Purbeck Beds Research Articles

THE SANDRINGHAM SANDS OF NORFOLK

Summary Improved knowledge of the stratigraphy and palaeontology of the Sandringham Sands of Norfolk permits the recognition of four divisions, named in ascending order: Roxham Beds (Middle Volgian), Runcton Beds (Upper Volgian), Mintlyn Beds (Ryazanian) and Leziate Beds (Valanginian). These marine strata span the Jurassic–Cretaceous boundary and represent the interval during which the topmost Portland Beds, the Purbeck Beds and the Hastings Beds of southern England were accumulated. The Sandringham Sands rest disconformably on the Kimmeridge Clay and nodule-beds and erosion surfaces mark the lower and upper limits of the Runcton Beds. Palaeontologically the Mintlyn Beds can be divided into a lower set of strata with Hectoroceras (unique in western Europe), preserved mainly at West Dereham, and an upper set with Surites and allies found in the King’s Lynn area. Strata above the Leziate Beds formerly included in the Sandringham Sands are united with the Snettisham Clay under the new name Dersingham Beds, which comprise an interfingering complex of marine Hauterivian–Barremian sands, silts and clays roughly equivalent in age to the Weald Clay of the south. Interpretation of the whole sequence is complicated by lateral changes in lithology, intraformational erosion and by regional overstep of the Carstone.

Palynology of the Jurassic‐cretaceous boundary in southern England

Abstract Miospore assemblages recovered from marine Upper Kimmeridgian and Portlandian and from nonmarine and lagoonal Purbeck Beds (which span the Jurassic‐Cretaceous boundary) in Dorset and Sussex are dominated numerically by Classopollis and/or Inaperturo‐pollenites and show a progressive diversification of species with time. Three microfloral suites containing a total of 77 species are recognized. Each suite is characterized by the entrance of new forms and by the frequencies of constituent species. The oldest suite is dominated by coniferalean pollen with less frequent pteridophyte spores and rare cycadalean pollen. The succeeding suites record a progressive diversification of pteridophyte and bryophyte spore types which are not, however, as abundant as coniferalean pollen. The over‐all species composition of the suites is believed to be controlled by evolutionary or large‐scale phytogeographic factors; but the detailed composition of each assemblage is controlled, in part, by local ecological‐enviro...

Detrital Portland chert and limestone in the Upper Purbeck Beds at Friar Waddon, Dorset

SUMMARYPebbles of derived Portland chert and limestone have been found in the Upper Purbeck Beds at Friar Waddon, Dorset. The Unio Bed at this locality is of unusual lithology with derived phosphatic fossils and with fish teeth. The presence of this chert in the Upper Purbeck Beds and its apparent absence in Lower Purbeck pebble beds suggests that a major phase of silicification in the Portland Stone took place at about Middle Purbeck times. Furthermore, erosion of the Portland Beds occurred in the Weymouth region, probably on the crest of the Weymouth anticline, during Upper Purbeck times.

Structures in the Upper Purbeck Beds at Peveril Point, Swanage, Dorset

AbstractStructures in the Upper Purbeck Beds at Peveril Point, Swanage, Dorset, are described. Interpretation of the overall structural pattern has been attempted by constructing a three-dimensional model of the Broken Shell Limestone surface, upon which the more complex folding of the shales and marbles has taken place. The regional setting of the structures is discussed.

Calciostrontianite in the basal Purbeck beds of Durlston Head, Dorset

SummaryA spherulitic mineral occurring as veins and nodules in celestine-bearing secondary limestones in the Lower Purbeck Beds at Durlston Head, Swanage, Dorset, is identified as calciostrontianite, a mineralnot previously recorded from Britain. Textural relationships indicate that it was formed from the alteration of celestine and was in part converted back into celestine. The calciostrontianite originated as an epigenetic mineral during the tectonic movement that gave rise to the Broken Beds.

Notes on some Ferruginous Strata in Buckinghamshire and Wiltshire

AbstractThe name Whitchurch Sands is proposed for patches of ferruginous strata scattered for 75 miles along the western edge of the Cretaceous from Stewkley, Buckinghamshire, to the Vale of Pewsey, in Wiltshire. They are interpreted as the vestiges of a marine-brackish formation of Middle Purbeck age indicating the transgressive front of an advancing sea thought to have come from the north and which left its mark farther south in the Cinder Bed of Dorset. In the past they have been regarded as Lower Greensand, Wealden, or Portland Sand. In places, e.g. Quainton, Buckinghamshire, they are overlain by undoubted Lower Greensand (Seend Ironsand) of Upper Aptian date. Both in faunal characters and in stratigraphical relations the Whitchurch Sands have much in common with the Serpulite of Hanover and the basal “Wealden” of the Bas Boulonnais.

Depositional Environment of Purbeck Formation

Depositional Environment of Purbeck Formation

Stratigraphy and structure of the Purbeck inliers of Sussex (England)

The Purbeck Beds exposed in Sussex are redescribed together with new sections exposed in gypsum mines workings and boreholes. Comparisons are made with Purbeck Beds elsewhere in southern England in boreholes and at outcrop. A new map of the Purbeck inliers is presented and the structure is discussed. Conditions of sedimentation are illustrated by isopachyte maps.

The structures in the Jurassic and Cretaceous Rocks on the Dorset Coast between White Nothe and Mupe Bay

The structures associated with the steep, northward inclined Portland and Purbeck Beds, and the nearly vertical or overturned Wealden Beds and Chalk, exposed on the Dorset coast between White Nothe and Mupe Bay, form the subject of an important paper by W. J. Arkell (1938). Arkell thought that the structures occurred on the steep north limb of a major monocline which was formed by lateral pressures from the south. In the present paper the minor structures in the Purbeck Beds are described in detail and it is shown that they can be related to the development of a fold formed by the increasing northward dip of the underlying massive Portland Beds. The direction of overturning of the minor folds indicates movement of the beds down the dip slope and not upward drag as postulated by Arkell. The formation of the sets of shear planes and faults recognised by Arkell in the Chalk is considered, and it is suggested that they can be divided into two genetic groups. The first group of structures formed during the closing of the syncline, while the second group brought about its modification by thrust displacements on southward inclined thrust planes and late faults. Arkell's conclusions concerning the causes of the reduction in the thickness of the incompetent Wealden Beds are summarised briefly, but by the construction of cross-sections of the main structure, it is demonstrated that the missing portions of the Purbeck and Wealden successions could have been displaced only by thrust movements on southward inclined major faults. A stage-by-stage synthesis of the development of the structures is presented and it is suggested that they were formed as the result of the accommodation of the blanket of largely unconsolidated sedimentary beds above a major thrust fault in the basement. At first a broad flexure developed and this became most pronounced in the highest division, the Chalk, because of the buckling and northward sliding of these beds off the rising land to the south. As the magnitude of the fault increased, successively higher beds fractured and eventually the fault extended to the surface and brought about the displacement and modification of the structures formed during the earlier phase of folding.

Ostracods from the Purbeck-Wealden Beds in Bornholm

An account of the Purbeck-Wealden beds in Bornholm and a brief summary of similar beds in the Scanian-Pomeranian area are given.Two faunas are described from the Jydegaard Formation. A Cypridea-fauna is correlated with Wealden 3 in Northwest Germany. It is followed by an ostracod-assemblage, known from Northwest Germany, indicating brackish-marine environments corresponding to Wealden 3 or 4 of the German subdivision. Two new ostracod species are described.Two other faunas have moreover been identified, a brackish one in the Rabekke Formation and a brackish-marine one in the inlier at Salene. The former contains elements suggesting a Middle Purbeck age.

The beds between the Kimeridge and Gault Clays in the Thame—Aylesbury neighbourhood

Some twenty square miles of the Thame Valley consist of Sandy Upper Kimeridge, Portland, Purbeck and Lower Cretaceous Beds. All the beds are thin and condensed. The Upper Kimeridgian Thame Sand passes laterally into silty Hartwell Clay; the Thame Sand is lenticular, and its base diachronous. Lateral variations of the lowest Portland beds, Glauconitic sand and Rubbly Limestone, are described, the two succeeding units of the Portland beds, the Crendon Sand and Creamy Limestone, being more uniform. The very thin Purbeck Beds have a wide outcrop. The unfossiliferous, ferruginous Lower Cretaceous Beds are divided into two formations; freshwater, mainly fine-grained, Wealden Beds which generally follow the Jurassic Beds conformably, overlain unconformably by coarse-grained marine Lower Greensand. North-south faulting, tilting and erosion took place in the Wealden-Lower Greensand interval. It is suggested that the Wealden Beds are more likely to be of Upper than Lower Wealden age, while the Lower Greensand is probably of Upper Aptian or lowest Albian age. Further erosion preceded the deposition of the Lower Gault Clay. Post-Gault east-west faults and warps, a sequence of river terraces 25, 50 and 100 feet above the River Thame, and late-glacial solifluxion deposits are described.

Correlation of the upper purbeck beds of england with the German Wealden

AbstractCertain ostracod species in the Upper Purbeck of England show a succession of morphological changes which correspond to those described by Wolburg from the Wealden of North‐West Germany. Thus stage by stage correlation is possible between the deposits in the two countries, and it is shown that the Upper Purbeck Beds in England correspond to most of the German Wealden W3 together with all of W4.

Lower Purbeck Beds of Swindon Facies in Dorset

IN the course of a yet unpublished study of the pseudomorphs after gypsum prevalent in the ‘Purbeck Caps’ and ‘Broken Beds’ of Lower Purbeckian age in Dorset, a marl has been discovered with a fauna resembling that of the freshwater facies of the ‘Swindon Series’. In the well-known Portisham quarry with the ‘fossil elephant’ (a silicified tree trunk1), the three-foot “impure marls with seams of chert” listed by Woodward1 as occurring 9 ft. 6 in. above the Portland Stone have been found to contain the Swindon ostracod Ulwellia papulata Anderson, and well-preserved charophytes in abundance. Gastropods are also abundant and perhaps have Swindon affinities. None of these has been recorded before in British strata of indisputably Lower Purbeck age, and a full palseontological investigation of the fauna is at present proceeding.

On the Occurrence of Celestine in the Caps and Broken Beds at Durlston Head, Dorset

An occurrence of celestine in the Lower Purbeck Beds at Durlston Head is described and is shown to be of syngenetic origin. Deposition probably took place near the margin of a basin of evaporite deposits into which strontium solutions were introduced. At a later date the celestine has been involved both in the brecciation that formed the Broken Beds and in complex local faulting.

The Ostracod genus Cypridea and the zones of the Upper and Middle Purbeckian

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.

The gastropods of the Purbeck Beds

Isolated occurrences of genera such as Viviparus and Valvata are known in Bathonian, Liassic and more doubtfully even earlier rocks, but the earliest assemblage of a dozen genera of unequivocally freshwater molluscs is found in the wonderful fauna of the Purbeck Beds. Attention was first called to it by Thomas Webster (1816, pp. 191–2) (see below, p. 81). His remark, “It is rather surprising that this very ancient freshwater formation should not have excited more attention,” might almost be said to be still true; for since J. de C. Sowerby figured a couple of species of Viviparus from the Purbeck Beds in 1826 and six species of lamellibranchs in that year and in 1836 (in Fitton's memoir), no further Purbeckian mollusca have. been adequately figured in this country or described in the English language. Edward Forbes was preparing a monograph on the invertebrate fauna of the Purbeck Beds when his work was cut short in 1854 by his death at the age of 39. All that appeared was a preliminary account, in which the genera Viviparus, Valvata, Lymnaea, Planorbis, Hydrobia, Physa, Melania, Cyclas , and Unio were recorded, and also many marine genera, but no species were mentioned. A number of MS. names were introduced by him on labels and in the Survey catalogues, and some of them have been used by other authors, but if any manuscript or type specimens existed they have disappeared. In 1856 Osmund Fisher published a detailed account of the stratigraphy of the Dorset Purbeck Beds

Notes on the age of the Swindon Purbeck Beds

Notes on the age of the Swindon Purbeck Beds

Ostracoda from the Portland and Purbeck Beds at Swindon

Ostracoda from the Portland and Purbeck Beds at Swindon

Three Tectonic Problems of the Lulworth District: Studies on the Middle Limb of the Purbeck Fold

Of all the folds thrown up across the Chalk plains of North-West Europe during the Alpine orogeny, the most interesting and the most perfectly exposed is the Purbeck anticline. In the cliffs of the Dorset coast the sea has laid bare all parts of the structure, from the core or crestal region, through the vertical and overturned middle limb and the “foresyncline” (Busk 1929, p. 29), into the foreland, where the Upper Cretaceous rocks are seen reposing with sharp unconformity upon a previously folded and eroded Jurassic foundation. The general and some special aspects of the fold and its associated faults have been discussed in a previous paper (Arkell 1936). The present paper results from a study of the middle limb, where it is dissected by the sea cliffs of the Lulworth district. These cliffs have long been renowned for their beauty, but their marvellous tectonic features, both great and small, have been neglected, with the result that their value to students of tectonics has not been fully utilized. The first three parts of the paper attempt a solution of the three most outstanding problems : contortions in the Purbeck Beds, the Purbeck Broken Beds, and the abnormal attenuation of the strata towards Durdle Door. The fourth part offers an interpretative essay and synthesis. The area dealt with is included in sheet 342 of the Geological Survey one-inch map and sheet 141 of the Ordnance Survey one-inch map. The contortions in the Purbeck Beds at Stair Hole are illustrated in

A Peneplain and Re-excavated Valley Floors in Dorsetshire

THE geological formation in the district around the ancient village of Worth Matravers (Geological Survey Map, “ Swanage,” Sheet 343), situated about ¾ mile from the Dorset coast, consists of Lower and Middle Purbeck Beds (Upper Jurassic). A considerable portion of these beds has been removed by denudation, with the result that the Upper Purbeck division has not been preserved near the coast, except westward at Worbarrow Bay and Lulworth Cove, and eastward at Swanage, the exposures at these localities being due to the high northerly dip. The Portland Stone immediately underlying the Purbeck Beds forms a line of inaccessible sea-cliffs extending from Winspit, one mile south-south-east from Worth Matravers, to Durlston Head, 4¼ miles in a direct line eastward.