West, Ian M. 2013. Geology of Lulworth Cove, Dorset, continued - Purbeck Group - East; . Internet site: www.southampton.ac.uk/~imw/Lulworth-Purbeck-East.htm. Revised - 16th December, 2013. (one of three webpages on Lulworth Cove)
Geology of Lulworth Cove continued, Purbeck Group of the East Side
By: Ian West
Romsey, Hampshire,
and Visiting Scientist at:
Faculty of Natural Science and the Environment Southampton University
Website hosted by iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory

MAIN LULWORTH COVE WEBPAGES:

LULWORTH COVE INTRODUCTION
LULWORTH COVE CONTINUED; PURBECK GROUP, WEST SIDE OF COVE
LULWORTH COVE CONTINUED: PURBECK GROUP, EAST SIDE OF COVE - THIS!

| Home and List of Webpages | Field Guide Maps and Introduction| |Stair Hole |Fossil Forest |Fossil Forest Purbeck Trees | Dungy Head |Durdle Door |Mupe Bay |Worbarrow Bay |Lulworth Bibliography | | Purbeck Palaeoenvironments | Lulworth Strata and Contained Fossils - more detail| |Durlston Bay - Peveril Point, Upper Purbeck Group |Durlston Bay, Middle Purbeck |Durlston Bay - Lower Purbeck |Durlston Bay - Central Zigzag Part & Coast Erosion |Durlston Head - Lower Purbeck Group & Portland Stone |Durlston Bay - Bibliography
Selected external links: |The Lulworth Estate |Jurassic Coast - World Heritage Site |

ALL LULWORTH COVE WEBPAGES:
Lulworth Cove Geology - Introduction
Lulworth Cove Purbeck Group, West Side of Cove
Lulworth Cove Purbeck Group, East Side of Cove
Lulworth Cove - Fossil Forest
Fossil Forest Purbeck Trees
Lulworth Cove - Stair Hole
Lulworth Cove - Dungy Head
Lulworth Cove - Durdle Door and Bats Head
Lulworth Cove Select Bibliography

Click here for the full LIST OF WEBPAGES

Click or double-click on images for full-size high resolution versions!

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In the corner of Lulworth Cove, Dorset, on Purbeck strata near East Point, 29th September 2012

The eastern part of Lulworth Cove, Dorset, seen from near East Point, Petrostrat field trip, 8th September 2012

Looking down to the dipping Cherty Freshwater Limestone at East Over, Lulworth Cove, Dorset, 1st October 2011

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1. SAFETY AT EAST OVER, PEPLER'S POINT AND EAST POINT



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1. INTRODUCTION:

1.1. Safety

Please see the safety notes in the main Lulworth Cove Introduction webpage. Take all normal geological precautions.

Attention is drawn to the fact that this eastern exposure of the Purbeck Group has been the site of a fatal accident, involving a geological party of a school teacher and three students. Do not loiter beneath overhanging strata or in places where there are any signs of rock-fall.

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1.2 INTRODUCTION continued:

1.2. SAFETY - EAST OVER


Accident Risk - South of East Over

Part of the cliff section to the south of the East Over promontory, where there is an overhang, has a particular risk. Unfortunately, the southern part of it is best avoided if possible in wet or frosty weather and with large parties. Specialists need to make a fleeting examination at this part of the section but they do so at their own risk. The foot of the cliffs should not be approached and in no circumstances should anyone go beneath or at the edge of an overhang. It is sad to report that there was a fatal accident in 1977 at the Hard Cockle overhang here, when a party seems to have sheltered from steady rain.

Cliff-fall accident at the southeast corner of Lulworth Cove, Dorset in 1977

Cliff-fall accident at the southeast corner of Lulworth Cove, Dorset in 1977 - continued

Accident at the southeast corner of Lulworth Cove. Anonymous. 1977. New Warnings after Cliff Fall Tragedy. Bournemouth Evening Echo, Feb. 21st, 1977. " Warnings came today about the "killer cliffs of Dorset" after yesterday's tragedy at Lulworth Cove when a landslip engulfed a school party in tons of rock and clay, killing a teacher and seriously injuring two others. " The cliffs are completely unstable for virtually the whole length of coastline from Swanage to Ringstead". .... The group [from Warlingham Comprehensive School, Surrey] had walked around the Lulworth beach to the East Point of the cove, and Miss Taite had stopped them under the 75-foot-high cliff [under or adjacent to the Hard Cockle Overhang] to point out the rock structure. They were drawing this in their notebooks when the cliff suddenly gave way. Tons of rock debris and clay [from a scar from the top Soft Cockle to the top Marly Freshwater Members] fell on to the party and two were buried. Mark Playdell ran a mile along the beach to the cafe to call for help, while the other three who were not badly injured, started to dig out their classmates. Ambulancemen, police and coastguards rushed to the scene, and two helicopters were sent from Lee-on-Solent. .... Two years ago a woman was killed by a cliff fall at Durdle Door while she was sunbathing on the beach below, and the same year a schoolboy with a field study group was seriously injured by a rock fall at Swanage. ..."

The southeast corner of Lulworth Cove, Dorset, seen from the East Over promontory at beach level

At present there is loose rock up on the cliff to the south of East Over. Here a fissure in the Hard Cockle Member high on the cliff that was just visible in 1892 and more obvious in 1910. It had opened to about half a metre by 1979. By 1997 there has been major movement resulting in a supply of loose rock on the cliff south of the beach. Thus the northern part of the southeastern cove, at East Over, could be used for study with caution if weather and tide conditions were appropriate, but it is not safe southeast of the Overhang (and particularly not under the overhang or adjacent to southernmost cliff).

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2. GEOLOGICAL LOCATIONS - PURBECKS, EAST SIDE OF LULWORTH COVE



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2.1. LOCATION (PURBECK FORMATION):

East Over

Details of East Point and East Over, Lulworth Cove, Dorset, shown on a map

View of East Point and East Over, Lulworth Cove, Dorset, in 1959

View of East Over and Pepler's Point, Lulworth Cove, Dorset, with paths up cliff shown

Overview of structures at East Over, Lulworth Cove, Dorset

The East Over Syncline at the east side of Lulworth Cove, Dorset, seen from above, October 2011

Purbeck Group at East Over to East Horn, Lulworth Cove, Dorset

Lulworth Crumple in Purbeck strata in the cliff near East Over, Lulworth Cove, Dorset

Overview of structures at East Over, Lulworth Cove, Dorset

The area south of the fault, shown on the above diagram, and just north of the East Over promontory needs modification now there is a good exposure of a small monocline, one of the Lulworth Crumples, in the back of the cliff.

View from the cliff top of Lulworth Crumples at East Over, Lulworth Cove, Dorset, in 2002

Small syncline at Lulworth Cove, Dorset

Small anticline at East Over, Lulworth Cove, Dorset

At the end of the promontory of East Over there is a small syncline and anticline in the Middle Purbeck Group. These are part of the "Lulworth Crumples". You can examine this fold well on the shore at low tide (take care with slippery algal-covered rocks). It is interesting that a very similar syncline, shown above, and associated anticline occur at Peveril Point, at the northern end of Durlston Bay, Swanage. The folds at Swanage are in the Upper Purbeck whilst those at East Over, Lulworth Cove are in the Middle Purbecks. The similarity is important in relation to Phillips (1964) gravity-slip theory. Contemplate the implications of similar structures having developed in the Swanage area with a dip of only 3 degrees, and in a thicker sequences of Purbeck and Wealden strata. Incidently the Lulworth Crumple type structures are not present at Mupe Bay and Bacon Hole or Worbarrow Bay but are present in the Isle of Purbeck further east from Herston to Peveril Point (and perhaps in other parts of the Isle of Purbeck). Contemplate the implications of this too!

East Over is where the soft Wealden strata end and the harder, thin-bedded Purbeck limestones project out westward into the cove. The name presumably refers to the fact that the walker will usually go up over the top here, and there is a steep, but easy, upward path. This leads to Pepler's Point, a promontory within the cove and overlooking the Purbeck outcrop.

A map and and some photographs of geological features of East Over are provided above. In the field, it is necessary now to decide whether to go up the hill, and perhaps onward to the Fossil Forest (if open) and Mupe Bay or to go along the beach towards East Point.

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2.2. LOCATION (PURBECK FORMATION):

Pepler's Point

Civil engineering students study the structure of Lulworth Cove, Dorset from Pepler's Point on the east side

The Pepler's Point monument stone near East Over, Lulworth Cove, Dorset

At Pepler's Point you will notice that the ground level is about the same as that on the Purbeck ridge extending eastward to Bacon Hole (near Mupe Bay ). This has significant geomorphological implications. It implies that there was no large Pleistocene valley at the mouth of Lulworth Cove before sea level rose to its present position. Most streams of any size in the region have some form of river terraces. That is to say, they have been progressively cut down and there is often a younger steeper and narrower valley within a broad one. Thus the old stream valley out through the cove entrance was almost gorge-like and relatively young, Devensian or later.

Notice also that from the Purbeck outcrop there is a gentle slope seaward towards the former Portland Stone outcrop. This is the opposite of the situation at Gad Cliff where the ground rises southward from Purbeck to Portland. This may be related to the former presence of the Ipswichian raised beach just south of Lulworth Cove. There is no firm evidence and it is good subject for discussion at Pepler's Point, if you have time.

From Pepler's Point it a very short walk to a footpath extending in the direction of East Point. This may not safe for student parties because on slipping it is possible to fall south to the sea or north to the cove. This path shows exposures of ripple-bedded Cypris Freestone Member of the Purbeck Group.

Just south of Pepler's Point is Potters Hole. This is small version of the Fossil Forest exposure. The approach to is involves a short walk on a seaward-slope on the cliff-top. Although easy, a slip could lead to a fall over the cliff. Thus it is best not done, except with a fixed safety rope. Potters Hole provides a good exposure of the basal Purbeck Caps and Dirt Beds and the stromatolites and Broken Beds above. The Fossil Forest further east is better but is sometimes closed because of army firing.



Footnote: The name "Pepler's Point"
It is named after Sir George Lionel Pepler, the tennant of Little Bindon. He was at one time the Chief Town Planning Inspector at the Ministry of Health. He became the president of the Town Planning Institute, which he founded. He was described as a very nice old man. Little Bindon is a cottage in the depression on the Wealden strata at the eastern part of Lulworth Cove. The name "Pepler's Point" does not appear on maps much before about 1980.


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PURBECK STRATIGRAPHY



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STRATIGRAPHY:

PURBECK GROUP -
Introduction - Isopach Map

An isopach (isopachyte) map for the Purbeck Group of southern England, based on, and modified after, Howitt (1964), with contours in feet, and with additional interpretation

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STRATIGRAPHY:

PURBECK GROUP -
Introduction

Simplified diagram of the stratal succession in the Lulworth Cove area of Dorset, England

Major lithostratigraphic units of the Lower Cretaceous of Dorset, England in approximate relationship to the chronostratigraphic stages

(To understand the Purbeck Group more fully the guides to the type section at Durlston Bay, Swanage. These contains more detail on the Purbeck facies, fauna, sedimentology and palaeoenvironments.

Durlston Bay - Peveril Point
Durlston Bay - Middle Purbecks and Building Stones
Durlston Bay - Lower Purbecks
Durlston Bay - Central Zizag Path area
Durlston Bay - Bibliography )

North-dipping Purbeck limestones at East Over, with view across Lulworth Cove, Dorset to the west side

East Horn of Lulworth Cove

East Point,  Lulworth Cove, Dorset, England in 1892

Cross-section through Purbeck Group in east side of Lulworth Cove

A simplified cross-section through the Purbeck Group is shown here for the east side of Lulworth Cove. This is based with modifications on a section in Damon (1884) and shows some of the major features of the Purbeck succession.

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STRATIGRAPHY:

Purbeck Group continued:
Limestone 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.

Folk's Classification of Limestones, a modification of the classic petrographic scheme of Folk (1962), with the addition of grumeleuse micrite

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 Pb 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.

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STRATIGRAPHY - PURBECK FORMATION:

Purbeck Group continued -
Clay Mineralogy

The clay mineralogy of the Purbeck of Lulworth Cove has been discussed in a recent 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.

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STRATIGRAPHY- PURBECK FORMATION :

Purbeck Group continued -
Upper Purbeck on the East side

Upper Purbeck

UPPER OSTRACOD SHALES MEMBER

(formerly " Upper Cypris Clays and Shales ")

See also:

Upper Purbeck strata at Peveril Point, Durlston Bay, Dorset.

Purbeck Marble at Peveril Point, Swanage

Shales, often pyritic and with some siderite and with glauconitic Viviparus limestone - Purbeck marble, as shown here (but from Peveril Point). This gastropod limestone (a biomicrite with glauconite) was quarried further east and extensively used for carvings in churches before alabaster was used as a replacement. Look for it as narrow projecting beds of rock on the foreshore at the top of the Purbeck succession.

It is a little confusing that a Unio Bed occurs within the Upper Cypris Clays and Shales and that Unio is also abundant in shales and limestones of the Unio Member beneath, and discussed below. The Unio Bed of the Upper Cypris Clays and Shales is a conspicuous greenish bed with " Unio " occurs in association with pyritic shales. It contains remarkable liquefaction structures in limestone. Reworked Portland and Kimmeridge detritus occurs in the region at this level and earthquake activity associated with the Late Kimmerian movements were probably responsible for the liquefaction (this is very well seen in Stair Hole but it extends to Mupe Bay).The pond-snail Viviparus , the freshwater bivalve Unio ,fish remains( Pycnodus, Lepidotus ) and ostracods such as the beaked freshwater form Cypridea are common in these beds. These strata are of freshwater lacustrine (lake) origin.

4.8 m.

UNIO MEMBER (as used by Damon, 1884)

Glauconitic limestone with Unio, from the Unio Member, near East Over, Lulworth Cove, Dorset

Unio in limestone, Upper Purbeck, Lulworth Cove

Modern Unio and Viviparus

(Unio Beds) Soft green, glauconitic shale full of Unio porrectus . Viviparus .Bands of hard limestone. " Beef " , shelly limestone and dark grey shales. Damon (based on Bristow) gives too great a thickness for this unit at Lulworth Cove (east side). It is less than 1 metre in thickness and lies directly on top of the Broken Shell Limestone. These strata originated in low salinity, lacustrine conditions. The bones of turtles and teeth, scutes and bones of crocodiles from this lake or lagoon can be found in this unit at various places. Examples are shown above with a modern "Unio" and Viviparus - type of pondsnail.

1.14 m.

Lulworth Cove, Dorset

Here, for examples, are some cross-sections of turtle bones (carapace or plastron?). Notice the dense external part and the honeycombed lower part, which reduced the weight. The original bony material white or buff in colour like modern bones but the phoshorite becomes darker with burial and time and the cavities have become filled with calcite. Thus these turtle remains like all the other fossil bones in the Purbeck Group are brown. These are not good specimens because they are broken across and seen in cross section and would be too difficult to extract. Occasionally better specimens may be seen with the shiny top surface preserved and with the characteristic grooves of turtle carapaces. Excellent complete carapaces have been found in the Purbeck Stone quarries of the Isle of Purbeck (near Swanage). At Lulworth Cove, look amongst the common fish teeth of these beds for striated crocodile teeth. These have been found at Stair Hole, Durlston Bay and other places in the Unio Member and other parts of the Purbeck Group.

The presence of crocodiles and turtles is not suprising in the warm lagoons and lakes of the Purbeck. They are fairly common from the Cherty Freshwater Member (Middle Purbeck) to the top of Purbecks, but are rare in the Lower Purbecks. This is because the salinities in the lagoon were often too high for the comfort of reptiles during earlier Purbeck time.

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STRATIGRAPHY - PURBECK FORMATION

East Side of Lulworth Cove -
Middle Purbeck Strata

(Safety Check - If you examine this cliff section in the field inspect the cliff for safety and do not approach if there are any signs of very recent rock fall.)

See also:

Middle Purbeck strata of Durlston Bay, Purbeck Type Section, Dorset.

Correlation of the Intermarine Member, Purbeck Group, from Ridgeway Railway Cutting to Durlston Bay, Dorset

The Middle Purbeck Building Stones of the Intermarine Member are very well-developed in the Swanage and adjacent area of the Isle of Purbeck. They are much reduced in thickness and poorly developed at Lulworth Cove and Stair Hole. This is because of the transition from "Basin" facies at Durlston to "Shelf" facies at Lulworth. The basin was downwarping at about twice the rate of the shelf but in both areas water depths in the lagoon were shallow. As you can see, there is some offlap from basin to shelf.

A thick seam of beef or cone-in-cone in the Chief Beef Member, Purbeck Group, at East Over, Lulworth Cove, Dorset

Three seams of beef, cone-in-cone, in the Chief Beef Member, Purbeck Group, at East Over, Lulworth Cove, Dorset

The form of fibrous calcite known as beef or cone-in-cone is most thickly developed for the Dorset region at East Over, Lulworth Cove. It is in the Chief Beef Member of the Middle Purbeck Group. The usual source of carbonate for this is the aragonite of the Neomiodon shells ( El-Shahat and West (1983).

The Cinder Bed, an oyster bed in the Middle Purbeck Group, East Over, Lulworth Cove, Dorset, a close-up view as seen in 2005

Cross-section of the Cinder Bed, seen in the field at West Over, Lulworth Cove, Dorset

The Purbeck Cinder Bed is quite conspicuous at East Over and because of the folding it crops out three times (look for the three outcrops!). It is blue grey in colour with small, bluish-black oysters of lagoonal type - Praeexogyra distorta. These superficially resemble the small oyster of the Texas coastal lagoons - Crassostrea virginica and it is likely that they lived in a similar but much broader environment. The lower photograph shows details in a cross-section, but this was taken on the other side of the cove, at West Over.

Under the Cinder Bed, and separated by an ostracod-rich shale with Cypridea is the white Cherty Freshwater Limestone. This is grumeleuse micrite (clotted micrite) or pelmicrite. It is important for containing pulmoniferous gastropods such as Physa, Ptychostylus and Planorbis. Viviparus is also usually present. The gastropods are often silicified but not conspicuous on a broken surface because they break through. A weathered surface is needed. The best place to find these, in fact, is not here but in this bed where soil-weathered on the coast between Durlston Head and St Albans Head. Silificied examples can be extracted from blocks by use of appropriate acid. Charopytes also occur and these too can be silicified, in which case they also can be extracted with acid. If you look carefully at loose blocks of the Flint Bed on the shore you may be able to see charophyte stems in cross-section with the standard 12 cells arranged in a circle.

Small calcite pseudomorphs after halite occur in the Cherty Freshwater Limestone, and these also may be seen in loose blocks (although probably more easily at the Fossil Forest exposure). They reveal that in spite of the very low salinity fauna there was a small of sodium chloride in the interstitial water from which halite could be precipitated on occasions when the bed dried out in the hot summer.

We will examine the cliff in photographs (of various dates). The first one which follows show the Cinder Bed as the upper and grey limestone of the pair at the top of the cliff section. The Flint Bed of the Cherty Freshwater Limestone Member is whiter, harder and more splintery. The underlying beds are more argillaceous. Find the Cinder Bed and the Flint Bed in the first photograph before continuing to ones below with more detail.

Lower Purbeck strata between East Over and East Point, Lulworth Cove, Dorset - mainly Hard Cockle Member and Soft Cockle Member, with MP Cherty Freshwater and Cinder at the top

Middle Purbeck strata, including the Cinder Bed and the Flint Bed of the Cherty Freshwater Member, at East Over, Lulworth Cove, Dorset, England, 18 September 2005.

Cinder and Cherty Freshwater at East
Over, 1997 or 8?

Cinder and Cherty Freshwater at East
Over - ferric iron

Silicified charophyte biomicrite from the Cherty Freshwater Member of the Middle Purbeck Group. DB89 of Durlston Bay

Also provided is a false colour image with micrite shown pale blue and surface ferric iron staining shown as the larger brown patches.

Partial Log of the Middle Purbeck Group

Cinder Downwards - modified but not fully updated. After El-Shahat (1977).

(Note 1. For an old log of all the Purbeck of Lulworth Cove see: the vertical section of Bristow and Whitaker in Horizontal Sections of the Geological Survey, Sheet 56. Note 2. Whether biosparite or biosparrudite is used for shelly limestones depends on just how critically Folk's Classification is applied. There is often casual use in the literature of biosparite for biosparrudite. Note 3. It is not certain that all beds have been thin-sectioned, so that some modification of limestone types listed here might be needed in the future. Note 3. The base of the log may not be the exact base of the Middle Purbecks.)

At the Top

21. Cinder Bed. Grey slightly argillacous limestone. Bivalve biomicrosparite with dark grey, calcitic shells of the small oyster: Praeexogyra distorta - 1.20m

20.Alternating shale and thin-laminated limestone, turtle remains and small phosphate nodules. Good Cypridea ostracods. In the recess under the Cinder. 0.48m

19. Pink limestone - bivalve biosparite (or biosparrudite). 0.05m.

18. Conspicuous, blocky, hard, light grey ("white") limestone - The Flint Bed of the Cherty Freshwater Member. Massive, charophyte biomicrite (or grumeleuse micrite or pelmicrite) with chert nodules. With freshwater gastropods and scattered, small calcite pseudomorphs after halite. 0.73m.

17. Earthy, argillaceous limestone (micrite) and shale with small chert nodules. 0.12m.

16. Earthy, friable (argillaceous?) limestone - biomicrite (or biomicrudite) with small white concretions.0.25m

15. Marl - calcareous clays with perished aragonitic bivalves. 0.20m.

14. Grey limestone - biomicrite (or biomicrudite). 0.16m

13. Marl - calcareous mudstone. 0.19m.

12. Hard, grey limestone - micrite. 0.30m.

11. Alternating argillaceous limestone - argillaceous micrite and marl or calcareous mudstone. 0.25m.

10. Calcareous shale and thin, laminated ostracodal limestone - micrite (or biomicrite). 0.13m.

9. Grey limestone - ostracod biomicrite. 0.18m.

8. Shale and thin-laminated limestone - micrite. 0.08m.

7. Laminated marls or calcareous mudstone. 0.18m

6. Soft limestone - micrite. 0.46m.

5. Hard, grey limestone - micrite in two units; near the top there is a band of fish scale. With burrows. 0.73m.

4. Brown marl - calcareous mudstone. 0.30m.

3. Grey, argillaceous micrite. 0.35m.

2. Calcareous clays, poorly exposed. 0.15m.

1. Grey, argillaceous micrite. 0.35m.

[end of partial log - exposures continue down into the Soft Cockle Member of the Lower Purbeck.]

(Exercise for students - from data on the website compare this part of the sequence the same part at Stair Hole).

You might, perhaps, want to compare the succession here with that at Durlston Bay , the type locality for the Purbecks.

Incidently, the silica of the Flint Bed is supposed to be derived from spicules of the unusual Purbeck "freshwater" sponge Spongilla purbeckensis. It is microscopic and you will not see it in the field. Changes in pH (silica is more soluble in alkaline than in acid conditions) are responsible for the precipitation of the chert, originally as opal. The limestone here is not weathered but merely broken by the sea. Because of this the fossil content, which is abundant, is not easy to sea. If you look with a hand lens at fallen blocks of the Flint bed you will see the coiled shells of freshwater gastropods in section and the minute small ovals which are sections through ostracods. Charophyte algae occur and can be recognisable by their twelve cells. They are not easily seen in the field just here but a fallen block in Stair Hole used to reveal them clearly.

Note the peculiar feature of silica pseudomorphs after halite (salt) crystals in blocks of the Flint Bed. There was some salt in the sediments in the summer when the lime-mud dried out. This is a little surprising in view of the freshwater character of the fauna. This matter will be discussed more elsewhere in these field guides.

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STRATIGRAPHY - PURBECK FORMATION:

East Side of Lulworth Cove - Purbeck Group -
Accident Risk - South of East Over

Purbeck Group at East Over to East Horn, Lulworth Cove, Dorset

Purbeck Group in East Over cliff

Part of the cliff section to the south of the East Over promontory, where there is an overhang, has a particular risk. Unfortunately, the southern part of it is best avoided if possible in wet or frosty weather and with large parties. Specialists need to make a fleeting examination at this part of the section but they do so at their own risk. The foot of the cliffs should not be approached and in no circumstances should anyone go beneath or at the edge of an overhang. It is sad to report that there was a fatal accident in 1977 at the Hard Cockle overhang here, when a party seems to have sheltered from steady rain.

Cliff-fall accident at the southeast corner of Lulworth Cove, Dorset in 1977

Cliff-fall accident at the southeast corner of Lulworth Cove, Dorset in 1977 - continued

Accident at the southeast corner of Lulworth Cove. Anonymous. 1977. New Warnings after Cliff Fall Tragedy. Bournemouth Evening Echo, Feb. 21st, 1977. " Warnings came today about the "killer cliffs of Dorset" after yesterday's tragedy at Lulworth Cove when a landslip engulfed a school party in tons of rock and clay, killing a teacher and seriously injuring two others. " The cliffs are completely unstable for virtually the whole length of coastline from Swanage to Ringstead". .... The group [from Warlingham Comprehensive School, Surrey] had walked around the Lulworth beach to the East Point of the cove, and Miss Taite had stopped them under the 75-foot-high cliff [under or adjacent to the Hard Cockle Overhang] to point out the rock structure. They were drawing this in their notebooks when the cliff suddenly gave way. Tons of rock debris and clay [from a scar from the top Soft Cockle to the top Marly Freshwater Members] fell on to the party and two were buried. Mark Playdell ran a mile along the beach to the cafe to call for help, while the other three who were not badly injured, started to dig out their classmates. Ambulancemen, police and coastguards rushed to the scene, and two helicopters were sent from Lee-on-Solent. .... Two years ago a woman was killed by a cliff fall at Durdle Door while she was sunbathing on the beach below, and the same year a schoolboy with a field study group was seriously injured by a rock fall at Swanage. ..."

The southeast corner of Lulworth Cove, Dorset, seen from the East Over promontory at beach level

At present there is loose rock up on the cliff to the south of East Over. Here a fissure in the Hard Cockle Member high on the cliff that was just visible in 1892 and more obvious in 1910. It had opened to about half a metre by 1979. By 1997 there has been major movement resulting in a supply of loose rock on the cliff south of the beach. Thus the northern part of the southeastern cove, at East Over, could be used for study with caution if weather and tide conditions were appropriate, but it is not safe southeast of the Overhang (and particularly not under the overhang or adjacent to southernmost cliff).

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LOCATION - PURBECK:

East Side of Lulworth Cove - Purbeck Group -
Lower - South of East Over

Lower Purbeck strata between East Over and East Point, Lulworth Cove, Dorset - mainly Hard Cockle Member and Soft Cockle Member, with MP Cherty Freshwater and Cinder at the top

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LOCATION - PURBECK OF EAST LULWORTH COVE

The Hard Cockle Member

Labelled photograph of Lower Purbeck strata, including the Soft and Hard Cockle Members, east side of Lulworth Cove, Dorset

South of East Over the Soft Cockle Member forms marly cliffs. Next to the south is a ledge of Hard Cockle Limestone. This limestone is seen at various places on the Dorset mainland. It often contains pseudomorphs after halite and ripple marks. It is a shallow water carbonate sand from the margin of the Purbeck lagoon and sometimes has oolites within it.

On the Isle of Portland, just across the sea from Lulworth Cove, it has a rich assemblage of dinosaur footprints. These are figured in my webpage on Portland Dinosaur Footprints .

Selectively enhanced image of theropod footprint with partially raised claws, Hard Slatt, Portland

Of special interest is the presence of very bird-like footprints of dromaeosaurs (a type of theropod) with raised central claws. These are the "Velociraptors" of Jurassic Park! A new interest is provided by the recent discovery that Archaeopteryx, the first known bird (and also from Upper Jurassic) also had this raised central claw. No footprints have yet been found at Lulworth Cove.

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LOCATION - PURBECK STRATA OF EAST HORN OF LULWORTH COVE

Basal Purbeck Strata - General

See particularly for details:

Purbeck Fossil Forest, east of Lulworth Cove.

East Over and East Point from near West Point, Lulworth Cove, Dorset, December 2005

Beyond the Hard Cockle Member, to the south are the Cypris Freestones, the Broken Beds (an evaporite breccia, since calcitised) and then the Caps and Dirt Beds. These basal Purbeck strata are best seen, and in greater safety at the Fossil Forest , a short distance further east. At that ledge the cliff slopes back and rock falls are much less of a threat.

Basal Purbeck strata at the Fossil Forest exposure, Lulworth Cove, Dorset

The general succession of basal Purbeck strata here is similar to that of the nearby Fossil Forest section, shown in this diagram. Note the stromatolites of the Hard Cap can be seen as large pillow-like objects on the sloping north face of the West Point peninsula. See the succession section of the Fossil Forest field trip for details of the basal Purbeck strata.

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EVAPORITIC BROKEN BEDS AND SOFT CAP

Purbeck Anhydrite Seismic Reflector - Equivalent

The southeast corner of Lulworth Cove, Dorset, seen from the East Over promontory at beach level

Broken Beds, a calcitised, tectonic evaporite breccia, seen here at the east side of Lulworth Cove, Dorset, November 2005

The Purbeck anhydrite seismic reflector seen as calcitised anhydrite, Lower Broken Beds, east side of Lulworth Cove, Dorset, October 2011, with Star Energy geolgists

The Broken Beds, now an oligomict limestone breccia, at the east side of Lulworth Cove, Dorset. This consists in its lower part of calcitised anydrite with ostracodal limestone blocks incorporated in the upper part. It originated as a tectonic evaporite breccia

Victorian geologists did not understand the limestone breccia, the Broken Beds, shown here. They were not aware that the lower part is calcitised anydrite (see the various papers of West. Now the horizon is very well-known to petroleum geologists and geophysicists as the Purbeck Anhydrite Seismic Reflector. It is commonly used to identify the approximate top of the Jurassic System, with the White Lias or Langport Group being the reflector which marks the base. A better view of this reflector, but still in calcitised condition, is at the nearby Fossil Forest. At both these localities the anhydrite was abnormally thin. However, by visiting the celestite locality at Durlston Head a more normal sequence in terms of thickness, can be seen. Those who are not petroleum geologists are likely to be more interested in facies, diagenesis and tectonics here. The Purbeck Group or Group, here contains much of great interest, even though the exposure is quite limited in size as well as being greatly condensed.

The Caps and Broken Beds are best seen at the Fossil Forest section or at Potters Hole. However in case the breccia is studied at Lulworth Cove the following information is given:

The Broken Beds are formed by a conspicuous limestone breccia of varying thickness, usually divisible into two distinct parts, the upper of angular limestone blocks, the lower of limestone blocks in soft calcitised anhydrite matrix.

The Upper Broken Beds are easily recognised by consisting of angular blocks of ripple-laminated limestone with no significant matrix (other than small broken blocks) between them. No chert occurs in this and there is no evidence that major evaporites were present in this part.

The Lower Broken Beds have much of the soft matrix which superficially resembles soft sandstone. It is porous calcitised anyhyrite which under the microscope shows pseudomorphs after anhydrite and the form of silica so often associated with evaporites - lutecite. Euhedral quartz crystals occur in places but some of the best evidence of evaporites comes from the chert. This consists of pseudomorphs after primary lenticular crystals of gypsum (if not immediately visible in a thin-section try a light etching with hydrofluoric acid - with care!). More lithified blocks of calcitised anhydrite occur in a matrix of soft calcitised anhydrite, indicating more than one date of calcitisation. For more details see West (1964; 1975). Calcitisation has been studied in term of stable isotopes and meteoric water seems to have been involved in this western area of the Broken Beds, whereas further east hydrocarbons were involved in the diagenesis.

This evaporite horizon thickens eastward and continues under southeast England. Anydrite, for example, occur in the Arreton Borehole in the Isle of Wight and is also present at this horizon at Portsdown, in the Southampton area (Hoe No. 1) and elsewhere. Where the anhydrite is hydrated back to secondary gypsum near the surface at Brightling and Mountfield in Sussed, there is mining of it by British Gypsum for plaster and plasterboard.

The breccia has been brecciated tectonically, frequently fracturing the chert. The date of movement of the overlying strata to the north (the direction is shown by folds within the Broken Beds) is either intra-Cretaceous or Tertiary but not yet firmly established.

The lower Broken Beds originated as gypsum in a hypersaline lagoon at salinities of about 124 parts per thousand. The environment was semi-arid at a latitute of about 37 degrees north.

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BASAL PURBECK STRATA continued:

HARD CAP WITH STROMATOLITES

East Point, Lulworth Cove, Dorset, in 1997

Large stromatolites in the Hard Cap at East Horn, Lulworth Cove, Dorset, 11th October 2011

The Hard Cap at the East Horn of Lulworth Cove is almost certainly similar to that at the Fossil Forest. It is of hypersaline origin, containing quiet-water lagoonal ooids and large stromatolites. The latter are not specifically around large trees, as far as is known, and are therefore slightly different from those in the Soft Cap. They resemble the Hard Cap stromatolites that are familiar features on the Isle of Portland and which are seen in many quarries. The Portland examples often contain "sand holes" or cavities left by the decomposition of narrow tree trunks, particular at Sandholes, southeast Isle of Portland.

The cliff face is too difficult for non-climbers at the far end of the East Horn of Lulworth Cove. Thus the Lower Dirt Bed and the junction of the Purbeck with the Portland Stone is not generally accessible just here. It can be seen at the Fossil Forest, but only with an easy scramble.

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BIBLIOGRAPHY AND REFERENCES

Go to Lulworth Cove Bibliography?

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ACKNOWLEDGEMENTS

I am very grateful to the many geologists who have accompanied me in Lulworth Cove over the years and discussed aspects of the geology. I am sorry that I cannot name them all. Discussion with students involved in research projects has been very helpful. I thank Louise Morris for useful contributions to the topic of the source of the Lulworth Cove beach pebbles. I very much appreciate the kindness of Professor Adam El-Shahat in allowing me to make use of his excellent work on the Purbeck Group in 1977. I thank Halfdan Carsten for his kind permission to use photographs taken on a field trip in Dorset in 2005. The support and encouragement during the development of these pages by the staff of the National Oceanography Centre, Southampton (formerly Southampton Oceanography Centre) is very much appreciated. The academic staff and students of various field trips at Lulworth Cove are thanked for their kind cooperation regarding photography on the cliffs. I particularly thank Professor Jonathon Bull, Associate Dean of the Faculty of Natural Science and the Environment at Southampton University for supporting the continuation of this website through Southampton University; this is much appreciated.

My wife Cathy has very kindly provided background support for the production of this and all the other webpages, and I very much appreciate her help.

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Copyright © 2013 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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Dr Ian West, author of these webpages

Webpage - written and produced by:


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

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


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