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INTRODUCTION:

General

Stair Hole, just to the west of Lulworth Cove, is a remarkable small cove with natural arches cut into steeply-dipping Portland and basal Purbeck limestones. Through these arches and a gap where one has collapsed the sea enters to erode the softer parts of the Purbeck limestones and shales. Soft multicoloured Wealden strata slump southwards towards the sea. The most remarkable feature of Stair Hole is seen in the eastern cliff. Here is a cross-section throught the Lulworth Crumple, where small folds in the Purbeck strata are present within the steep northern limb of the Purbeck Monocline.

This place is a classic site, painted by Turner in about 1811, visited, photographed and sketched by thousands of people and shown in many geology and geography textbooks.

For more information, when at Lulworth Cove, visit the Lulworth Cove Heritage Centre with pamphlets, books and displays of rocks, fossils and minerals.

INTRODUCTION:

Geological and Other 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.

INTRODUCTION:

Aerial Photographs

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INTRODUCTION continued:

Safety

Many general features of Stair Hole can be seen from the viewpoint at the back (north) of Stair Hole. The high eastern viewpoint provides good views of various aspects of Stair Hole, but the Lulworth Crumple is not so well-developed in the western cliff which can be seen from here. The high western viewpoint should be treated with caution and may be unsuitable for school parties. There is risk of rock fall at the base of the cliffs of Stair Hole at both eastern and western ends. It is not safe to climb the central ridge.

STRATIGRAPHY:

Purbeck Group - Introduction

Although, some of the basal Purbeck strata cannot be easily seen without rather hazardous climbing, Stair Hole has good exposure of the Cypris Freestone Member upwards. The basal Purbeck strata including the Caps, the Great Dirt Bed and the Broken Beds are present in the central bastion. They can be seen to some extent in the cave at the western end of Stair Hole, providing the tide is low. The Soft Cap in the central bastion contains a small quantity of celestite and also some small gastropods, but climbing this is not recommended.

These photographs show the Cinder Bed and the Flint Bed of the Cherty Freshwater Limestone Member at the eastern side of Stair Hole. The upper Purbeck can be seen north of the main vertical cliff, away from the main area of risk. The strata are also visible at the western end of Stair Hole.

The same beds are shown in more detail. The Cinder Bed can be recognised by the numerous small, blue-grey oysters. The Flint Bed is almost white with conspicuous chert nodules.

Blocks of the Flint Beds, of the Cherty Freshwater Member, contain remains of charophytes. With a hand-lens the 12-celled cross-sections of charophyte stems can be seen. They show that the water depth of the freshwater lake was not excessive and in the photic zone. In sea-washed blocks ostracods and freshwater gastropods can seen in cross-section.

The bed of shale bed (SH 68) with some chert and some thin laminated argillaceous micritic limestone and which occurs under the Flint Bed contains planorbid freshwater pondsnails.

Incidently before leaving this part of Stair Hole notice the joint directions in the Flint Bed. The Cinder Bed does not show the same type of jointing. Bluish calcitic shells of the small oyster Praeexogyra distorta occur in the Cinder Bed of Stair Hole. The matrix is microsparite - fairly fine grained calcite - and the rock can be classified in Folk's classification of limestones as a biomicrudite.

These small oysters are quite different from the large oysters of the Jurassic marine clays beneath. There is evidence of marine water in the form of rare echinoid remains ( Hemicidaris purbeckensis ), but the oysters have some similarity to the small Crassostrea virginica of the coastal lagoons of the Gulf of Mexico. What used to be called " Ostrea distorta " can be regarded as a euryhaline lagoonal oyster. Its tolerance of salinities lower than seawater can be demonstrated by its occurrence in brackish water beds in the Intermarine Member.

Here, the Flint Bed is on the left and the junction of the Marly and the Cherty Freshwater Members can be seen. A peculiar jointed bed (bed 65 of Stair Hole log) is marked by the empty can. It is laminated in its upper part (the strata are vertical but young towards the north - that is - to the left in the left image) but micritic and slightly argillaceous in its lower part. Ensom, has described sediment-filled cracks at a similar horizon at Worbarrow Tout.

STRATIGRAPHY:

Purbeck Group - Broken Beds and Cypris Freestones

The Broken Beds is the name of a breccia, well-developed in the Lulworth area near the base of the Purbeck Group. It is a tectonic breccia with movement northward and it has been fractured at and above the level of the major Purbeck evaporites, the Purbeck Anhydrite, of southeast England. It is described in more detail with regard to the Fossil Forest .

STRATIGRAPHY:

Purbeck Group - Hard Cockle Member

The Hard Cockle Member is a relatively resistant limestone. It is similar in facies in Stair Hole as it is in Lulworth Cove.

STRATIGRAPHY:

Wealden - Introduction

The Lower Cretaceous Wealden strata are exposed in a patchy and irregular manner in the northern slumped slopes of Stair Hole. The Wealden is overturned to some extent. It consists of greyish and pink fluvial sands and clays. Lignite is common in much of the sequence. An oil sand occurs low in the sequence, not far above (stratigraphically) the Purbeck Group. This oil sand is shown in a separate section below, under the heading "Petroleum Geology".

STRATIGRAPHY:

Wealden - Remains of Tempskya, the Tree Fern

Silicified remains of the tree-fern Tempskya are widespread in the British Wealden (Watson and Alvin (1996). Radley (2005) reported a gastrolith of Tempskya from a Wealden plant bed at Yaverland, near Sandown, Isle of Wight. An example found on Weston Beach, east of Sidmouth is also shown above. The occurrence at Stair Hole, although not necessarily in situ seems to have come from near the base of the Wealden strata, close to the Upper Purbeck outcrop.

PETROLEUM GEOLOGY

Oil Sand and Oil Traces at Stair Hole, Lulworth Cove

The Wealden sands and the Purbeck Group are most notable for oil indications in the Lulworth Cove area. Good oil sands can found in a faulted block of Wealden near the base of the formation. It is easy to find and it is quite near small faults near the Purbeck-Wealden junction. A photograph above shows an oil geologist from PGS Reservoir Ltd. holding a sample. (Lees and Cox, 1937, mentioned below, found good oil indications at the cliff top just to the west of Stair Hole, but houses have since been built there and those exposures are either not accessible or not obvious now.

It was back in 1937 that geologists of the D'Arcy Exploration Company, later to become part of BP, investigated the Dorset coast in a search for oil (Lees and Cox, 1937). They said " The Purbeck Beds at their outcrops on the Dorset coast show a number of impressive oil indications; these include bituminous limestone, as at Peveril Point, and fractured limestones at Mupe Bay, Lulworth Cove, and Durdle Door which show oil residues on the faces of cracks and which ooze a little soft bitumen in warm weather. " Their plate 11 has an arrow pointing to this part of Stair Hole. Here in a crack or joint of a block of Purbeck limestone is one of these oil residues of the type which started the oil exploration in this region.

Eventually the early finds of oil indications led to the discovery of the great Wytch Farm Field, the biggest onshore oilfield in northwest Europe.

The oil seep at Stair Hole, together with other oil seeps of the Dorset coast, has been studied geochemically by Bigge and Farrimond (1998). All these oil seeps have been degraded but in different manners.

PETROLEUM GEOLOGY

Oil migration and Seepage in Early Wealden Times

It is generally accepted that the oil in Dorset was generated by Cretaceous times. There was sufficient depth of burial of the Liassic source rocks (about 3km) by this time in the offshore basin, and the oil has migrated into traps of Cretaceous age, rather than Tertiary structures. Reworking of older Jurassic clasts first occurs in the Upper Purbeck (just under the Purbeck Marble here with oil) because at this time the Inversion boundary fault had moved for the first time on any scale. The inversion uplifted round about 100 metres (because there is reworked Portland detritus and some Kimmeridge in the Purbeck Unio Beds). This initial fault movement had just started when the lower Wealden beds were deposited. It is not surprising that there seems to be an "oil river" in the lower Wealden at nearby Mupe Bay (even though geochemists have disputed this). A penecontemporaneous seep would be expected in this area and at this time. It may not have continued because Wealden clays sealed the fault.

This does not necessarily imply that the oil sand of Stair Hole received the oil at the surface. This could have happened, but the Wealden sand may well have been under very shallow burial. The Mupe Bay "oil river" is at about the same horizon, but is more convincing evidence.

STRUCTURAL GEOLOGY:

The Lulworth Crumple

The Lulworth Crumple is a minor fold structure within the major monocline of Alpine (Tertiary) age in the Lulworth Cove area. It is particularly well seen in the east side of Stair Hole.

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The Lulworth Crumples are minor folds within the north limb of the major monocline. There are several of these and although they are best seen in the Lulworth area, the place where deformation due to Tertiary tectonics is most severe. Similar small folds, however, occur along the outcrop as far east as Peveril Point, Swanage. Specific examples have been exposed near Blashenwell (in the Purbeck marble), at Herston (west of the school) and are complex at Peveril Point itself (Cosgrove and Hearn, 1966). Undoubtedly there are others. The relative relationships to the major fold are shown diagrammatically in the cross section in the main Lulworth Cove introduction . The images here are of the most well-known and the best-exposed, which is in the eastern cliff of Stair Hole. They show both the broad structures and the minor structures within it. Evidence of shortening of the Middle and Upper Purbeck Group is most obvious. Local compression can be seen. Phillips (1964) has shown, as in the diagrams below, there are also extensional features, resulting from compression, at the lower levels of the Purbeck Group within the structure (i.e. south).

Above is a diagram from Phillips (1964), from which I have removed the sliding and collapse. See also the interpretation by Bevan (1985).

This lower resolution image shows both the photograph diagram to enable comparison. The images are not as good as the separate ones.

Abbreviations: DD = Durdle Door, WSH = West Stair Hole, EL = East Lulworth Cove, BH = Bacon Hole. Purbeck Members: A = Caps Member, B = Broken Beds and Cypris Freestones Member, C = Hard Cockle Member lower part, D = Hard Cockle Member upper part limestones, E = Soft Cockle Member, F = Cherty Freshwater Member and Cinder Bed, G = Intermarine Member and Chief Beef Member, H = Shales not shown.

There are several small faults and shears within the structure. The direction of movement within these is quite variable. Occurring within the eastern cliff face are the Stair Hole "Snakes Heads". These resemble on a small scale the snake's head structures of thrusts (see for example McClay, 1989, fig. 3.13, p. 56), although the strata here are vertical. They are occurring near the top of the Intermarine Member, with the Cinder Bed visible on the right. The strata are younging towards the left.

It is important to note that the complex structures are not entirely confined to the Purbeck Group. The Wealden strata, stratigraphically above the Purbeck, are exposed in an irregular manner in the collapsed slopes of the back (north side) of Stair Hole. These are overturned in much of the exposure, and they also show much faulting. It is not initially obvious because the Wealden does not contain conspicuous thin-bedded limestones. Nevertheless it should be appreciated that the Lulworth Crumples involve Wealden strata to some extent, in addition to the Purbeck limestones and shales.

STRUCTURAL GEOLOGY:

Lulworth Crumples - Theory - Gravity Slip

Gravity collapse was suggested by Lees (in Phillips, 1964). Downward collapse against the planes of thrust faults during the final stages of uplift of the south side has been considered.

STRUCTURAL GEOLOGY:

Lulworth Crumples - Theory - Upward Drag

Upward drag during the major folding (i.e. Alpine folding) of the monocline was suggested (Arkell, 1938).

STRUCTURAL GEOLOGY:

Lulworth Crumples - Theory - Upward Faulting of Purbecks

I argued for upward movement of the Middle and Upper Purbecks a long time ago (West, 1964) and still have the same view. It is obvious that the Middle and Upper Purbeck Group have been shortened by the folding (and some faulting) within the Lulworth Crumple

The Durdle Door area, further west shows a lower structural level in the Tertiary monocline. This is because there is an easterly plunge. Here, the higher Purbeck strata, the very beds which are ruckled and shortened at Stair Hole, are faulted out. There is an unconformity beneath and to the north and this would have prevented downward movement of the Middle and Upper Purbecks. The missing strata at Durdle Door could only have gone upwards.

This diagram, which is partly schematic (and also intended for the quite different purpose of showing possible oil migration) shows how the Lulworth Crumples could have been formed by the squeezing out of Purbeck strata from the zone of compression. It is unlikely that this is the entire explanation and that, as discussed by West (1964), bed-over-bed drag was also involved.

STRUCTURAL GEOLOGY continued:

The Lulworth Crumples - Recent Work using Seismic

A key work on this and related subjects, is the seismic study of Underhill and Paterson (1998) - Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck - Isle of Wight Disturbance, Journal of the Geological Society, London, vol. 155. This paper provides various accurate sections through the Purbeck Disturbance, and although much concerned with other matters, it also provides evidence relating to the Lulworth Crumples. See also the related work of Underhill and Stoneley (1998).

STRUCTURAL GEOLOGY continued:

The Lulworth Crumples - More Notes

Above is a diagram from Phillips (1964), from which I have removed the sliding and collapse. See also the interpretation by Bevan (1985).

This lower resolution image shows both the photograph diagram to enable comparison. The images are not as good as the separate ones.

Abbreviations: DD = Durdle Door, WSH = West Stair Hole, EL = East Lulworth Cove, BH = Bacon Hole. Purbeck Members: A = Caps Member, B = Broken Beds and Cypris Freestones Member, C = Hard Cockle Member lower part, D = Hard Cockle Member upper part limestones, E = Soft Cockle Member, F = Cherty Freshwater Member and Cinder Bed, G = Intermarine Member and Chief Beef Member, H = Shales not shown.

ACKNOWLEDGEMENT

I am very grateful for helpful discussion with many groups of geologists and students who have visited Stair Hole with me. I thank Christian Zozaya for kindly allowing reproduction of an old photograph of Stair Hole. I am very much obliged to Dr. Travis Mason and the Channel Coastal Observatory for permission to use aerial photographs of the Lulworth Cove area, and elsewhere. I am very grateful for the support of the Dean and other Staff of the Faculty of Natural and Environental Science at Southampton University.