Ian West,
Romsey, Hampshire

and: Visiting Scientist at:Ocean and Earth Science ,
Faculty of the Natural and Environmental Sciences Southampton University,
Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.
Website archived at the British Library

Home and List of Webpages| Kimmeridge - Kimmeridge Bay and Introduction to Kimmeridge |Kimmeridge - Fossils |Kimmeridge - West of Kimmeridge Bay |Kimmeridge - Hen Cliff, Cuddle and Yellow Ledge |Kimmeridge - Blackstone, Oil Shale at Clavell's Hard |Kimmeridge - Burning Beach, Burning Cliffs |Kimmeridge - Rope Lake Head to Freshwater Steps |Kimmeridge - Egmont Bight to Chapman's Pool |Kimmeridge Clay Boreholes at Swanworth Quarry
|Kimmeridge - Bibliography - Start
| Selected external links: | Jurassic Coast (DCC) | Exmouth to Milford-on-Sea 1800-2000, Kimmeridge section - old photographs collected by Doreen Smith

Kimmeridge Field Guides

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(You are advised, if posssible, to download this educational site to: SurfOffline or WebCopier or similar software to keep a permanent offline copies, but note that updating of the live version takes place periodically. Offline copies of this and other Wessex Geology webpages are always retained on offline webpage software of this type, in addition to various online and offline backup systems, and is also archived.)

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Access

Some of the locations described here are often closed to the public, and this is unfortunate but necessary. Notices show the location of restricted areas. There is access to the cliff top above Brandy Bay and Gad Cliff when the Range Walks are open. Hobarrow Bay and Brandy Bay beaches are not open, but can viewed with binoculars or photographed from the cliff top. This website enables you to see the beaches and cliffs in some detail, and you are also welcome to make your own offline copy. In any case the geological detail shown here is greater than that which you would observe on a single trip to Brandy Bay. Photographs shown here have been taken over many years by various people on various field trips, one or two having been organised by scientific societies. Separate photographic records, not shown here, are in the possession of various geologists. The cliffs and coast, discussed here, have been studied geologically since about the early 19th century.

The objective of this website is to provide geological information for the geologist and the general public. It is an illustrated description and not a walking guide or an itinerary and does not provide a field trip programme. It simply gives as much geological data and photographs on the area as is possible to obtain over a number of years. Some of the localities are easily accessible and some are almost inaccessible. By providing this geological information any need to go to the less-accessible parts should be greatly reduced.

In any case, the tide is a major limiting factor, and risks should not be taken. When the tide is rising it is not possible to proceed any significant out of Kimmeridge Bay. There are potential tidal trap points in the Broad Bench, Hobarrow Bay and Long Ebb area.

With regard to the Army Ranges, at certain times Range Walks are open, giving access to coastal paths. The Army Ranges, now, with periodic access for the public, are very good with regard to conservation and public access. Adequate notices provide information. The dates when the Range Walks are open can be obtained from the internet. Brandy Bay beach is not within part of a range walk and is therefore not normally available for public access. It can be seen from above, though, and specialist work occasionally takes place on the cliffs. The Range Walks are open at most, but not all, weekends and at these times the red flag is not flying. They are also open on a few occasions (usually public holidays) in addition the weekends. Before visiting please check the opening schedule:

Access Times at Defence Training Estate, Lulworth and Tynham Village, 2011 ( or 2012 etc.)
[ Ditto for 2012. etc]

or Search for: "Lulworth Ranges"

Note that this website is descriptive and only provides geological information; it does not suggest that you proceed to any particular locality or to any place shown. It should be obvious to you from notices and general observation where you can and where you cannot go.

INTRODUCTION:

Safety; the Hazards of Kimmeridge Cliffs

Note that there is some significant danger with regard to the Kimmeridge cliffs. One danger, the main one, is risk of rock fall; the second hazard is that of being cut off by the tide. This applies particularly to the cliffs east of Kimmeridge, although it could also happen at Brandy Bay and elsewhere. In general, keep out as far from the cliffs as possible. Some debris falls every day and if you are close to the cliff you are at risk of being hit by a rock-fall.

Another hazard is that of slipping on wet or slimy rocks and receiving some injury from a fall. If a walker is alone there could be a serious problem if a person was injured in a fall and unable to return. A mobile phone and a torch should be taken and a whistle might be useful.

The Hobarrow and Brandy Bay area are in the Army firing range and, even when the range walks are open, they are normally closed off. Abide by the warning notices and do not touch any dubious metal objects. Usually when the ranges are closed, at least part of the area is under observation from a warden's hut in Kimmeridge Bay. There is also a CCTV camera adjacent to Broad Bench. In general, the greatest risk with regard to Kimmeridge cliffs is that of rock-fall, rather than anything else. The cliffs are in general (and there are exceptions) a little less vertical to the west of Kimmeridge Bay than to the east, where the risk is more substantial.

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Kimmeridge Safety Addendum:

Photographs shown on this website have been taken over a period of many years by the author and various other people. Some photographs are from organised field-trips, which may be those of the present author or of other geologists. Many, though are from informal, private coastal walks, or from private, research field-trips. The photographs are for geological purposes only and are there to show rocks, not people or techniques. They are not intended to show safety procedures and no activities shown are necessarily intended to be copied. This website is about geology for geologists. The cliff, sea, tide and weather conditions vary greatly so always make your own assessment of the cliffs and conditions on the day, and arrange your coastal procedures in accordance. Always consult tide tables before field work at or near Kimmeridge. No responsibility at all is taken for any activities of field parties or individuals going to the Kimmeridge coast for their own purposes or objectives. As at other geological sites a risk is present and the possibility of an accident, although a rare occurrence, cannot be eliminated.

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INTRODUCTION:

Geological Map

INTRODUCTION:

Aerial Photograph and Bathymetry Image

PALAEONTOLOGY:

Lower Kimmeridge Ammonites - Aulacostephanus

The image above is of the ammonite Aulacostephanus pseudomutabilis , Lower Kimmeridge Clay, autissiodorensis Zone, Weymouth, Dorset. Ammonites of the genus Aulacostephanus can be recognised by the laterally compressed shell, the subinvolute coiling (i.e. partial overlapping) of the whorls, the fasciculate ribbing branching out from bullate tubercles on the umbilical (i.e. inner) shoulder. There is a prominant smooth ventral band, as shown in the image above. The genus is known from Europe and Russia and belongs to the superfamily Perisphinctaceae.

Examples of this genus (although most likely crushed and fragmentary) in the Lower Kimmeridge Clay which is well exposed from about the centre of Kimmeridge Bay to Broad Bench and beyond to the west. The tubercles and the branching ribs aid recognition; if an ammonite which fallen into the sediment on its venter, as occasionally happened, the smooth band on the venter will be easy to observe. This is not present in most other Kimmeridgian ammonite genera.

PALAEONTOLOGY contin.

Ammonites of Brandy Bay (Aulacostephanus to Pectinatites Zones)

Crushed ammonites are common on the ledges of Brandy Bay. The casual visitor is most unlikely to see any uncrushed specimens, but the well-known fossil collector Steve Etches has found uncrushed specimens. The usual ammonites, in the bay are species of Pectinatities from the Upper Kimmeridge Clay, but some other genera also occur. The crushing is very thorough and reveals the great extent to which the Kimmeridge Clay sediment has been compacted, perhaps more than ten times in many cases.

A specimen of Aspidoceras sp. was pointed out by Steve Etches in a block of argillaceous dolomite, associated with a ledge, in the middle part of Brandy Bay, March 2012.

PALAEONTOLOGY:

Bivalves

The robust bivalves, commonly known as Trigonia, but more strictly Myophorella, are very common in the shallow water facies of the Corallian and the Portland Stone. They are not abundant in the deeper water Kimmeridge Clay. The occasional specimen does occur, as shown here. Normally they are present as isolated examples in the Kimmeridge Clay, whereas accumulations ("Trigonia Beds") occur in the Corallian.

PALAEONTOLOGY:

Coprolites

STRATIGRAPHY:

Sequence - Introduction

The cliff section in Brandy Bay has a moderate dip. The result of this is that it is possible to walk quite quickly through a substantial stratigraphical sequence. In three minutes you can walk past strata that would almost 30 minutes to traverse from Kimmeridge Bay eastward to almost Clavell's Hard. Unfortunately, the cliffs often have a talus apron at the foot, and it is not as easy to recognise the stratigraphical position as in the classic section to the east. It is not worth studying the Brandy Bay section until the eastern section and its stone bands are familiar to you. Access here is also much more difficult, so all considered, it is really a place for the specialist. The general coastal walker can see something of the cliffs of Brandy Bay from the coastal footpath when the ranges are open.

LOCATION:

Broad Bench (SY 898787)

(See also the: Kimmeridge Bay webpage)

Broad Bench provides an excellent exposure of the Flats Dolomite Bed with expansion megapolygons. It was described by Cox and Gallois (1981) as "The Flats S.B.: persistant tabular cementstone cut by polygonal network of low angle thrusts." It is not actually a cementstone but a ferroan dolomite. These authors showed it to be at the top of the eudoxus Zone. It represents an horizon of overlap of ammonites of the Aulacostephanus eudoxus group and of the Aulacostephanus autissiodorensis group.

Ca _51.09 Mg _44.37 Fe _4.47 Mn _0.07 CO ₃

There is 95 wt% dolomite in this bed, compared to only 77wt% in the Maple Ledge. The fact that this rock is a relatively pure carbonate is relevant to the thrusting if that process was due to burial diagenesis. There is no significant clay to buffer expansion growth or retard water flow (however, there is much kerogen). Fe ²⁺ is fairly low at less than 0.5 wt%. It is interesting though that siderite, as such, occurs in sediments 6m above the Flats. Sr ²⁺ in the Flats is 314 ppm, about the same as Washing Ledge. As noted below, this seems to be a fairly high figure for a local Jurassic dolomite.

LOCATION:

Hobarrow Bay (map ref. SY 896790)

Hobarrow Bay is in the firing range area and is not normally accessible to the public at beach level. The cliffs can be seen in general from the footpath on the cliff-top, but detailed study would really require beach access.

The Kimmeridge Clay is not completely exposed at Kimmeridge. The lowest part has been studied in boreholes and is exposed in a rather thinner facies in the Osmington Mills and Ringstead area. At Kimmeridge, the type locality, the lowest part visible is at Hobarrow Bay. There is a fault, shown in photographs above which throws the Flats 9.75m up Arkell (1947). Notice that the Maple Ledge dolomite has a central thin bed of bituminous shale, about which it is in general symmetrical (this is not surprising because it is a product of burial diagenesis). The Flats has the small thrusts which when seen in cross-section in the cliff appear as thickenings of the bed. These are incipient "snake heads" where the hanging- wall bulges in a snake-head like form above the foot-wall, as a result of the blind thrust.

LOCATION - STRATIGRAPHY:

Hobarrow Bay - Aulacostephanus eudoxus Zone (KC 32)
(lowest strata seen - under the Flats just west of the Hobarrow Bay Fault)

Because of the up-faulting there is some exposure here of the shales under the Flats Dolostone Bed. These belong to the upper part of the eudoxus Zone with Aulacostephanus eudoxus (see photograph above, showing an uncrushed specimen from the Kimmeridge Clay of Black Head , near Osmington Mills). The shales here are notable for Nannocardioceras. These ammonites were formerly known as Amoeboceras. Arkell (1947) referred to a band 3m below the Flats with small crushed forms of this genus, both smooth and faint-ribbed types.

At the base of the cliff is a bituminous shale with widely-spaced fossiliferous concretions - the Nannocardioceras Cementstone of Cox and Gallois (1981). The Nannocardioceras Cementstone can be found at Black Head and Osmington Mills and Ringstead Bay. The Nannocardioceras beds can be traced from the Dorset coast to the Warlingham Borehole (Surrey) and on to the Wash area Cox and Gallois (1981).

The Lower Kimmeridge Clay, particularly the eudoxus Zone, commonly contains the small, curved and ribbed oyster - Nanogyra virgula (Exogyra virgula in old literature). This is not normally found in the Upper Kimmeridge Clay (above Blake's Bed 42 and above the Yellow Ledge). A specimen of this oyster found in a drainage ditch of the Weymouth Relief Road is shown above. With it are numerous shells of the bivalve Protocardia. These are common in various parts of the Kimmeridge Clay. The original aragonite shell is preserved and so too, are the original growth lines. Shell colour is not normally seen and the brownish stain is the result of weathering of pyrite.

Cox and Gallois (1981) have recorded that below the Flats Stone Band, about 3 metres up from the beach shingle, there are shelly oil shales or bituminous shales with Amoebites, Nannocardioceras, and large Nanogyra virgula. Incidently at Black Head, to the west of Osmington Mills Cox and Gallois (1981) have reported on the occurrence of a thin Virgula Limestone, also in the eudoxus Zone. This is a soft muddy limestone, just a few centimetres thick, and composed largely of Nanogyra virgula. The eudoxus Zone is near the base of the cliff of Kimmeridge Clay at Ringstead and is characterised by several bituminous shales. This section also contains many large Nanogyra virgula according to Cox and Gallois (1981).

LOCATION:

Long Ebb Ledge (map ref. SY893791)
(and adjacent cliffs)

Long Ebb is a long ledge of the Flats Dolomite Bed and is situated at the junction of Hobarrow Bay and Brandy Bay, to the west of Kimmeridge Bay. The Flats Dolomite Bed here has a surface marked by a polygonal pattern of small thrusts or teepees of diagenetic origin, as is the case at Charnel and Broad Bench. The dip, though, is to the northwest and the ledge is part of the northern limb of a small anticline; Broad Bench is part of the southern limb. This anticline is also seen in the western side of Kimmeridge Bay and the oil well is in the centre of it.

The cliffs at Long Ebb are of Lower Kimmeridge Clay of the Aulacostephanus autissiodorensis Zone. They include shales above and below the Washing Ledge Dolomite Bed, so conspicuous in Kimmeridge Bay and here too with a central parting. The autissiodorensis Zone is very obviously cyclical, as discussed elsewhere. It may lead to a false impression that the Kimmeridge Clay is in general a very cyclical formation. It is not generally cyclical in the neat and simple manner as seen here. See the cliff section at Cuddle, for an example of a sequence which is much more irregular.

The beds seen in the cliff here descend to the shore in Brandy Bay and most are quite well-exposed in the exposed shale on the foreshore. Remember, though, that this is in a firing range and the shore is not for public access even when firing is not taking place.

LOCATION:

Brandy Bay
(north of Long Ebb)

Brandy Bay (SY 983792 - SY 889795), to the west of Kimmeridge Bay and adjacent to Gad Cliff, is not accessible to the public at beach level (there is even a CCTV camera on the cliff above Broad Bench observing access to this bay!). It is within the military firing range and beyond the limit of the Range Walks. However, part of Brandy Bay (photographs above) is quite well-seen from the cliff top footpath from Kimmeridge Bay to the top of Gad Cliff. This is one of the Range Walks and is open when firing is not taking place, usually at weekends (most) and certain holiday periods.

As shown by the photographs above the Upper Kimmeridge Clay is exposed, dipping from 10 to 20 degrees northward. The sequence includes autissiodorensis, elegans, scitulus, wheatleyensis and hudlestoni Zones. The Flats forms the reef of Long Ebb, and from there in a northwestward direction the following stone band markers are present: Washing Ledge (dolomite), Maple Ledge (dolomite), Cattle Ledge, Grey Ledge, the Blackstone (oil-shale), the White Stone Band (coccolith limestone - laminite). Less conspicuous, and above the White Band, just under Gad Cliff, are the Middle White and Freshwater Steps Stone Bands. See Cox and Gallois (1981) for details. These authors mention that the thickness of the beds up to the Cattle Ledge (scitulus) is comparable to that exposed in the eastern limb of the Kimmeridge Bay Anticline (i.e. in the cliffs east of the bay), but that above this the Brandy Bay sequence is about 30 percent thinner than the comparable beds that crop out between Cuddle and Freshwater Steps. They suggested tectonic attenuation, but it is more probable that this is the result of depositional differences and the proximity of the Brandy Bay section to the Purbeck Fault-Monocline axis. There is much evidence of penecontemporanous thickness changes at Worbarrow Bay and Mupe Bay . The end-Jurassic and early Cretaceous phase of Late Kimmerian movements, so obvious in the Purbeck and the Albian etc of this area, may have commenced in Upper Kimmeridgian (scitulus) times (with some earlier phases in the Lias).

The most common macrofossils in the Brandy Bay section are various crushed ammonites, particularly Pectinatites, including Virgatosphinctoides and Arkellites. The notable fossil collector, Steve Etches of Kimmeridge village has recorded some interesting fossil finds from Brandy Bay, including fish remains of the Pachycormidae, Gyrodus, Lepidotes, an ammonite with an egg sac within the body chamber, and Trachyteuthis with soft parts preserved in a calcareous nodule. He has also found pterosaur bones there, and a dinosaur limb bone. There is no reason to suggest that the fossils are any more abundant at Brandy Bay than in other parts of the Kimmeridge Coast, as you will note from the listings in Etches and Clarke (1999).

Notable in Brandy Bay are the cyclical beds of the Aulacostephanus autissiodorensis Zone. These are easily seen from the cliff top and easily found because they commence with the Flats Dolomite Bed (top Aulacostephanus eudoxus dolomite) at Long Ebb. One image above shows the cyclical beds inverted, so as to place the Flats at the base and the sequence in natural order, and also mirrored and stretched for comparison with tabulated magnetic susceptibility cycles.

This rythmic or cyclical sequence is well exposed at low to mid tide if seen on the shore of Brandy Bay. Details of magnetic susceptibility, total gamma ray, uranium, potassium, thorium etc in these beds have been given in the large Kimmeridge Clay log of Coe et al. (2001).

Weedon et al. (1999) agreed with House (1985) and others in attributing them to the 38ka (38 thousand years) orbital obliquity cycle (this is the result of the periodic changes in the tilt of the Earth's rotational axis relative to its plane of rotation around the sun, the tilt generally varying between 21.8 to 24.4 degrees over about 40 thousand years; it is one of three main cycles in Milankovitch cyclicity, and the rival mechanism is the 20ka precession cycle). Weedon et al. have studied the cycles systematically by magnetic susceptibility and reference should be made to that paper for more information.

The sequence visible on the shore is a natural representation of this cyclity as shown by relative erosion of the beds within the cyclical sequence. It is showing the sedimentation effects, resulting from changes in palaeoclimate and palaeoceanography, of astronomical forcing. A major factor was probably the changes in the earth's axis in relation to the plane of the orbit at a particular time (during magnetic polarity chron M23 in the late Jurassic).

However, it should be noted that cyclicity within the Kimmeridge Clay formation is very well seen in certain parts of the sequence (particularly near the car park!). It would incorrect to assume that the whole sequence is highly cyclical. Some parts clearly show cyclicity, but other parts do not demonstrate it. This is also true for much of the Jurassic sequence in Dorset. Of course, one obvious reason for very imperfect cyclicity is that the Kimmeridge succession is placed almost exactly at the margin of the English Channel Inversion. It is a place of penecontemporaneous tectonism. This explains why the Kimmeridge Clay almost doubles its thickness from Ringstead Bay to Kimmeridge. It also explains why the Brandy Bay section is not exactly the same in terms of thickness and lithology (although similar) to that in the cliffs east of Kimmeridge Bay. This is not a criticism of studies of cyclicity; it is simply a statement that this is a place of Late Cimmerian tectonism, not a place of stability recording eustatic changes in the late Jurassic. Therefore two processes near separating, if that is possible.

In the shale ledges between the Washing Ledge Dolomite Bed and the Maple Ledge Dolomite Bed some large specimens of ammonites of the genus Aulacostephanus can be seen. In this shale sequence they are almost always in crushed and flattened condition. Solid well-preserved ammonites, like those of parts of the Lias are rarely found here. The original aragonitic shells is preserved though. To recognise Aulacostephanus look for the bullae or tubercules close to the umbilicus. If you are fortunate enough to see part of the venter (not usually visible) look for a smooth band.

LOCATION - STRATIGRAPHY:

Brandy Bay - Unidentified Ledge

The ledge above is shown to illustrate the problem of identification. This ledge is in the northern part of Brandy Bay. It beneath the White Stone Band and the Basalt Stone. It is above Flats Stone Band and the Washing Ledge. It shows the unusual feature for the diagenetic dolomites of possessing some lamination. Thus it has some partial similarities to pectinatus beds but it is obviously Upper Kimmeridgian below the half-way point of the hudlestoni Zone. Is it Grey Ledge (wheatleyensis) or the Rope Lake Head Stone Band (low hudlestoni). You will see from the log that you can decide this by its relationship to the Blackstone. However, the photograph does not make this easy.

LOCATION - STRATIGRAPHY

Brandy Bay - Oil Shale - Kimmeridge Blackstone

Please see also the webpage on:
The Kimmeridge Blackstone - at Clavells Hard and Rope Lake Head, East of Kimmeridge Bay

The Blackstone, the main oil-shale is about 60cm in thickness here. It contains, as usual, large pyritic concretions and the characteristic minute plates of the pelagic crinoid Saccocoma. Between the Blackstone and the coccolith limestone, the White Band, there is, on a small scale, some alternation between oil-shale facies and coccolith limestone facies. Obviously both facies are the result of deposition of planktonic debris, but this variations from carbonate to dominantly organic material is of interesting palaeoenvironmental significance.

About 80cm below the Blackstone is the Bubbicum, another oil shale which is slightly thinner than the economically useful Blackstone. It has not been worked commercially in the cliffs to the east of Kimmeridge Bay. Pyrite is less obvious in the Bubbicum than in the Blackstone, so it is easy to distinguish the two.

LOCATION-STRATIGRAPHY:

Brandy Bay -
Slumped Carbonate Bed
(Low hudlestoni Zone)

The slumping in this thin white, partially laminated, limestone was noticed by Dr. David Martill, during the field trip with Ian West, Steve Etches and Ramues Gallois on the 11th March 2012. The precise stratigraphical position of the bed is not absolutely certain. It seems to be the equivalent of the Rope Lake Head Stone Band (coccoliths and dolomite) or the Short Joint Coal (unusual limestone associated with bituminous shale). The matter will be considered in more detail later.

There is small scale slumping in a coccolith micrite not far above the Blackstone in Brandy Bay. This was found by Dr. David Martill of Portsmouth University. The slumping seem to be almost confined to the coccolith bed (rather similar to the White Stone Bands above). It is in the expected direction, that is southward towards the Kimmeridge Basin (English Channel Inversion). It has long been realised that the Kimmeridge coastal section is at the very edge, almost at the hinge line, of the basin. This explains the much greater thickness and some facies differences from the shelf facies at Ringstead. It also explains the minor differences between the Brandy Bay section (usually with slightly thinner units) and the main Kimmeridge cliff section to the east (but effectively slightly south of Brandy Bay). The Kimmeridge type section has long been recognised as as having been affected by local tectonism (as is the Purbeck Group or Formation). It certainly shows good cyclicity in part (particularly in the autissiodorensis Zone which is the part near the car park). However, it is not an undisturbed deepwater facies. It would also be incorrect, however, as to regard as greatly disturbed; it is a mixture of undisturbed and disturbed facies.

The thickness and facies irregularities of the Dorset Jurassic are very interesting. The here Jurassic is prone to show local tectonic effects here and there from the Lias upward (i.e. Junction Bed, condensed Inferior Oolite etc). Near to the Late Cimmerian (or Late Kimmerian) phase these effects are more pronounced. The most extreme evidence is from the Kimmeridge Clay phosphatic ammonites which are reworked into the Upper Purbeck Unio Bed in the Weymouth area West and Hooper (1969). So within about 10 or 15 million years the Kimmeridge Clay was strongly uplifted on the high, north of the Inversion. The effects shown in the photographs here are very small but they are clear proof that the Kimmeridge Clay conditions in southeast Dorset were at times a little unstable, at least on a small-scale.

Examine carefully the excellent cliff log by by Coe et al. to see just which parts of the sequence are cyclical and which are not.

As a follow-up to the discussion of minor slumping in an apparent coccolith micrite, above, it should be noted that the main coccolith micrites, the White Stone Band etc. may not be totally free of minor liquefaction or slumping. An example is shown above. In addition, local expansion of thickness into a small channel was observed by Dr. David Martill. Some Late Cimmerian movement would be expected to have taken place during the sedimentation at the northern margin of the English Channel Inversion.

LOCATION-STRATIGRAPHY:

Brandy Bay, Coccolith Micrites
(The White Stone Band, Middle White Stone Band and Freshwater Steps Stone Band etc.)

LOCATION - STRATIGRAPHY:

Coccolith Laminites - Details

LOCATION:

Gad Cliff (SY 880795)

Portland Stone forms the upper part of this spectacular precipice. In spite of the occurrence of the stone over Portland Sand and Kimmeridge Clay there are few rotational slips or major topples. The cliff differs greatly from the cliffs of the Isle of Portland in this respect, probably because of the significant landward dip. Mostly there is fall of individual rocks to the shore or small areas of cliff fall, as is clearly visible. At the foot of the promontory in the distance is a large block known as Wagon Rock. The height of the cliff here is 134 m.

The undercliff here is in the Army firing range and is inaccessible, which makes it a good nature reserve. There is a Range Walk, open at times, along the cliff top and various photographs have been taken from this. When the walk is open you can continue westward to Worbarrow Bay.

LOCATION-STRATIGRAPHY:

Gad Cliff - Kimmeridge Clay Beneath

The upper part of the Kimmeridge Clay as seen in some limited exposures under Gad Cliff belongs the Kimmeridgean Stage sensu anglico. It might be useful, by following the publication of Gallois and Etches (2001) , to mention its international correlatives. It is Portlandien Superieur sensu gallico , i.e. in northern France. In the Tethyan Province to the south the equivalent is Upper Tithonian (Simplisphinctes and Paraulacosphinctes transitorius Zones), and with regard to Russian classification it is Middle Volgian (Dorsoplanites panderi and Virgatites virgatus). This might be of use to non-British readers.

With regard to the details of the sequence, Gallois and Etches (2001) have recently described the upper part of the Kimmeridge Clay for several Dorset localities. They have provided a small columnar section for a exposures of the Fittoni and Rotunda zones near Waggon Rock, under Gad Cliff, although it should be emphasised that these beds are better seen in the Hounstout area, east of Kimmeridge. The exposures at Gad Cliff are weathered and separated by fallen debris from the Portland strata above. Because this locality is closer to the Purbeck Monocline (and associated fault) and the structural high to the north it is not suprising that the sequence is thinner (from penecontemporaneous thinning) than elsewhere in the Kimmeridge region. The beds are dipping at 25 degrees (angle) to 376 degrees (direction) and may be affected by minor faults and shearing. This, of course, is the usual pattern because the coast is oblique to the main structure; as Mupe Bay and Lulworth Cove is approached the Upper Jurassic and Lower Cretaceous strata are all more affected by Late Kimmerian and tectonic thinning.

According to Gallois and Etches (2001) the lowest part of the sequence (beds KC 54 to 58) can be matched in detail with the type-section further east. The higher sections contain lithologies and fauna that are similar to parts of the Lower Hounstout Silt, the Hounstout Clay and the Upper Hounstout Silt but are too deeply weathered and subject to mass-movement for those authors to have been able to correlate them in detail with the Hounstout section. Consult their paper for further information on the lithologies and faunas of the particular bed in the Kimmeridge cliffs.

STRATIGRAPHY:

Gad Cliff - the Portland Succession

In order to understand the Portland Group of Gad Cliff it is necessary to see the general classifications of subdivisions of these strata. The diagram on the left is based on the classic work of Arkell (1933 and later) and shows the general uppermost Jurassic/basal Cretaceous succession in the Isle of Purbeck , as compared with that on the Isle of Portland. The Purbeck sequence follows the Portland strata. Gad Cliff contains a variation of the Portland succession of the Isle of Purbeck type shown here. More detail will be given below. On the right are shown two alternative schemes of different date for the terminology of the Portland and Purbeck successions. Although the Arkell scheme is still in common use, Townson (1975) introduced a largely new terminology, shown alongside here (diagram after Bosence, 1987, from Townson, 1975). Note that there are some correlation problems, and see Wimbledon (1986). Whichever scheme is used the work of Townson, 1975) should be consulted for sedimentological information.

Arkell (1933, 1935, 1947) provided some useful detail on the Gad Cliff Portland sequence. He commented that for two kilometres the highly inclined Portland Stone and the basal part of the Purbeck Formation have been eroded into jagged bastions. These overhang a slope of Portland Sand with Kimmeridge Clay at the foot. In this area the name Portland Sand is a misnomer and it mostly consists of marl and dolomite with very little sand. The undercliff contains fallen debris but has few landslides, compared to the cliffs of Portland. Gad Cliff is a striking feature and owes its grandeur to the fact that, whereas at St. Aldhelm's Head the strata are almost horizontally, at Gad Cliff they dip inland at 25-35 degrees.

In the west to central part and high in the cliff two conspicuous white bands can be seen. The upper of these is not present in the eastern part near Brandy Bay because of erosion. The higher one is the Shrimp Bed (V) (a bed with the crustacean Callianassa) and marks the top of the Portland Stone; the lower is about on the horizon of Bed M of the Cherty Series.

LOCATION - STRATIGRAPHY:

Gad Cliff - Portland Sequence - Details

A list of the succession of Portland Group strata at Gad Cliff is provided below. This should be useful as a convenient guide for the general reader, but research workers, who will no doubt also consult the original publications, should treat it with caution for the following reasons. It is largely based on the old work of Arkell (1933, 1935, 1947) and partially updated by reference to the sedimentological study of Townson (1975). The faunal data is probably sound and the measurements fairly accurate (although converted from feet and inches to metric). The references in Arkell to "cementstones" and "sandstones" are not all reliable. Some "sandstones" have been corrected to dolomites, but while most of the "cementstones" probably are argillaceous limestones, some may be dolomites. It is recommended that research workers check them by XRD. A further factor to bear in mind is that the measurements from bed K upwards were made at the western end of the Gad Cliff section at Pondfield and Worbarrow Tout, whereas the lower units were measured under Gad Cliff. In relation to the major structure the western sections are in effect northward and down-dip compared to the high Gad Cliff section. By analogy with the Kimmeridge Clay some limited (Late Kimmerian) thinning northward would be expected and thus the thicknesses of the higher units might be slightly underestimated. Obviously any research worker involved with the detail here should try to obtain new measurements, with good sedimentological records, if that is possible at a difficult section within a firing range!

Note that the the lettering of the beds B to V of the Portland Freestone correlates with Arkell's section at St. Aldhelms Head, but the beds of the Portland Sand are numbered independently because of difficulty of precise correlation.

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THE WINSPIT MEMBER OF THE PORTLAND LIMESTONE FORMATION - THE PORTLAND FREESTONE

V. Shrimp Bed. White micrite or lime mudstone, with the usual fossil content. In two blocks.- 2.1m.
U. Titanites Bed. Fossiliferous, bioclast grainstone (biosparite or biosparrudite) limestone, with giant Titanites 0.3m from the top, and many bivalves. - 2.7m.
T (part). Band of small chert nodules. - 0.3m.
T. (main part) Pond Freestone. Ooid grainstone (oosparite) with a rich shell bed in the middle containing "Trigoniae" etc. - 1.2m.
S. Chert Bed. Three thick and several thinner bands of chert in limestone, fine bioclast packstone or wackestone (biomicrite). - 1.5m.
R-O. Under Freestone and Caps. White, oolitic freestone, (ooid grainstone or oosparite and/or bioclast grainstone) in two blocks. - 2.3m.

Total of Winspit Member (Portland Freestone) - 10m.

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DANCING LEDGE MEMBER AND DUNGY HEAD MEMBERS OF THE PORTLAND LIMESTONE FORMATION - THE CHERTY SERIES

N. (part) Band of small chert nodules. - 0.3m
N. (part) Buff Rhaxella wackestone (?) referred to by Arkell as sandy limestone or calcareous sandstone. - 1.5m.
N. (part) Buff Rhaxella wackestone (?) referred to by Arkell as sandy limestone. With undulating but mainly horizontal veins of thick tabular chert, as in bed N at Worth (Swanworth Quarry?). - 2m. (N - total - 3.8m)
M. & L. White limestone, Rhaxella wackestone (?), with two bands of chert nodules, and crossed by numerous parallel oblique veins of black secondary chert which cut the bedding at an angle of about 45 degrees. - 2.1m.
K. Limestone, Rhaxella wackestone (?), with a conspicuous and persistent vein of tabular black chert, 15cm to 25cm thick. Forms good datum for correlating Gad Cliff with Worbarrow Tout. - 1.2m.
J. to E. undivided. Cherty limestones, Rhaxella wackestone (?), an almost indivisible and inaccessible mass, which forms the main feature of the precipices of Gad Cliff. - 9.1m.
C. Limestone, Rhaxella wackestone (?), with much chert, a well-marked block. - 1.2m.
Soft Rhaxella wackestone (?), referred to by Arkell as "sandstone, weathering to a sand", with two or three rows of chert nodules. - 0.9m.

Total thickness of the Dancing Ledge and Dungy Head Members (Portland "Cherty Series"). - 18.3m.

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THE PORTLAND SAND FORMATION

GAD CLIFF MEMBER (THE BLACK DOLOMITES AND PARALLEL BANDS)
(these have been referred to wrongly in the old literature, including that of Arkell, as the "Black Sandstones".)

A.15-17. Hard black dolomite, riddled by cavities with shapes resembling chert nodules, but filled with black calcite; showing burrow structures (Thalassinoides?). Honeycombe weathering. In three blocks, as at St. Alban's Head, the two lowest with the abundant perisphinctid ammonite Glaucolithites. The lowest bed (1.5m) is the "Upper Parallel Band". Total - 6.9m.
14. Soft grey mud and mudstone, hardens on contact with the sea. 0.9m.
13. Middle Parallel Band: Dolomite ("cementstone" of Arkell). - 0.3m.
12. Lower Parallel Band (equivalent of): Grey marl with a thin, soft dolomite (?) ("cementstone" of Arkell) bed in the middle, and a 0.3m dolomite (?) ("cementstone") bed at the base. - 1.2m.

Total of the Gad Cliff Member - 9.1m.

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PONDFIELD MEMBER (ST. ALBAN'S HEAD MARLS)

11. Grey-blue shaly and sandy marls and shales with rows of cementstone nodules, especially in the upper part. - 6.7m.
10. Exogyra Bed. Tough, pale-grey, sandy, marl or argillaceous limestone full of flattened pellets of white marl, and crowded with small valves of Nanogyra nana, with abundant Plicatula boisdini de Loriol, etc. - 0.6m.
9. Brachiopod and Exogyra Beds. Tough grey sandy marls (hardening on contact with the sea-water) with rows of cementstone nodules and in the bedding-planes impersistent layers of larger Nanogyra nana crowded together, with abundant Oxytoma octavia, Ostrea expansa, Plicatula boisdini and locally (in one seam, 1.8m from the top) numerous Rhynchonella sp. and Terebratula bononiensis Sauvage and Rigaux. - 3m.
8. Tough grey sandy marl. - 1.5m.
7. Two bands of tough grey marlstone with softer parting. -0.6m.
6. Yellow sandy marl (10cm) overlying grey marl (20cm) full of Nanogyra; also Oxytoma octavia, Serpulae, minute Terebratulid. - 0.3m.

Total of Pondfield Member (St. Alban's Head Marls) - 12.8m.

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BLACK NORE MEMBER - UPPER PART (THE WHITE CEMENTSTONE AND EMMIT HILL MARLS)

5. Tough, grey-centred, brown-weathering, marly, sandy cementstone weathering to yellow marly sand. 1.8m.
4. Grey sandy marls, some bituminous, with three rows of cementstone nodules (0-15cm). - 9.1m.
3. Marly cementstone band, breaking into nodules. 0.15m - 0.22m.
2. Grey sandy marls, as above. 1.06m.

Total of Black Nore Member - Upper Part. - 12.2m.

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THE MASSIVE BED

1. Tough grey, cream-weathering sandy marl and marlstone, with obscure nodular structure. Stands out as a hard band, but not so conspicuously as it does farther east. - 1.5m.

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Total thickness of Portland Sand sequence shown above. 35.7m.

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[Beneath - Top of Hounstout Marls, dark grey-blue marl seen to 4.6m. The scheme of Townson (1975) continues the Portland Group down to include the Hounstout Clay and the Rhynchonella Marls]

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Please see also the webpage on:

The Kimmeridge Blackstone - at Clavells Hard and Rope Lake Head, East of Kimmeridge Bay

Bibliography and References - Kimmeridge

Please see separate

Kimmeridge - Bibliography - Start
Kimmeridge - Bibliography Continued

ACNOWLEDGEMENTS

I particularly thank Alan Holiday for his help in kindly providing some good photographs of the Kimmeridge area. Pari White and Michael Bauer have very kindly contributed several very good photographs and I much appreciate. I am very grateful to the expert on Kimmeridge, Dr. Ramues Gallois for showing me interpretations of offshore geology in the area, and for helpful discussion. Steve Etches, well-known for his extensive Kimmeridge Clay fossil collection, has greatly helped in the field and has shown me fossil material from the localities discussed here. I very much appreciate the very useful discussion of sedimentological and other features in the by Dr. David Martill of Portsmouth University. He has been particularly helpful in discovering sedimentological and other features in the coastal section. His guidance has led to many improvement in the Brandy Bay section of this webpage. I am much obliged to Dr. Bertrand Ligouis and his research team for permission to use photographs relating to his geoarchaeological research on the Kimmeridge oil shales. I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.

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Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.

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at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.