West, Ian M. 2019. Mupe Bay, Arish Mell and Bacon Hole; Geology of the Wessex Coast of England (UNESCO World Heritage Site, Jurassic Coast). Internet site. http://www.southampton.ac.uk/~imw/mupe.htm. By Dr. Ian M. West, Geology of the Wessex Coast, Romsey and Southampton University. Revised version: 17th August 2019.
Dr. Ian West,
Geology Wessex Coast, Romsey
and Visiting Scientist at:
Faculty of 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.
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LULWORTH COVE WEBPAGES:
LULWORTH COVE INTRODUCTION
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Lulworth Cove - Stair Hole
Lulworth Cove - Fossil Forest - General
Fossil Forest - Purbeck Trees
Lulworth Cove - Dungy Head and St. Oswald's Bay
Lulworth Cove - Durdle Door to Bats Head
Lulworth Cove - Mupe Bay and Bacon Hole
Lulworth Cove Select Bibliography
See also associated webpage: Worbarrow Bay.
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Mupe Bay (or Mupes Bay or Mewps Bay) and Bacon Hole is within an Army Firing Range, open at certain times. Is essential to abide by the safety rules and regulations of the Army Ranges; these are well-displayed on various notices in the area. There are recognised and marked Range Walks and off these there is danger of the presence of unexploded shells. Suspicious objects should not be handled or tampered with in any way. Accidents involving shells or other missiles are exceptionally rare within the Ranges, although a case has been recorded long ago (on Whiteways Hill). Nevertheless care should be taken.
Danger from cliffs is in practice probably greater than that from shells. Climbing of cliffs, particularly chalk cliffs, has led to many accidents, many rescues and, sadly, some fatalities in the Lulworth area. Rock falls have killed students and a teacher at Lulworth Cove and there is significant risk in the Mupe Bay area. As the photographs below show, major falls have occurred on the Chalk cliffs on the north side of Mupe Bay and there is dangerously unstable chalk still hanging on those cliffs. Keep away from the quite obviously dangerous parts of the Chalk cliffs. Rock falls can occur on the relatively lower cliffs of Purbeck strata in Bacon Hole, particularly in the area near Smugglers' Cave. Wear hard hats and take care, especially in wet or frosty weather, which may cause debris to fall.
It is hazardous to try to climb down the cliff from the top above the southwestern promontory of Bacon Hole, or from other places. A usually feasible access route is via the footpath down to Mupe Bay and then along the beach.
There is risk of accident by trying to traverse the foot of cliffs too far at low tide and being cut off by the rising tide. This risk is probably greatest at the foot of the Chalk cliffs between Mupe Bay and Arish Mell. It is unlikely to be a major hazard on the main shingle beach of Mupe Bay or in Bacon Hole, except perhaps in extreme weather and tide conditions. Rock-climbing attempts have been made in the past to traverse between the Fossil Forest ledge and Bacon Hole; this is dangerous; it is not a feasible scramble and should not be attempted by other than experienced rock climbers, if at all. Take care in the course of ordinary geological scrambling, beware of being caught by a large wave when on rocks near the sea (two rock climbers have been drowned near the Fossil Forest).
There is the usual risk of slipping on algal-covered rocks at low tide. Adders can live on the cliff-top grassland, but are only a minor risk. Keep clear from cliff edges particularly when on the Chalk hills. The path up Bindon Hill at the edge of Mupe Bay has stone steps but is very steep and a little exposed, as is also that from Arish Mell to the hill top (above Cockpit Head). Care should be taken at both these localities and persons who are older or unfit should proceed at a slow and comfortable pace. Note that although Bindon Hill is only about 150m high the steepness of the slope and proximity of a dangerous precipice to the sea gives an almost a "mountain" feeling to these trails. Some walkers use poles here.
Take a mobile phone and bear in mind that the Army Range notices provide an emergency number (see photograph above).
For more information on geological risks see the Safety Webpage. A description in the present webpage of a geological locality does not imply that it is safe to approach the place, or that there is permission to do so, even if it has been studied in detail in the past. Field leaders should make their own assessments of risk appropriate to terrain, weather conditions on the day and tide on the day, and the ability and fitness of the field party.
Mupe Bay is within an Army firing range, that is usually open on most Saturdays and Sundays (but not all). It is open at certain holiday seasons as in August, Christmas and Easter. En route to Lulworth Cove you will there are notices which state whether the Range Walks are open or not. Lulworth Cove car park provides the easiest access to Mupe Bay. It is possible to walk from Worbarrow Bay but this is a longer excursion.
You can combine your trip to Mupe Bay with a visit to nearby Fossil Forest. It is unwise to try to do too much in a short time. The Lulworth Cove area can easily occupy two days or more. Note that toilets, shops and an interpretation centre are present at Lulworth Cove. There are no facilities at Mupe Bay.
Bacon Hole and Mupe Bay are just off to the right (east) of the map shown. Take the path from Lulworth Cove and head eastward. Smugglers Cave is at Bacon Hole.
INTRODUCTION continued:
The paths around Mupe Bay and Bacon Hole are shown in the above aerial photograph. You can walk eastward to this area from Lulworth Cove when the Army Ranges are open and the red flag is not flying. The access is normally at weekends, although not every one, and also at certain holiday times of the year. See the Army Lulworth Ranges Information webpage.
There is one footpath down to the beach. It is not feasible to descend the cliffs at other places, and there is both risk of falling and risk from unexploded missiles. From the beach there is access to Purbeck exposures and Wealden exposures, and also to Upper Cretaceous, Chalk etc in the northeast of Mupe Bay. The basal Purbeck section can be seen by walking round ("rock hopping") Bacon Hole to the Smuggler's Cave area at the southwestern end.
Geological Maps
The British Geological Survey map, 1:50,000, Solid and Drift, 2000 Edition, Swanage Sheet, 343 and part of 342, is well worth purchasing. It can be obtained from the British Geological Survey website and is very inexpensive, costing only 12 pounds sterling.
STRATIGRAPHY:
Strata of the Mupe Bay Region - Introduction
This image shows the simplified succession of strata in the Mupe Bay and Lulworth Cove region. The thicknesses given are only approximate and apply to Lulworth Cove. At Mupe Bay most units are slightly thicker and they continue to thicken in an eastward direction. FF - Fossil Forest horizon, LGS - Lower Greensand (very thin here), PL - plenus marl, a grey marl marking the boundary between Lower and Middle Chalk.
A simplified geological map of the Lulworth Cove area, including Mupe Bay, based on Townson ( 1975b) and the British Geological Survey map of Swanage (sheet 343 and part of 342). The dip in the Portland, Purbeck and Wealden strata is steep and to the north. The synclinal axis can be recognised by the long narrow Tertiary outcrop north of the cove. This axis runs east-west. The main axis of the very asymmetrical anticline is under the sea, at the Lulworth Banks, south of the map. The steeply dipping strata is part of the north limb which runs along the coast. The locations of some of the many notable geological features here are shown. Localities east of those described in this particular guide are in the military firing ranges.
Map and Cross-Section
Left: A detailed geological map of the Mupe Bay area based, with additions and minor modifications, on part of a map of Nowell (1997) . Note that two quartz grits are present at Mupe Bay and shown on the map. This matter is discussed further below. Note the minor fault cutting through Mupe Ledges. These reefs are formed by Middle and Upper Purbeck limestone beds, the shales between having been eroded away by the sea. The Cinder Bed is easily found here.
Right: This is an old north-south cliff section of Mupe Bay, drawn to scale, and showing the general features. Later work has led to some additional details and perhaps some minor modifications.
Wealden Group - Introduction
The Lower Cretaceous, Wealden Group consists of mottled reddish, purple and buff sands and clays as elsewhere. These are mainly fluvial with some coarse sandstones or grits and black lignitic branches and trunks of trees. Fossils, other than plant material, are rare. The most notable feature is a conspicuous conglomeratic oil sand (to be discussed below). It is situated immediately north of the steps down to the beach.
Wealden Group - Thickness
Arkell (1947) noted that the Wealden here is only about 229m (about 750 feet) in thickness, not much more than half as thick as it is only 2.5 km away across the bay at Worbarrow. Now it is clear that the gap between Mupe Bay and Worbarrow Bay marks a major (probably oblique) boundary between a thin shelf or high facies, as at Lulworth Cove and a relatively thicker basinal facies to the east-southeast (towards Swanage). This conspicuous change is easily seen in the Purbeck Formation and the Wealden Group, but Kimmeridge, Portland, and other Cretaceous units are also affected. The details, however, are complicated and a matter for continuing discussion.
Most geologists have regarded the thinning to have been piecemeal, a general sedimentary thinning. Nowell (1997) , however, as noted above, showed two quartz grits on his geological map (see above). He considered that the lower one is the equivalent of the quartz grit seen at Lulworth Cove. He thought that the upper one is not present at Lulworth Cove because of erosion before the deposition of the Lower Greensand and Gault. If Nowell is correct, then the Wealden Group has mainly been thinned westward to Lulworth Cove by erosion after Wealden deposition, and not by sedimentary thinning. This view has been criticised by Radley (1998) . For further information see the Nowell paper and the discussions in the Proceedings of the Geologists' Association.
Multibeam Bathymetry Information on Wealden Thinning
(These are later illustrations and comments, 19th July 2010)
The multibeam bathymetry image of Worbarrow Bay, shown above should be compared to the BGS map, a small modified section of which is shown further above. The multibeam bathymetry image confirms that that the outer Portand and Purbeck stone outcrop is not broken by any major fault, although the BGS map shows one at Arish Mell. The bathymetry image reveals that the Kimmeridge Clay outcrop contains a number of north-south trending faults (as elsewhere). The westward overstep of the Wealden by the Albian in Worbarrow Bay is clearly shown. The Wealden is much thinner at Mupe Bay. Note that the continuation of the Coarse Quartz Grit of the Wealden across the bay is very well-shown on the bathymetry image. This confirms the discussion of this topic by Arkell (1947), pp. 158-159. He noted that the Wealden strata are of about 229 metres thick at Mupe Bay in contrast to 425 metres at Worbarrow Bay cliff section. The reduction has been to almost half. The Coarse Quartz Grit at Mupe Bay is 30 metres above the middle of the Wealden at Mupe Bay, whereas it is almost at the middle in the cliffs at Worbarrow Bay. Arkell commented that some 30 percent of the upper division of the Wealden was removed before the deposition of the (locally thin) Lower Greensand. (note that in the illustrations, and in some cases in the text, distinction is not always made between sub-Albian and sub-Aptian unconformities. They are related and closely adjacent phases of the Late Kimmerian unconformity. The well-known, sub-Albian is the major one.)
PETROLEUM GEOLOGY - MUPE BAY:
Strata - Wealden Conglomeratic Oil Sand - the Mupe Bay Palaeoseep
As mentioned above, a conglomeratic oil sand, outcropping near the steps is one of the most interesting features of the Wealden section. It has long been regarded as the consequence of a natural example of "palaeo-pollution", i.e. a river into which oil seeped in Cretaceous times. West (1975, p.211) , believed the seepage to have occurred up an unexposed intra-Cretaceous fault.
See Wimbledon, Allen and Fleet (1996) for some more recent studies. Selley and Stoneley (1987) and Stoneley (1992) followed this theory further but Miles et al. (1992) argued against the view. See also Cornford et al. (1988) for organic geochemistry of the Mupe Bay oil sand.
This diagram, based on Selley and Stoneley (1987), was captioned as "Geophantasmogram to illustrate the evidence for late Jurassic and Early Cretaceous movement along the Purbeck disturbance or fault, and the genesis of the Mupe Bay palaeoseep." Note that this theory is slightly different from that of West (1975) which invoved an additional, local fault transverse (i.e. roughly north-south) to the main east west structures shown here. According to Miles et al. (1992) penecontemporaneous oil seepage is a myth. However, the latest new information on the topic below shows that there really was transportation of oil-cemented clasts into the bed, and that there is also a modern active oil seep, here, at Mupe Bay!
In spite of some arguments against the Mupe Bay conglomeratic oil sand being evidence of a palaeoseep, it has generally been considered that oil migrated from the Portland - Isle of Wight Basin (offshore) in Cretaceous times. This is the conventional theory for the origin of the oil in the Wytch Farm Oilfield (with some additional Tertiary migration). It is now important to note that the original theories of early Cretaceous oil seepage and migration at Mupe Bay just here have been proven again.
See the key paper by Emerton et al. (2013) - A Magnetic Solution to the Mupe Bay Mystery. The abstract is given below, as a convenient summary, but the full paper is available online and this should be read. The reference and abstract (with highlights) is given below:
Emerton, S., Muxworthy, A.R. and Sephton, M. A. 2013. A magnetic solution to the Mupe Bay mystery. Marine and Petroleum Geology, vol. 46, pp. 165-172. September 2013. By Stacey Emmerton, Adrian R. Muxworthy and Mark A. Sephton. Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
� The clasts hold a magnetic signal unlike the matrix indicating two phase staining.
� The clasts show similar directions and thus were soft when deposited into the bed.
� The clasts indicate the onset of oil migration in the Early Cretaceous.
� The matrix is consistent with today's magnetic field and is an active oil seep.
An outcrop of Wealden beds at Mupe Bay (UK) is associated with a key piece of evidence for the timing of hydrocarbon migration in the Wessex Basin. A conglomeratic bed contains oil-stained clasts and matrix that appear different upon superficial observation. Conventional interpretations assign differences to the erosion and transport of oil-cemented clasts by Wealden rivers before their incorporation into a later stained conglomeratic bed. This scenario constrains the onset of oil migration in the basin to the Early Cretaceous; however, arguments have been put forward for single phase staining.
Magnetic information may provide new ways to examine the Mupe Bay record of oil migration. Migrating fluids such as hydrocarbons have been shown to cause chemical conditions suitable for the alteration or formation of authigenic magnetite resulting in associated chemical remanent magnetization (CRM). Magnetic characterization reveals both the matrix and clasts contain multi-domain magnetite but abundant hematite only exists within the clasts. Hysteresis parameters show the matrix has more multi-domain and likely larger magnetic grains than the clasts.
Magnetic directions are different in the clasts and matrix supporting a two-phase oil-staining event. Moreover, paleomagnetic directions for the clasts after tilt correction (82.6�N and 155.2�E) are consistent with biodegradation processes in the Early Cretaceous. Consistent directions in separate clasts imply the biodegradation and magnetite formation took place following transportation and incorporation of the oil-cemented clasts into the conglomerate bed. Magnetic directions reveal that the Mupe Bay matrix has a remanent magnetization corresponding to today's magnetic field, confirming the matrix represents an active oil seep.
This study represents an unprecedented use of magnetic data to date the onset of oil migration in a basin. The classic two-stage oil-staining scenario, which constrains the onset in the Wessex Basin to the Early Cretaceous, is supported by our data.
Portland Group
Portland Freestone Member
The Portland Freestone is usually accessible in sloping ledges at the southwestern promontory of Bacon Hole. It is not a very steep slope, but if it seems at all hazardous do not ascend it. Only the uppermost part of the Portland Freestone is readily accessible to the average geologist, although obviously rock climbers could reach more. Thus it is not easy to make a good comparison with the well-known and quarried rock faces of the eastern Isle of Purbeck (Winspit, Seacombe etc) or of the Isle of Portland. The nearest place to the west where the Portland Freestone and Portland Cherty Series is exposed is at Dungy Head, but there the section is much tectonised. At Worbarrow Tout and Pondfield Cove across the bay from Bacon Hole, only the top of the Portland Freestone is really accessible. Study of the western end of Gad Cliff really requires a boat, although it was accessed on foot by Arkell (1935) (Steve Etches can probably get to it!).
LOCATON - BACON HOLE:
Basal Purbeck Exposure
Bacon Hole - Basal Purbecks - Introduction
The basal Purbeck strata are only accessible at the southwest corner of Bacon Hole (i.e. as far as you can go). They do not crop out elsewhere in this region, except within Mupe Rocks, but they are not generally directly accessible. The exposure where they can be seen, at the southwestern corner of Bacon Hole, and is on and adjacent to a large sloping slab of limestone. The slab is largely of Portland Freestone Member, its angle controlled by the northward dip. This limestone is oolitic, although not so porous as on the Isle of Portland, and seems harder. It contains the poorly preserved remains of large ammonites (Titanites) and moulds of thick-shelled marine molluscs. Directly above it comes the basal Purbeck (basal Lulworth Formation) succession. This is accessible with care up the slab and to the right, by an easy scramble. Beware of falling over the cliff to the seaward and watch out for risk falling rocks; do not attempt this if you are unsure of your safety on this slab.
The details of the section are shown in the left column of a diagram here. In the field, look at the junction with Portland, and the surfaces just below it. An interesting and peculiar feature is the occurrence of pseudomorphs after halite in the uppermost part of the Portland Stone. These have clearly originated during a drying out of the very early Purbeck lagoon.
The basal Purbeck, Hard Cap shows some ripple-laminated peloidal and oolitic limestone, with some stromatolitic and some ostracodal limestone. Interestingly there are some small slump or liquifaction structures, probably related to the location of this site to the faulted margin of the English Channel Inversion (note the thickness changes from here to Worbarrow Tout). Above the Hard Cap is the Great Dirt Bed. This is easily recognised up on this ledge by its dark colour and its content of limestone pebbles. No trees seem to have been found here (this is at the southeastern edge of the forest).
Many Purbeck features are accessible from the beach at Smugglers' Cave (adjacent northward). Beware of falling rocks here. The Great Dirt Bed and overlying Soft Cap is accessible at the left hand side of the cave mouth. Stand back on the beach and look at the Broken Beds. Notice how the strata have been asymmetrically folded in a manner that suggests that the overlying strata have moved northward. There has been dispute over this matter, back in the past, when the evaporitic origin of the Broken Beds was not understood (Arkell, 1938a). The northward movement has been discussed by Selley and Stoneley (1987), who suggested that the overlying strata slid over the evaporitic beds northward from the axis of a rollover anticline during Cretaceous times. Although the existence of such an anticline has not been firmly proven the theory is quite convincing. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Fitton (1836) described this as a nook or recess, called by the boatmen Bacon-hall. He noted that "notwithstanding the apparent firmness of this rocky coast, great changes are constantly in progress there. An old boatman who conducted me, without having been asked a question, expressed in very strong terms his surprise at the alterations produced, to his own knowledge within the last thirty years." Personally I have seen little change from 1955 except for some slumping of Wealden strata over Purbeck limestones and shales. Thus some unusually significant coastal changes seem to have happened here between about 1800 and 1830, but it is not known what they were.
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LOCATION - BACON HOLE - SOUTHWEST CORNER
Basal Purbeck Sequence - More Detail
LOCATION BACON HOLE - SOUTHWEST CORNER:
Basal Purbeck Group (Lulworth Formation - lowest part)
BASAL PURBECK
Breccia Pipe and Hydrofracturing?
There is a peculiarity in the occurrence of Broken Beds type of breccia in one of the Mupe Rocks at Bacon Hole. Limestone breccia occurs higher than it normally does, and an isolated occurrence is present at the top of the rock. The Broken Beds breccia has an evaporite breccia (with carbonate and silica replaced evaporites) in its lower part (see West (1975) and other West papers). The upper and greater part of the Broken Beds does not contain obvious evidence of appreciable evaporites, but is a breccia of laminated peloidal and ostracodal limestone, like the 'Cypris' Freestones. Lake (1986) has interpreted the unusual breccia feature as an upward injected "pipe". He has suggested that dehyration of the gypsum, once present in these beds, to anydrite has released significant quantities of water, thus increasing fluid pressure. The overpressured fluids then could have led to hydrofracturing ("fracking" of natural origin) and injection of the breccia pipe.
This is a reasonable hypothesis but it is difficult to prove whether this did or did not happen. In its favour is the increase in former evaporite thickness in the Mupe Bay to Worbarrow Bay region and beyond. This where the cliff sections show change from marginal shelf at the Fossil Forest etc (upthrow side of the Late Cimmerian fault) to basin facies (English Channel Inversion) of the south-eastern Isle of Purbeck. Against it is the problem that the theory is dependent on restriction of the high pressure fluids by the underlying Great Dirt Bed acting as an underseal (Lake, 1986). It is rather doubtful as to whether the few centimetres of carbonaceous palaeosol could have held the high pressure fluids from downward movement. The theory, though is not impossible and there are other peculiarities in the region, such as the highly fractured Portland Freestone of Swanworth Quarry that could be explained by overpressure.
Other explanations for the "pipe" include some tectonic activity of a peculiar type. There is a small fault on the west side of the rock in question, and so some faulting activity is not out of the question. It is also known that elsewhere occasionally the Broken Beds brecciation is not necessarily stricty confined to one horizon.
The possibility also exists that the Broken Beds once contained not just the obvious calcium sulphate evaporites at the base, but also possibly halite or at least, strongly saturated brine, in the upper part. There are some grooves in places in the brecciated limestones, but true slickensides are not normally developed. This lack of slickensides in such a tectonically brecciated unit needs explaining. Separation of the limestone blocks by salt could account for this. However, this is just speculation, and at the present the explanation is not proven. The work of Lake (1986) is useful though and should be taken into account when considering complications of the tectonic-evaporite origin of the Broken Beds.
PURBECK GROUP
Lulworth - Durlston Formations - Overview
LOCATION - BACON HOLE:
Middle Purbeck (Lulworth Fm. - Durlston Fm.)
Cinder Bed
There may be some expectation that the well-known Cinder Bed, with the small bluish oyster, Praeexogyra distorta will be obvious to see at Bacon Hole. It is 2.95m thick in Durston Bay, but here it is only 1.3m in total. It is not particularly resistant and tends to weather in the cliffs. It does not form a strong ledge in the sea. The very white Flint Bed of the Cherty Freshwater Limestone is much more obvious. This should be looked for initially and the more crumbly Cinder Bed will be found above it. It crops out at shore level at the junction of Bacon Hole and Mupe Bay where there are ledges extending eastward (the photograph is probably from here) and also reaches the shore at two sites further east in Bacon Hole.
As can be seen in a photograph above the small oyster shells have been reworked by wave action. This is, as usual for this bed, a thanatocoenosis. This is not strange, but it is odd that no biocoenosis of Praeexogyra distorta has been found in southern England.
This bed, the Cinder Bed, changes in thickness in a manner that approximately corresponds to the overall Jurassic thicknesses in this region [it is Berriasian, just above the Jurassic]. Here, at Bacon Hole it is seen at the margin of the Inversion Structure, the offshore Portland - Isle of Wight Basin. The obvious explanation would be that slow progressive subsidence of the basin was proceeding as normal under tectonic control up into at least the early Berriasian. Others have suggested, though, that this simple explanation is not the reason, but that the Purbeck sequence represents the results of an end-Jurassic subsidence. A late subsidence process could produce similar results. Just which process was responsible remains an open question. The continued basin subsidence is probably best regarded as the explanation unless there is evidence to contrary.
LOCATION BACON HOLE:
Middle Purbeck Group - General
Bacon Hole provides a very section of the Purbeck Formation, which is unusual in continuing up without a major break into the Wealden strata. The stacks are in the basal Purbeck Formation and the cliffs on the left show Middle and Upper Purbeck limestones and shales. The white Cherty Freshwater Limestone and the bluish-gray and softer Cinder Bed are easily recognised. The Cinder Bed contains the small oyster - Praeexogyra distorta, which is very obvious. The beds can be seen near the foot of the cliff, and is shown in a photograph above.
(details to be added)
In the Cherty Freshwater Member at Bacon Hole the Purbeck charophytes were first discovered. Edward Forbes recorded (1851) in these cherts "for the first time in the Oolitic Series, gyrogonites, the spore vesicles of Characea."
MIDDLE PURBECK (TOP LULWORTH FM.)
Cherty Freshwater Limestone
DURLSTON GROUP:
Unio Beds
Mupe Rocks (more details)
The images above show the Mupe Rocks, as seen from Bacon Hole and viewed in a southeastly direction The rocks trend west-east, and Worbarrow Bay is in the distance beyond them. These erosional relics are of north-dipping Portland Freestone Member and basal Purbeck Group (base of the Lulworth Formation), Caps and Broken Beds. The Caps here are undergoing an important facies change here between the Fossil Forest (shelf facies with soils and trees) and Worbarrow Tout (more basinal facies without trees). Incidently, the rocks shown here include Arish Rock and the eastern-most is End Rock. See Brannon (1860) or Legg (1992, p.5) for a reproduction of Brannon's etching of Mupe Rocks, which gives their names. They have not changed much in appearance since 1860. The conspicuous Wreckneck Rock is shown below.
Wreckneck Rock, the western most of the large Mupe Rocks show a good cross-section through the Broken Beds. This is a former evaporite breccia near the base of the Purbeck Formation from which residual evaporites have now been dissolved. Note the effects of tectonism on the strata above the former evaporite unit and, in particular, the "Cypris" Freestones displaced upwards in the central part of Wreckneck above the breccia.
MUPE BAY MUPE BAY LOCATIONS:
The Chalk and the Chalk Cliffs of Bindon Hill
The cliffs on the south side of Mupe Bay show Chalk dipping very steeply northward. Some brief description of these given here is based mainly on older literature (but see comments further down, though, on the newer lithological terminology of the Chalk by Mortimore ).
Arkell (1947) commented that the Cenomanian basement bed consists of hard dark green and glauconitic sand. This bed and the overlying Cenomanian (Lower Chalk) is almost unfossilferous here. It is much faulted. The lower part of the Cenomanian consists of grey chalk with a few flints while the upper part is a cyclical sequence of grey chalk and bluish marl. The Plenus Marl marking the top of this Stage is 2.1m thick here.
The Labiatus Zone of the Turonian is 14.9m thick. It is hard nodular and wave-battered. The characteristic fossil, although scarce, can be found (if access to it is not obstructed by cliff falls). The Terebratulina Zone, forms long ledges and is still harder. Fossils are rare in this. The Planus Zone was formerly accessible up to a cave referred to by Arkell. This part of the Chalk is grey and well-weathered yielding a fair list of fossils, including most of the characteristic species (Arkell, 1947). The Bicavea Bed is present on the lowest ledge where the Planus Zone is exposed.
Beyond here, the cave on the west side of Black Rock (the conspicuous offshore rock about half-way along the Chalk cliff), is cut in the Cortestudinarium Zone. Thus only the lowest part of the Upper Chalk is present at the foot of the cliff, but obviously higher parts will be present higher in the cliff and are completely inaccessible.
In terms of structure there are various interesting faults and changes of dip present in the Chalk here, but these are less accessible than in the past because of the landslides. At the northern end of the main pebble beach of Mupe Bay the Upper Greensand and basal Chalk dip 45 ° N. At a higher stratigraphic level the dip then increases gradually to 53° (Arkell, 1947) N., until in the upper part of the Lower Chalk a group 5 fault inclined 35° N. reduces the dip by about 20° as at Lulworth Cove. The fault-plane breaks up near the beach into several branches. There is the same formation of lenses along the fault-plane and fracture-cleavage in the hanging wall, as at St. Oswald's Bay and elsewhere, and some south-dipping shears (Arkell's group 3) are developed both above and below the fault. At this locality the Arkell's group 5 fault is at a rather lower horizon in the Chalk than elsewhere. Except possibly at Durdle Door it is not elsewhere known to be present below the Plenus Marl. Some minor normal faulting occurs in the Plenus Marl, after which the section is interrupted by the eastern of the large landslides. Under the landslide there is presumably a fault of group 4 (the major fault system running east-west through the monocline and steepening the dip on the north side), for beyond it nodular Middle Chalk emerges, dipping 70-75° N, an increase in dip of 40° in a few metres. In the Planus Zone the dip increases to vertical, accelerated by at least one minor fault of Arkell's group 4. Compare the dip changes here with those in Lulworth Cove where overturning occurs high in the Chalk cliff north of an F4 fault. There is a cave and arch at the farthest point accessible on foot at normal tides before the way was recently blocked by landslides. These show, in both dip and strike section, shears of group 1 (dipping 55° -60° S.) and group 3 (dipping 35° S.), with in addition some vertical slickensiding along bedding-planes. Arkell (1947) reported that here shears of group 1 occasionally turn up through 25� and run into the bedding-planes.
It is important to note that in the above discussion the traditional stratigraphical terminology is used. There are many new formation and member names in use, mostly originating with the work of Mortimore. When time permits information on these will be added here. In the meantime these terms will be found in Mortimore and Duperret (2004) (see fig. 1 on page 4). Some lithological detail on the Lewes Chalk (Turonian-Coniacian) has been given in Mortimore and Pomerol (1987).
ARISH MELL
Arish Mell - a Small Bay in Chalk Cliffs,
Between Mupe Bay and Worbarrow Bay
(Location Map follows:)
Arish Mell is a small and interesting bay and beach that is about one mile directly south of Lulworth Castle and East Lulworth. The castle and village originated at the head of the small valley which goes down to the sea. This was presumably the original direct access to the coast for the estate. Lulworth Cove is now the important coastal location, but it is further from the Castle. It has the advantage of giving protection to boats and small ships in rough weather which Arish Mell does not. Nowadays Arish Mell is relatively inaccessible because it is within, and quite close to the main active area of an Army tank firing range. It has in a sense become separated from the Castle by the Army Range. It is not a completely closed place though and the Range Walks are open at most weekends, and there is footpath acess to the area. There is no public access to the beach and cliffs of Arish Mell at present. In this respect it differs from the other beaches and bays within the Army Range. However, the adjacent footpath provides a very interesting route that is well-worth a visit. Structural geological features can be seen in Arish Mell. A good record of the fossil content and Chalk Zones have been given by Rowe (1902). The geology of Arish Mell is quite well-known.
An image above shows the scene looking westward from Rings Hills, above Worbarrow Bay (The photograph is by Gareth Lloyd). Notice the triangular Chalk cliff on the west side of Arish Mell; this is Cockpit Head. The top right photograph is of Upper Chalk zones at Cockpit Head, as shown on a old photograph (now tinted) in Rowe (1902), taken by Professor Armstrong from as near Cover Hole on the east side of Arish Mell as the tide would permit. At the point marked X Rowe discovered a specimen of Belemnitella mucronata.
The photograph beneath shows the seaward entrance to Arish Mell, as viewed from Worbarrow Bay.
On the Mupe Bay (western) side of the headland the Chalk originally proved good for fossils, although with much slickensiding. Rowe found it to be the best weathered locality in the Lulworth region. The cliff is protected by a barrier of fallen blocks of chalk which prevent it being pounded by the shingle. He found Actinocamax quadratus, Terebratulina rowei, Offaster pillula, Bourgueticrinus, Porosphaera and bryozoa. He reported Echinocorys vulgaris var. gibbus in profusion. Corals, bryozoans and brachiopods here are reddish in colour.
The west cliff of Arish Mell (Bindon Hill) consists of Quadratus and Mucronata chalk dipping 75° -80° N.N.W. Arkell (1947). It is sliced through and through by shear-planes, the majority of which are inclined about 35° N. or N.N.W. Arkell found it difficult to make out any prevalent direction of movement, for the bands of flint that are shifted south along some of the slide-planes are shifted north again on others.
Now there is no direct access to the cliffs in Arish Mell. However, there is good view of the western cliff. The shear planes here seem to dip north, and in spite of Arkell's (1947) comments, the dominant sense of movement seems to northward. Having looked at the shear planes in the Needles, the stacks of the Isle of Wight which are also in Portsdown Chalk (Belemnitella mucronata Zone), there is an impression of similarity. However, I have made no serious and no statistical study; I merely draw attention to the apparent similarity. It might be of interest if the structures in the steeply dipping Chalk were broadly similar both east and west of the peculiar area with the Ballard Down Fault. Both at the Needles and at Arish Mell there seems to be northward and downward movement; does this hint that the Ballard Down Fault might be of related origin?
With regard to fossils, they are not abundant there but the diligent work by Rowe resulted in sufficient finds to establish the approximate zonal limits, as shown on the map and photograph. The Actinocamax quadratus Zone is 111 metres thick, a substantial thickness. The Belemnitella mucronata zone (Portsdown Chalk) is rather more fossiliferous and Rowe and Sherborn (1901) found 12 examples of Belemnitella mucronata and also Echinocorys, Magas pumilis, Crania egnabergensis var. costata, Terebratula carnea, Rhynchonella limbata and Kingnea lima .
This Campanian Chalk of Arish Mell is higher stratigraphically than that exposed in any other sea cliffs east of Bat's Head (near Durdle Door), and the corresponding difference in its tectonics led Arkell (1947) to consider that it is near the zone of packing which is known from the Middle Bottom section (p. 286) to lie in the axial plane of the foresyncline. However, further complications may haye been introduced by the change of strike from E. to E. 18° N. which takes place at this point. The comparison with the steeply, dipping and hard Campanian Chalk of the The Needles, Isle of Wight may worth investigating.
In the east cliff (Cover Hole) the Planus and Micraster Zones forming the headland with a natural arch are relatively normal, dipping 65° N., with shears of Arkell's group 1 dipping 65° S and showing distinct upward displacement. The group 1 shears are cut and shifted by conspicuous shears of group 3a, dipping 10°-12° S.
The change of strike in the Chalk at Arish Mell is associated with a sudden eastward thickening of the Wealden and Purbeck Beds, which takes place somewhere between Mupe and Worbarrow Bays. A major north-south fault runs through the centre of Arish Mell and Worbarrow Bay. This was a compartmentalizing fault where the axis of the monocline was displaced (Nowell, 2001) . In terms of Late Jurassic and Early Cretaceous sedimentation it marks the approximate boundary between thicker basin facies to the southeast (i.e. Kimmeridge etc) and the shelf or high facies of Lulworth and the Fossil Forest.
Footnote: Arish Mell
Arish Mell is listed as Arish Mill as on a map in Robinson (1882) . A curiosity here is drinkable water coming out of the foot of the Arish Mell cliff. Inland of here, between 1794 and 1817, was the Trappist monastery farm supported by the landowner Mr Thomas Weld. Robinson repeats an account of a visit to this monastery made in 1800. The visitor was unimpressed. After 11 miles horse ride from Dorchester he commented, by no means in the best of tempers, that the monastery is built of very rude materials and in a very rude style.
"Its immediate neighbourhood presents a picture of bleak desolation. The hills are destitute of wood, and the east wind, sweeping from the Channel, perishes the early shoots of vegetation. Ringing at the gate of the monastery, we were received by the porter. It is impossible to give an accurate idea of the hideousness of this man's dress, which was composed of a tunic made of coarse, thick and woollen cloth: over his shoulders he wore a cape made of the same material; this was partly thrown back, so that his face was visible; but the other monks, who were clad precisely in the same manner as the porter, covered their visages so that nothing but the eyes and noses could be seen. Their stockings are made of coarse cloth, and their shoes are wooden, and about three inches thick in the sole. After being asked whether we had any women in our party, and being answered in the negative, the porter attended us to the refectory. .... The appearance of the soup, I must confess, turned my stomach. The bread was absolutely black. .....
Passing from the chapel, through a cloister, we visited the burying ground, which occupies a small inner court, overgrown with rank weeds and tall luxurient grasses. Two graves, already tenanted are marked by two wooden crosses; and one grave is always kept open ready to receive the next deceased. Our conductor assured us that each individual of rthe fraternity prayed sincerely that he himself might become the occupant. At this I am not surprised; for such misery and such a degradation of human nature is exhibited within the precincts of these walls I never elsewhere witnessed.
None of the brotherhood except the porter, are permitted to speak, unless by special permission of the superior. The stillness of the place was awful. ....
When taking leave of this gentleman, he cast his eyes on the ground, with modest humility, half extended his dirty paw.. A few shillings was the toll levied on our exit from this gloomy abode of ignorance and nastiness, which I quitted with a sigh, breathed in compassion of the lot of those whom vice or folly drive for the expiation of real or fancied iniquities into the community of La Trappe. "
The Chalk zones on the east side of Arish Mell as shown by Rowe (1902).
I am very grateful to Gareth Lloyd for the photograph of Arish Mell and Mupe Bay in the distance. I much appreciate permission from the Geologists' Association to use the old photographs of Rowe. The geoarchaeologist Pari White has been very helpful in contributing photographs and this is very much appreciated.
Please also go to the separate Lulworth Bibliography.
References - Mupe Bay and Bacon Hole - Specific
Arkell, W. J. 1935. The Portland Beds of the Dorset Mainland. Proceedings of the Geologists Association, vol. 46, part 3, pp. 301-347. By W.J. Arkell, M.A., D.Sc., F.G.S. Received 3rd September 1934; Read 7th December, 1934.
[This is a key paper on the Portland Stone Formation of the Dorset mainland. It has good and very informative photographs and diagrams. A significant part of it has been reproduced in Arkell (1947). In addition to the information on the Portland Group there are brief summaries of the sequences of basal Purbeck strata at Holworth House, Poxwell, Chalbury Camp etc. Although in traditional quarryworkers terms they are useful as confirmation of later work on these sequences(West, 1975). It is of passing interest to notice that William Joscelyn Arkell managed to get to some rather inaccessible places, like the eastern end of Gad Cliff in the 1930s, presumably before he had health problems.]
Arkell, W.J. 1938a. Three tectonic problems of the Lulworth district: studies of the middle limb of the Purbeck Fold. Quarterly Journal of the Geological Society, London, 94, 1-54. By William Joscelyn Arkell, M.A., D.Sc., F.G.S. No abstract. Example extract: Introduction etc.: "Of all the folds thrown up across the Chalk plains of North-West Europe during the Alpine orogeny, the most interesting and the most perfectly exposed is the Purbeck anticline. In the cliffs of the Dorset coast the sea has laid bare all parts of the structure, from the core or crestal region, through the vertical and overturned middle limb and the "foresyncline" (Busk 1929, p. 29), into the foreland, where the Upper Cretaceous rocks are seen reposing with sharp unconformity upon a previously folded and eroded Jurassic foundation. The general and some special aspects of the fold and its associated faults have been discussed in a previous paper (Arkell, I936). The present paper results from a study of the middle limb, where it is dissected by the sea cliffs of the Lulworth district. These cliffs have long been renowned for their beauty, but their marvellous tectonic features, both great and small, have been neglected, with the result that their value to students of tectonics has not been fully utilized. The first three parts of the paper attempt a solution of the three most outstanding problems: contortions in the Purbeck Beds, the Purbeck Broken Beds, and the abnormal attenuation of the strata towards Durdle Door. The fourth part offers an interpretative essay and synthesis. The area dealt with is included in sheet 342 of the Geological Survey one-inch map and sheet 141 of the Ordnance Survey one-inch map.
Contortions in the Purbeck Beds: (a) Description. The contortions in the Purbeck Beds at Stair Hole are illustrated in Pl. I. The top of the Portland Stone and the hard "Cap" limestones of the basal Purbeck Beds dip northward at 40�, forming the arches through which the sea enters. In the sides of the little recess the rest of the Purbeck Beds, above the top of the Cypris Freestones, consisting of alternations of limestone and marl, shale, or clay seen in dip section, begin to assume from below upwards an increasingly large knee-shaped anticlinal bend. The apex of the bend is directed upwards and outwards, towards the north, with the short horizontal limb to the south. The longer limb stands vertical or slightly inverted, and where the beds composing it plunge below the beach they are directed to meet the top of the Cypris Freestones at an angle of 50�. In the Corbula and Beef Beds (upper part of the Middle Purbeck) and Upper Purbeck strata, the sharp apex of the anticline becomes blunter with increasing size. In these beds, near the top of the east cliff, a second but smaller sharp anticline develops above the horizontal limb of the other. This is of the same type as the larger flexure, with the apex pointing in the same direction, but it is more overturned northwards, and it does not affect strata below the Cinder Bed. There are signs of a third fold just below the top of the cliff, but the upward continuation has been eroded away. The intervening synclines are closed and approaching recumbent. This structure, which may be called the Lulworth crumple, is continued in the direction of strike for a visible distance of a mile and a half, but the other sections are not so good. On the west side of Stair Hole (Pl. I and Fig. 4) only the main anticline and syncline are seen, at a higher elevation above the beach, showing that the crest-line is rising westward. At Dungy Head and Durdle Door only the lower part of the structure survives, truncated by the cliff top...."[continues].
Arkell, W.J. 1938b. The Purbeck Broken Beds. Geological Magazine, 75, 333-334.
Arkell, W.J. 1947. The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey, 386 pp.
Baylis, S.A. and Clayton, C.J. 1996. Characterisation of an outcropping reservoir � the oil seeps of Dorset. In: J.R. Underhill, Editor, The Development and Evolution of the Wessex Basin and adjacent areas. Geological Society., London, Special Publication 59.
Bigge, M.A. and Farrimond, P. 1996. M.A. Biodegradation of seep oils in the Wessex Basin�a complication for correlation. In: J.R. Underhill, Editor, The Development and Evolution of the Wessex Basin and Adjacent Areas. Geological Society, London, Special Publication 59.
Brannon, P. 1858. The Illustrated Historical and Picturesque Guide to Swanage and the Isle of Purbeck with a Clear Digest of the Geology and a Minute Description of the Coast from Bournemouth Bay to White Nore. Published - Poole and London.
Bruce , P. 1989. Inshore Along the Dorset Coast. Boldre Marine, Kestral Cottage, Shirley Holms, Lymington, Hampshire, SO41 8NH, Tel. 0590 - 683106. 115 pages + maps. Price was �9.95. This is useful for topographic details and names of rocks, coves and embayments etc.
Cornford, C. 1988. Geochemistry and Volumetrics of Dorset Oil Seeps. Chris Cornford Integrated Geochemical Interpretational Ltd., Hallsannery Field Centre, Bideford, Devon EX39 5HE. Tel: 02372-71749. Conference - "Regional Geology of the southern United Kingdom; Petroleum-explanation perspectives" held in London September 1988. - Abstract. Analysis of the steranes and triterpanes in the oil seeps of the Dorset coast of Southern England (Mupe Bay, Osmington Mills) shows both the oils to derive from a similiar mid-late mature source rock. Measured maturity of the Jurassic source rocks at outcrop are uniformally immature (0.3 -0.5%R) despite a history of Mesozoic burial and Tertiary inversion and uplift. Two migration episodes, the first during the Lower Cretaceous,, were identified at Mupe Bay. -- Locally calibrated time/temperature thermal geohistory modelling can just simulate source rock maturites appropriate to the seep oils if paleo-geothermal gradiants and surface temperature were higher in the Jurassic than at the present day. -- A volumetric balance between oil in place in the Wytch Farm and adjacent fields and the available area of source rock draining into the structure suggests that an anomalously thick and/or organic rich Rhaeto-Liassic section must have developed in the Bournemouth Bay Basin, and that Mesozoic geothermal gradients were higher in this specific area.
Cornford, C., Christie, O., Endresen, U., Jensen, P. and Myhr, M-B. 1988. Source rock and seep oil maturity in Dorset, Southern England. Advances in Organic Geochemistry, 1987, 13, 399-409. Abstract: Analysis of the steranes and triterpanes in the oil seeps of the Dorset coast of Southern England (Mupe Bay, Osmington Mills) shows both the oils to derive from a similar mid-late mature source rock. Measured maturity of the Jurassic source rocks at outcrop are uniformally immature (0.3-0.5%R) despite a history of Mesozoic burial and Tertiary inversion and uplift. -- Evidence from oil cemented sandstone clasts within oil stained Lower Cretaceous fluviatile channel sands at Mupe Bay, suggest that oil was already seeping during the Lower Cretaceous. Detailed work comparing the extracts of the oil-stained clasts and the surrounding oil-stained sandstones show the clasts to contain oil derived from a slightly, but significantly, less mature source rock. This is consistent with lower maturity oil seeping first into the sand that was incorporated as rip-up or bank collapse clasts into the channel sand. At a later stage, with further source rock burial, more mature oil entered the matrix sandstone. -- Locally calibrated time/temperature thermal geohistory modelling can just sismulate source rock maturities appropriate to the seep oils if paleo-geothermal gradients and surface temperatures were locally higher in the Jurassic that at the present day. -- A volumetric balance between oil in place in the Wytch Farm field and the available area of source rocks draining into the structure suggests an anomalously thick and/or organic rich Rhaeto-Liassic section must have developed in the Bournemouth Bay basin, and that Cretaceous palaeo-geothermal gradients were higher in this specific area. - End of Abstract. - Six kilometres to the east, Farrimond et al. (1984) reported " mature " oil-type extracts from a relatively tight coccolith limestone band sealed within the immature oil-shale at Kimmeridge Bay. They supposed that this must have migrated into the band at, or shortly after, deposition suggesting even earlier movement of oil within the basin. -- Alternatively an active seep may exist at Kimmeridge Bay, as reported at the Freshwater Steps location (R. Burwood, personal communication, 1987), though the authors were not able to find it. " extract from p. 404. -- From the sterane and triterpan ratios (Table 2), there is a small but consistent maturity increase from the Mupe Bay clast oil, via the Mupe Bay matrix oil, to the oils at Osmington Mills. extract from p. 404 (my comment - But this implies that the Osmington oil is Tertiary following their Fig 14 in which they imply that the Mupe matrix oil is Tertiary. -- The largest clast is 1.2 m along its major axis. -- The usual group of steranes (C27-C29) and triterpanes (C27-C33) were determined using molecular ion transitions.
Ensom, P. 2010. Lithological Log of the Purbeck Group of Bacon Hole:
Ensom, P. 2010. The Purbeck Limestone Group (Tithonian-Berriasian) Succession Between Bacon Hole and Mupe Bay, near West Lulworth, Dorset. Proceedings of Dorset Natural History and Archaeological Society, vol. 131, pp. 140-144. By Paul Ensom, who has also logged the Worbarrow Tout, Purbeck section, across the bay.
[This includes a graphic log of 157 beds at Bacon Hole, shown in 3 pages.]
This log is Appendix 2 in: Ensom, P. 2010. Lithostratigraphic sections through the Purbeck Limestone Group (Tithonian-Berriasian) at five Regionally Important Geological/Geomorphological Sites (RIGS) on the Isle of Purbeck, and at Bacon Hole near West Lulworth, Dorset, southern England. Proceedings of the Dorset Natural History and Archaeological Society, vol. 131, 2010, pp. 127-144.
Ensom, P. 2010. Lithostratigraphic sections through the Purbeck Limestone Group (Tithonian-Berriasian) at five Regionally Important Geological/Geomorphological Sites (RIGS) on the Isle of Purbeck, and at Bacon Hole near West Lulworth, Dorset, southern England. Proceedings of the Dorset Natural History and Archaeological Society, vol. 131, 2010, pp. 127-144. By Paul Ensom.
The creation of a virtual field trip principally featuring the Purbeck Limestone Group on the Isle of Purbeck, Dorset, southern England by Dorset's Important Geological/Geomorphological Sites Group, led to the production of logs of the strata exposed in the five Regionally Important Geological/Geomorphological Sites. The logged sections of these are published along with one for the Purbeck Limestone Group exposed between Bacon Hole and the southern margin of Mupe Bay near West Lulworth. Insects recovered from one of the sites are listed. Reference is made to the distribution of silicified wood based on observation in the field. The presence of microinvertebrates, dinosaur tracks, and of reddened strata at certain horizons is recorded
Hesselbo , S.P. 1988. Sequence boundaries in the basal Wealden Beds ( Lower Cretaceous; Wessex Basin) at Mupe Bay, Dorset. B.S.R.G. - British Sedimentological Research Group, 1988, Cambridge, no page numbers. (Two erosion surfaces are angular unconformities - one is the palaeo-oil seep -fluvial above)
Hesselbo, S.P. and Allen, P.A. 1991. Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous south Dorset. Journal of the Geological Society, London, 148, (1) 105�113. By Stephen P. Hesselbo and Phillip A. Allen, Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR UK. Abstract: The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern England at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and fades relationships, are interpreted as unconformities. The lower unconformity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces are treated as candidate sequence boundaries (WBI-WB3), with the thin intervening ?lagoonal deposits representing peaks of transgression. The pattern of one minor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to the Early Cretaceous sequence stratigraphy suggested in recent 'global cycle charts'. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas. [Comments: They discussed the origin of the conglomeratic oil sand at Mupe Bay. They recognised unconformities and have considered these as candidates for major subdivisions within the Wealden. Two major erosion surfaces at Mupe Bay, one the celebrated oil sand channel, are interpreted as unconformities. These authors attempt to establish a sequence correlated with Early Cretaceous sequence stratigraphy suggested in recent global cycle charts. The authors also comment, that subsidence histories and stratal geometries show that the main extensional activity on the fault system took place in early Jurassic and late Jurassic/early Cretaceous times and that the faults are probably active throughout deposition of the Purbeck and Wealden Beds. They record soft sediment deformation at Mupe Bay.]
Kinghorn, R.F., Selley, R.C. and Stoneley, R. 1994. The Mupe Bay Oil Seep Demythologised. Marine and Petroleum Geology, 11, (1), p 124-125. Discussion of paper by Miles et al. . As previously suggested, geochemical differences between the clasts and matrix can be attributed to different stages of maturation of the source, hence two distinct episodes of oil migration, before and after formation of the conglomerate. The sand clasts would not have survived transportation unless previously saturated by oil. Authors' reply p. 125-126.
Lake, S.D. 1986. Brecciated pipes in the Broken Beds, Purbeck Limestone Formation. Proceedings of Dorset Natural History and Archaeological Society, 107 (for 1985), 191-192. By S.D. Lake, at the time, Department of Geological Sciences, University of Durham, South Road, Durham.
Previously unrecorded breccia pipes extend up from the underlying Broken Beds into the undisturbed sedimentary cover. These have been identified at Bacon Hole and Durdle Door in south Dorset. The likely cause for the formation of the Broken Beds is re-evaluated. The results imply both diagenetic and tectonic effects during the Tertiary uplift of the Isle of Wight/Purbeck monocline alonga former Mesozoic roll-over anticline. [end of abstract]
[some incomplete extracts follow:]
" ---- Recent fieldwork by the author led to the discovery of breccia pipes that extend, in some cases, at least 10 metres from the Broken Beds into the overlying Lower Purbeck sequence. Therefore these hypotheses listed by West (1960) must somehow explain this phenomena or alternative hypotheses put forward.
The occurrence of the breccia pipes is particularly well exhibited at Bacon Hole (SY842797) where the largest upstanding stack has a peculiar knoll at the top (Fig. 1). This is particularly well seen in Plate 1 of Arkell's (1947) Memoir. On close examination this knoll displays the typical limestone brecciation associated with the Broken Beds, yet lies at least 5m above the main Broken Beds horizon. Examination from the seaward (southern) side of the stack particularly well displays the upward extension from the main Broken Beds horizon. A small scale example is well displayed on the eastern side of the stack, the brecciation extends some 4m into the overlying "Cypris" Freestone Member (Fig.1). The width of both of these pipes is approximately 4m.
Durdle Door eastern promontory (SY807803) also displaysa similar feature. The bedding planes of the 'Cypris' Freestones show a hemispherical brecciated body at beach level between the beach and the promontory. This brecciated body bears remarkable similarity to the underying Broken Bed horizon yet lies some distance above it. The location and size of this brecciated body is interpreted as reflecting another breccia pipe and not a separate Broken Bed horizon, such as those identified at Worbarrow Tout by West (1975) and Ensom (1985). Clearly these breccia pipes are important in the evolution of the Broken Beds horizon.
[continues]
In the light of this work [previous papers by West] a further contribution can be made resulting from the observed breccia pipes and new structural evidence. The evolution of the Broken Beds can perhaps be envisaged as follows: Primary gypsum was intercolated with calcareous shale and pelletoidal limestones in the lowest portion of the Broken Beds, whilst ostracodal biosparites, intrasparites and pelletoidal limestones occurred towards the top of the Broken Beds unit (West, 1964, 1975). Increasing depth of burial of the unit resulted in dehydration of the gypsum to anhydrite. This undoubtedly occurred prior to the Eocene and the initiation of uplift. The depth of burial was probably less than 1500 m on based on known sedimentary thickness of the overlying units. This structural evidence is also in agreement with theoretical studies which demonstrate that dehydration of gypsum occurs at a temperature of 42 degrees centigrade. Similar consideration of the present day geothermal gradient (30 degrees C / Km) allows a minimal depth estimate of dehydration. Coincident with this increasing burial, calcitisation of the lower Purbeck evaporite sequence took place in response to decarboxylation of organic matter. The dehydration resulted in an approximately 38% volume decrease (Blatt et al., 1972) thus resulting in abnormal fluid pressures. Fluids were undoubtedly derived from the nearby algal limestones and clays. Locally the dirt beds (the Great Dirt Bed in particular) that lie at the base of the Broken Beds sequence may have acted partly as an impermeable layer preventing downward percolation of fluids. This combined with increasing hydrostatic pressure associated with burial may have caused the pressured fluids to have outflow diapirically or resulted in hydrofracturing into the overlying 'Cypris' Freestones Member prior to flexuring.
[continues with text]
[See also the Fig 1 - a diagram of a breccia pipe in an offshore Mupe Rock at Bacon Hole.]
[Reference and - END OF ARTICLE]
Lees, G.M. and Cox, P.T.C. 1937. The geological basis of the present search for oil in Great Britain by the D'Arcy Exploration Company Limited. Quarterly Journal of the Geological Society of London, 93, 156-194.
Legg, R. 1992. Tyneham: Dorset's Ghost Village. Dorset Publishing Company, at the Wincanton Press, National School, North Street, Wincanton, Somerset, BA9 9AT. ISBN 0948699 30 2. 128pp.
Mortimore, R.N. and Duperret, A. 2004. Coastal Chalk Cliff Instability. Geological Society, London, Engineering Geology, Special Publication, 20, 173pp. [Not specifically on the Dorset area, but with much content that is relevant to the Dorset Chalk cliffs. Particularly see fig. 1 on page 4 in a paper by Mortimore et al. on Coastal cliff geohazards in weak rock: the UK Chalk cliffs of Sussex. This fig 1 is a very useful table correlating traditional Chalk stratigraphy, stages, key marker beds etc and newer formation and member names. Researchers on the Chalk will need this or a similar diagram for translating terms.]
Mortimore, R.N. and Pomerol, B. 1987. Correlation of the Upper Cretaceous White Chalk (Turonian to Campanian) in the Anglo-Paris Basin. Proceedings of the Geologists' Association, 98, (2), 97-143. [See fig. 9 on p.111 - Dorset and Isle of Wight field sections compared with the Marchwood and Winterborne Kingston Borehole Logs. There are logs with some lithological details for Ballard Head, Mupe Bay and White Nothe, Dorset.]
Miles, J.A., Downes, C.J. and Cook, S.E. 1992. The fossil oil seep in Mupe Bay, Dorset: a myth investigated. Marine and Petroleum Geology, 10, 58-70. -- The conclusion is that the clasts are not contemporaneous "oil-cemented" boulders as previously supposed. The only difference noted in this study between the clasts and their host sandstone is in their grain sizes and the degree of biodegradation of the oils. This difference in biodegradation can logically be ascribed to the difference in permeability associated with the change in grain size , and the resulting penetration of meteoric water. The source of the oil in both of the sandstones can be correlated to the Lias. Aromatic biological markers for thermal maturity, resistant to biodegradation, indicate a maturity equivalent to about 1% vitrinite reflectance or higher for both oils. These conclusions remove the constraint of early Wealden generation of oil previously imposed on mburial history reconstructions for the timing of oil generation and migration in south Dorset. It is further proposed that the Osmington/Bran point - Lulworth Cove - Mupe Bay - Worbarrow Bay area was the site of a large oilfield, with reservoirs within the Wealden and extending down to the Bencliff Grits ( and Bridport Sandstones?), of the same order of size as the Wytch Farm accumulation. This oilfield was uplifted and breached during the erosion which followed Tertiary compression. -- The above is the second half of the authors' abstract. The first half is introductory. -- See discussion by Kinghorn.
Nowell, D.A.G. 1997a. Structures affecting the coast around Lulworth Cove, Dorset and syn-sedimentary Wealden faulting. Proceedings of the Geologists' Association, London, 108, 257-268. Pre-Albian northwest-southeast faults have been inferred along the coast between Durdle Door and Worbarrow Tout. From the westward increase in the thickness of Wealden units between Mupe Bay and Lulworth Cove followed by the dramatic thinning across the cove it is suggested that these faults were synsedimentary growth faults during the early Cretaceous. Also the Wealden is shown to contain two main quartz grit bands, only the lower of which is seen in Lulworth Cove due to increasing westwards Albian erosion which mainly explains the westward thinning of the Wealden. These faults may have continued to be active throughout the rest of the Cretaceous. During the later Alpine formation of the Purbeck monocline, southward dipping faults also developed. Their alignment may have been controlled by the reactivated northwest-southeast faults. To the west of Lulworth Cove much of the succession between the Portland Stone and the Chalk is cut out by these east-west thrusts.
Nowell, A.G. 2001. 'The seismic expression of synsedimentary channel features within the Chalk of southern England' by D.J. Evans and P.M. Hopson - comment. Correspondence section of the Proceedings of the Geologists' Association, London, 112 (2), 183-184.
Radley, J.D. 1998. "Structures affecting the coast around Lulworth Cove, Dorset and syn-sedimentary Wealden faulting" by Nowell (1997): comment. Proceedings of the Geologists' Association, London, 109, 237-238. This is a discussion of the paper - Nowell, D.A.G. 1997a. Structures affecting the coast around Lulworth Cove, Dorset and syn-sedimentary Wealden faulting. Proceedings of the Geologists' Association, London, 108, 257-268. Radley criticises the matter of Wealden correlation. He questions the use of lumps of lignite as marking an identifiable horizon. A major problem is the correctness or not of the correlation of quartz grits put forward by Nowell, which is against the usual view and that held by Radley. The recognisable character of the Coarse Quartz Grit is argued by Radley who suggests that it may be isochronous. The Mupe Bay conglomeratic oil sand (or "Oily Boulder Bed") is also referred to, although there has been separate controversy over this ( see Miles, J.A., Downes, C.J. and Cook, S.E. 1992).
Robinson, C.E. 1882. A Royal Warren; or Picturesque Rambles in the Isle of Purbeck. The Typographical Etching Company, 23 Farringdon Street, E.C., London. 186pp. By C.E. Robinson, M.A., Barrister-at-Law, Author of the "Cruise of the Widgeon"; "The Golden Hind, Thessale, and other poems" etc. The etchings by Alfred Dawson.
Rowe, A.W. and Sherborn, C.D. 1901. The zones of the White Chalk of the English Coast 2, Dorset. Proceedings of the Geologists' Association, 171-76.
Selley, R.C. and Stoneley, R. 1987. Petroleum habitat in south Dorset. Pp. 139-148 in: Brooks, J. and Glennie, K. (eds.), Petroleum Geology of North West Europe. Graham and Trotman. Abstract: An analysis of the surface geology and of surface and subsurface petroleum occurrences is used to unravel the complex history of the generation, migration and entrapment of oil in south Dorset. Early basin subsidence lead to growth faults downthrown to the south. Field evidence suggests that the Purbeck - Isle of Wight disturbance controlled the deformation of the Broken Beds of the Purbeck Beds, and that these brecciated limestones slid northwards towards the fault down the northern limb of a rollover anticline. This fault also appears to have controlled sedimentation in a Wealden palaeovalley. Liassic shales began to generate oil in the Early Cretaceous. Some oil escaped up faults to the surface, generating the Mupe Bay palaeoseep, but much was trapped in the Bridport Sands. By the end of the Cretaceous, oil was migrating north across the Purbeck - Isle of Wight flexure. Inversion through the Tertiary sealed the faults, trapping petroleum in Wytch Farm and adjacent traps. Palaeogene uplift and cooling allowed the development of the fault-sealed Kimmeridge Bay underpressured system. Adjacent fault blocks to the north and south may still be petroliferous at deeper levels.
Stoneley, R. 1992. Review of the habitat of petroleum in the Wessex Basin: implications for exploration. Proceedings of the Ussher Society, 8, 1-6. Petroleum exploration involves the examination, integration and application of all available geological, geophysical and geochemical data, to assess the likelihood of finding oil or gas in a particular area. It requires the geologist to predict quantitatively the occurrence and distribution of oil and gas in order to guide expenditure on drilling costly wells. Given the predictive nature of the exercise, it is inherently impossible to arrive at definitive estimates of petroleum occurrence until all exploration has been completed. Parts of the Wessex Basin of southern England provide a superb case-history of exploration principles, thinking and uncertainties.
Townson, W.G. 1971. Facies Analysis of the Portland Beds. Unpublished D.Phil. Thesis, Oxford University., 284pp.
Townson, W.G. 1975 Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, 131, 619-638. Abstract: The "Portland Beds" of Dorset (Portlandian of English usage) are described in terms of a Group comprising two Formations and seven Members. Facies and thickness variations indicate the presence of a swell separating an East from a West Basin. The swell may be due to the movement of Triassic salt. The environmental history of the Portland Group is described in terms of three cycles consisting of major regressive and minor transgressive phases superimposed on an overall regression. The lower cycle consists of siliciclastics and dolomite deposited in a relatively deep marine environment. The dolomite formed by in situ replacement of lime mud. The middle cycle consists of cherty fine-grained limestones deposited on the outer part of a carbonate shelf. The abundance of replaced sponge spicules adequately accounts for the amount of chert. The upper cycle consists of cherty limestones passing up into shallow-water grainstones. Ooid shoals developed over the swell. These marine limestones are overlain by stromatolites and evaporites which formed on the basin margin.
West, I.M. 1975. Evaporites and associated sediments of the basal Purbeck Formation (Upper Jurassic) of Dorset. Proceedings of the Geologists' Association, London, 86, 205-225. It was shown in this paper that the oil sand is situated at a locality that has been undergoing uplift both before and afterwards. See also Drummond. West suggested that there was seepage of oil up the fault-plane to the Wealden land surface resulting in impregnation of unconsolidated sand sediment, followed by erosion, local transport and deposition of oil-bound clasts. A small erosion surface occurs at the base of the Purbeck Formation in this area. The limestones beneath the Lower Dirt Bed become reduced in thickness so that at Dungy Head the Lower Dirt Bed rests on the Portland Freestone (see also Townson 1975 for Portland details and thickness changes). Although not referred to in the above paper, note also that the Purbeck Formation becomes reduced in thickness here so that beds of gypsum in the Soft Cockle Member do not persist west of Worbarrow Tout ( except for a limited thickness on the Isle of Portland).
Wimbledon, W.A., Allen, P. and Fleet, A.J. 1996. Penecontemporaneous oil-seep in the Wealden (early Cretaceous) at Mupe Bay, Dorset, U.K. Sedimentary Geology, 102, 213-220. Abstract: Investigation of an oil-globule enveloped in laminated silty clay shows that oil, derived from the Lias, impregnated early Wealden alluvial sediments penecontemporaneously and later. Some of the earlier oil reached the surface of the floodplain and seeped from the eroding bank of an ephemeral river. Lag-sediment in the channel, comprising debris from the oily alluvium, was invaded by later oil (or oils). The palaeoseep was geologically short-lived. Relevant geochemical and petrological data and the geological context are presented. Process-models based on fault and/or water-table controls on oil migration are briefly proposed.
(See also: Erratum to �Penecontemporaneous oil-seep in the Wealden (early Cretaceous) at Mupe Bay, Dorset, U.K.� [Sediment. Geol. 102 (1996) 213�220], Sedimentary Geology, Volume 105, Issues 1-2, August 1996, Page 115)
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