, representing the two sexes. The smaller microconch forms had similar ribbing to their aperture which bears a marked ventral horn; several examples showing this were found, together with the larger macroconchs which develop a strengthened rib style on their body-chamber and which have a plain mouth border. This type of dimorphism is unique to Pectinatites and its recognition was instrumental in leading the Director early on in his PhD work to recognise that these British ammonites were unrelated to the Lower Tithonian ammonites of southern Germany as had been supposed by earlier workers such as Spath (1936) and Arkell (1947). The promontory here known as Cuddle is the westernmost point at which the Blackstone (the principal oil-shale horizon) had been worked (on the cliff top). In the cliff here were two cementstone bands, Cattle Ledge and above it Grey Ledge. These had formerly both been worked at this point and provided the first hydraulic cement known in Britain. The cement was used in the construction of the quay at the western end of Hen Cliff, the stone blocks of which were provided by fallen material from Yellow Ledge. Although ammonites were common in the shales between Yellow Ledge and Cattle Ledge, they were almost unknown in the beds between Cattle Ledge and Grey Ledge. Because these upper

beds were largely devoid of bituminous and oil-shales, but were largely soft mudstones, they did not form ledges and so material could only be collected from the cliffs where it was deeply weathered. This facies change was due to a temporary lower sealevel which resulted in breaks in the succession when looking at more marginal areas of deposition. In the realisation that the Kimmeridge Clay was the principal oil source rock in the North Sea, the Geological Survey instituted a major investigation of the Formation across its onshore outcrop during the 1970s. This resulted in many boreholes and provided much new information. One of the results was that it was recognised that lithological packages tended to be widely traceable laterally and this led the Gallois and Cox (1976) to set up a numbering scheme for the Formation by dividing it up into small units of similar lithology which they called ‘Beds’. They were believed to have chronostratigraphical significance. Unfortunately this scheme was originally proposed for the Kimmeridge Clay successions in the eastern part of Britain which are in parts markedly attenuated. The result was that when the authors came to apply the scheme to Dorset (Cox and Gallois, 1981) they found that they could not everywhere convincingly correlate the bed numbers with the East Anglian succession. For example the 21 m or so of the Hen Cliff Shales of Dorset are represented by only some 3.35 m of shale in the Wash area. Higher up the beds between the Cattle Ledge and Grey Ledge Stone Bands, some 22 m of rock, are totally unrepresented in the Wash area and thus cannot have a number (Cope, 2009). There are thus serious limitations to parts of this bed numbering scheme. The scheme also relies on the correct identification of the beds studied and this has led to previous incorrect correlations caused by mismatching of lithologies, rather than using the ammonite faunas for correlation (Cope, 2009). A further problem arises higher up where Gallois (1998, 2000) introduced new bed numbers for higher horizons unrepresented elsewhere, which differ from the bed numbering scheme used by Morgans Bell et al. (2001) which was based on the earlier scheme of Coe (1992). So there are two bed numbering schemes for the beds above bed 46 in the Pectinatus Zone. Ascending the succession as the party moved eastwards, a number of pieces of railway lines were seen to be protruding from the cliff. These are remnants of former workings in the Kimmeridge oil shale, which was quarried along a length of the cliff (a photograph appears in the old Geological Survey Weymouth and Purbeck Memoir (Strahan, 1898)). As well as following the outcrop along the cliff there were also several adits into the cliff, now mostly collapsed. Coastal erosion had recently exposed an intact railway truck used in these works; this had been recovered and it was planned to exhibit it in the proposed new fossil museum at Kimmeridge, to be built primarily to exhibit the magnificent Kimmeridge Clay fossil collection of Mr Steve Etches. The party crossed over Grey Ledge, here marking the base of the Wheatleyensis Zone, and passed round the promontory of Clavell’s Hard, this marking a point at which it is possible to be cut off by a rising tide. To the eastern side is a cliff in the Wheatleyensis Subzone of the Wheatleyensis Zone. Examples of large Pectinatites of the grandis group were seen on the ledges and samples of the Blackstone, the principal oil-shale, were collected. A strong breeze precluded ignition of the shale but application of a flame to a thin flake provided smoke and a sulphurous smell. On an earlier Association field trip in 1896, one of its Directors, W. H. Hudleston (1896), had reported that the pyritised ammonites at the top of the Blackstone were similar to Ammonites pectinatus (later to become the type species of Pectinatites) of the Oxford area. This was the first correlation of the attenuated Kimmeridge Clay successions of the Midlands with those of Dorset and was a remarkable observation for its time. In fact the true correlation with the Pectinatus Zone is with beds above the White Stone Band at Kimmeridge, but

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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Hudleston must be credited for the first recognition of (what was later to be known as) Pectinatites in Dorset. In recognition of that observation the Director had named the zone above the Wheatleyensis Zone as the Hudlestoni Zone (Cope, 1967). Towards Rope Lake Head the party encountered its first coccolithic limestone, the Rope Lake Head Stone Band. The succeeding shales had abundant ammonites of the Reisiformis Subzone of the Hudlestoni Zone, both horned microconchs and larger microconchs with strongly ribbed body-chambers were noted. At Rope Lake Head the succession upwards could be seen along the coast. It was immediately apparent that there is a major lithological change at this point, with calcareous mudstones making up the succession and the absence of oil-shale horizons was notable. The succession continued through the Basalt Stone Band, so called by Arkell (1933) because of its superficial resemblance to a basalt flow or sill, upwards to the White Stone Band, blocks of which could be seen on the beach at this point. This is a closely laminated coccolith limestone and the succeeding two hard bands, the Middle White Stone Band and the Freshwater Steps Stone Band are also coccolithic limestones. The absence of oil-shale horizons through much of the Hudlestoni Zone reflected a period of somewhat shallower water, during which time the sea had receded from shallower areas, as for example around the London landmass, so that the Hudlestoni Zone had not been recognised in many of the Midlands successions and there is a non-sequence between the Wheatleyensis and Pectinatus zones in the Midlands region. Unfortunately, following the loss of the stairway up the cliff on the western side of Freshwater Steps, there is no way up the cliff farther to the east. Only at lowest spring tides is it possible to get round the promontory at Freshwater Steps, so a return to Kimmeridge along the coast was necessary. This piece of coast is probably one of the least accessible coastal regions of southern England. On the return around Clavell’s Hard promontory it was noted that the tide was slightly higher than it had been on the outward walk, although low tide was still some hours away. It was explained that there are double tides in this area, so that after ebbing for some two hours, there is a short period of tidal inflow before a further ebb. After returning to the cars, lunch was taken at Church Knowle before driving via Corfe Castle and Kingston, through the village of Worth Matravers to the car park near Renscombe Farm. The descent was made to Chapman’s Pool (Fig. 2) following the track down into the valley from Renscombe Farm and thence to the path that reaches the beach where the stream runs out to sea. Fortunately the path was essentially dry, as the descent can be tricky after rain. The view from the coast westwards took in Hounstout Cliff where the top part of the Bolonian Stage and most of the Portlandian Stage were visible. This is the thickest succession of marine rocks of this age, certainly in Europe and possibly the world. The ammonite fauna of these highest beds of the Kimmeridge Clay was correlated by Neaverson (1925) with his Pallasioides Zone, founded in the Aylesbury district. Casey (1967) showed that in that area the Rotunda Zone belonged above the Pallasioides Zone rather than below it as Neaverson (1925) had supposed. Cope (1978) identified Pavlovia pallasioides for the first time in Dorset and confirmed Casey’s conclusions about the relative positions of the Pallasioides and Rotunda zones. Cope (1978) found in the beds above the Rotunda Zone a new ammonite fauna including a new genus, Virgatopavlovia, the basis of the Fittoni Zone at the top of the Bolonian Secondary Standard Stage. Virgatopavlovia occurs farther west in Dorset, at Dungy Head and on Portland Isle (Cope, 1978) but has not been recorded elsewhere. The top part of the Kimmeridge Clay has been divided into what we would now recognise as members of the Kimmeridge Clay

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Formation (Buckman, 1926) and Arkell (1933); the latter defined the boundaries between each division. These divisions have been accepted over the years as readily recognisable and useable. However, an internal BGS Technical Report, (Gallois, 1998) and subsequently published papers (Gallois, 2000; Gallois and Etches, 2001) claimed that Arkell’s divisions were neither recognisable in the boreholes inland at Swanworth Quarry, Worth Matravers, nor were the lines of seepage, cited by Arkell as separating his divisions, always recognisable or reliable lines of division. Gallois (1998) thus set up a new series of divisions for these beds. However, Morgans Bell et al. (2001) who provided the official report for the NERC Rapid Global Geological Events Special Topic (RGGE) on the integrated stratigraphy of the coastal cliffs and the Swanworth boreholes were able to identify Arkell’s divisions without difficulty in the boreholes. Furthermore, in over 50 years of regular visits to these sections the Director had found the lines of seepage persistent and reliable horizons at all times of the year (Fig. 3). The seepages are due to oil-shale aquicludes that are otherwise rare in this part of the Kimmeridge Clay. The claim by Gallois and Etches (2001, p. 170) that different lines of seepage were identified by different authors is not accepted, as the seepage lines are quite discrete and well-separated. The differences quoted are due entirely to differences in measurements of the section. Arkell (1933) described the Hounstout Marl as a ‘sandy marl’ and a disagreement over lithological description should not over-ride priority in nomenclature, so that the term ‘Upper Hounstout Silt’ of Gallois (1998) should be regarded as a junior synonym of the Hounstout Marl of other authors. The Hounstout Clay of Gallois (1998) is bound to lead to confusion, as it is not the Hounstout Clay of Arkell (1933) and other authors, but the Hounstout Clay and the Rhynchonella Marls of Arkell (1933) and other authors. Gallois’ (1998) Lower Hounstout Silt is nothing other than the Lingula Shales of Arkell (1933) and other authors. Morgans Bell et al. (2001) describe their lithology as dark silty clays and mudstones and there seems no merit in introducing a new member-level name for this as was done by Gallois (1998). A further problem arises lower down in the succession and the beautifully exposed section through the upper part of the Pallasioides Zone gave members of the party an opportunity to draw their own conclusions on the debate. When Cox and Gallois (1981, p. 44) described the Kimmeridge section they noted a ‘Plaster of partially phosphatised Pavlovia with belemnites and oysters’ some 3.1 m below (what is) Blake’s (1875) Bed 2 at Chapman’s Pool. No particular importance was attached to this observation by Cox and Gallois (1981) and no break noted at this level. However, 17 years later, Gallois (1998, p. 28) assigned a new significance to this horizon: ‘At Chapman’s Pool, a thin (up to a few centimetres thick) gritty, shelly, silt-rich mudstone with abundant belemnites and oysters, and phosphatised bivalves and body chambers of pavloviid ammonites rests on a bioturbated surface. It marks an important sedimentary break and faunal change at the base of the Rotunda Zone. . . .It has been named the Chapman’s Pool Pebble Bed’. Gallois (1998, p. 38) assigned this bed to the base of his bed KC55 and noted ‘the basal pebble bed is especially well exposed at Chapman’s Pool’. This information only came into the public domain following the publication of Gallois’ (2000) report on the Swanworth Quarry boreholes in the PGA and the subsequent Gallois and Etches paper (2001) also in the PGA on the stratigraphy of the uppermost parts of the Kimmeridge Clay. The Director has been familiar with the Chapman’s Pool section for over 50 years and did not see how he could have missed such an ‘especially well exposed’ horizon. Following publication of the latter paper, he had visited Chapman’s Pool in 2001 together with Steve Etches (who had never seen this ‘pebble bed’) to see if the horizon could be found. Despite perfect exposure, it could not be

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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found. Since then he had visited Chapman’s Pool on at least six occasions, usually with parties and found no trace of this bed. On one of these occasions the bay was virtually clear of shingle and the mudstones below Blake’s Bed 2 beautifully exposed across the foreshore. Again on this 2012 Geologists’ Association field trip the cliff section was absolutely clean and it was possible to measure down accurately from the base of Blake’s Bed 2 to examine the relevant part of the section in detail in bright sunlight. No trace of any such ‘pebble bed’ horizon was found. The fully detailed Geological Magazine RGGE report (Morgans Bell et al., 2001) does not record this pebble bed, but refers the reader to Gallois and Etches (2001) for details. This seemed to be a rather odd thing to do, so Helen Morgans-Bell (first author this paper) was contacted to seek clarification. She replied (in litt. to JCWC, March 2008) that a visit there with three of her co-authors to try to locate the bed had failed to find any trace of it. Williams et al. (2001) analysed quartz content of the Kimmeridge Clay from the Swanworth No. 1 and Metherhills boreholes. For the Pallasioides Zone that content lay entirely within the range of 4–7.5% and there was no sign of any quartz spike at the level that might be expected for a ‘gritty shell-rich pebble bed’ some 3.1 m below Blake’s Bed 2. Isolated quartz spikes with levels approaching 20% quartz were, however, recorded in the Wheatleyensis and Scitulus zones of the Bolonian and the Eudoxus Zone of the Kimmeridgian. What is also disconcerting is that the Pallasioides/Rotunda zonal boundary has been moved downwards (Gallois, 1998), without any apparent concern for the ammonite fauna of the beds in question. It should be noted that the ammonite faunas recorded by Gallois (1998, 2000) and Gallois and Etches (2001) were neither taken from the boreholes, nor from the coastal exposures, but solely from the records of Cope (1978) and woven into the new bed numbering scheme. However, the occurrence of ammonites recorded by Gallois (1998, 2000) and Gallois and Etches (2001) is in part selective. Gallois’ (1998) Bed KC55 embraces both the top of the Pallasioides Zone and the base of the Rotunda Zone, but their records for Gallois’ Bed KC55 do not include the Pallasioides Zone species as recorded from the lower 2.5 m of this bed by Cope (1978); these include Pavlovia pallasioides, P. composita, P. composita waddingtoni and Pectinatites (Pectinatites) circumligatus. Thus the beds up almost to the base of Blake’s Bed 2 (the top 0.9 m of Gallois’ Bed KC55) contain Pallasioides Zone ammonites, with Pavlovia pallasioides itself occurring to within 1.8 m of the base of Blake’s Bed 2 according to Cope (1978, p. 501 and text- Fig. 11). Pavlovia concinna and P. rotunda both first appear in Blake’s Bed 2 (the topmost part of Gallois’ (1998) Bed KC55), marking the base of the Rotunda Zone as defined by Cope (1978). Such ‘important sedimentary breaks’ (Gallois, 1998, p. 28) as Gallois claims at the base of his bed KC55 must be proven rather than assumed, as appears to have been the case here; such a claim is not supported

by the ammonite fauna. Cope (1978, text-Fig. 11) records no break in ammonite fauna at that level. In any case if body-chamber fragments had indeed been found 3.1 m below Blake’s bed 2, there must be considerable doubt that they could be specifically identified, as one pavloviid body-chamber fragment is very much like any other. In looking back over my records (Cope, 1978) of the ammonite fauna, I discovered a discrepancy in my recorded occurrence of Pavlovia pallasioides. The range of that species is shown correctly on the detailed description of the section (1978, p. 472), in the taxonomic description of the species (1978, p. 501) and in the ammonite range chart (1978, text-Fig. 11). It should therefore be clear that the range quoted in the stratigraphical section (1978, p. 526) is incorrect. To double-check this I re-examined my collections in the National Museum of Wales (NMW 77.12G.) and reconfirmed the range of the species, but also discovered a younger specimen of Pavlovia pallasioides from only 1.2 m below the base of Blake’s Bed 2, apparently overlooked earlier. Thus there is no palaeontological basis for moving this zonal boundary from that earlier established by Cope (1978). So how is one to explain the observations made in these various publications? Over the years isolated pockets of phosphatised fossils in other parts of the Kimmeridge Clay section have been observed, especially in the Wheatleyensis Zone. On a subsequent visit these have apparently disappeared following coastal erosion. Such a pocket could well explain the original observation by Cox and Gallois (1981), but cannot explain other details. Following discussion of the ‘pebble bed’ the party made its way south-eastern corner of Chapman’s Pool where Blake’s Bed 2, at the base of the Rotunda Zone yielded abundant crushed, though wellpreserved, ammonites at the base of the Rotunda Zone. The route back to the vehicles followed the road from behind the boathouse until this met the steep zig-zag path up West Hill, thence across the fields, following the footpath to the car park. Most members of the party enjoyed an early evening drink at the Square and Compass at Worth Matravers that also provided an opportunity to see the inn’s small fossil museum. 3. Sunday September 30 (Fig. 4) Sunday morning proved dry and bright, but with little sun. The party met in the car park at Lulworth Cove and followed the coastal path westwards. Near the top of Hambury Tout the party paused to take in the view. Eastwards the very slight southerly dip of the Portland Stone on St Aldhelm’s Head was observed, this being the only part of the southern limb of the Purbeck Anticline on land. To its west the flat dip of Emmet’s Hill was noted and in the foreground it was possible to see the steeper dip of the Portland Stone at the entrance to Lulworth Cove. The Director pointed out that the distance between the Portland Stone and the Chalk was

Fig. 4. The Dorset coast between Bat’s Head and the Fossil Forest, showing localities referred to in the text.

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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considerably less here than where we had first observed on the Chalk ridge above Kimmeridge the previous morning. The separation is some 4 km at the eastern end of the Isle of Purbeck around Swanage, but at Lulworth the distance has been reduced to some 250 m. There were three factors in this separation: (i) the westwardly decreasing thickness of the intervening strata (in particular the Wealden Group — for details see Radley (2012)), (ii) the westwardly increasing dip and (iii) the westwardly increasing unconformity beneath the Gault; so that 60 m of Lower Greensand north of Swanage is reduced to some 15 cm on the eastern side of Lulworth Cove and it is absent on the west side of the cove. To the west the very gentle southerly dip of the southern limb of the Weymouth Anticline was noticeable by the dip slope of the Purbeck Group on the Isle of Portland, some 15 km distant. To its north the scarp slope of the northern part of the island consisted of the Portland Group overlying the Kimmeridge Clay. That Formation lies beneath Chesil Beach and the higher ground to the north is the Corallian Group outcrop around Wyke Regis. Having taken in the regional picture, the party followed the path down to Durdle Door, the natural arch in the Portland Stone and basal part of the Purbeck Group. The outcrop of the Portland Stone could be traced off shore by a series of named offshore rocks running westwards as far as Bat’s Head, where the near vertical White Chalk cliff was seen to be cut by southward dipping Group 3 shears (Bevan, 1985). Within Durdle Cove the Wealden Group is obscured by landslipping after recent rains and the Gault above was also poorly exposed; the junction between the two was hidden. The party made its way up the section to the Upper Greensand, here slightly overturned and thus appearing to dip southwards. Shingle levels on the beach were extraordinarily high so that the lower, more argillaceous, parts of the Upper Greensand were not exposed. Higher horizons showed well-cemented glauconitic sandstones and the Chert Beds, the silica of the cherts having originated from siliceous sponges and subsequently redistributed during diagenesis. The shape of some of the chert masses suggested that they had been formed preferentially within burrow systems, particularly those of decapod crustaceans, belonging to the ichnogenus Thalassinoides. Above the Upper Greensand it was possible to see the Cenomanian Basement Bed; here a non-sequence has cut out the Lower Cenomanian and phosphatized pellets and rounded ammonite fragments were seen to be weathering proud of the surface. In the upper part of the bed, several specimens of the echinoid Holaster were seen. Overlying the basement Bed the remainder of the Grey Chalk Subgroup, belonging to the Zig Zag Chalk Formation was visible. A re-entrant in the cliff showed exposure of the Plenus Marl Member at the base of the succeeding Holywell Nodular Chalk. The Chalk here exhibits a line of small caves excavated along a flat-lying fracture. This belongs to Group 3a (Bevan, 1985) and is a reverse fault. The party was able to follow the White Chalk succession upwards as far as the small cove beneath Scratchy Bottom where the Seaford Chalk Formation is well exposed. This locality frequently yields common echinoids such as Micraster of the coranguinum groups and Echinocorys, but on this occasion proved unfossiliferous. The party made its way back to the peninsula of Wealden Group and crossed the col to descend into Man o’War Cove. Here it proved possible to examine the Lulworth Formation of the Purbeck Group. Particular interest was shown in the Broken Beds, a jumble of blocks of limestone some 3 m thick, lying between apparently normal limestone horizons. The origin of the brecciation had long proved contentious and early explanations all involved tectonics, but this cannot explain the occurrence of Broken Beds with similar characteristics to these that occur on Portland Isle where the dip rarely exceeds 28. A more acceptable explanation was first suggested by Hollingworth (1938) who

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recognised that there had originally been beds of evaporates interbedded with the limestone; upon dissolution of the evaporites the lithified limestones had collapsed, causing brecciation. The Lulworth Formation was followed back across the cliff until the Cinder Bed, marking the base of the Durlston Formation, was seen; as usual the Cinder Bed was composed of masses of the oyster Praeexogyra distorta, though farther east it contains other fossils, notably the echinoid Hemicidaris at Durlston Bay. Although the Cinder Bed is virtually in the middle of the Purbeck Group, it is here almost the highest horizon of that Group to be exposed, before the Wealden Group outcrop. The absence of most of the Durlston Formation (and probably a considerable thickness of the Wealden Group too) is due to faulting. It is implied that pressure on this limb of the fold was so intense that these incompetent strata were translated into the hinge region of the fold where pressures were less. Upwards in the Wealden Group succession the party examined the coarse pebbly sandstone in the middle of the succession. Such hard bands create prominent features along the Wealden Group outcrop in Purbeck; farther east the Geological Survey (Sheet 342/ 343, 2000) had mapped several such bands, east of easting line SY 93, but only one appears to be present from Worbarrow Bay westwards and this thickens in a westerly direction. Dark coloured clasts in this bed included chert from the Upper Palaeozoic of south-west England and tourmaline derived from the Dartmoor Granite aureole of Devon. This material was brought in by rivers draining the Upper Palaeozoic Cornubian highlands of south-west England and the abundant black flecks in the sediments are carbonised plant fragments, much of them in the form of charcoal, showing wildfires affected the plants around the Wealden basin. The striking yellow efflorescence so obvious on many surfaces is the mineral jarosite, an iron potassium sulphate. The base of the Gault was again hidden here by slips, but the Upper Greensand and the Basement Bed of the Chalk were better exposed. The group followed the beach around the cove to the point where the wave-cut platform of Chalk was crossed at the eastern limit of the cove. Here the distance between the lower part of the Purbeck Group seen on the Man o’War rock and the Chalk was only some 50 m and this point is the closest these two Groups are found anywhere in Dorset. Compression was strongest here and clearly much of the Purbeck and Wealden Groups, together with some (at least) of the Gault and Upper Greensand had been faulted out. On negotiating the shore beneath Man o’War Head into St Oswald’s Bay, the important role that bedding plane slip had played in these structures was evidenced by abundant slickensides. Many of the flint nodules had been drawn out into elongated accumulations of flint particles. A steep reverse fault (Group 4 of Bevan, 1985) was clearly visible in the cliff here and brought Lewes Nodular Chalk into the cliffs. Solution pipes in the Chalk were very obvious features of the cliff here; the base of some of them was only a few metres above beach level. Within the pipes could be seen horizontally bedded sediments, mostly bright orange in colour. As these were vertical pipes into near-vertical Chalk they must be post-orogenic in origin, which meant they were likely to be Pliocene or younger. House (1996) had collected lignitic material from one of these pipes, but the only pollen grains yielded were Recent, suggesting possible contamination. Cope (2012) suggested these could be of Pleistocene age. Continuing eastwards around St Oswald’s Bay, the cliffs showed a northward dipping reverse fault (Group 5 of Bevan, 1985), that brought Holywell Nodular Chalk in juxtaposition with the Chalk Basement Bed and the Upper Greensand. Within the Upper Greensand were exposed clear Thalassinoides burrows, their form reproduced perfectly by a chert replacement of the original burrow systems and weathered proud of the glauconitic sandstone matrix.

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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The Gault was again poorly exposed and search of the cliffs failed to find its basal pebble bed. Slipping had also caused the Wealden Group to be poorly exposed and the party made its way across the faulted outcrop of the Purbeck Group to examine the Portland Stone Formation beneath Dungy Head. Beneath the Portland Freestone Member the Cherty Member was well exposed. The Director explained that the Portland Sand was exposed farther along the coast, but that cliff falls had made the beach very difficult and had covered the dark sands at the base of the succession which had yielded Virgatopavlovia at the top of the Bolonian (Cope, 1978). The party looked at the cliff formed by the Cherty Member and noted ammonites protruding from the rock at one level. When Blake (1880) had described the Portland Beds of Dorset he had recorded a basal shell bed at the bottom of the Cherty Member on the Isle of Portland. His description of the sections in Purbeck also noted a shell bed at the base, as on Portland. In fact no such bed exists in Purbeck, as noted by Arkell (1933, p. 488). When the identity of the ammonites of the Portland Group became known, it became clear that the ammonite species of the Basal Shell Bed of Portland Isle were the same as those occurring in Arkell’s (1935) beds J and J’in the middle of the Cherty Member of Purbeck (Cope and Wimbledon, 1973; Wimbledon and Cope, 1978). These beds are the ones containing ammonites that we were now looked at on Dungy Head. The lithological similarity to the Basal Shell Bed of Portland was pointed out by Townson (1975). On Portland Isle the lower part of the Isle of Purbeck’s Cherty Member is represented by the Portland Clay (Wimbledon and Cope, 1978). The party climbed the steep cliff path over the Wealden Group outcrop and made its way back to Lulworth and lunch. In the afternoon the party first climbed to view Stair Hole from its western side, allowing full appreciation of the ‘Lulworth Crumple’. The view eastwards took in the whole of Lulworth Cove and it was apparent that its narrow entrance through the Portland Group was due to the resistance of these rocks, relative to those of the Purbeck and Wealden Groups. Erosion had slowed against the Chalk Group forming the back of the cove, so that the cove was essentially circular. At Stair Hole this evidence of the strength of the Portland Stone was illustrated again, though the rampart was partially breached; this contrasted with the wasting of the cliff behind in the softer Purbeck and Wealden groups. Many texts quoted Stair Hole as a mini-Lulworth Cove in the making, but it was important to realise that the processes in its formation were not the same. In particular, as noted by Jones et al. (1984), a stream runs into Lulworth Cove and this must, before the cove’s existence, have entered the cove over a waterfall on Portland Stone cliffs. Thus breaching of the Portland Stone there was a combined effect of marine coastal and fluvial erosion. The party then took in the ‘Lulworth Crumple’ on the eastern face of Stair Hole. This affected principally the Worbarrow Tout, Stair Hole and Peveril Point members of the Purbeck Group. A structural analysis by Underhill and Paterson (1998) explained the ‘crumple’ as an entirely tectonic feature resulting from flexural slip on the hanging-wall of the Purbeck Fault. Previously Phillips (1964) had suggested that these structures are consistent with those produced by gravitational forces. Over the years the Director has visited the site with many structural colleagues and research students and the independent consensus was that the fold geometry was consistent with gravity-produced structures. There is no doubt that the Purbeck Fault was active during Purbeck and Wealden Group times (as both groups are absent north of the fault here) and such movements could have been instigators of penecontemporaneous gravity induced folding (such as is seen to the east in Bacon Hole). Underhill and Paterson (1998) had measured the bedding planes of limestones involved in the ‘crumple’ and had plotted a stereographic projection of poles to

bedding planes (1998, Fig. 19); they had concluded from the parallelism of all the structures in the area that the ‘crumple’ was formed during the Alpine deformation. However, Late Cimmerian folding is remarkably parallel to Alpine folding in the area and the observations of those authors do not preclude a Late Cimmerian origin for the ‘crumple’. Indeed in Bacon Hole (SY 839 797) only just over 1 km east of Lulworth Cove the lower part of the Lulworth Formation displays unequivocal evidence of penecontemporaneous deformation (Cope, 2012, Fig. 69). The party then made its way down to Lulworth Cove and walked round the western side of the cove as far as the tide would allow. The lowest horizon that it was possible to examine was the Broken Beds, here better developed than at Man o’War Cove. The wet weather of the preceding months had resulted in extensive slipping of higher horizons within the Purbeck Group and many of the small-scale structures that were formerly well seen in the cliff here were no longer visible. The succession was followed upwards to the Chalk, at which point time had intervened and with a long journey home for many members of the group, the weekend’s activities were concluded. References Arkell, W.J., 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford xii + 681 pp., 41 pls. Arkell, W.J., 1935. The Portland beds of the Dorset Mainland. Proceedings of the Geologists’ Association 46, 301–347, pls. 19–26. Arkell, W.J., 1947. The geology of the country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain, xv + 386 pp., 19 pls. Barton, C.M., Woods, M.A., Bristow, C.R., Newell, A.J., Westhead, R.K., Evans, D.J., Kirby, G.A., Warrington, G., 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Memoir of the British Geological Survey. viii + 161 pp. Bevan, T.G., 1985. A reinterpretation of fault systems in the Upper Cretaceous rocks of the Dorset coast, England. Proceedings of the Geologists’ Association 96, 337– 342. Blake, J.F., 1875. On the Kimmeridge Clay of England. Quarterly Journal of the Geological Society of London 31, 196–237. Blake, J.F., 1880. On the Portland rocks of England. Quarterly Journal of the Geological Society of London 36, 189–236, pls. 8–10. Blake, J.F., 1881. On correlation of the Kimmeridge and Portland rocks of England with those of the Continent. Part I. The Paris Basin. Quarterly Journal of the Geological Society of London 37, 497–587. British Geological Survey, 2000. 1:50,000 Geological Map Sheet No. 342/343. Buckman, S.S., 1926. Type Ammonites, 6, 30. Callomon, J.H., Cope, J.C.W., 1993. The Jurassic Geology of Dorset: guide to excursions. In: Arkell International Symposium 1993, University College London. Casey, R., 1967. The position of the Middle Volgian in the English Jurassic. Proceedings of the Geological Society of London 1640, 128–133. Casey, R., 1971. Facies, faunas and tectonics in late Jurassic-early Cretaceous Britain. In: Middlemiss, F.A., Rawson, P.F., Newell, G. (Eds.), Faunal Provinces in Space and Time. Geological Journal, Special Issue, 4. Seel House Press, Liverpool, pp. 153–168. Casey, R., 1973. The ammonite succession at the Jurassic-Cretaceous boundary in eastern England. In: Casey, R., Rawson, P.F. (Eds.), The Boreal Lower Cretaceous. Geological Journal, Special Issue, 5. Seel House Press, Liverpool, pp. 193–266. Coe, A.L., 1992. Unconformities within the Upper Jurassic of the Wessex Basin, Southern England. University of Oxford (DPhil Thesis). Cope, J.C.W., 1967. The palaeontology and stratigraphy of the lower part of the Upper Kimmeridge Clay of Dorset. Bulletin of the British Museum (Natural History). Geology Series 15, 1–79, pls. 1–33. Cope, J.C.W., 1978. Ammonite faunas and stratigraphy of the upper part of the Upper Kimmeridge Clay of Dorset. Palaeontology 21, 469–534, pls. 45–56. Cope, J.C.W., 1993. The Bolonian Stage: an old answer to an old problem. Newsletters on Stratigraphy 28, 151–156. Cope, J.C.W., 2008. Drawing the line: the history of the Jurassic-Cretaceous boundary. Proceedings of the Geologists’ Association 119, 105–117. Cope, J.C.W., 2009. Correlation problems in the Kimmeridge Clay Formation (Upper Jurassic, UK): lithostratigraphy versus biostratigraphy and chronostratigraphy. Geological Magazine 146, 266–275. Cope, J.C.W., 2012. Geology of the Dorset Coast. Geologists’ Association Guide, vol. 22. , viii + 232 pp. Cope, J.C.W., Wimbledon, W.A., 1973. The ammonite faunas of the uppermost Kimmeridge Clay, Portland Sand and Portland Stone of Dorset. Proceedings of the Ussher Society 2, 593–598. Cox, B.M., Gallois, R.W., 1981. The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences.In: Report of the Institute of Geological Sciences, London, 80/4. , pp. 1–44.

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004

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Oppel, A., 1865. Die tithonische Etage. Zeitschrift der Deutschen geologischen Gesellschaft 17, 535–558. d’Orbigny, A., 1842–51.In: Pale´ontologie Franc¸aise: Terrains jurassiques, I. Ce´phalopodes, Paris. Phillips, W.J., 1964. The structures in the Jurassic and Cretaceous rocks on the Dorset coast between white Nothe and Mupe Bay. Proceedings of the Geologists’ Association 75, 373–405. Radley, J.D. (Ed.), 2012. The non-marine Lower Cretaceous Wealden strata of southern England. Proceedings of the Geologists’ Association, 123, 233–386. Riley, L.A., 1977. Stage nomenclature at the Jurassic-Cretaceous boundary, North Sea basin. In: Proceedings of the Mesozoic Northern North Sea Symposium 1977. MNNSS/4, Norsk Petroleumsforening, Oslo, pp. 1–11. Salfeld, H., 1913. Certain Upper Jurassic strata of England. Quarterly Journal of the Geological Society of London 69, 423–432, pls. 41–2. Scotchman, I.C., 1989. Diagenesis of the Kimmeridge Clay Formation, Onshore UK. Journal of the Geological Society, London 146, 285–303. Spath, L.F., 1936. The Upper Jurassic invertebrate fauna of Cape Leslie, Milne Land. Meddelelser om Grønland 99 (3) 1–180, 50 pls. Strahan, A., 1898. The geology of the Isle of Purbeck and Weymouth. Memoir of the Geological Survey. Townson, W.G., 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society 131, 619–638. Underhill, J.R., Paterson, S., 1998. Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck-Isle of Wight Monocline. Journal of the Geological Society 155, 975–992. Weedon, G.P., Coe, A.L., Gallois, R.W., 2004. Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England. Journal of the Geological Society, London 161, 655–666. Williams, C.J., Hesselbo, S.P., Jenkyns, H.C., Morgans-Bell, H.S., 2001. Quartz silt in mudrocks as a key to sequence stratigraphy (Kimmeridge Clay Formation, Late Jurassic, Wessex Basin, UK). Terra Nova 13, 449–455. Wimbledon, W.A., Cope, J.C.W., 1978. The ammonite faunas of the English Portland Beds and the zones of the Portlandian Stage. Journal of the Geological Society, London 135, 183–190, pls. 1–3. Zhamoida, A.I., Prozorovskaya, E.L., 1997. Resolutions on the Jurassic-Cretaceous boundary position in the Boreal Realm and on the status of the Volgian Stage. Resolutions of the Interdepartmental Stratigraphic Committee and its Permanent Commissions 29, 5–7, St Petersburg (in Russian).

Please cite this article in press as: Cope, J.C.W., Field meeting in the Isle of Purbeck, September 2012, to examine the Upper Kimmeridge Clay and the Lulworth district. Proc. Geol. Assoc. (2013), http://dx.doi.org/10.1016/j.pgeola.2013.07.004