1. Safety at West Over

This western stretch of rocks and ledges is not as easy as the main part of Lulworth Cove. It is not recommended for the less agile, and it has some particular risks associated with high tides and storm conditions.

First, in general, please go to:

Lulworth Cove Introduction - Safety .

Specific risks at this western promontory of the Lulworth Cove include risk of being hit by a falling rock from the Purbeck limestones, falling when scrambling over slippery Purbeck limestone blocks or ledge at the foot of the cliff, and being washed away by storm waves. See section below:

2. PURBECK GROUP:
2.1 Introduction

2.2 PURBECK GROUP continued:

Carbonate Petrography - Introduction

Purbeck limestones can be identified in general descriptive terms in the field with the aid of a hand lens and dilute hydrochloric acid. There are obviously bivalve shelly limestones in the Middle Purbeck for example. There are glauconitic, bivalve shelly limestones in the Upper Purbeck. The Cinder Bed is an oyster shelly limestone. With a hand lens it is easy to recognise ostracodal limestones in the Cypris Freestone Member. The stromatolites of the Caps are very conspicuous in the field. Some limestones are more difficult to identify and all the limestones provide much more information in thin-section. Because of the presence of dolomite, siderite, aragonite, quartz, gypsum, celestite, pyrite etc in various Purbeck strata it is recommended for the researcher to have X-ray diffraction data at hand if that is possible (the content of Sr etc is most useful if trace element data is also available). Some introductory information on petrography is now given.

Thin-section petrography of the Purbeck carbonates is not dealt with in detail here. It is convenient, though, to refer to a few limestone types. Most geologists probably use the well-known Folk's Classification, although some, particularly in the oil industry, prefer Dunham's Classification.

For reference, a simple, student-level version of Robert Folk's (1962) scheme is given here. It is only satisfactorily used with good thin-sections, but with experience you can often make an intelligent guess at the Folk's name using a hand-lens. A trap for the novice is that many limestones which might appear to be micrites are are in fact pelsparites or pelmicrites. Another trap is that some apparent micrites are in in fact dolomicrites, not limestone micrites. However these are mainly in the Soft Cockle Member. The Cinder Bed has unusual petrography: it is biomicrosparite (or biomicrosparrudite).

A minor modification to the scheme has been made so as to include Carozzi's grumeleuse micrite. Carozzi's classic work was on the Purbecks of the Jura Mountains, so it not surprising that grumeleuse micrites are important in the equivalent strata of Dorset. Some of these are compacted pelmicrites or pelsparites; do not confuse grumeleuse micrites with thrombolites, which show poorer sorting and are normally found within stromatolites. Both grumeleuse micrites and thrombolites, together with pelsparites, pelmicrites, quiet-water oosparites, and ostracodal biosparites and biomicrites (especially in Cypris Freestones Member) are very common in the basal Purbecks. With them are typical evaporite replacement features, such as lutecite, length-slow chalcedony, euhedral quartz, nodular structure, celestite and pseudomorphs after lenticular gypsum and anhydrite. Secondary limestone replacement of evaporites are present. The lower part of the Broken Beds is really an unusual type of cargneule, not reddish and polymict as in the Pyrenees and Alps but oligomict and grey.

Higher in the Purbecks (Middle Purbeck upwards) biosparudites and biomicrudites are very common. Quartz sand increases in quantity and glaucony is common in the Upper Purbeck. The well-known Purbeck Marble is a "biomicrite", in more precise terms a biomicrudite.

2.3. PURBECK GROUP continued:

Clay Mineralogy

The clay mineralogy of the Purbeck of Lulworth Cove has been discussed in a paper by Schnyder, J., Ruffell, Deconinck, J-F and Baudin, F. 2006 on the conjunctive use of spectral gamma-ray logs and clay mineralogy in defining late Jurassic–early Cretaceous palaeoclimate change (Dorset, U.K.). The SGC log will be of particular significance to specialists concerned with boreholes. The clay mineral log is more direct interest to field studies of the Purbeck Group.

To understand the clay mineralogy, it is necessary to note the general pattern of Purbeck palaeoclimatology (already well-established at Durlston Bay). The Lulworth Cove section shows clearly the Lower Purbeck the semi-arid phase, and then a transitional phase until the Cinder Bed, after which subhumid conditions existed. The clay mineral assemble for the Lower Purbeck is illite/smectite mixed layer with illite, but, interestingly, no kaolinite (except for some in the Caps). As at Durlston Bay there is episodic palygorskite [the Mg clay mineral] in the Lower Purbeck. Supplementary information is provided on TOC [total organic carbon]. This is generally low in the Lower Purbeck but in the Middle Purbeck Mammal Bed it rises to almost 7%. If you look at this bed in the field you can see the organic matter and it seems to be carbonaceous rather than bituminous (i.e. this is not a potential oil source rock). The paper contains other informaton on K, U, Th and ratios of these, but the values are not remarkable. The Purbeck above the Cinder Bed is not studied in detail.

2.4. PURBECK GROUP continued

Purbeck Strata of the West Side of Lulworth Cove

2.5. PURBECK GROUP continued

Unio Member

The Unio Member of the Purbecks is well-exposed in a conspicuous wall on the west side of the cove, just north of West Over. Here there is steeply north-dipping, glauconitic, shelly limestone with Unio and with fish teeth. This is a good location for teaching the use of the compass-clinometer to beginner students (another good place is at East Over).

The Purbeck succession is similar to that at the eastern side of the cove and at Stair Hole.

2.5. PURBECK GROUP continued

Unio Member - Seismite
(Earthquake Bed, of about 140 million years ago, mid-Berriasian)

[penecontemporaneous, Biscayan or Late Cimmerian, tectonism, pre-Atlantic Ocean]

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Recent storms have been provided quite good exposures of the Biscayan seismite, well-known at Stair Hole, but now also well-exposed at western Lulworth Cove. The effects actually extends eastward to at least Mupe Bay, but the original earthquake would probably affected most of the eastern-extention of Nova Scotia of which much of Lulworth Cove was a part.

Lulworth Cove is situated almost exactly where the northern boundary of the early (Jurassic - Lower Cretaceous) Purbeck - Isle of Wight basin concides with the northern Tertiary Inversion Structure. It is a classic place for this type of structure. In the past, this was not necessarily understood and the place was simply explained as just a Tertiary, compressive structure [and it reasonable that this old and simple explanation may still be used by non-technical parties]. It is in reality much more complicated than this, and the earlier, Late Cimmerian or Biscayan, extensional phase has to be seen through the more obvious Tertiary compaction story. Unfortunately, even the Tertiary story may be sometimes be given in simplified "Miocene" terms, but actually much tectonism is late Eocene, as shown by the Isle of Wight. Some of the faults are largely Tertiary, but some are re-activated Late Cimmerian or Biscayan. The feature discussed just here is purely Biscayan.

The Unio Bed Seismite, assocaited with an oil seep, was formed during the major earthquake when the Weymouth area was suddenly uplifted in late Purbeck times, i.e. early Cretaceous, or Biscayn times.

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A seismite, a contorted earthquake bed, is a conspicuous unit in the Unio Member at both Lulworth Cove and Stair Hole. It very easy to find. At Lulworth Cove it is, as seen, the thin representative of an unconformity in the Weymouth area.

At Upway the stratal equivalent contains clasts of Portland Roach from the Portland Stone Formation, showing that almost the whole of the Purbeck sequence is missing nearby. The erosional gap was probably over some part of the Weymouth Anticline, north of the Isle of Portland. The bed is incorporated in the Purbeck (or Durlston) sequence because it is in a carbonate-shale facies. Really, this bed is the start of the Wealden freshwater facies, and above it come the freshwater Purbeck Marbles (gastropod limestones). The Wealden freshwater facies is the result of development of tectonic barriers - this bed actually shows the effects of the starting earthquake (but this fact has been rather ignored!).

[More detail - See: West, I.M. and Hooper, M.J. 1969. Detrital Portland Chert and Limestone in the Upper Purbeck Beds at Friar Waddon, Dorset. Geological Magazine, vol. 106, no. 3, pp. 277-288. Clasts of Portland Oolite occur in the same Unio Bed at an old quarry at Friar Waddon, north of Weymouth. Therefore, an unconformity beneath the Unio Bed had cut down through the (probably upfaulted) complete Purbeck sequence, somewhere between Upwey and the Isle of Portland. So the seismite is a bed overlying an unconformity at some places. The earthquake, resulting in the seismite at Lulworth Cove, would have been caused by movement of strata at this phase of development of the unconformity.]

3.1. PURBECK LOCATIONS:

West Over

West Over is less pronounced as a promontory than is East Over. This is because the prevailing wind direction is from the southwest. Naturally most erosion takes place towards the northeast. As you might expect, on the east side of the cove the Wealden deposits (Wessex Formation) have been eroded back further in relation to the Purbeck Group than on the west side. Because the erosion is generally more rapid there the Purbeck strata have been cut by almost vertical cliffs just to the south of West Over.

Landslides of mud and limestone in the 1970s led to the destruction of the coastguard huts on the cliffs here. These old slips are not very active now but are still wired off. A new phase of erosion has commenced at the foot of the cliff though. Thus the toe of the landslide is being removed and eventually a new phase of slipping will take place. While slipping of mud and rock could present a hazard to a person it is usually just a risk to property. A more serious hazard to persons occurs where vertical cliffs are undercut, as south of West Over, and where rock falls are common. Sadly, though part of the cliffs from West Over to West Point have been the site of a tragic accident when two boys watching a storm were washed away by the waves.

At low tide and in normal weather conditions it is possible to walk over the shore rocks to the Purbeck outcrop, although this is not necessarily recommended and it may not be possible at high tide. It is probably unnecessary for general parties concerned with geomorphology and the major geological features. Many of the basal Purbeck features are better seen at the Fossil Forest exposure. The Hard and Soft Cockle, the Marly Freshwater and the Cherty Freshwater can be seen quite well and in safer conditions than at the base of Stair Hole. Take care with algal-covered, slippery rocks. The cliffs are mostly fairly low but be careful to avoid getting beneath any loose debris and do not go into the cave in the Broken Beds. Attempting to climb to West Point is not advised.

3.2. LOCATION - PURBECK GROUP:

Cinder Bed at West Over

The well-known oyster bed, the Cinder Bed is at about the middle of the Purbeck Group. At one time it was thought to be at about the Jurassic-Cretaceous boundary (Casey, 1963), now know to be lower and round about the level of the top of the Lower Purbeck, Soft Cockle gypsum. On grounds which relate to chronostratigraphy and not lithostratigraphy, the Purbeck strata were divided into "Lulworth Beds" below and "Durlston Bedsj" above. Later, these were taken, contrary to the rules of lithostratigraphic nomenclature, to be Formations; i.e. the "Lulworth Formation" and the "Durlston Formation". The Cinder Bed is included as the basal part of the Durlston Formation. This Lulworth Formation and Durlston Formation seems to have been established in the literature. Thus the Purbeck unit is considered as the Purbeck Group and consists of two formations. The Purbeck Group is partly Upper Jurassic and partly Lower Cretaceous (Berriasian). The photograph below shows the Cinder Bed which now marks the base of the Durlston Formation.

The Middle Purbeck Cinder Bed is easily recognised at West Over. It has the usual dark bluish grey shells of the small lagoonal oyster Praeexogyra distorta (pronounced "Pre - Exogyra). It is a wackestone according to Dunham's classification but it is only just matrix-supported, so that in places it may become a packstone. In Folk's it is really a biomicrosparrudite (in simple language - "a biomicritej"). Thin-sections show that the matrix is usually microspar not micrite and the allochems are of rudite size. I have not examined it petrographically though at this locality.

In the field at West Over an interesting aspect is that a broken section through an echinoderm is visible. Echinoderms occur in the Cinder Bed but are difficult to find. This may well be an example of the famous Hemicidaris purbeckensis , which at one resulted in the whole of the Purbeck strata becoming listed as Jurassic (but thi is not now the case).

Considering specific beds, the probable equivalent of the Mammal Bed of Durlston Bay is dark carbonaceous marl with freshwater gastropods. It represents the marshy margins of a freshwater lake. At Durlston Bay is the source of the famous fossil mammal discoveries of Victorian times. Further down in the Lulworth Formation (i.e. Lower Purbeck) there are interesting features in the Soft Cockle Member.

The Pellety Bed - the intraclast bed of the uppermost Soft Cockle dolomites and limestones

The uppermost part of the Soft Cockle Member consists of dolomites with pseudomorphs after halite and probably some limestones. These carbonates alternate with softer marls. They are seasonal salt-lake deposits, resulting from the late restriction of the hypersaline Purbeck lagoon. The lakes dried completely at times and thus the halite crystals were produced as the white dolomitic sediment (with palygorskite in places). This was the last phase of dry conditions and the Marly Freshwater Member which follows has a good freshwater fauna.

The most conspicuous bed of this sequence is the Pellety Bed. This is browner than the other beds and composed of flat clasts of argillaceous limestone or marlstone with some rounded ends and some angular fractures. The Pellety Bed is a good marker horizon in the Lulworth area.

The Pellety Bed of Lulworth Cove is probably the equivalent of bed 77 of Worbarrow Tout in Ensom's (1985) Worbarrow Purbeck log. Note that intraclasts are also present at the top of 69b. Look for the algal stromatolite and serpulid limestone - 76b of Worbarrow, which may be present in the Lulworth Section. It is very interesting to observe that this flat-pebble conglomerate is not present at Durlston Bay. See the extract from the classic log of Clements (1969; 1992) also provided above. The absence at Durlston Bay is because this locality has a thicker Purbeck sequence which represents that of the "basin" as opposed to the shelf facies of Lulworth. The salt lake at Durlston was not subjected to storm erosion by the lake margin waves; it was probably still under water. If the correlation suggested above for Worbarrow Bay is correct then it is appropriate to consider DB 59 and its vicinity for comparison at Durlston. It ties in very well with serpulid beds just below (and these in turn probably relate to the stromatolite bed of Perryfield Quarry, Portland - see House, 1968 ). Thus the lateral equivalents are well-known but more research is needed on the details. A further point of interest is that the high Soft Cockle dolomites, of which the Pellety Bed is the base are important fossil insect horizons at Durlston Bay and elsewhere (look for insects at Lulworth Cove). See Coram (2002) and other publications by Coram and by Jarzembowski.

The flat pebble clasts in flat pebble conglomerates are one of several type of intraclasts. It should be noted that Folk's (1962) defination of intraclasts is very broad, including almost everything of intrabasinal carbonate detrital origin from many peloids to bahamites, grapestones to plasticasts and tidal flat clasts (i.e. flat pebble conglomerates). This approach may seem rather confusing but the original definition made it clear that Folk intended the term to be generic: "Intraclast should be used as broad class term without specifying the precise origin" Folk (1962), p. 64.

To summarise, the intraclast bed or flat pebble conglomerate of the upper Soft Cockle Member of the Lulworth area is almost certainly a storm accumulation on the supratidal flats of dried lime mud ripped up from the desiccated playa lake to the southeast. This debris was built up quite rapidly on the Lulworth Shelf which formed a sabkha-like margin to the terra firma of the South Dorset High, just to the north of Lulworth Cove. Worbarrow Bay was at the very gentle break of slope between the shelf and the "basin" (just slightly lower) and there the storm channeled the underlying carbonate. On the Dorset mainland the distribution of the flat pebble conglomerate approximately mirrors that of the Great Dirt Bed with pebbles, lower down, and of course the GDB fossil forest (Worbarrow Bay is the southeastern limit of this). These are all shelf features, not present in the "basinal" facies of the Durlston Bay type section of the Purbecks. In contrast the Soft Cockle gypsum beds of Durlston Bay just reach Worbarrow Tout, where they show abundant supratidal enterolithic features (as you would expect), but they do not extend into the Lulworth area (the dry margin of the lagoon).

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3.3. LOCATION - PURBECK FORMATION:

West Side of Lulworth Cove - Lulworth Crumple

The diagram above is modified, with additions, from one of Phillips (1964). The photographs shows detail within this section and include the "southward overthrust" of Arkell (1938) (fig.5). Another, taken in 2002, reveals the details of a "knee-bend" of a Lulworth Crumple.

Phillips commented that on the west side of Lulworth Cove there are several shallow asymmetric folds with steeper lower limbs in the Cypris Freestones. These folds plunge at 10 degrees in a direction N.285 degrees E. approximately parallel to the strike, though this varies slightly because of undulations which plunge at 30 degrees in a direction N.338 degrees E. A number of tighter minor folds which have been overturned down-dip are present in some of the limestone bands in the lower part of the Hard Cockle Member. There is a broad fold in the main Hard Cockle limestones which are almost vertical just above beach-level. Shear fractures dipping at 56 degrees S. curve into the vertical bedding planes and show marked displacements downward on the south side.

Several small faults, steeply dipping to the south, occur in some of the limestones of the Soft Cockle Member, and a displacement of 3.7m has occurred on the largest fault. The beds on the north side of the large fault dip at 65 degrees N. at beach-level but become vertical in the cliff face, a change which indicates tilting of the beds to the north. Arkell regarded this fault as a southward overthrust formed by the dragging of the overlying beds up the steep limb of the anticline (Arkell, 1938). Phillips (1964) considered that the northward tilting of the beds could not be explained by this interpretation and he thought it more likely that these faults are steep thrust faults formed as a result of the downward sliding of the beds. He thought that the curved southward dipping shear planes in the vertical part of the main Hard Cockle Limestone may have been formed as steep northward dipping thrust faults that were subsequently rotated and modified during the formation of the fold.

3.4. LOCATION - PURBECK FORMATION:

West Side of Lulworth Cove - Purbeck Group -
Broken Beds (updated)

The Broken Beds near the base of the Purbeck Group are shown here as seen in the cliff near West Point, Lulworth Cove. To explain the sequence a diagram that related to the similar section at the Fossil Forest (further east) is also provided.

The Broken Beds have originated as a sequence of calcium sulphate (initially primary lenticular gypsum) overlain by thin-bedded ostracodal and peloidal limestones of the Cypris Freestones Member. This evaporite-limestone was tectonically brecciated by movement northward of the overlying Cypris Freestones. Small drag folds show the direction of movement

3.5. LOCATION - PURBECK FORMATION

The Great Dirt Bed near West Point

The Great Dirt Bed is a well-known palaeosol (ancient soil) of rendzina (carbonate-rich) type. It is well seen at the Fossil Forest section and on the Isle of Portland, as in one of the photographs above. It occurs in the higher part of the Caps, the basal limestone of the Purbeck Group that are characterised by stromatolites, pelletoidal and fine-grained oolitic limestones of lagoonal origin. The trees of the Fossil Forest and the Portland exposures were rooted in this soil.

This dark carbonaceous bed with black and cream-coloured pebbles and some irregular clasts is visible on the dipping limestones near West Point. It is not easily accessible and it is much more easily studied at the Fossil Forest exposure. It is dangerous to climb up on the projecting rocks of West Point.

We will consider briefly the palaeopedology. This ancient soil (palaeosol or paleosol) was once thicker and is now compacted. It is a rendzina, a calcareous soil of a type that develops on limestones. See the paper of Dr. Jane Francis (1986) - The calcareous paleosols of the Basal Purbeck Group (Upper Jurassic), southern England for more information. It has a simple A/C rendzina profile, consisting of a dark organic rich horizon, with both lignite and charcoal overlying limestone bedrock that was already lithified by early cementation. It is interesting that the upper O-horizon of undecomposed plant litter is lost, but some organic matter has been reworked in places into the overlying limestone.

Francis (1986) was able to classify the the Great Dirt Bed into particular catergories of rendzinas. Because it has formed on lithified parent rock it can be described as a lithomorphic rendzina. Because of the presence of secondary carbonate (i.e. caliche), resulting from seasonally arid conditions, it can also be termed a xero-rendzina.

At this particular site near West Point of Lulworth Cove it is not sufficiently accessible for detailed study. Furthermore there do not seem to be any well-developed stromatolites with tree moulds just here. To understand this bed more fully it is best to visit the Fossil Forest exposure, and read the key publications by Jane Francis.

3.6. LOCATION - PURBECK FORMATION:

West Point

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BIBLIOGRAPHY AND REFERENCES

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