INTRODUCTION, GENERAL, - POXWELL QUARRY AND THE POXWELL ANTICLINE

The Poxwell area is a very well-known Portland - Purbeck locality. It achieved some geological fame as one of the initial sites for oil exploration in southern England in 1936. It has an anticlinal structure and is near to Osmington Mills, where some oil is present in Corallian strata.

There is a small but good quarry section showing the Portland - Purbeck junction very clearly. At one time I was involved in petrographic studies here. There are some other small exposures nearby, but they are not good now. The nearby cutting on the main road to Weymouth at the bend provided at one time a good section of the Middle Purbeck and associated strata. This was studied by Professor Sylvester Bradley, the well-known Purbeck ostracod expert and who at one time lived at Poxwell.Various geologists, including Arkell and House over the details of faulted structure of the south limb of the Poxwell Pericline. Because of the interesting structure of a faulted anticline, the area became a training ground for students. Thus there are various mapping reports and other local studies relating to the area.

INTRODUCTION:

Safety

This account of Poxwell Quarry and adjacent area is a description not a field trip itinerary. It may not be necessary to visit the site. Should you do so, please respect private property and farm land and cause no disturbance to local people and livestock in fields etc. Notice that it is not an easy place to park a car without obstructing gates, and it is at a curve in a fast main road to Weymouth. There is risk with regard to road traffic. Any attempt to examine the visible remains of the former road-cutting section may be hazardous because of traffic. The cutting is largely overgrown now.

At the quarry, take care with any loose rock and be aware of the risk of falling debris. In particular never hammer chert because it produces very dangerous splinters and geologists have received serious injuries from hammering flint or chert.

INTRODUCTION:

Location

Poxwell is a small village situated about 7km northeast of Weymouth. The map reference of the nearby quarry is SY 743835. The quarry is about half a kilometre south of the village on A353 (Weymouth Road). There is a farm entrance way near the quarry but more than one or two cars here would he at risk of blocking access of heavy farm machinery which enters and leaves from here. The entrance is in use and must not be be blocked. This is not a suitable place, either in terms of parking or of size of exposure, for coach parties. There is a footpath from here to Moign that is suitable for one or two walkers. A former road cutting is now overgrown and is a dangerous place in terms of fast passing road traffic.

INTRODUCTION:

Aerial Photographs

Shown above is an aerial photograph of part of the Poxwell Anticline (Poxwell Pericline) with the location of the quarry shown. Also visible is part of the line of old Cypris Freestone quarries on the south limb of the anticline.

INTRODUCTION:

DIGS Group.

Preservation of Poxwell Quarry as a geological exposure is in the care of the: Dorset DIGS Group. This very active organisation preserves important geological exposures in the county of Dorset.

INTRODUCTION:

Geological Maps

Part of the 1949 edition Geological Survey map of the Poxwell area is shown above. Preferably purchase the new edition: British Geological Survey, West Fleet and Weymouth, England and Wales Sheet 341 and part of 342, 2000 Edition, 1:50,000 series. This is obtainable from the BGS Bookshop Online, and at low cost. The difference between the maps for the specific area of Poxwell is not great. There are changes of small detail and changes of nomenclature. It is always good policy to possess both the current BGS map of an area of interest and some older editions for comparison.

The map shown above is based, with modifications on part of a geological map of House (1989). The Abbotsbury-Ridgeway Fault system is shown, but the details of the faults may not be accurate.

STRUCTURAL GEOLOGY

Poxwell Pericline

Structure and Deep Cross-Section

The interpretation of the Poxwell anticline and associated faults has not been simple. See the publications of Strahan (1898), Arkell (1947), House (1961), Mottram and House (1956), Ridd (1973), Taitt and Kent (1939). In parrticular see the recent work by the British Geological Survey amd the cross-section on the BGS map.

The Poxwell Anticline is one of three, the Sutton Poyntz, Poxwell and the Chaldon Herring Anticline in an east-west line 10km. long and 3 km. north of the coast. Exposed in the folds are strata of Middle Jurassic to Late Cretaceous age. Albian strata (Upper Greensand and Gault Clay) rest unconformably on the older units ( Ridd, 1973).

Introduction to the Portland-Purbeck Section

Above is a panoramic view of Poxwell Quarry in 1983, modified after Bainbridge (1983). Some more recent photographs follow.

The Poxwell Quarry on the east side of the road at Poxwell has long been known as an interesting Portland Stone and basal Purbeck locality (Arkell, 1949). It is situated at about 500 metres south of Poxwell Church.

Most of the quarry is in Portland Stone of white or cream appearance. This contains moulds of typical large Portland molluscs. The sequence continues up into the Transition Beds which marks the base of the Purbeck sequence.

The Purbeck exposure is limite. About 3.3 metres of Purbeck limestones and dirt beds (palaeosols) were exposed here, and still are, although in rather weathered condition now. The Transition Bed is very conspicuous and contains the usual moulds of small gastropods. It can be divided into two units. The Basal Dirt Bed and the Lower Dirt Bed can easily be seen, although they are thin. It is a locality for fossil tree remains, coniferous tree, not cycadophytes, and they are preserved in normal silificied condition. An upright example in place could be seen for many years. It is interesting that one silicified tree trunk here attained more than 1.8m. in length. I have not seen this one and it has probably gone long ago.

STRATIGRAPHY

Portland Stone Succession

The succession of Portland Stone in Poxwell Quarry is of interest. The section was logged by Bainbridge (1983) . Details, based on Bainbridge's work are shown above. A feature of particular interest is the occurrence of Atyxiella portlandica, the well-known "Portland Screw" gastropod. As far as I know this does not occur in the Portland Stone on the mainland, only on the Isle of Portland. Apart from that feature the Portland Freestone is of chalky, micritic facies, similar to that at Chalbury Camp, Upwey, Ridgeway, and Portesham. This is relatively soft and quite unlike the hard building stone of the Isle of Portland. It has been used at Poxwell for lime-burning, not building, and there is the remains of a kiln still there.

STRATIGRAPHY - BASAL PURBECK STRATA

The Transition Bed

At the base of the Purbeck Group on the Isle of Portland, and on the Dorset mainland is a thin bownish limestone, often about 30cm thick, which is known as the Transition Bed. It represents a transition from the Portland Freestone to the overlying Purbeck Group. The bed is notable for containing no large Portland molluscs but instead has hollow moulds after small gastropods and bivalves, particularly Hydrobia and Valvata. These were lagoonal-type molluscs and are associated with ostracods. The thin bed is normally fused directly on to the Portland Stone but is quite different in appearance and faunal content.

At Poxwell Quarry, the Transition Bed is very well-developed and very conspicuous. It is the easiest bed to see, at a first glance. It was recorded by West (1975) here as conisting of two fused beds of limestone. Clements (1967) used a bakelite impregnation technique on this bed to produce casts of gastropods etc. He also obtained objects which he initially thought were some type of sponge spicule. They proved, instead, to be peudomorphs after lenticular crystals of gypsum of the type described in many parts of the basal Purbeck formation in the early papers of (West, 1964; 1965; 1975 etc). The Transition Bed is the first stratum of mildly hypersaline lagoonal facies following on directly from the Portland Freestone marine environment. On the Isle of Portland it contains dinosaur footprints on its upper surface (Francis) .

Ostracods are common in the Transition Bed at Poxwell. The gastropods frequently have complete appertures and there is no evidence of any significant transport (Clements, 1967). This was clearly a very quiet and to some extent, hypersaline, environment. The end of the Portland Freestone phase of deposition was extremely quiet. Notice incidently, that there is no indication of any major structural control on facies at the Transition Bed level. There is no sign of an early Poxwell Pericline, although it would not of course be expected at this date (it is mainly Tertiary).

POXWELL QUARRY

Purbeck Succession - Silicified Trees

Silificied tree trunks of the usual type of basal Purbeck coniferous tree, occur above the Lower Dirt Bed at Poxwell. A prostate example is shown, with acknowledgement to K.M. Bainbridge (1983) who took the photograph. Tree remains occur at this level not only here in the Weymouth area, but also at Chalbury Camp, Upwey and Portesham.

Fragments of silicified wood from the basal Purbeck Formation can be found in ploughed fields of the Osmington and Poxwell area ( Bainbridge (1983). These fragments must almost certainly have come from the tree horizon immediately above the Lower Dirt Bed because silificied trees from above the Great Dirt Bed are not known here. The cells can be clearly seen in cross-section with small cells marking the unfavourable season for growth. The climate was seasonal and semi-arid and at a palaeolatitude of about 33 degrees north (i.e. Mediterranean, like Cyprus or parts of Tunisia). Evaporites provide evidence concerning the semi-aridity.

Purbeck Formation - Zonal Scheme

The type-section of the Purbeck Formation is at Durlston Bay.

See Durlston Bay and other Durlston Bay webpages). The key work on this is the detailed log by Roy Clements (1993), which should be consulted for information on lithology and faunal content of indivual beds. At Portesham only the lowest part of the Purbeck sequence is visible. At present the top of the Portland Stone, the Purbeck Caps and marl which is equivalent to the Great Dirt Bed can be seen together with a section of the 'Cypris' Freestones at the back of the quarry.

More could be seen in the past. Woodward (1893) gave the following thicknesses, in downward sequence:

Soft Cockle Member - 5 ft. (1.52 m.) (it is uncertain whether this is the total of the SCM or just what was seen).

Hard Cockle Member - 5 ft., 9 inches, (1.75 m.)

Lower Insect Beds - 4 ft. (1.2 m.)

'Cypris' Freestones - 22 ft. 11 inches (7 m.) to 23 ft. 11 inches (7.28 m.)

Strata equivalent to the Broken Beds and the Hard and Soft Cap (details shown in a log given here) - 23 ft. 10 inches (7.28 m) to 25 ft. 4 inches (7.7 m.)

(I measured (in about 1960) approximately 6.35 metres for this basal part of the sequence, but the measurements will vary slightly from place to place within the quarry, and accuracy will depend upon state of exposure. I cannot be sure that I placed the junction of the Cypris Freestones with the BB equivalent accurately because of the poor state of the exposure. Thus Woodward's measurements are probably better).

The Purbeck Formation consists of limestones, shales and marls of lagoonal and lacustrine origin. There some relatively thin parts, such as the Cinder Bed or Cinder Member, which were deposited in marine salinity. Most of the formation, however, originated in very variable salinity conditions and ammonites are absent. Thus the zoning on marine fossils is not useful in the Dorset area. Instead ostracods ["water fleas"] are used because the minute shell so these are common in the Purbeck Formation. In parts, as in the "Cypris" Freestones they are actually rock-forming. Zonal schemes based on ostracods have been clearly shown in a diagram of Horne (2002) and which includes his 1995 scheme. A modified version of this is provided above.

Note that the scheme is based on species of Cypridea, the freshwater (or low salinity) ostracod. This has a distinctive beak and a variable pattern of turbercles or surface ornament. Because substantial parts of the Middle Purbeck and Upper Purbeck are "freshwater" ostracods of this genus are easily found there. The Lower Purbeck is more problematical because it is largely of hypersaline origin and contains much gypsum and replaced gypsum and some replaced halite. It is so evaporitic that it is a commercial source of gypsum for plaster in mines and Sussex, and formerly on the Dorset coast. In boreholes anhydrite is commonly found in the lower part. The hypersaline Lower Purbeck Formation does not often contain freshwater strata. Portesham Rocket Quarry and the Ridgeway Railway cutting are the main localities for low salinity Lower Purbecks. Even then it is only certain beds, mainly the equivalent of the Great Dirt Bed, which contains a freshwater fauna. Thus the field evidence for Cypridea dunkeri is limited. Portesham Quarry is probably the only place in Dorset where you can easily find this fossil. Further inland, however, there is an unusual Purbeck facies at Swindon which contain abundant Cypridea dunkeri. This will be discussed in a separate section below.

PURBECK STRATIGRAPHY continued:

Sedimentological and Faunal Logs of the Basal Purbeck Strata

The small Poxwell Quarry is of particular interest to enthusiasts regarding Portland - Purbeck stratigraphy. It is does not provide a long section, but it is good because it is very clear and does not require much expertise to interpret. It is also similar to Chalbury Camp, Upwey, the Ridgeway Railway Cutting, Friar Waddon and Portesham. It is quite different from the Portland and Purbeck of the Isle of Portland or of the Fossil Forest, Lulworth Cove area, and is also quite different from the basinal facies of the Purbeck Formation as seen at Durlston Head and adjacent areas. Holworth House section at Ringstead is a linking section connecting Poxwell Quarry to both the basal Purbeck of Isle of Portland succession and to the basal Purbeck of Lulworth Cove sequence.

The most striking and obvious feature is that, as at Chalbury Camp, Upwey and Portesham, the fossil forest here is not the same one as at the Fossil Forest exposure Lulworth Cove. It is instead based on the Lower Dirt Bed, not the Great Dirt Bed. The trees were found quite commonly during quarrying and examples have been seen in more recent years in the quarry and elsewhere in the area. They are not the cycadophytes, known from just above the Lower Dirt Bed on the Isle of Portland, but are coniferous (Cypress-like) trees resembling those found in large numbers above the Great Dirt Bed on Portland (so numerous that one of the quarries stores away stacks of silicified tree trunks in a disused underground quarry).

The trees are well rooted in the Lower Dirt Bed, a rendzina palaeosol. Just above this is some gypsum replaced by silica. Thus, as was usual in Dorset the trees were submerged into hypersaline water, near the salinity (about 124 ppt) for gypsum precipitation. This explains the usual occurrence of trees encrusted in thrombolites, which lack almost any fauna. Althought the horizon is lower the general setting is almost the same as that of the familiar Fossil Forest (Lulworth Cove and Isle of Portland) level above the Great Dirt Bed.

The Great Dirt Bed palaeosol at Poxwell is not distinctive as at Portland and the Fossil Forest, Lulworth. It is a marl and not so obviously a palaeosol. The same situation occurs at Upwey, where it is quite difficult to recognise the equivalent of the Great Dirt Bed. At Portesham the equivalent of the Great Dirt Bed is not a palaeosol at all but a marl with a peculiar mixed fauna and flora indication of both low and high salinity conditions.

The sequence at Poxwell does extend, in a good exposure, much above the equivalent of the Great Dirt Bed. However, the upper and harder part of the Cypris Freestones have been very extensively quarried in an east-west direction along the south limb of the Poxwell Anticline. This is a relic of very old quarrying. Attempts are being made to compare this stone with the limestone of Cypris Freestone type that is present in the historic building of Wolfeston Riding House, Charminster, Dorchester. The stone may not necessarily have come from this locality and could have come from Portesham or Upwey, but it is similar in lithological characteristics (ripple cross-bedded, pelletoidal, ostracodal, lagoonal limestone, with some mud clasts).

The Portland Stone is of relatively soft "chalky facies". That is to say it is micritic limestone or lime-mudstone, not a hard, oolitic building stone, as on the Isle of Portland. Thus it has been used at Poxwell for lime-burning and there is still the remains of a kiln there. The uppermost bed of the Portland Stone is a little harder (as at Upwey) but probably still not good enough for use a building stone. Portland bivalves and the occasional ammonite occur in the Poxwell Quarry.

PURBECK STRATIGRAPHY continued

Purbeck Palaeogeography

Poxwell Cutting (30/741834)

There follows the details of the Poxwell Road cutting measured in 1936-1938 by Professor Sylvester-Bradley and Brian Mottram (published as an Appendix to Sylvester-Bradley (1949) in Proceedings of the Geologists's Association, vol. 60, 1949, part 2, pp. 151-153. Some notes have been added by me in square brackets. Succession is downward from bed 50 in the Upper Purbeck down to bed 1 in the Lower Purbeck. The ostracod nomenclature is that used by Sylvester-Bradley and is now out-of-date (note for example that C. dunkeri, as used here does not carry a Lower Purbeck implication). Gastropod names are unchanged, but refer particularly to Arkell (1941). "Ostrea distorta", the well-known Purbeck lagoonal oyster of the Cinder Bed is given the modern name: Praeexogyra distorta here.

POXWELL ROAD CUTTING continued

THE ROAD CUTTING LOG OF SYLVESTER-BRADLEY

.

C. setina Zone, Upper Purbeck
Subzone of Cypridea rectidorsata

Bed 50. Slipped clay, not in position, but yielding many ostracods. Cypridea tuberculata var., C. propunctata, C. (Pseudocypridina) rectidorsata, Cyprione sp., Metacypris sp.

Bed 49. Sandy bed. Unio valdensis, Cypridea propuctata, C. (Pseudocypridina) rectidorsata, Cypridea spp. nov. A and B, Cyprione sp. Metacypris sp.. 6 inches [O.15m.]. [equivalent of the Unio Bed or part of the Unio Member]. [Sandy Unio Beds are known at Ridgway - see Fisher (1856).

[APPARENT BASE OF UNIO MEMBER?]

Bed 48b. Calcite mudstone [micrite]. About 1 ft. [about 0.3m.].

Bed 48a. Shaly marl. Cypridea propunctata, C. (Pseudocypridina) rectidorsata, Cypridea spp. nov. A and B, Cyprione sp. Metacypris sp. seen to 6 inches [0.07m].

Gap

46. Shaly marl. Cypridea propunctata, Cypridea sp. nov. C and D. C. (Ulwellia) sp. nov. A. Cyprione sp. Metacypris sp., "Candona" sp. "Cypris" sp., fish teeth. About 1 ft. [about 0.3m].

C. granulosa Zone. Middle Purbeck.
Subzone of Cypridea fasciculata.

45c. Hard Marlstone. 1 to 2 feet [0.3 to 0.6m]
[Is this Arkell's Bed 29? It is probably the equivalent of Fisher's Ridgway Bed 40, "Hard grey marlstone, conchoidal fracture in brown sand with narrow lines of beef" - 2ft 6ins (0.76m). This is listed as the "Upper Broken Shell Limestone". However, it is not of the shell debris facies as at Durlston Bay. See bed 44 which might be the equivalent of the BSL]

45b Shale. Cypridea (Ulwellia) sp. nov. A, Cyprione sp., Metacypris sp.; the charophyte Clavator bradleyi [a high level in the Purbecks for charophytes].

45a. Calcite mudstone [micrite]. About 1 ft. [0.3m] [Probably Fisher's Bed 42 - Extremely hard grey marly rock - 8 inches or 0.2m]

[BROKEN SHELL LIMESTONE - but too thin!]

44. Hard crystalline limestone. C. (Ulwellia) sp. nov. Fish. 6 inches [0.15m]. [Probably Fisher's - Hard limestone composed of bivalves, like Swanage Paving Stone - 0.10m., i.e. a biosparudite. Perhaps this is the BSL]

[CHIEF BEEF MEMBER (Note the highest occurrence of aragonitic bivalves here)]

[The Chief Beef Member is only about two metres thick in the Ridgway Cutting.]

43. Sandy bed with rotted [i.e. aragonitic] bivalves (? Corbula), C. (Ulwellia) sp. nov., Fish teeth. 7 inches [0.18m]

42. Rotted bivalves [aragonitic], reduced to a white powdery marl, with clay and beef. Ostracods indet. [might be Arkell's bed 26 with "white limy deposit"]

[Note: Boundary of Chief Beef Member and Corbula Member is uncertain; it might be here or it might be lower.]

[CORBULA MEMBER?]

41. Corbula limestone. Cypridea fasciculata, C. (Ulwellia) sp. nov. A. Candona bononiensis, fish teeth. [Note the Corbula, but this does not necessarily mean that this unit is part of the Corbula Member. It might still be Chief Beef Member.] 8 inches [0.20m or 20cm]

40. Beef and clay. Planorbis fisheri [distinctive very low salinity, "freshwater" gastropod and charophytes present; compare to DB 180, DB 166 and DB 164 of Durlston Bay; these have the very low salinity Ptychostylus.and are the only parts of the Corbula Member with such a low salinity fauna], Hydrobia chopardiana, C. (Ulwellia) dunkeri, C. (Pseudocyprina) sp. nov. A, Cyprione sp., Metacypris sp., "Cypris" sp., "Candona" bononiensis, ostracod gen. et sp. nov. [Charophytes:] Clavator grovesi, Perimneste horrida. Thickness something under 6 feet [1.8m].

39. Perished [aragonitic] shells and beef. Squashed Cypridea sp., fish teeth. Perhaps 3 feet [about 1 metre]

38. Clay and beef. Fish teeth. Branched hollow tubes. [What are the branched hollow tubes?]. About 12 feet [about 4m.].

37. Shell limestone. [presumably a usual Purbeck bivalve biosparrudite]. 6 inches [0.15m. or 15cm]

36. Marl and beef. Hydrobia chopardiana, ? "Candona" sp. Acrodus ornatus ?, Lepidotus sp., Hydrobia parridens?. [a fish bed, cf. Fisher Bed 62 with teeth and Turtle, in the Corbula Member] 1ft 6in [0.46m or 46cm].

35. Compact marly limestone. Ostracods indet. 1 foot, 3 inches [0.076m, 7.6 cm]

34. Shell limestone. Macrodentina textilis, "Candona" ansata. 1 inch. [0.025m , 2.5cm]

33. Clay, marl and beef. Valvata helicoides, Cypridea sp., Macrodentina textilis, fish teeth [another fish bed] 1 foot. [0.3m. , 30cm]

32b. Thin earthy shell limestone. 2 inches [ 0.02m, 5 cm]

32a. Ferruginous clay. Macrodentina textilis, fish teeth. 0 to 1 inch [0 to 2.5cm, 0 to 0.025 m.]

31. Limestone. Neomiodon and broken shells. "Candona" ansata, abundant fish teeth. About 1 foot [Approx. 0.3 metres, approx. 30cm.]

30. Limestone and marlstone. Macrocypris ? 9 inches [0.23m, or 23 cm]

29. Perished [aragonitic] shells with much beef. Physa bristovii, ? Valvata, Cypridea posticalis, Cypridea sp., Metacypris sp. fish teeth. Charophytes: Clavator reidi?, Clavator grovesi. [a low salinity "freshwater" bed]. About 1 ft [about 0.3m or 30cm].

28. Mottled clays. Cypridea sp. indet. About 1 foot, 6 inches [about 0.45m or 45cm.]

27. White marl and beef. Ostracod indet. 4 inches [0.10m or 10cm.]

26e. Sand and sandy clay. Fish teeth. About 5 inches [about 0.13m or 13cm.]

26d. Sandstone. 2 inches [0.05m or 5cm]. [Sandstone is fairly unusual in the is part of the Purbeck Group].

26c. Gritty limestone. Bivalve casts (? Corbula), Cypridea sp. 11 inches. [0.28m or 28cm]

26b. White argillaceous sand. The gastropod Valvata helicoides? Fish teeth. About 3 inches [About 0.08m or about 8 cm.]

26a. Reddish sandy clay. Fish teeth. About 6 inches [About 0.15m or about 15cm.]

25. Shell limestone [presumably a bivalve biosparrudite]. Cypridea fasciculata, C. cf propunctata. 9 inches [0.23m or 23cm.]

24. Beef and clay. Cypridea posticalis, ? "Candona" sp. Charophyte: Clavator reidi.

Gap [of unknown size] [no bed 23 listed]

22. [PLANORBIS BED OF THE INTERMARINE MEMBER] Clay, with thick layers and thin seams of perished [aragonitic] bivalve shells. [Very low salinity fauna - "freshwater" lake clay deposit] Planorbis fisheri, Valvata helicoides, Hydrobia chopardiana, Cypridea sp. nov. E. C. (Ulwellia) dunkeri, Metacypris sp. fish teeth. [Charophytes:] Clavator grovesi, Perimneste horrida. Fossil wood. [Compare to the Planorbis Bed, DB 115 of Durlston Bay, 0.43m, shales and calcareous clays, which contains Planorbis fisheri, Valvata helicoides, Hydrobia, Cypridea etc.] About 1 foot [About 0.30m or about 30cm.]

21. Marl. Preexogyra distorta, Cypridea fasciculata. About 1 foot [Approx. 0.3m or 30cm.]

20. CINDER BED [SOFT CINDER]
With Praeexogyra distorta, more abundant at the top. 1 foot, 7 inches [0.48m or 48cm.]

19. [HARD CINDER.] Limestone [with the small oyster] Praeexogyra distorta. 1 foot, 3 inches [0.38m or 38cm] [Total Cinder Bed thickness 0.86 m or 86cm., i.e. rather less than a metre. Probably about the same thickness in the Ridgeway Cutting, with Protocardia recorded by Fisher in the Hard Cinder (normal for the Cinder Bed). [The total Cinder Bed is 2.95m, i.e. about 3 metres in the basinal facies of Durlston Bay.]

18c. Clay. Cypridea fasciculata. Fish teeth. 1 inch. [Approx 0.025m or 2.5cm]

18b. Thin marl and limestone. [oyster] Praeexogyra distorta, [ostracod] Cypridea fasciculata. 5 inches. [ 0.13m or 13cm.]

[CHERTY FRESHWATER MEMBER]

18a. Thin-bedded limestone with finely broken shells. Cypridea fasciculata. 8 inches [0.20m or 20cm.]

17. Marly Shale. ["Freshwater" shale]. ["freshwater2 gastropods] Valvata helicoides, Planorbis fisheri, Hydrobia chopardiana, Viviparus cariniferus, Physa bristovii. [Ostracods] Cypridea fasciculata, Metacypris sp. Candona boniensis. [Charophytes] Clavator grovesi, Perimneste horrida. 5 inches [0.13m or 13cm.].

16c. Shell limestone [probably bivalve limestone, as usual]. [Ostracods:] Cypridea fasciculata, Metacypris verrucosa. 0 to 1 inch. [0 to 0.025m or 0 to 2.5cm.]

16b. Marl. About 4 inches [0.10m or 10cm.]

16a. Softer Marl. [Gastropds - low salinity "freshwater" fauna] Hydrobia chopardiana, Planorbis fisheri, Valvata helicoides, Viviparus subangulatus, Ellobium jaccardi.
[Ellobium jaccardi is a small, low-salinity gastropod with deep cancellate ornament. A body whorl of 1.5mm long was collected, presumably from this horizon, by Sylvester-Bradley and is in the Bradley Collection. A cast of another has been found in the Cherty Freshwater Member at Portesham (Groves collection, British Museum). The species is common in the Freshwater Limestone of Northwest Germany (Koert). The type is from the Couches nympheenes of Villers-le-Lac in the French Jura. These notes are from Arkell (1941), p. 110. See also data on Ellobium durlstonense.]
[Ostracods:] Cypridea cf. propunctata, C. cf. valdensis, Metacypris forbesi, "Candona" bononiensis.
[Charophytes:] Clavator grovesi, Perimneste horrida.

Subzone of Cypridea granulosa (sensu stricto).

15. The Biscuit Bed (Bed of mud clasts or rip-up clasts).
Calcite mudstone and chert, the top of which is piped by funnel-shaped inclusions of marl, thus weathering to a "biscuit like" surface (seen also at Durdle Door). [This is the result of penecontemporaneous erosion of semi-consolidated marl].
[Fauna - low-salinity, "freshwater" gastropods] Viviparus subangulatus, Planorbis fisheri, Valvata helicoides, Physa bristovii, Hydrobia chopardiana, [bivalve] Neomiodon sp. and other bivalves.
[Ostracods:] Cypridea cf. propunctata, C. sp. nov. E, C. cf. valdensis, C. (Ulwellia) dunkeri?, Cyprione sp., Metacypris forbesi, M. verrucosa.
[Charophytes:] Clavator reidi, C. grovesi.
Thickness 2ft, 1inch [ 0.63m. or 63cm.]

14. Clay and perished [aragonitic bivalve] shells.
[Ostracods: Cypridea cf. valdensis, C. Ulwellia dunkeri, Metacypris forbesi, M. verrucoas.
[Charophytes:] Clavator reidi, Perimneste horrida.
[Thickness] 4 inches [0.10m or 10cm.]

13. Blue-hearted calcite mudstone.
[Ostracods:] Cypridea cf. propunctata, C. cf. valdensis, C. (Ulwellia) dunkeri, Metacypris forbesi, M. verrucosa.
[Charophytes:] Clavator grovesi, C. reidi.
[Thickness:] 1 foot, 2 inches [0.36m or 36cm.]

12b. Soft shaly marl, with a layer of lignite consisting of small branched twigs [Note that there is a plant bed 85 in the Ridgway Cutting, but it has large plant impressions].
[Ostracods:] Cypridea posticalis, C. (Ulwellia) dunkeri, Metacypris verrucosa, all in great abundance.
Fish scales.
[Thickness:] 1 foot, 6 inches [0.45m or 45cm.]

12a. Marlstone.
[Ostracods:] Cypridea granulosa (rare), C. posticalis, C. (Ulwellia) dunkeri, Metacypris verrucosa, all in great abundance.
Fish scales.
[Thickness:] 1 foot. [0.3m. or 30cm].

11b. Clay.
[Ostracod:] Cypridea granulosa (rare).
[Thickness:] 1 foot. [0.3m. or 30cm].

11a. Marl.
[Gastropod:] Hydrobia chopardiana.
[Ostracod:] Metacypris sp.
[Charophyte:] Clavator grovesi.
[Thickness:] 2 inches [0.05 m. or 5cm.]

10. Blue-hearted calcite-mudstone, shelly at the base.
[Gastropod:] Hydrobia chopardiana.
[Ostracod:] Cypridea granulosa (common).
1 foot, 11 inches [0.58m or 58cm.]

9. Soft marl and clay.
[Thickness:] 1 foot, 4 inches [0.41m or 41cm.]

8. Blue-hearted calcite mudstone, shelly at the base.
[Ostracod:] Cypridea granulosa (abundant).
[Thickness:] 1 foot, 1 inch [0.33m or 33cm.]

7b. THE CYPRIDEA BED.
Limestone almost composed of Cypridea granulosa.
[Compare to Ridgway, Bed 93, Yellowish Cypris Stone?]
[Thickness:] 4 inches. [0.10m or 10cm.]

7a. Shelly Limestone. Neomiodon casts [moulds?]
[Ostracod:] Cypridea granulosa.
[Thickness:] About 5 inches. [About 0.13m or 13cm.]

6. Clay, marly at the top.
[Thickness:] 9 inches [0.23m. or 23cm.]

5. Irregular marl.
[Thickness:] 1 to 6 inches [0.025m or 2.5cm to 0.15m or 15cm.]

4. Root Bed. Marlstone piped by slender root-like markings.
[Thickness:] 1 foot, 5 inches [0.43m or 43cm.].

3. Marly clay. [Ostracod:] Cypridea granulosa.
[Thickness:] About 1 foot [about 0.30m or about 30cm.]

C. purbeckensis Zone

2. Limestone, with ostracods.
[Thickness:] 11 inches [0.28m or 28cm.]

1. Marls, calcite mudstone and marlstone, seen intermittently for about 6 feet [1.83m ]

[The Lower Purbecks seem not have been exposed substantially. Thus there is no further continuation of the log.]

Poxwell Anticline - a Potential Oil Field

In the late 1930s the importance to the country of making major oil discoveries on the south coast of England was obvious. Second World War ships ran on oil in accordance with Winston Churchill's policy. the D'Arcy Exploration Company was very successful in Iran (Persia) and was leading to the development of the British Persian (BP) Oil Company or Anglo-Iranian Oil Company. However, almost all oil was imported to Llandarcy in South Wales (partly named after the D'Arcy Exploration Co, but with a Welsh-sounding prefix).

The famous anticlines in the Iranian fold belt are long and narrow and closed at both ends and in Tertiary and Cretaceous strata. They are usually faulted. In the 1930s D'Arcy Exploration Company geologists on leave in Dorset found an oil seep in the Corallian (Bencliff Grit) east of Osmington Mills. This is now very well-known now but it was an exciting discovery then. They realised that just inland from Osmington Mills is a small closed and narrow anticline, the Poxwell Pericline. the initial theory was that the anticline would be similar to the Ringstead Anticline (with the oil at Osmington Mills) but the potentially oil-bearing Corallian strata would be capped by a substantial thickness of Kimmeridge Clay (Ridd, 1973). The well was drilled to 507m. It did not lead to an oil discovery. The results of the exploration were published in a short paper by Taitt and Kent (1939).

POXWELL BOREHOLE continued:

Borehole Log

There was much surface investigation, and three very shallow boreholes was put down to try to investigate the structure before the the main drilling operation. In the light of the evidence obtained in the three shallow boreholes a location was chosen (lat. 50 degrees, 39 minutes, 4.7 seconds N., long. 2 degrees, 21 minutes, 23 seconds W.), with the object of penetrating the Corallian in the crestal area of the anticline and on the upthrow side of the Ridgeway Fault. Drilling was commenced on 24th May 1937, and completed at a depth of 1,666 feet [508 m.] two months later.
The following is a brief account of the formations penetrated in the Poxwell deep borehole. It is based, with some addition notes, on Taitt and Kent (1939).

POXWELL BOREHOLE LOG.

Portland Beds: True thickness: 162 feet [49m.] in the borehole, 152 ft [46m.] true thickness.

Portland Stone. Soft buff limestone with rare chert bands. 0 to 28ft [8.5m] in borehole [not normal to the strata].

Portland Sand. Dark grey sandy clay and argillaceous sandstone. 28 - 160 ft in bh.

Kimmeridge Clay: 842 ft in borehole [247m]. True thickness: 700 ft [213m.] [conversion: 0.83]
Dark calcareous clays, rather silty in the upper part. 160 - 485 feet [325 feet, 99m.] in borehole, not at right-angles to the strata. [true thickness conversion: 82m. thick].
Oil shale with Saccomoma [Kimmeridge Blackstone] - depth in the borehole (oblique to bedding) 485-490 ft. [Thickness along borehole 5 ft or 1.5m, corrected to true thickness 1.26m. This is probably too thick for the Blackstone itself, about 60cm at Kimmeridge in the basin, and may include adjacent oil shale.]
Dark grey calcareous clays; abundant Aulacostephanus [Lower Kimmeridge Clay ammonite] in cores between 590 [180m] and 600 feet 183m.] in the borehole; Aulacostephanus with Amoeboceras and Aptychi between 750 [229m.] and 770 ft [235m.] in the borehole; Rasenia [ammonite from near the base of the Kimmeridge Clay] at 890 ft. [271m.] in the borehole, and Ostrea fragments [presumably the flat oyster Liostrea delta of the basal Kimmeridge Clay] below 965 ft [294m.]. Depths 490 - 1002 ft [149m - 305m in borehole, [apparent thickness in borehole - 185m, converted to vertical thickness 151m.]

Corallian Beds. 305 ft drilled. True thickness: 245 feet [75m.]

Ringstead and Sandsfoot Beds.

Brown and dark green sandy clay with scattered ironshot ooliths. 1002-1067 ft in the borehole. [not corrected for angular difference]
Rich oolitic ironstone [like the Abbotsbury Ironstone] with Chlamys midas. 1067 - 1075 ft in borehole. i.e. 8 ft in the borehole. Corrected - 6.4 ft = 1.95 m. [almost 2m of oolitic iron ore, an unusual feature, not seen on the coast at Ringstead].
Oolite of calcareous ooliths in a dark green matrix. 1075-1090 feet in borehole.

Trigonia clavellata Beds.

Hard grey crystalline limestone with abundant Trigonia and bands of marl and sandstone. 1090-1101 ft in the borehole. 1090-1101 ft in borehole.

Osmington Oolite Series.
Pale nodular limestone and dark clay ("Nodular rubble" [sponge spicule beds]). 1101-1117 in borehole.
Nodular oolitic limestone and marl. 1119-1142 ft in bh.
Light oolitic limestone. 1142-1152 ft in bh.
Dark grey oolitic marl with oolitic limestone nodules and pisolite. 1152-1176 ft in bh.

Bencliff Grit.

Soft grey sandstone and marly sandstone [notice not yellow underground and unoxidised]. 1176 - 1200 in bh. 24 ft in bh = 7.3m. Corrected 5.8m. vertical thickness approx.

Nothe Clay.

Dark grey silty clay with Trigonia hudlestoni. 1200 - 1238 ft in bh.

Trigonia hudlestoni Bed

Oolitic limestone with abundant Chlamys fibrosa, Pleuromya sp. and Gryphaea sp. 1238-1256 ft in bh.

Nothe Grit.

Calcareous sandstones and sandy limestones with large Gryphaea sp. and Cercomya undulata. 1256-1307 ft. in bh.

Oxford Clay.

223 ft in borehole. True thickness 185 ft [56m.]
Dull grey silty clay with Cardioceras sp. at 1320 ft. and again at 1445 ft.

Great Oolite Series - 136ft drilled.

Forest Marble - 74 ft. drilled.

Hard light grey raggy limestone with Chlamys sp. 1530-1535.5 ft. in bh.
Hard grey-green marl with conchoidal fracture. Rare small Rhynchonella sp. in the lower part. 1535.5 - 1602.5 ft in bh.
Hard grey green marl with conchoidal fracture. Rare small Rhynchonella sp. in the lower part. 1535.5 - 1602.5 ft in bh.
Grey marl and limestone with abundant Goniorhynchia boueti, Ornithella cf. digona and Chlamys sp.

Fuller's Earth. 62 ft [18.9m.] drilled.

Hard light grey marl with limestone nodules. 1604-1612 ft in bh.
Hard light grey marl with conchoidal fracture, with scattered plant fragments and Oppelia fusca at 1646ft. 1612-1666. [end of borehole log]

ACKNOWLEDGEMENTS

I am much appreciative of help from many people at Poxwell quarry and vicinity over the last half century. The late Professor Sylvester Bradley, who had been brought up at Poxwell, helped with discussion, although I was more specifically involved with excavating part of Portesham quarry with him. I am very grateful to Alan Holiday and Jo Thomas and the DIGS Group with their help regarding this quarry, and the history of old quarry working for Cypris Freestones. DIGS has kept the old quarry in surprisingly good condition and the main units are still clearly visible. I thank former students from Southampton University for much valuable help with field studies at Poxwell and in the adjacent area. In particular the leading work is that which was undertaken by K.M. Bainbridge in 1983 and still exists as a substantial report. I am particularly grateful to him and would like to be in contact again.

REFERENCES AND BIBLIOGRAPHY

See also:

Purbeck Formation - Bibliography |
.
Anderson, F.W. 1958. In: Wilson, V., Welch, F. B. A., Robbie, J.A. and Green, G.W. 1958. Geology of the Country around Bridport and Yeovil. Memoir of the Geological Survey of Great Britain, Explanation of sheets 327 and 312. 118-129. Purbeck ostracod zones given for the Purbeck outcrops in the east of the area.
.
Arkell , W.J. 1941. The gastropods of the Purbeck Beds. Quarterly Journal of the Geological Society, London , vol. 97, part 1, 79-128.

Arkell , W.J. 1947. The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain. 386pp. With Wright, C.W.and Melville, R.V. 2nd edition - 1952 with Addenda and Corrigenda. For the Poxwell road cutting log see page 143.

.
British Geological Survey. [BGS] 1974. 1:50,000 geological map - Weymouth, Sheet 342. [This covers Weymouth and Portland with Lulworth Cove in central part (a large part of this map is sea, the geology of which is not shown). It extends from west of Weymouth to east of Kimmeridge and northward to Winfrith Newburgh. This map, based on a survey of 1896. The new map, referred to below, provides much more detail.]

British Geological Survey. 2000. Geological Map: West Fleet and Weymouth. 1:50,000 Series, England and Wales Sheet 341 and part of sheet 342. Solid and Drift, with seafloor geology, cross-sections and other data. New map. [This commences just to the west of Durdle Door, Lulworth and includes Bats Head. It extends from here westward to Abbotsbury, including all the Fleet Lagoon and the Isle of Portland. It provides a cover of most of the Weymouth Anticline. The continuation southward onto seafloor geology is something that the previous Weymouth and Fleet maps do not have. There are large, north-south cross-sections based on boreholes and seismic data. These show deep faulting and such features of interest as the thickness and distribution of evaporites in the Permo-Trias. It has good structural contour maps for the top Penarth Group and the top Corallian and other information, such as sea-floor sediment data for the offshore area around Weymouth and Portland.

.

British Geological Survey (BGS). (Compiled by M.A. Woods) 2011. Geology of South Dorset and South-East Devon and its World Heritage Coast.

Special Memoir for 1:50,000 geological sheets 328 Dorchester, 342 West Fleet and Weymouth and 342/343 Swanage and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 330 Newton Abbott. Compiled by M.A. Woods. By Barton, C.M., Woods, M.A., Bristow, C.R., Newell, A.J., Westhead, R.K., Evans, D.J., Kirby G.A., and Warrington, G. Contributors: Biostratigraphy - J.B. Riding; Stratigraphy - E.C. Freshney; Economic Geology - D.E. Highley and G.K. Lott; Engineering Geology - A. Forster and A. Gibson. British Geological Survey, Keyworth, Nottingham, 2011. 161 pp. This is the new version of the Geological Survey Memoir for the Dorset Coast etc. and replaces Arkell (1947) and the earlier memoir by Strahan (1898). It covers a wider area than these old memoirs, though, and includes all of "Jurassic Coast", UNESCO World Heritage Coast. It is a key reference work. Available from BGS Online Bookshop at 24 pounds stirling (in Jan. 2012).

.
Bainbridge, K.M. 1983. The Geology of the Country around Osmington, Dorset. Unpublished, Undergraduate Mapping Report of Southampton University. 136pp. [A very good mapping report, supervised by Ian West in the early 1980s. I am not at present in contact with K.M. Bainbridge, but would like to make contact to discuss Dorset geology. His work records features not at present visible, and there is renewed interest in the Poxwell Quarry.]

.
Damon, R. 1884. Geology of Weymouth, Portland, and Coast of Dorsetshire, from Swanage to Bridport-on-the-Sea: with Natural History and Archaeological Notes. New and Enlarged Edition (2nd Ed.), Weymouth, R.F. Damon, London, Edward Stanford. 250p. With a colour geological map of part of the Dorset coast, and including a log of the Purbeck strata of Durlston Bay, Swanage, by H. W. Bristow and Prof. E. Forbes (although note that it contains a small error).

.
Fisher , O. 1856. On the Purbeck strata of Dorsetshire. Transactions of the Cambridge Philosophic Society, 9, 555-581. [Classic early log of the Durlston section with useful faunal information. See Wilding, 1988 for biography of Osmond Fisher. With regard to Poxwell, the log of the Purbeck Formation for the Ridgeway (Ridgway) Railway Cutting is relevant.]
.
Francis , J. E. 1983. The Fossil Forests of the Basal Purbeck Formation (Upper Jurassic) of Dorset, Southern England. Unpublished Ph.D. Thesis, University of Southampton.

Francis, J. E. 1986 The Calcareous Paleosols of the Basal Purbeck Formation (Upper Jurassic) Southern England. p. 112-138 in: V.P. Wright, (Ed.) Paleosols: Their recognition and interpretation. Blackwell, Oxford. (Basal Purbeck Dirt Beds of Lulworth and Portland)

Francis, J.E. 1983. The Dominant Conifer of the Jurassic Purbeck Formation, England. Palaeontology, 26, 277-294. [Key paper. Protocupressinoxylon purbeckensis a cypress or juniper-like tree. Cycadophytes also occurred but were much less common.]

.
House, M.R. 1961. The structure of the Weymouth Anticline. Proceedings of the Geologists' Association, vol. 72, pp. 221-238. [relevant to the Poxwell Anticline].

House, M.R. 1989. Geology of the Dorset Coast. Geologists' Association Guide, 169pp, 39 text-figs and 34 monochrome plates. September 1989. Paper-back. ISBN 0 7073 0485 7. See particularly pp. 94-95 with Fig. 26 - Geological sketch map of the Poxwell Anticline, and Fig. 27 - Geological Cross section from the Poxwell borehole site southwards to the coast (modified from Mottram and House, 1956).

House, M.R. 1993. Geology of the Dorset Coast. Geologists' Association Guide No. 22. 2nd edition, 164pp., 43 text-figs (mostly cliff diagrams and maps) and 32 plates, some in colour. Paper-back. ISBN 0 7073 0485 7. See particularly pp. 94-95 with Fig. 26 - Geological sketch map of the Poxwell Anticline, and Fig. 27 - Geological Cross section from the Poxwell borehole site southwards to the coast (modified from Mottram and House, 1956).

.
Lees , G.M. and Cox, P.T. 1937 . The geological basis for the search for oil in Great Britain by the D'Arcy Exploration Co. Ltd. Quarterly Journal of the Geological Society, London, 93, 156-190. By George Martin Lees, M.C. D.F.C. Ph.D. F.G.S., and Percy Thomas Cox, M.A. F.G.S. Read March 10th, 1937.

Abstract
Seepages of oil at the surface or in coal mines have been known in Great Britain for several centuries. Records of the use of oil from the Coalport seepages date from 1684. James Young in 1847 extracted wax, burning oil, and lubricating oil from a crude oil obtained in the Biddings colliery, near Alfreton in Derbyshire. In 1902, some boring was carried out in the Sussex Weald, stimulated by the accidental discovery of natural gas at Heathfield, but this enterprise was soon abandoned.
In 1918, a serious search for oil was made under the pressure of war-time conditions, and Messrs. S. Pearson & Son, Limited, between 1918 and 1921, under contract for H.M. Government, drilled 11 test-wells: seven in Derbyshire, two in Staffordshire, and two in Scotland. In every case the objective was an oil accumulation in Carboniferous beds. A small production was obtained from their first well, Hardstoft No. 1 in Derbyshire, and showings of oil were struck elsewhere, notably in the D'Arcy well in Scotland. Although the results up to that time may not have been conclusive, they were not considered sufficiently encouraging to justify further Government expenditure, and operations were suspended. Two wells were drilled later (1924–26) on the Hardstoft structure on the initiative of the Duke of Devonshire, owner of the property, but neither obtained any oil-production.
Other attempts during the last 20 years to find oil by drilling have been those at Kelham, near the site of a coal exploration borehole, which found a show.
[end of abstract]

[an example extract follows]
(c) Jurassic:
Though not so prolific in oil seepages and bitumen shows as the Carboniferous, the Jurassic system of Great Britain yields abundant evidence of oil generation within its beds. Liassic shales are bituminous, in the sense that they contain material of kerogen type, which will yield oil on heating, throughout the length of their main outcrop from northeastern Yorkshire to Dorset. The Jet Rock of Whit by and the Lower Lias outcropping near Charmouth and Lyme Regis are among the richer examples. Veinlets of hard bitumen, which must originally have been fluid, occur in the Belemnite Marls, the Black Ven Marls, and in parts of the Blue Lias of Dorset. Analysis has proved traces of soluble oil, up to 0.1 per cent, in Liassic shales and sandstones of the south coast exposures. In Kent, at Chilham, boring is reported to have found bitumen along joints in Liassic limestones. In the Eskdale and Robin Hood's Bay area of Yorkshire, small pockets of liquid oil occur, filling cavities in calcareous concretions in the outcropping Lias, and the ironstone of the Cleveland district is said to contain a trace of oil at depth (Strahan 1920, p. 62). Middle Jurassic rocks have yielded little evidence of the. presence of petroleum. Tarry matter has been recorded (Strahan 1920, p. 62) as filling joints in the Bath Oolite penetrated by borings in Kent.
The Oxford Clay contains many horizons which are comparatively rich in insoluble organic matter of kerogen type and traces of oil are known to occur in it. The following data refer to a specimen collected by Mr. C. T. Barber near Stalbridge in Dorset :
Oil yield (Imperial gallons per ton) .. 12,2
Specific gravity of oil at 60°F.. 1.002 . .
Setting point of oil (rotating thermometer method) .. 52degrees F.
Sulphur .. 4.95%
Yield of oil obtainable by extraction with benzine .. 1.75%
Ash content of clay .. 71.4%
The presence of a rich impregnation of oil in the Corallian Bencliff Grits outcropping near Osmington Mills in Dorset was recently discovered by Mr. A. H. Taitt. A specimen of the sand from the exposed cliff face has given 12.1 per cent of oil, and where these beds pass below high-tide level at Bran Point there is a small but active seepage of free oil. This seepage is, so far as we are aware, a new discovery, and its existence does not appear to be known even to the local inhabitants. Following are two analyses of the oil [the second one is in brackets]:
Specific gravity at 60° F. - 0.941 (0.944)
Hard asphalt - trace
Soft asphalt - 6.47 (5.90)
Soft wax - 8.11 (11.04)
Distillate to 200° C. at 5 mm. Hg. pressure - 21.4% (19.2%)
Specific gravity of distillate - 0.907 (0.910)
Specific gravity of residue - 0.955 (0.957)
Sulphur - 0.25% (0.30%)
This Corallian oil may be indigenous to the group of beds in which it occurs or it may have been derived from another stage of the Jurassic and have lodged in this porous sandstone. The high sulphur content of oil distilled from both the Oxford and the Kimeridge clays does not favour a relationship between the Corallian oil and the kerogen material of these clays, but we are not yet able to offer a definite opinion upon this point.
The oil shales of the Kimeridgian have been described by Strahan (1920, pp. 18-40), and attempts have been made to exploit them commercially. The oil derived from this shale differs from that obtained from the Scottish shales in its higher sulphur content. Kimeridge shale oil generally contains from 2 to 7 per cent of sulphur, whereas the Scottish shales rarely yield as much as 2 per cent. Oil distilled from an Oxford clay, is, as is shown above, similar in this respect to Kimeridge oil. It is noteworthy that the free oil in the Corallian sand has less than 0.3 per cent of sulphur.
Portland beds have yielded traces of oil on analysis from outcrops at Tisbury, Portash, Portland, and Kimeridge.
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. Strahan (1898, p. 107) recorded bituminous limestone in Purbeck beds near Holworth House and in the Chaldon anticline; oil residues similar to those of Lulworth Cove were found in Purbeck beds. Purbeck limestones outcropping at Upway gave a colour on extraction with chloroform, though they lacked visible oil content; this is also the case in the Purbeck inliers of the Weald. Lower Purbeck beds in both the Portsdown and the Henfield borings contain traces of liquid oil.
Some of the Purbeck oil and bitumen may be indigenous to that series, which also contains some oil shales. There is, further, the possibility of upward migration from the underlying Kimeridge beds, although analysis of the Purbeck residues shows that their sulphur content is generally less than 1 per cent. It is of interest in this connexion that an oil shale from the Sussex Purbeck outcrops yielded on distillation 20 gallons per ton of oil with a specific gravity of 0.878, setting point at 77° F. and low sulphur content.
(d) Wealden:
The Wealden beds of the south of England, in spite of the generally accepted theory of their freshwater origin, contain a remarkable number of richly oil-impregnated sands. The senior author of this paper first discovered such impregnation in sands at Worbarrow Bay (PI. XI), and subsequently he and his colleagues found rich oil impregnation in all Wealden exposures between this place and Dungy Head (PI. XII).
At Dungy Head the lowest Wealden sandstone is richly impregnated, chocolate-brown to black in colour, and sticky, due to its oil content. This sandstone again outcrops in the cliff face between Dungy Head and Lulworth Cove at a point marked by a navigation beacon. It is here about 12 feet in thickness and is chocolate-brown in colour, but on account of its exposed position the oil content is no longer so fresh as at Dungy Head. On the western side of Lulworth Cove, a prominent black sand exposed in the cliff owes its colour to carbonaceous matter, and not to oil impregnation, but on the eastern side of the cove an oil sand occurs a short distance above a black carbonaceous bed in the lower part of the series and a second oil sand is present in the higher Wealden beds.
In Mupe Bay an especially strongly impregnated conglomera. tic sand occurs about 90 feet above the Purbeck beds, and a second oil sand towards the top of the series. Coarse sandstone boulders, of which one sample contained 7.9 per cent by weight of oil, are found in less strongly impregnated, more finely grained sand or clay of the lower horizon.
In Worbarrow Bay, where the Wealden series is well exposed~ there are several sands containing oil. Analysis of one about two-thirds up from the base of the formation showed 2.83 per cent of oil with a specific gravity of 0.96.
In the Wealden exposures of Kent and Sussex, important oil shows are rare, but at Chilley, near Pevensey, there is a sand of the Tunbridge Wells group which, on extraction with benzine, yielded 12 per cent by weight, or about 25 per cent by volume, of black, viscous oil with a specific gravity of 0.995 and a sulphur content of 0.66 per cent. Mantell, in his Geology of the South-East of England (1833, pp. 172-3) recorded that a bituminous sand containing 15.4 per cent of bitumen was struck during the excavation of a drainage trench in the marshy ground near Chilley. Tunbridge Wells sandstones exposed in an old quarry near Hailsham also give a colour on extraction with chloroform, and an auger-hole drilled some two miles farther south at Down Ash, between Hailsham and Chilley, encountered a sand lightly impregnated with oil. Colour has also been obtained from tests with chloroform of the Hastings Beds at Hastings.
The natural gas occurrences in the Heathfield district have been described by Strahan (1920, pp. 111-4), Milner (1922, pp. 148-9) and others. In the evidence given before the Royal Commission on Coal Supplies of 1904 (Second Report, p. 112), it was stated that abundant petroleum was found in wells drilled for gas in this neighbourhood, but we have no confirmation of this. The gas was obtained either from the Hastings Beds or from the upper parts of the Purbeck and may have been indigenous to one or other of these series (both of which contain a considerable amount of lignitic matter), or it may have come from a lower Purbeck or Kimeridge source- The latest analysis of this gas, quoted by Strahan (1920, p- 113), shows it to contain 2-94: per cent of ethane, 2-90 per cent of nitrogen or - other inert gas, and 1 per cent of carbon monoxide, in addition to 93-16 per cent of methane. Natural gas has been found elsewhere in the Weald, in the Hastings Beds at Cuckfield, and at Hawkhurst.
[continues with Part 4, The Exploration Programme.]

Notes:
These authors suggested that the sharp folding of South Dorset and violence of the Cretaceous unconformity may be due to the presence, at no great depth, of a plastic salt series which has allowed the top group of rocks to move out of harmony with those below. The age of such a salt series may be Trias or even Permian. Certainly the unexpected complications proved at Poxwell and descrbed in this paper and the complications which undoubtedly occur in the neighbouring Sutton Poyntz anticline, where we have found Cornbrash and older rocks exposed lend support to this suggestion. The collapse of a fold floating, as it were, on a plastic salt series, during a quiescent period following one of compression is understandable [but this is probably not the correct story - i.e. major collapse after Tertiary compression is not probable; the authors did not necessarily fully understand the Late Cimmerian (pre-Albian) extension phase at this date. This is a major complicating factor in the Poxwell - Sutton Poyntz areas.]

.

Lees, G.M. and Tait, A.H. 1946. The geological results of the search for oil fields in Great Britain. Quarterly Journal of the Geological Society, London, 101, 255-317.

.
Mottram, B.H. 1950. Notes on the structure of the Poxwell Pericline, and the Ridgeway Fault at Bincombe Tunnel, Dorset. Proceedings of Dorset Natural History and Archaeological Society, vol. 71, (for 1949), pp. 175-183.

Mottram, B.H. and House, M.R. 1956. The structure of the northern margin of the Poxwell Pericline. Proceedings of Dorset Natural History and Archaeological Society, vol. 76 (for 1954), pp. 129-135. With a geological cross-section from the Poxwell borehole site (1937) south to Ringstead. This section has been reproduced in House (1993) Fig. 27, p. 95.

.

Pugh , M.E. 1966. The Petrography of the Lower Purbeck Limestones of Dorset. Unpublished M.Sc. Thesis, Chelsea College, London.

Pugh, M.E. 1968. Algae from the Lower Purbeck limestones of Dorset. Proceedings of the Geologists' Association, London, 79, 513-523.

.
Ridd, M.F. 1973. The Sutton Poyntz, Poxwell and Chaldon Herring Anticlines, southern England: a re-interpretation. Proceedings of the Geologists' Association, vol. 84, part 1, pp. 1-8.
Abstract:
Examination of the literature concerning these structures suggests that they are not rejuvenated pre-Albian anticlines as was previously thought. Instead they developed during the Tertiary on what in pre-Albian times was the faulted, south-dipping limb of the Upton Syncline. During the Tertiary, northward-directed folding and thrusting occurred and the Kimmeridge Clay apparently acted as a zone of detachment; therefore the previously inferred reversal of movement on the deeper part of the Abbotsbury Fault need not have taken place.
[This is a key paper on the Poxwell Anticline, in terms of oil reservoir potential. Ridd correctly indicated that the Pre-Albian subcrop map for the structure showed the Upton Syncline extending to the Abbotsbury Fault. In other words there was no predecessor Poxwell anticline in early Cretaceous time. This reduced the potential for oil retention, since there would have been no trapping of oil that had been generated in Pre-Albian times. His cross-section does not accord with that of the BGS (Weymouth Geological Map - 2000 edition) in that he postulated a thrust with detachment in the Kimmeridge Clay. He was not aware of the Mercia Mudstone salt under the area. His thrusts are at rather low angles. These are the main areas of disagreement with later work.]

.
Sylvester-Bradley, P.C. 1949a. The ostracod genus Cypridea and the zones of the Upper and Middle Purbeckian. Proceedings of the Geologists' Association, 60, 125-151. By P.C. Sylvester-Bradley, B.Sc., F.G.S. Received 14th November 1947.
Summary: The nomenclature of the genus Cypridea is clarified, and a valid genolectotype is selected. The shell characters of the genus are described. Stratigraphically significant species of the Upper and Middle Purbeck Beds are described, and their distribution incorporated in a revised zonal scheme.
With Appendix:
A Section of the Purbeck Beds at Poxwell.
Poxwell Cutting (30/741834), Dorset.
(From a section measured in 1936-38 with the help of Mr Brian Mottram. Dr. W.J. Arkell kindly allowed me to compare my section with that which he measured for the Geological Survey Memoir. He also identified the gastropods. Professor T.M. Harris identified the Charophyta. The unnamed ostracods are to be described in due course by Mr. F.W. Anderson. The zonal arrangement follows that proposed by the author.)
[There is a good, detailed listing of 50 beds with their faunal content. The succession is well-described and could be compared in detail with the well-known Clement's Log of the Durlston Bay Purbeck sequence. Most of the log at Poxwell is concerned with Middle Purbeck and only a small part of the uppermost Lower Purbeck is included. See also Arkell (1947) where a shorter version of a log of the Poxwell Road Cutting is given. This has less detail and lists 31 beds.]

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Strahan , A. 1898. The Geology of the Isle of Purbeck and Weymouth. Memoirs of the Geological Survey. Her Majesty's Stationery Office, London. 278 pages with a map.

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Taitt, A. H. and Kent, P.E. 1939. Note on an examination of the Poxwell Anticline, Dorset. Geological Magazine, vol. 76, pp. 173-181.
The discovery of an active oil seepage in Corallian sandstones at Osmington Mills, near Weymouth, led in 1937 to an examination of the adjacent Poxwell or Moigns Anticline by geologists of the D'Arcy Exploration Company, a subsidiary of the Anglo-Iranian Oil Company, Ltd. [now well-known under its modern name - "BP"]. The investigations, carried out by Dr. C.T. Barber in the earliest stages, and subsequently by the junior author, consisted of surface mapping assisted by pits, trenches, auger holes, and shallow bore-holes, and culminated in the drilling of a test well in the crestal area of the anticline. This note summarises the more important information obtained with regard to stratigraphy and tectonics.
The general form of the Poxwell Anticline, which was ably described by Strahan (1898), is a dome, greatly elongated in an east-west direction, separated in the west by a small saddle from the larger Sutton-Poyntz anticline. In order to locate a bore-borehole to test the oil-producing possibilities of the Corallian it was, however, necessary to obtain informtion concerning the Ridgeway fault, which separates the the Jurassic and Cretaceous rocks on the northern flank of the structure.
Three shallow bore-holes were drilled along a north-south line across the presumed trace of the fault near Poxwell village. The most northerly borehole (lat. 50 degrees, 39 minutes, 14.3 seconds N, long. 2 degrees, 21 minutes, 52 seconds W.) penetrated 120 feet of Chalk, of which the lowest 22 feet was of the Micraster cortestudinarium Zone. A bore-hole 120 feet south of this, 40 feet deep, drilled through 20 feet of Chalk and Purbeck Limestone debris before encountering solid Chalk of the Rhynchonella cuvieri Zone. No. 3 bore-hole, 166 feet farther south, started in Purbeck and entered the Portland Sand at 171 feet, and at 230 feet the Upper Greensand, in which formation the well was completed at a depth of 432 feet (Fig. 2).
The true thicknesses, allowing for dip of the formations encountered in No. 3 borehole were Purbeck [but not complete Purbeck sequence] 80 feet [24 metres], Portland Stone 40 feet [12 metres], Portland Sand 42 feet [13 m.], and Upper Greensand c. 100 feet [c. 30m.].
The thickness of the Portland Stone, obtained by allowing for the dip of 42 degrees, shown in the cores of the Purbeck, proved to be less than anticipated. Dr. Arkell suggested that the apparent attenuation might be the result of normal faulting, but augering over the mapped Portland Stone outcrop on both flanks of the anticline showed that the recorded outcrop was too wide. The amended boundaries agreed with an approximate thickness of 40 feet for the Portland Stone.
In the light of the evidence obtained in the three shallow boreholes a location was chosen (lat. 50 degrees, 39 minutes, 4.7 seconds N., long. 2 degrees, 21 minutes, 23 seconds W.), with the object of penetrating the Corallian in the crestal area of the anticline and on the upthrow side of the Ridgeway Fault. Drilling was commenced on 24th May 1937, and completed at a depth of 1,666 feet [508 m.] two months later.
The following is a brief account of the formations penetrated in the Poxwell deep borehole.

Portland Beds: True thickness: 162 feet [49m.] in the borehole, 152 ft [46m.] true thickness.

Portland Stone. Soft buff limestone with rare chert bands. 0 to 28ft [8.5m] in borehole [not normal to the strata].
Portland Sand. Dark grey sandy clay and argillaceous sandstone. 28 - 160 ft in bh.

Kimmeridge Clay: 842 ft in borehole [247m]. True thickness: 700 ft [213m.] [conversion: 0.83]
Dark calcareous clays, rather silty in the upper part. 160 - 485 feet [325 feet, 99m.] in borehole, not at right-angles to the strata. [true thickness conversion: 82m. thick].
Oil shale with Saccomoma [Kimmeridge Blackstone] - depth in the borehole (oblique to bedding) 485-490 ft. [Thickness along borehole 5 ft or 1.5m, corrected to true thickness 1.26m. This is probably too thick for the Blackstone itself, about 60cm at Kimmeridge in the basin, and may include adjacent oil shale.]
Dark grey calcareous clays; abundant Aulacostephanus [Lower Kimmeridge Clay ammonite] in cores between 590 [180m] and 600 feet 183m.] in the borehole; Aulacostephanus with Amoeboceras and Aptychi between 750 [229m.] and 770 ft [235m.] in the borehole; Rasenia [ammonite from near the base of the Kimmeridge Clay] at 890 ft. [271m.] in the borehole, and Ostrea fragments [presumably the flat oyster Liostrea delta of the basal Kimmeridge Clay] below 965 ft [294m.]. Depths 490 - 1002 ft [149m - 305m in borehole, [apparent thickness in borehole - 185m, converted to vertical thickness 151m.]

Corallian Beds. 305 ft drilled. True thickness: 245 feet [75m.]

Ringstead and Sandsfoot Beds.

Brown and dark green sandy clay with scattered ironshot ooliths. 1002-1067 ft in the borehole. [not corrected for angular difference]
Rich oolitic ironstone [like the Abbotsbury Ironstone] with Chlamys midas. 1067 - 1075 ft in borehole. i.e. 8 ft in the borehole. Corrected - 6.4 ft = 1.95 m. [almost 2m of oolitic iron ore, an unusual feature, not seen on the coast at Ringstead].
Oolite of calcareous ooliths in a dark green matrix. 1075-1090 feet in borehole.

Trigonia clavellata Beds.

Hard grey crystalline limestone with abundant Trigonia and bands of marl and sandstone. 1090-1101 ft in the borehole. 1090-1101 ft in borehole.

Osmington Oolite Series.
Pale nodular limestone and dark clay ("Nodular rubble" [sponge spicule beds]). 1101-1117 in borehole.
Nodular oolitic limestone and marl. 1119-1142 ft in bh.
Light oolitic limestone. 1142-1152 ft in bh.
Dark grey oolitic marl with oolitic limestone nodules and pisolite. 1152-1176 ft in bh.

Bencliff Grit.

Soft grey sandstone and marly sandstone [notice not yellow underground and unoxidised]. 1176 - 1200 in bh. 24 ft in bh = 7.3m. Corrected 5.8m. vertical thickness approx.

Nothe Clay.

Dark grey silty clay with Trigonia hudlestoni. 1200 - 1238 ft in bh.

Trigonia hudlestoni Bed

Oolitic limestone with abundant Chlamys fibrosa, Pleuromya sp. and Gryphaea sp. 1238-1256 ft in bh.

Nothe Grit.

Calcareous sandstones and sandy limestones with large Gryphaea sp. and Cercomya undulata. 1256-1307 ft. in bh.

Oxford Clay.

223 ft in borehole. True thickness 185 ft [56m.]
Dull grey silty clay with Cardioceras sp. at 1320 ft. and again at 1445 ft.

Great Oolite Series - 136ft drilled.

Forest Marble - 74 ft. drilled.

Hard light grey raggy limestone with Chlamys sp. 1530-1535.5 ft. in bh.
Hard grey-green marl with conchoidal fracture. Rare small Rhynchonella sp. in the lower part. 1535.5 - 1602.5 ft in bh.
Hard grey green marl with conchoidal fracture. Rare small Rhynchonella sp. in the lower part. 1535.5 - 1602.5 ft in bh.
Grey marl and limestone with abundant Goniorhynchia boueti, Ornithella cf. digona and Chlamys sp.

Fuller's Earth. 62 ft [18.9m.] drilled.

Hard light grey marl with limestone nodules. 1604-1612 ft in bh.
Hard light grey marl with conchoidal fracture, with scattered plant fragments and Oppelia fusca at 1646ft. 1612-1666. [end of borehole log]

[This is interesting in showing a very well-developed oolitic facies in the upper part. There is a rich oolitic iron ore with Chlamys midas in the Ringstead and Sandsfoot Beds. This is of Abbotsbury Ironstone facies, but within the Corallian not the Kimmeridge Clay. Another interesting aspect is the presence of green material, probably berthierine in the iron-rich Ringstea and Sandsfoot Beds. All this is suprising in view of the proximity to the coast section at Osmington Mills.]
[The subdivisions of the Corallian sequence are given as top and bottom depths in a borehole that is inclined in relation to bedding, and the figures are in feet. Therefore they need conversion before they become useful in comparision with the Osmington Mills coast section.]
[The paper continues for three more pages of text and one diagram. These further details are not reproduced here, but the short paper is in a well-known journal and probably very easily obtainable.]
[Of interest is mention of the early recognition by Lees and Cox of the thick Triassic salt sequence beneath the Poxwell area. This was understood even though it was not penetrated in the borehole. Here is a short extract from p. 180.]

There are a number of points of interest about this succession. The Kimmeridge Clay shows marked attenuation as compared with the standard succession at Kimmeridge, so that the Gravesia and Subplanites zones [i.e. Pectinatites Zones](from the "Blackstone" oil shale to the highest Aulacostephanus) measure less than 100 ft [30m.], as compared to 210 ft. [64m.] in the type section.

The unexpected development of Oolite and Oolitic Ironstone in the upper part of the Corallian is comparable with the facies at Abbotsbury, but the Ironstone here occurs at a lower horizon. The remainder of the Corallian is, as would be expected, closely similar to the outcropping beds at the well-known Osmington Mills sectin, 1 and a half miles distant. The Nothe Beds yielded a specimen of the lamellibranch Cercomya undulata, which was interesting in showing punctation, a feature usual in France, but previously unrecorded in England.

The beds below the Corallian were greatly reduced in thickness or absent. [continues]

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Taitt, A.H. and Kent, P.E. 1939. Note on an examination of the Poxwell Anticline, Dorset. Geological Magazine, vol. 76, pp. 173-181.

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Terris, A.P. and Bullerwell, W. 1965. Investigations into the underground structure of southern England. Advancement of Science, London, vol. 33, pp 232-252.

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Underhill , J.R. 2002. Evidence for structural controls on the deposition of the late Jurassic- early Cretaceous Purbeck Limestone Group, Dorset, southern England. Pp. 21-40 in: Milner, A.R. and Batten, D.J. (Editors) 2002. Life and environments in Purbeck times. Special Papers in Palaeontology, No. 68, Palaeontological Association, London, 268pp. Abstract: Integration of field studies with subsurface data demonstrate that deposition of the late Jurassic - early Cretaceous Purbeck Limestone Group occurred under semi-arid conditions during an important phase of syntectonic extensional activity in the Wessex Basin. Use of structural restorations and neotectonic analogue studies support the introduction of a new model to account for the regional and local along-strike variability in deposition of the Purbeck Limestone Group. It is proposed that the segmented nature of the episodically-active, basin-bounding Purbeck Fault explains observed thickness and sedimentological variations, including lateral facies changes and clast reworking. In addition to affording a tectonic framework in which to understand the rich palaeontological assemblages, the new depositional model also provides a mechanism for explaining the rapid death and unusual preservation of conifer forests through rapid submergence following co-seismic activity on the normal fault system. [By John R. Underhill, Department of Geology and Geophysics, University of Edinburgh.]

Underhill , J.R. and Stoneley, R. 1998. Introduction to the development, evolution and petroleum geology of the Wessex Basin. In: Underhill, J.R. (Ed.) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publications, 133, 1-18.

West, I.M. 1964a. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proceedings of Yorkshire Geological Society, 34, 315-330. [Petrographic evidence of vanished evaporites in the Caps and Broken Beds - pseudomorphs, celestite, lutecite etc. Diagenetic history established.] Abstract: Five stages have been determined in the diagenetic history of the calcium sulphate beds of Lower Purbeck age in Dorset, particularly by reference to abundant pseudomorphs and other relict textures and structures preserved in late-formed gypsum and in secondary silica and calcite. The weight of the overburden is thought to have controlled the changes. Occurrences are described of minerals, including celestite, calciostrontianite, lutecite and quartz remaining after the removal in solution of gypsum. A description is also given of secondary limestones which occur particularly in the Caps and Broken Beds. They are shown to be almost entirely replacements of original sulphate deposits. [End of abstract].

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. Abstract: Four facies of limestones, each with particular contents of calcitised evaporites and of skeletal debris were recognised. They are compared with sediments of modern evaporite-depositing environments. The lowermost limestones, stromatolitic and pelletoid with foraminifera, probably originated in intertidal to shallow subtidal, moderately hypersaline, water. Overlying pelletoid limestones with algal-mats and some gypsum are products of high-intertidal flats. The main evaporite beds were originally gypsum, probably formed in supratidal to intertidal, very hypersaline, palaeoenvironments. The gypsum was converted to anhydrite and later brecciated in part, forming the Broken Beds. Extensive calcitisation produced porous unfossiliferous limestones. Ostracodal limestones above probably originated in shallow, only moderately hypersaline water. All the basal Purbeck strata were formed in and around a large shallow gulf with extensive tidal flats and with water of varying but predominantly high salinities. At times of uplift, thin soils developed on the former margins of the gulf. Forests were able to exist there because, although the area was within the semi-arid zone, it was probably very near to the boundary of the warm-temperate zone. End of Abstract.
[This paper deals specifically with the Portesham section - see p. 216 and p. 220.]

West, I.M. 1979. Sedimentary Environments and Diagenesis of Purbeck Strata (Upper Jurassic - Lower Cretaceous) of Dorset, U.K. Unpublished Ph.D. Thesis, Southampton University, 181 p.

West, I.M., 1979. Review of evaporite diagenesis in the Purbeck Formation of southern England. In: Symposium "Sedimentation Jurassique W. Europeen." A.S.F. Publication Speciale, No.1, March, 1979. 407-416.

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Woodward , H.B.1895. The Jurassic Rocks of Britain. Vol 5. The Middle and Upper Oolitic Rocks of England (Yorkshire excepted). Memoirs of the Geological Survey of the United Kingdom. 499pp. [See pages 254-255 on the Purbeck strata of Worbarrow Tout.]
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Whitaker , W. and Edwards, W. 1926. Wells and Springs of Dorset. Memoir of the Geological Survey of England and Wales. [This includes the log of the Friar Waddon Borehole, Dorset with Purbeck strata.]

Woodward , H.B. 1895. The Jurassic Rocks of Britain. Vol 5. The Middle and Upper Oolitic Rocks of England (Yorkshire excepted). Memoirs of the Geological Survey of the United Kingdom. 499pp.