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The Purbeck Formation (or Purbeck Beds) consist mainly of fossiliferous thin-bedded limestone and shale, 119m thick at the type section. It has long been recognised as a distinctive unit. it differs obviously from the massive Portland limestone beneath not only by the thin-bedding but also by the generally non-marine fauna and rarity of thick-shelled molluscs and absence of ammonites. The lagoonal and lacustrine fauna with vertebrates of continental origin contrasts with the marine faunas of the underlying Jurassic. The separation from the non-marine clastic Wealden Group above is lithological and there is little faunal difference.
The basal part of the Purbeck Formation is Jurassic and the greater part Lower Cretaceous, Berriasian. It is the consequence of a major regression at the end of the Jurassic Period. The relatively deep marine Kimmeridge Clay facies shallowed, through the Portland Sand to the shoal oolites of the Portland Stone. The extensive shallow area of carbonate sand in warm Mediterranean climatic conditions, became restricted from the open sea. The large lagoon was so shallow that the floor was frequently exposed to produce desiccation cracks and to preserve dinosaur footprints. At times the surface had a salt crust in the dry summer, often trapping insects and plant debris; at other times greater exposure resulted in the formation of palaeosols, the Purbeck Dirt Beds. Coniferous trees growing in these were "pickled" in hypersaline brine and then silicified. Water levels rose with influx of marine water from a considerable distance, but the depth was rarely more than about one metre, so that thin beds were formed by the filling of the lagoon and the next phase of exposure. This special environment so near to mean sea-level at times accumulated shell-beds in the shallow water; at other times there were stromatolites, evaporites and clay sediments. Freshwater environments occurred when restriction from the sea was greatest. These preserved freshwater molluscs, ostracods and charophytes. Dinosaurs and early mammals lived on adjacent land areas and their remains are found in the sediments.
The Purbeck Formation is about 119 m thick at the type-section of Durlston Bay, Swanage. The thin-bedded alternation of limestone and calcareous shale or marl is a general characteristic. There have been many rapid salinity fluctuations, the major ones being on a scale of about half a metre to a metre, smaller oscillations being on a scale of a few centimetres. Although not simply interpreted, the salinity fluctuations have some relationship to the rapid lithological changes. Superimposed on this is, however, a progressive change from strata of hypersaline origin with evaporites in the lower part to alternating freshwater, brackish and marine strata in the middle part to dominantly freshwater with influx of land-derived clastics and Fe in the upper part. Brackish water shell deposits (of Neomiodon) form the Purbeck biosparrudites or shell limestones of the Purbeck building stone. A notable horizon in the middle is the Cinder Bed, which contains the small lagoonal oyster Praeexogyra distorta, with some normal marine fauna.. This conspicuous marker and transgressive horizon was once regarded as the Jurassic/Cretaceous boundary (Casey,1953) but the matter is not so simple and the boundary is appreciable lower (Feist et al, 1995).
There are many excellent exposures between Portesham, northwest of Weymouth and the Durlston Bay type section, at Swanage. Three examples are referred to below.
The Type Section - Durlston Bay
Durlston or Durlstone Bay ,Swanage is the type-section of the Purbeck Formation and the geology has been much studied since it was first described by Thomas Webster in 1816. This classic Purbeck section of lagoonal and lacustrine limestones alternating with shales and marls is the thickest exposed on the coast. Shelly limestones, the Purbeck Stone and Purbeck Marble, have long been quarried. Remains of dinosaurs, mammals, turtles, pterosaurs, crocodiles, fish, isopods and insects have been found here. In the 19th Century famous discoveries at Beckle's Mammal Pit were even used for creationist versus evolutionary arguments. Ostracods are common throughout and are used to determine palaeosalinities. Charophyte algae occur in low salinity beds. Evaporites, including gypsum and celestite, are common in the lower part. 246 numbered beds have been described in some detail by Clements (1969; 1993). Some of these subdivided Many of the beds are also named and the ostracod and gastropod content of each is known. Petrographic, magnetostratigraphic, geochemical and clay mineralogical data exists for large parts of the succession.
The Fossil Forest Exposure
East of Lulworth Cove, the Fossil Forest section is famous for the basal Purbeck strata with the Caps, the Great Dirt Bed with tree remains, stromatolites above and the evaporitic Broken Beds.
The Isle of Portland
Some similar features in the Basal Purbeck Formation can be seen at many localities in the norther part of Portland. The evaporitic Broken Beds are not developed here, although traces of evaporites are present. The Isle of Portland is the place where the best silicified tree remains have been found as a result of quarrying operations. Specimens are preserved on display. .
The Members of the Purbeck Formation
(This is only a summary. More detail, with diagrams and photographs, will be found in other Purbeck locality descriptions)
The formation is divided into a number of subdivisions with specific characteristics. These are now regarded as members and are easily recognised in the field.
1. (At the base) The Caps and Dirt Beds Member
The Purbeck Formation follows from the marine oolites of the Portland Stone through a thin Transition Bed. The oosparites with thick-shelled molluscs give way to mostly thin-bedded pelletoidal limestones and hummocky stromatolitic (thrombolitic) limestones with either no fauna or very limited faunas of mostly ostracods. These basal limestones of the Purbeck Formation have long been known as the Caps because they cap the quarried Portland Stone. The limestone also include fine-grained oolites, quite different from the medium-grained Portland oolites, and these are of special interest in containing nuclei of gypsum crystals, since replaced by calcite. The gypsum and the lack of normal marine or of freshwater faunas has indicated a hypersaline origin (West, 1975). The stromatolitic limestones ("tufa" in the old literature) in many cases encrust tree remains, often preserved in silicified form.
Interbedded with these limestones are the "Dirt Beds", calcareous, carbonaceous palaeosols of rendzina type. The most notable of these is the Great Dirt Bed. It is about 0.25m thick but variable to some extent. This palaeosol contains subrounded pebbles of limestone. The pebbles usually consist of limestone types similar to those in the underlying Caps. A proportion of the pebbles are blackened, as in similar facies elsewhere (Strasser, 1983) . Remains of coniferous trees (Francis, 1983) and cycadophytes (Buckland and De la Beche, 1837) are rooted in the dirt beds. This dirt bed, like the others (the Basal Dirt Bed and the Lower Dirt Bed) contain fusain or fossil charcoal, as shiny millimetre-size rectangular blocks (Francis, 1986). The fires seem to have involved only the undergrowth and not significantly charred the trees.
2. The Broken Beds
The Broken Beds is an old traditional name for this peculiar limestone breccia which puzzled many geologists before it was studied microscopically and geochemically. It extends from Durlston Bay, Swanage, where it is thickest and westward to Durldle Door where it is very thin. It is particularly well-seen at the Fossil Forest section. Much of the lower part of the breccia and the upper part of the underlying Soft Cap consists of clasts and matrix of very porous limestone, often soft and sandy in appearance. This is calcitised anhydrite, an unusual type of limestone produced when calcium sulphate is changed to calcium carbonate and any remaining sulphate dissolved . This lower part of the breccia (and the bed immediately beneath) originally consisted of beds of gypsum, deposited in a type of salt lake or lagoon. These evaporites of calcium sulphate occur in boreholes in the Isle of Wight and southeast England at this horizon as anhydrite. The upper and thicker part of the breccia consists of angular blocks of pelletoidal limestone with numerous small shells of ostracods. The breccia has been fragmented by tectonic action at a later date because the soft evaporites provided a plane of weakness and the overlying brittle limestone has been drawn in. The breccia is a special type of cargneuele or rahwacke, residues of tectonised evaporites that are common in the Alps and the Pyrenees.
3. The "Cypris" Freestones
This consists predominantly of ripple-lamined ostracodal and pelletoidal limestones. It mostly originated in water of about twice sea-water salinity (West, 1975). In the dry summers (the climate was very seasonal) the lagoon sometimes partially dried up leaving halite crystals in the fine carbonate sand flats. These were later dissolved away and filled with sediment so that now natural casts are preserved. The isopod Archaeoniscus brodiei occurs at certain horizons.
3. The Hard Cockle Member
The Hard Cockle Member in the Lulworth area consists of sandy bivalve limestone. The molluscs were mainly Protocardia purbeckensis, a small cockle shell, hence the name. This evidence of evaporites scattered through these beds, particularly pseudomorphs after halite. At the type section, in Durlston Bay, the Hard Cockle Member consists not of limestones but of marls and marlstones, with Protocardia purbeckensisand with fossil insects in some beds. Solitary wasps occur at one horizon (Rasnitsyn, Jarzembowski and Ross, 1998). There are no freshwater fossils or normal marine faunas and hypersaline lagoonal conditions were dominant.
4. The Soft Cockle Member
The Soft Cockle Member also contains Protocardia purbeckensis and consists of softer marls than the Hard Cockle Member. It is notable for containing secondary gypsum after anhydrite after primary gypsum. Sabkha facies are common and good examples of enterolithic veins are well-displayed at Worbarrow Tout. The sequence displays sabkha cycles. Insect remains occur at various horizons and small separate stromatolites occur (House, 1968). Thin lacustrine dolomite beds with palygorskite and pseudomorphs after halite occur in the upper part of the Soft Cockle Member. There is some celestite replacing calcium sulphate evaporites. Serpula coacervata, from which the German Serpulite takes its name, occurs commonly in the Soft Cockle Member.
5. The Marly Freshwater Member
The Marly Freshwater Member is notable for lacustrine deposits with freshwater fossils. Mg is still relatively abundant in this unit, but evaporites are generally absent. The most notable bed is the Mammal Bed, an horizon from the remains of 14 genera of mammals have been found. The main excavations were in Beckles Mammal Pit in the cliff top. The Mammal Bed has been described by Clements (1993) as dark-grey, shelly, carbonaceous, calcareous clay and shale which rests on an irregular surface of, and in part grades into the bed below. Gastropods are very abundant and ostracods are common. Mammals include: the Multituberculate - Plagiaulax becklesii ; the Pantotheres - Amblotherium and five other genera; the carnivorous Triconodonts and Symmetrodonts - Triconodon mordax, Trioracodon ferox, Spalacotherium and Peralestes , according to Arkell (1947). In terms of palaeoenvironment, the Mammal Bed is the deposit of a shallow lake with freshwater molluscs and accumulated vertebrate debris, perhaps washed into the marginal shore region.
6. The Cherty Freshwater Member
The Cherty Freshwater Member consists of cherty limestones, mainly bioclastic and pelletiodal ("micrite" in the field), with some shales and marls. Most of the strata within this member are lacustrine with low-salinity (freshwater or near-freshwater) gastropods such as Viviparus, Ptychostylus, Planorbis and Physa. Low-salinity ostracods such as Cypridea are common and charophyte algae occur in certain beds. An apparent anomaly is the occurrence of small pseudomorphs after halite in the Flint Bed (Meyer, 1872; Radley, 1992). This is probably because the lake in which these beds were deposited was not absolutely devoid of sodium and chloride ions and of extreme desiccation in the summer probably dried out and a little salt was precipitated in the lime-mud.
The chert is a replacement of carbonate sediment and shells and the silica is usually regarded as having been derived from the freshwater sponge Spongilla purbeckensis, which occurs in the flint bed. It is not abundant, however, and other sources of silica may have been involved. Diagenesis probably occurred as result of pH changes (c.f. Peterson and Von der Borch, 1965).
At Sunnydown Farm, many remarkable vertebrate discoveries have been made in the Cherty Freshwater Member (Ensom, 1988; Ensom, 1996; Kielen-Jaworowska and Ensom, 1992). Dinosaur footprints, amphibian and mammal remains have all been found in the Sly Bed (equivalent of DB 102 of Durlston Bay type section) just above the New Vein. These are discussed in a separate palaeontological section of the proposal.
7. The Cinder Member
The Cinder Bed consists of beds of bluish-grey shelly limestone with small oysters - Praeexogyra distorta (example in image is from Stair Hole). The Soft Cinder, is more argillaceous and overlies the Hard Cinder . This conspicuous unit is easily distinguished by its colour and its relative lack of jointing and a tendency to erode to a roughly rounded surface (unlike the blocky Flint Bed a little further down). In terms of palaeoecology it was probably similar in habitat to that of the small oyster Crassostrea which lives in coastal lagoons of the Gulf of Mexico.
Echinoid spines ( Hemicidaris purbeckensis ) occur in part of the Cinder Bed which show that salinity was normal marine for a short time. The complete echinoid is very rare. Remains of aragonitic bivalves are also present. Although not now regarded as representing the Jurassic/Cretaceous boundary the Cinder Bed is a very extensive and useful marker for the middle part of the Purbeck Formation. When it was regarded as the boundary the unsatisfactory (non-lithostratigraphic) terminology of Lulworth Beds, below, and Durlston Beds, above was used by some, and reference to this will be found in literature of the 1960s and 70s.
The Cinder Bed indicates an incursion of the sea into the lagoon. It is the result of a brief marine transgression. In the shallow environment the water became just about marine enough for echinoids (Hemicidaris purbeckensis ), but not for long. The marine conditions were not stable for long enough for ammonites to live in this area. The small oyster could tolerate lower salinities than truely marine and still flourished when the water became rather brackish. This was not the only marine incursion; there were about 37 others at least during deposition of the Purbeck Formation.
8. The Intermarine Member
The Intermarine Member lies above the Cinder Bed and contains the main limestones of the Upper Building Stones. It consists of beds of hard shell-debris (biosparrudite) limestones with some pyritic shales in the central part. The old traditional name presumably applies well to the upper part of brackish water The lower part (Downs Vein etc, just above the Cinder Bed), though, is of almost freshwater origin and has similarities to the Cherty Freshwater Member in containing pond-snails like Physa bristovii and Ptychostylus , and charophyte algae. It, thus, starts as freshwater lake deposits but marine water gradually gained access to what then became a large lagoon. Austen (1852) and Fisher (1856) referred to the unit as the "Turtle Beds" because turtle remains are common.
The shelly limestones are mostly coarse enough to be termed "biosparrudites "(coarse shell debris with a sparry cement) in accordance with Folk's (1962) classification, but sometimes referred to for simplicity as " biosparites " (shell debris sands) as in Clements' (1969; 1973) log. They represent debris mostly from the brackish water bivalve Neomiodon . The shells were originally aragonite but have been replaced by calcite now. They have been accumulated by storms with easterly winds at the western margin of the Purbeck basinal area. The shell beds originated as extensive white shell sediments. Some of which were usually at or just below water-level and others formed shell beaches, above water-level for much of the time (El-Shahat and West, 1983). Water-levels fluctuated, probably seasonally and over longer periods in the seasonal subhumid climate, and dinosaurs left their footprints in the damp shell deposits which were sometimes cemented by carbonate at an early stage, thus favouring preservation.
9. The Scallop Member
These are hard, sandy, shell limestone or biosparrudite which contain Chlamys , a bivalve suggesting near-marine salinities. This unit indicates a significant marine incursion. Seawater entered the lagoon on a scale almost like that at the time of deposition of the Cinder Bed.
10. The Corbula Member
These limestones and shales are notable for a varied content of gastropods and bivalves. Apart from the small Corbula alata bivalves include Neomiodon , a brackish water genus which is a major constituent of limestones in the Middle Purbeck Formation. The oyster of the Cinder Bed, Praeexogyra distorta also occurs in this unit as does a " Pecten ". This unit originated in nearly marine salinities.
11. The Chief Beef Member
The Chief Beef Member is a dark shaly unit with beef that lies underneath this, and well above the Upper Building Stones (or Intermarine Member). The sequences consists mainly of organic-rich dark soft shale with bands of hard shell-limestone (biosparrudite) with Neomiodon. Beef is the form of diagenetic fibrous calcite that develops by diagenesis in organic-rich shales under burial and may show cone-in-cone structures. As usual, it is associated with aragonite which is usually the source of the carbonate for the diagenesis. There are layers of white, disintegrating, aragonitic Neomiodon shells. The organic rich shale here has preserved the aragonite to some extent, although elsewhere in the Purbeck Formation it has mostly been lost quite early. This is a brackish-water lagoon sediment.
12. The Broken Shell Limestone Member
This is a conspicuous biosparrudite, 2.9m thick at the type-section at Durlston Bay. It has been used for building stone purposes in the past and forms the Peveril Ledges. Unusual and as yet not satisfactorily explained circular depressions occur on the top surface.
13. The Unio Member
Immediately above the Broken Shell Limestone are thin-bedded limestones and shales with the freshwater bivalve Unio and with vertebrate remains, particularly of turtle and crocodile.
14. (At the top) The Upper Ostracod Clays and Shales Member
The uppermost part of the Purbeck Formation consists of shales with the ostracod Cypridea and some thin but conspicuous limestones. The Unio Bed is notable for only for the molluscs but also for its green colour produced by glauconite. Reworked glauconite occurs in this bed with other reworked detritus (a result of Late Kimmerian movements); more unusual is the authigenic glauconite occurring in this and other beds in this member. The Pureck Marble consists of two or three beds of Viviparus biomicrite, that is famous for its use as an ornamental stone in churches and cathedrals in historic times.
Remains of charophytes, the aquatic plants known as stoneworts, occur in parts of the Purbeck Formation usually in association with the ostracod genus Cypridea and with low salinity gastropods. The main horizon at which they occur is in the white limestone of the Cherty Freshwater Member of the Middle Purbeck Formation. The oogonia or "gyrogonites", to use an old word, are small ovoid bodies with spiral markings. Examples are shown in the left image (which is plate 10 from Harris, 1939) and can be obtained in silicified form by dissolving away appropriate semi-silicified parts of the Flint Bed (of the Cherty Freshwater Member). Specimens 1 to 12 are of Flabellochara grovesi (Harris) Grambast ("Clavator grovesi") and 13 to 18 are of the larger Clavator reidi. These images are of drawings on photographs ("improved" photographs).
They are not easily seen in the field. In the central image silicified stems are shown in cross-section and longitudinal section from bed DB 89, Cherty Freshwater Member, Durlston Bay. In the right images is a longitudinal section showing node and internode from the Cherty Freshwater Member, Worbarrow Bay, El-Shahat bed 22b. The cavities within the cortical tubes are infilled with sparry calcite while the walls of the stems are silicified. Both photomicrographs are from El-Shahat, 1977.
Cross sections like the central image can be seen in the field, although these need a good handlens, good light and a careful search of loose Cherty Freshwater limestone blocks on the beaches. The small, circular, 12-celled, cross-sections of stems may be seen, with luck, in fallen blocks of the white Flint Bed at Durdle Door, Stair Hole, Lulworth Cove, the Fossil Forest, Mupe Bay and Worbarrow Bay. They also occur, however, in the basal Purbeck at Portesham (in an unusual charophyte chert) and in the Upper Purbeck at the Poxwell road cutting, and there are various minor occurrences in the Middle Purbecks at Durlston Bay and elsewhere.
The charophytes are useful for stratigraphical correlation. They are useful for recognising low salinity horizons but there has been some discussion about their upper salinity tolerance. For palaeoenvironmental purposes, they could provide evidence on palaeosalinity and might be informative with regard to water-depth.
. Charophytes - Aquatic Plants - Introduction
Charophytes are small branching algae, normally living in carbonate-rich freshwater. Modern examples are known as "stoneworts" because they have a partial carbonate skeleton. In appearance they look like small subaqueous "horsetails". The three images above are of a small meadow-like assemblage of charophytes on the bottom of a shallow pond in a limestone environment at Guardiola de Bergueda in the Catalan Pyrenees. The water is only about 0.4 m deep and the water surface is indicated in the photographs by some traces of floating oil.
The Characeae or Charophyta are a strange and isolated group of aquatic plants growing entirely under water. Modern examples prefer ponds or lakes, although they are occasionally found in running water, and have a partiality for somewhat brackish conditions, such as freshly dug ditches in marshes near the sea. In overgrown waters they soon give way before more vigorous vegetation (angiosperms not present in Purbeck times), whilst in a newly formed pool they are frequently the first plants to appear (Allen, M.A. 1950, British Stoneworts (Charophyta), Haslemere Natural History Society, Arbroath. 52 pp.). They are recognisable without difficulty by their translucent green colour (somewhat greyish in some species dure to their marked afinity for lime), their flexibility and the whorled arrangements of the branchlets, as the lateral members are termed.
These are advanced forms of erect algae that can cover the entire bottom of a pond. They have a musky odour, and are consequently known as muskgrass, and can distintegrate to a powder of carbonate on drying out. They can therefore provide carbonate sediment. They promote water clarity, enhanced fish population and stabilise the lake floor. In clear still water masses of orange-red antheridia (the male reproductive organs) can be seen (Allen, 1950). The average height of these plants is from 0.30m to 0.46m, but a few are only several centimetres. Study of them is, therefore, microscopic.
. Purbeck Charophytes - Historical Review
For modern work on Purbeck charophytes see the important paper of Feist, Lake and Wood (1995). This provides key charophyte records in Sussex and correlation with the Dorset succession. It also provides an up-to-date set of references. For older work on charophytes of the Dorset Purbeck sequence the key publication is that of Tom Harris, Professor of Botany at Reading University, who published in 1939 "British Purbeck Charophyta" (British Museum, Natural History, 83 pp. + 17 plates). The following notes are summarised from a section of this publication.
The Purbeck charophytes were discovered by Edward Forbes. He recorded (1851) in the Middle Purbeck cherts of Bacon Hole, near Mupe Bay "for the first time in the Oolitic Series, gyrogonites, the spore vesicles of Characea". During the second half of the nineteenth century the stratigraphy of the Purbeck was studied by the staff of the Geological Survey. Vertical sections were published by Bristow (1857), where the main Charophyte horizon is clearly shown in the Middle Purbeck. The same bed with Charophytes was referred to in the Memoirs of the Geological Survey.
In 1890 Wethered described some "Chara" from the Middle Purbeck Chert collected at Lulworth. He figured some thin-sections through the chert in which the nodes and internodes of Clavator reidi are clearly recognisable, but did not give a botanical description.
Beside the references to the charophytes of the Middle Purbeck cherts, there have been records from the classic Fossil Forest locality near Lulworth. A map included in the Reid and Groves notes (referred to by Harris, 1939) bears a manuscript entry by C.D. Sherborn " Tree trunks with beds of chert containing Chara. Strahan (1898, p. 79) wrote " a seam of dark chert in the tufa (Purbeck "Cap") surrounding one of the tree stools in the cliff half a mile east of Lulworth is of purely freshwater origin. It contain Valvata naticoides in abundance and an occasional stem of Chara" and again (p.83) when discussing the Fossil Forest "tufas" (thrombolitic or stromatolitic limestones) " Though no plant structure is now recognisable in the limestone, Chara, a freshwater plant, which flourished both in purely fresh and slightly brackish water, is not uncommon in the chert-bands; the power of this plant to precipitate lime is well-known. "
The record of Valvata naticoides is probably sound but it is not clear evidence of any persistent freshwater conditions. In the same bed, the Soft Cap, on the central ridge of Stair Hole, the present author (IMW) has found chert nodules with pseudomorphs after small lenticular gypsum crystals with Hydrobia and with a small vug of celestite (strontium sulphate, commonly replacing calcium sulphate in the Purbecks). Gastropods and evaporites are reminiscent of the Portesham charophyte chert (West, 1961) and with the fluctuating salinities of the Purbecks charophytes are not out of the question at this level. I have not seen them myself, as yet, at the Fossil Forest.
Harris (1939) looked carefully for the charophytes of the basal Purbecks of the Fossil Forest but did not find any clear examples. The best indications were indeterminable fragments of cylindrical tubes, but tubules occur commonly in the basal Purbeck thrombolites and are probably not charophytes. More searching may resolve this mystery. A small low-salinity pool immediately above the Great Dirt Bed, the soil horizon, could have had charophytes, even though the general conditions following this were of a hypersaline lagoon precipitating gypsum (preserved as pseudomorphs in much of the chert).
The main charophyte remains of the Purbecks come from the Cherty Freshwater Member of the Middle Purbecks. Loose blocks of this lie on the Fossil Forest ledge, but as Harris (1939) pointed out, it is very unlikely that anyone could confuse this with basal Purbeck chert.
During the years 1913-1916 Reid and Groves made a considerable collection of Middle Purbeck chert and began to work out the fossils. They published a preliminary paper in 1916 in which the main features of Clavator reidi were brought out, but Reid's death brought the work to an end. Clavator reidi was referred to at some length by Groves in 1924a, 1924b and 1933; but these accounts add nothing important to the joint paper by Reid and Groves.
Harris used the material of Reid and Groves for further detailed study and, in addition, collected new material with the assistance of Professor Peter Sylvester Bradley. The book - British Purbeck Charophyta - by Harris (1939) presents the results of acid-etching and thin-section study of all these samples.
. Purbeck Charophytes - Occurrences
The following list is only a summary and is not claimed to be comprehensive, but it includes most of the known British Purbeck charophyte localities. Please refer to the individual publications for details. Particularly see Feist et al. (1995) for recent work and see Appendix 1 - Distribution of Charophytes in Boreholes in the Purbeck and Wealden of Southern England, and Appendix 2. - Distribution of Charophytes in Purbeck and Wealden Outcrops Southern England.
Most specimens from Dorset, listed here, are from the Middle Purbeck Cherty Freshwater Member, where they are very common. Some have been found at Durlston Bay in the higher but still Middle Purbeck, Intermarine Member. There are some at Durlston Bay in the Upper Purbeck Unio Member and Upper "Cypris" Clays and Shales. The equivalent of these horizons elsewhere have not been properly searched and may also yield them. Charophytes are surprising rare in the Marly Freshwater Member. They are absent in most of the hypersaline Lower Purbecks with an exceptional occurrence in the basal Purbecks at Portesham, and possibly also at the Fossil Forest. Charophytes are common in the Lower Purbecks (or possible Portland equivalent) at Swindon. Charophytes occur in some parts of the Purbeck Formation in the Vale of Wardour.
Portesham - ploughed fields - Cherty Freshwater Member? - C. reidi, F. grovesi (also A. clavatoris) Reid and Groves (1918), Harris (1939).
Portesham Charophyte Chert - Basal Purbeck - West (1961; 1975); Barker et al. (1975) - silicified "Clavator westi" (rare basal Purbeck occurrence of charophytes in Dorset. Compare with Swindon).
Near Coryates (Portesham District). Loose chert in ploughed field. - C. reidi - Harris (1939).
Friar Waddon, near Upwey - Cherty Freshwater Member - C. reidi, abundant, P. horrida common, F. grovesi rare, A. clavatoris rare. Anderson coll. Harris (1939).
Upwey Railway Cutting - Cherty Freshwater Member - C. reidi abundant, Algacites clavatoris rare - Sylvester Bradley and Harris Collection, Harris (1939). (also charophytes in limestones blocks west of Water Works)
Bug's Lane, Moign Down - Cherty Freshwater Mb, not in situ - SB and H.
Moigne Down by waterhole. - Cherty Freshwater Mb - Flabellochara grovesi (Harris) ("Clavator grovesi"). SB and H.
Osmington Mills (Bradley's Excavation) - Middle Purbeck limestone with chert - F. grovesi, C. reidi (P. horrida rare).
Osmington Mills (Bradley's Excavation) - Upper Purbeck above Unio Bed. - Gyrogonites like C. reidi but with 12 ridges in lateral view. Also F. grovesi, C. reidi stem.
Poxwell Road Cutting - PC 21, 26, 34, 36, 37, 43 - various horizons - Flabellochara grovesi (Harris) ("Clavator grovesi"), Perimneste horrida - probably mostly Middle Purbeck - see Sylvester Bradley's paper on Poxwell.
Durdle Door West - Cherty Freshwater Mb. - Flabellochara grovesi (Harris) ("Clavator grovesi") - Middle Purbeck chert - SB and H collection.
Durdle Door east - Cherty Freshwater Mb. - Flabellochara grovesi (Harris) ("Clavator grovesi") - Middle Purbeck chert and shale beneath. Harris (1939).
Stair Hole - Cherty Freshwater Mb. - stems sometimes visible in fallen Flint Bed material. (IMW).
Lulworth Cove, East - Cherty Freshwater Mb. - lower bed and top bed of Middle Purbeck chert. Harris (1939).
Fossil Forest - Cherty Freshwater Member - Cherty Freshwater Member, Flint Bed - SB and H. Also, uncertain record from the basal Purbeck Caps.
Bacon Hole - CFM? - beds 46 and 48 of Sylvester Bradley and Harris collection.
Worbarrow Tout - Cherty Freshwater Member. - Flint Bed - El-Shahat bed 22b = Ensom bed 114, - C. reidi - bed 64 and 66 of SB and Harris. (possibly Under Flint 112 and Flint Bed 114? ).
North of Anvil Point - Cherty Freshwater Member - C. reidi - Harris (1939).
Lower Purbecks - a large number of samples searched unsuccessfully - Anvil Point, Pondfield Cove and Gad Cliff, Bacon Hole, and Poxwell Lodge Quarry. Harris (1939).
Dorset - Durlston Bay
Upper "Cypris" Clays and Shales - DB 224.
Unio Member - DB 223 - Clements (1993).
Intermarine Member - DB 115, DB 116, top DB 129 - Clements (1993) .
Cherty Freshwater Mb. - 3 limestones - Harris (1939), DB 89, DB 97, DB 99, DB 89 - El-Shahat (1977), DB 90, DB 97, DB 99, DB 102, DB 106, - Clements (1993).
Marly Freshwater Mb. - DB 79 - Clements (1993).
Soft Cockle Member (near top), - DB 70 - Globator protoincrassata(Mojon) - Feist et al., 1995.
Surrey and Sussex
Warlingham Borehole, Broadoak Calcareous Member - Porochara maxima (Donze), <>Globator rectispiraleFeist , Globator praecursor(Mojon), Clavator reidi - Feist et al. 1995.
Fairlight Borehole, Broadoak Calcareous Member - Globator protoincrassata(Mojon) , Globator praecursor(Mojon), Clavator reidi- Feist et al. 1995.
Fairlight Borehole, Robertsbridge Faunicycle, Lower Purbeck, nine faunicyles above the upper limit of Cypridea dunkeri papulata. - Flabellochara grovesi (Harris) ("Clavator grovesi")
Swindon, basal Purbeck or Portland, exposure 3 of Sylvester Bradley - Swindon Series, Cythere Marls and Chara Marls - Clavator reidi, Feist et al., 1995.
Swindon, basal Purbeck or Portland, ranges through major part of Purbeck succession - Clypeator discordis Shaikin - Feist et al., 1995.
Vale of Wardour
Teffont Evias, Vale of Wardour - Middle Purbeck, TE 11, TE 13. - C. reidi - SB & H in Harris (1939).
Kings Cross, near Haddenham - "Chara" recorded by A.M. Davies (1899).
Purbeck Charophytes Zones
Shown here is a simplified diagram based on Feist et al. (1995), text-fig. 3., but with the probable position of the Caps and Broken Beds indicated. It must emphasised, however, that there is no direct charophyte evidence as yet from Dorset for the exact positioning of the Jurassic Cretaceous Boundary as shown here, and from various points of view this is still a disputable matter. There seems general agreement that it is not, as was once thought (Casey, 1963), at the Cinder Bed, but somewhere near the base of the Purbeck Formation. It is a pity that it is not yet known for certain whether the Caps and Dirt Beds are Jurassic or Cretaceous. Hunt (1985; 1987) on the basis of palynomorphs found a floral change near the base of the "Cypris" Freestone Member which he took to be the base of the Berriasian (the J-K boundary). Hoedemaeker's (1991) interpretation ties in with this. Allen and Wimbledon (1991) and Ogg et al. (1991), however, placed the Jurassic-Cretaceous boundary a little lower, at about the base of the Purbeck Formation. The matter is not yet resolved and further charophyte discoveries in the basal Purbeck would help. Further study of the Portesham charophyte chert and associated charophyte-bearing marls would probably clarify correlation. See Feist et al. (1995) for more detailed discussion of zones and correlation.
Purbeck Charophytes - Habitats and Water Depths
Usually modern charophytes grow in a depth of 0.3 to 0.6m. (Allen, 1950). In Swiss lakes where the water is very clear, though, they have been found as low as 18m in depth. A figure of 0.6m is much about the typical thickness of a Purbeck limestone bed. The Flint Bed, for example, at Durlston Bay is 0.84m and the Durlston charophyte-bearing bed DB 89 is 0.46m. The lack of thick lacustrine limestones, considered in association with the frequent occurrence of surface exposure features like desiccation cracks, pseudomorphs after halite and dinosaur footprints, suggests very shallow lakes and lagoons. The charophytes do not seem to provide direct evidence of water depth but in general their occurrence is compatible with the evidence for very shallow water conditions. This too would have suited the aquatic pulmoniferous gastropods of the Cherty Freshwater Member, which presumably browsed on charophytes and associated algae.
Harris (1939) commented the "great abundance of charophytes at the horizon of the Middle Purbeck Cherty Limestone [Cherty Freshwater Member] suggests that there was, in Dorset, a very large lake shallow enough (1 to 10m deep) for these gregarious plants to grow over large areas. The rather local occurrence of the different species suggests that they were deposited near the place where they grew." Some further comments of Harris (1939) on the habit of Clavator are as follow:
" Although much broken, the material of C. reidi provides some idea of the habit of the plants; C. grovesi [Flabellochara grovesi (Harris) ("Clavator grovesi") ] seems to have agreed with it, though rather larger in its vegetative parts. From creeping stems with short internodes erect stems of considerable length arose. These were provided with nodes at intervals of about 1 cm. from which whorls of six short leaves sprang, and often also a single lateral branch. In certain parts of the plant the leaves bore the reproductive organs, antheria or oogonia in a single file along their inner sides. The plant grew in a calcareous freshwater lake, and was heavily calcified, so that it must have been very hard while alive."
The comment " that there were erect stems of considerable length" is interesting. Unfortunately, the meaning of "considerable length" is not clear, except that it probably indicates that the plants grew to a greater height than the small modern examples shown above. Further studies might help in establishing the approximate water-depth in which the Flint Bed was deposited. Although not very thick, it is continuous across much of Dorset from Portesham to Durlston Bay and this is not true for most of the individual bed of the Purbeck. It resembles the near-marine Cinder Bed in having this continuity. The Flint Bed might have originated in water a little deeper than that of the other beds, not just about 0.6 m but possibly even as deep as 2 metres or more. The limited thickness of the bed and the occurrence of very shallow water and drying out indications in association makes a 10 metre depth unlikely. At the moment, though, water depth considerations are just speculation, but eventually some more definate evidence may be found.
Purbeck Charophytes - Seasonality
Many modern charophytes have a short season. They flourish in India and South Africa in pools that dry-up completely in hot weather (Allen, 1950). The oospores can remain viable for a long time. Allen considered that drying up favoured the growth of charophytes in the next wet season. They grow quickly.
The strange occurrence of pseudomorphs after halite in the Flint Bed of the Cherty Freshwater Member at the Fossil Forest and elsewhere is evidence of the drying out of the carbonate mud in a dry season. It is not known whether this occurred annually but in the seasonal climate of the Purbecks (Francis, 1984) it is very likely. The palaeolatitude of about 37 degrees north suggests that the rainfall was probably greater in winter. It is not clear when the charophytes grew. Did they flourish in the Spring before the lake dried out or almost dried out in late summer (although note that lack of red-bed facies suggests that the water table did not descend far below the sediment surface)? Finally, were the extensive subaqueous charophyte meadows browsed by any reptiles, and, if so, by which?
Purbeck Charophyte Palaeosalinities
Most Purbeck charophytes are associated with very salinity faunas, such as those of the Cherty Freshwater Member. Here there are usually pulmoniferous gastropods (pond snails) and ostracods such as Cypridea which are associated with such gastropods. As noted above, charophytes are rare in the Lower Purbeck strata which are dominantly hypersaline with evaporites here and there. They do not normally occur with gypsum but the Portesham charophyte chert (West, 1961; Barker et al. 1975) is an exceptional case. It can be explained by the seasonal conditions of the Purbeck environment in which a wet-season, low salinity fauna and flora was preserved in a dry-season gypsum crust. It should be noted, that the Flint Bed of the Cherty Freshwater Member, which contains most charophytes and has good evidence of a low salinity fauna, also has chalcedony pseudomorphs after halite (Meyer, 1872; Radley 1992). These lakes were obviously not completely free of sulphate or chloride ions.
Purbeck palaeoenvironmental analogues occur in South Australia. Some of these contain charophytes and are saline. They have been described by Burne, Bauld and De Decker (1980). They commented on a possible comparison with Purbeck charophyte-bearing sediments:
"The evaporite-bearing Purbeck Beds of Dorset contain a regressive cycle which differs from the sabkha cycle of Shearman (1966) in that it contains no units of subtidal or lagoonal origin and maximum regression is represented by 'dirt beds' or soils, which indicate deposition under semi-arid rather than desert conditions (West, 1975). In one of these 'dirt beds' at Portesham, West describes and association of charophytes and chert pseudomorphs after gypsum. Ostracods, molluscs, and land plants also occur in the bed. The original gypsum was in the form of discoidal crystals. West considered the environmental problems posed by this association and, although he was unaware at the time that some of the lakes adjacent to the Coorong do contain charophytes, concluded that the environment and hydrology of the coastal lagoons of South Australia, as described by Alderman and Skinner (1957), might explain the association of charophytes and evaporites. "
The authors mention that the palaeolatitudes of the Upper Jurassic of Dorset comparable to the present day latitude of the saline lakes of South Australia, which suggests a similar Mediterranean type climatic regime.
(Further discussion will be added later with regard to whether the Purbeck lakes with charophytes were almost freshwater but desiccating in summer with local development of a gypsum crust or some halite crystals or whether the lakes were to significantly saline. - More later!)
Purbeck Charophyte References
Allen, G.O. 1950. British Stoneworts (Charophyta). Haslemere Natural History Society. Arbroath, T. Buncle and Co. Ltd. 52 pp.
Barker, D., Brown, C.E., Bugg, S.C. and Costin, J. 1975. Ostracods, land plants and charales of the basal Purbeck Beds of Portesham Quarry. Palaeontology, 18, 419-436.
Burne, R.V., Bauld, J. and De Deckker, P. 1980. Saline lake charophytes and their geological significance. Journal of Sedimentary Petrology, 50, 281-293. Abstract: Charophytes are found in some empheral saline lakes in Australia. Healthy green charophytes, bearing oogonia, are frequently observed at salinities ca. 1-2 times that of seawater. Field observations of salinity tolerance are confirmed by experiments which demonstrate photosynthetic 14 C-carbon dioxide fixation at these salinities. The lakes containing these charophytes have various hydrological settings but all are influenced by inputs of continental groundwater of varying salinity. Different evaporite minerals are deposited depending upon local desiccation-evaporation balances. In coastal lagoons adjacent to the Coorong, charophytes are found associated with calcite, aragonite and protodolomite while in the continental lakes around northern Spencer Gulf the observed associated are with gypsum and halite. Lake deposits containing charophyte oogonia and discoidal gypsum provide a recent analog for some ancient evaporite units which are not adequately explained by the arid, coastal marine evaporite model based on the present-day Persian Gulf sabkhas. For example, the Purbeckian evaporites of Dorset (England) and the Tertiary evaporites of the Paris Basin (France) both contain charophyte- and evaporite-bearing units which are similar to the deposits of these Recent Australian lakes. We suggest that sediments containing associations of charophyte oogonia and evaporites formed as a result of varying salinity within lakes of the semi-arid, Mediterranean-type, climatic zone. (There are other papers by Burne on related topics that relevant to Purbeck palaeoenvironments).
Clements, R.G. 1993. Type-section of the Purbeck Limestone Group, Durlston Bay, Swanage, Dorset. Proceedings Dorset Natural . History Archaeological Society, 114 for 1992, 181-206. (The classic section log widely used as a reference for research on the Durlston Bay type section)
Colin, J-P., Feist, M., Grambast-Fessard, N., Cherchi, A. and Schroeder, R. 1985. Charophytes and ostracods from the Berriasian (Purbeckian facies) of Cala d'Inferno (Nurra region, NW Sardinia). Bolletin della Societa Palaeontologica Italiana, 23, 345-354.
El-Shahat, A. l977. Petrography and Geochemistry of a Limestone Shale Sequence with Early and Late Lithification: the Middle Purbeck of Dorset,England. Unpublished Ph.D.Thesis , University of Southampton. 295 pp. (on the type section of Durlston Bay)
Feist, M. and Batten, D. 1990. Comparitive charophyte and palynofloral biozonation of the British Purbeck and Wealden succession of southern England. 17-18 In: Proceedings of the International Symposium of the IGCP-245, Nonmarine Cretaceous Correlations. Alma-Atma, 72 pp.
Feist, M., Lake, R.D. and Wood, C.J. 1995. Charophyte biostratigraphy of the Purbeck and Wealden of southern England. Palaeontology, 38, Part 2, 407-442. Abstract: The distribution of charophyte assemblages in the Purbeck and Wealden sequence of southern England has been established from borehole samples from the Weald and from outcrop material collected in Dorset, Wiltshire and the Isle of Wight. Of the twenty-one taxa represented, three are new: Globator rectispirale, Clypeator britannicus and Sphaerochara andersonii; three new combinations are proposed: Globator praecursor, Globator protoincrassatus and Atopochara triquetra. The Chinese Valanginian species Flabellochara xiangyunensis is recognised for the first time in Europe. In the context of the phylogeny of the Family Clavatoraceae, G. rectispirale represents the Jurassic ancestor of the Globatorlineage and a separate origin is suggested for both Flabellochara and Clypeator. The correlation established with the Tethyan realm locate the Jurassic Cretaceous boundary within the Lulworth Formation of the Purbeck Group; in this context, the whole 'Purbeck' sequence of Swindon (Wiltshire) is attributed to the Upper Tithonian. The distribution of the Clavatoraceae indirectly confirms the contemporaneity of the Boreal Galbanites kerberus and Titanites anguiformis with the Tethyan 'Durangites' ammonite zones. For the Wealden Supergroup, the charophyte data affirm the Hauterivian-Barremian boundary near the upper division of the Weald Clay and the Upper Barremian is identified at the base of the Vectis Formation of the Isle of Wight.
Feist, M. and Schudack, M. 1991. Correlation of charophyte assemblages from the non-marine Jurassic-Cretaceous transition of NW Germany. Cretaceous Research, 12, 495-510.
Forbes, E. 1851. On the succession of strata and distribution of organic remains in the Dorsetshire Purbecks. Reports of the British Association for the Advancement of Science (1850), Abstracts, pp. 79-81.
Groves, J. 1924a. Clavator Reid and Groves. Journal Botany, 62, 116-117.
Groves, J. 1924b. A sketch of the geological history of the charophyta. In: Groves, J. and Bullock-Webster, G.R. British Charophyta, vol. 2, pp 72-90. Ray Society, London.
Groves, J. 1933. Charophyta. Fossilium Catalogus, 2, Plantae, pars 19, 74 pp. Berlin.
Groves, J. and Bullock-Webster, G.R. British Charophyta, vol. 2, pp 72-90. Ray Society, London.
Harris, T.M. 1939. British Purbeck Charophyta. British Museum (Natural History). London, Printed by Order of the Trustees of the British Museum, Issued April 22nd, 1939. 83 pp + 17 plates. (by Professor Thomas Maxwell Harris, Professor of Botany in the University of Reading - Prof. Tom Harris, well-known for studies of Mid. Jurassic plants).
Platt, N.H. 1991. Lacustrine carbonates and pedogenesis: sedimentology and origin of palustrine deposits from the Early Cretaceous Rupelo Formation, W. Cameros Basin, N. Spain. Reprinted from Sedimentology, 1989, 665-684. Pages 323-342 in the present book. In: Wright, V.P. and Tucker, M.E. (1991) Calcretes. Int. Ass. Sedimentol., Reprint Series, 2, 352. Blackwell Scientific Publications. Oxford. ( Charophytes, ostracods, gastropods and rare vertebrates. Silicified evaporites found near the top of the sequence. Stable isotope analysis. del 13C for -7 to -11 and del 18 O from -3 to -7.5. Palustrine limestones formed through pedogenic modification of lake carbonate. Low gradient, low energy, unstratified lake. Berriasian. Dark intraclasts like black pebbles.)
Reid, C. and Groves, J. 1916. Preliminary report on the Purbeck Characeae. Proceedings of the Royal Society, London, B, 89, 252-256.
Strahan, A. 1898. The Geology of the Isle of Purbeck and Weymouth. Memoirs of the Geological Survey, England and Wales. 278pp.
Sylvester-Bradley, P.C. 1941. The Purbeck Beds of Swindon. Proceedings of the Geologists' Association, London, 51, 349-372. (Records of Clavator reidi, Flabellochara grovesi (Harris) ("Clavator grovesi") etc in basal Purbeck or possibly Portland equivalent beds).
Sylvester-Bradley, P.C. 1949. A section of the Purbeck Beds at Poxwell. Proceedings of the Geologists' Association, London, 60, 151-3. (Middle Purbecks with charophytes etc. Parts of this are still exposed).
West, I.M. 1961. Lower Purbeck Beds of Swindon Facies in Dorset. Nature, 190, p. 526.
Wethered, E. 1890. On the occurrence of fossil forms of the genus Chara in the Middle Purbeck Strata of Lulworth, Dorset. Proceedings of the Cotteswold Nat. Field Club, 10, 101-103.
Bibliography, References and Internet Links on Purbeck Formation
Go to Bibliography and References relating to the Purbeck Formation.
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