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.
Integrated field surveys, continuously cored boreholes and seismic-reflection surveys in Fenland and the adjacent marine embayment of The Wash have revealed a network of concealed valleys. These are interpreted as a pre-glacial, cold-climate fluvial system that entered the North Sea via a gap in the Chalk escarpment at the mouth of The Wash. The pre-glacial Fenland valley system was overridden by an ice sheet during the Anglian Stage (Marine Isotope Stage (MIS) 12) when it became infilled with glacial deposits, mostly till and varved clay. East of the Chalk escarpment, a pre-glacial network of dipslope valleys drained directly to the North Sea. This was also overridden by Anglian ice and locally over-deepened, presumably by subglacial meltwater. The over-deepened parts have been referred to as tunnel valleys by various authors, but they can be more accurately described as glacially modified fluvial valleys. The analysis of high-resolution seismic-reflection profiles in the North Sea and adjacent areas has revealed complex anastomosing networks of tunnel valleys that were formed subglacially without a fluvial precursor or superimposed on possible fluvial systems that were glacially modified to the extent that they cannot now be confidently identified as fluvial in origin. In comparison, the concealed fluvial valleys of East Anglia and Fenland have been subject to minor glacial modifications, and are morphologically distinct from the North Sea examples. They should not be referred to as tunnel valleys. Prior to the Anglian glaciation, the rivers that flowed from the Midlands to the North Sea passed through one of two gaps in the Chalk escarpment, the Goring Gap in the south and a similar gap that is now concealed beneath the Wash. The field evidence does not support the presence of an additional gap in the intervening Bury St Edmunds area. A wide embayment to the north of this in the Thetford to Swaffham area in which the crest of the Chalk escarpment has been displaced eastwards by up to 20 km is an Anglian glaciation erosion feature.
The Lyme Regis (1901) Borehole was one of numerous coal-exploration boreholes
drilled in southern England during the late 19th and early 20th centuries. It is one of
the few deep boreholes (>200 m depth) in the east Devon-west Dorset area and,
unlike more recent hydrocarbon-exploration boreholes, was continuously cored. The
borehole was sited [NGR SY 3364 9297] on the floodplain of the River Lim on the
outcrop of the Jurassic Blue Lias Formation, and was continuously cored to a final
depth of 396.85 m within the Triassic Mercia Mudstone Group. Selected samples and
some of the cores were examined by the Geological Survey geologists Jukes-Browne
and Woodward who were working in the area at the time of drilling. The former
published a description of the succession based on his and Woodward’s notes and the
driller’s log, and correlated it with the succession of Triassic and Jurassic rocks that
are almost wholly exposed in the cliffs between Sidmouth and Lyme Regis. A recent
revision of the stratigraphy of the coastal successions has enabled that proved in the
borehole to be reassessed and placed more accurately into its regional stratigraphical
context.
The unglaciated part of southern England was subject to periglacial climates that lasted for more than 20 ka on at least eight
occasions during the last 750 ka. There are large outcrops of Jurassic mudstones in south-west England, principally the Lias Group
and the Oxford Clay and Kimmeridge Clay formations, and extensive exposures of these beds in the cliffs on the Dorset coast.
Notwithstanding the susceptibility of this type of mudstone to permafrost damage and deformation, there is no published record
of large-scale folding in the region that has been attributed to periglacial disturbance. Three examples of folding are described
here, in the Lias Group at Charmouth and Seatown in west Dorset, and in the Kimmeridge Clay on the Isle of Portland that are
attributed to intermittent downhill creep of surface layers up to 20 m thick when in a partially frozen condition. The style of folding
in the mudstones and the geometry of the disturbed deposits indicates that they are not tectonic in origin, nor were they formed
by valley bulging or landsliding. These are the first large-scale structures of their kind to be recorded in southern England: similar
folds elsewhere have been interpreted as valley bulges or tectonic in origin. At the Seatown and the Isle of Portland localities, the
deformed mudstones have been preserved beneath younger landslides. The absence of similar structures elsewhere on the Dorset
coast is attributed to the rapid removal of similarly weakened materials by marine erosion at times of high sea level during the last
c. 6000 years.
Abstract At its maximum development in the type area on the Devon coast, the Upper Greensand Formation comprises up to 55 m of sandstones and calcarenites with laterally and stratigraphically variable amounts of carbonate cement, glauconite and chert that were deposited in fully marine, shallow-water environments. The formation is divided into three members, in ascending order the Foxmould, Whitecliff Chert and Bindon Sandstone, each of which is bounded by a prominent erosion surface that can be recognised throughout the western part of the Wessex Basin. The full thickness of the formation, up to 60 m, was formerly well exposed in cliffs in the Isle of Purbeck in the steeply dipping limb of the Purbeck Monocline. The upper part of the succession is highly condensed in comparison with the Devon succession and exhibits lateral variations over distances of hundreds of metres that are probably related to penecontemporaneous fault movements. Much of the fauna is not age-diagnostic with the result that the ages of parts of the succession are still poorly known. However, the Isle of Purbeck sections contain diverse ammonite faunas at a few stratigraphically well-defined levels that enable the succession to be correlated with that of east Devon and west Dorset.
The Upper Greensand of south-west England can be divided on bulk lithology into two roughly equal parts, each 25 to 30 m thick.
The lower part, the Foxmould Member, consists of weakly cemented glauconitic sandstones with low carbonate contents. The
member weathers, largely by oxidation, to soft, loose, yellow and foxy brown sands. In contrast, the overlying Whitecliff Chert
and Bindon Sandstone members consist of calcareous sandstones and sandy calcarenites with numerous chert-rich horizons.
Dissolution, particularly during the warm humid climates of the Eocene and the periglacial climates of the late Pleistocene, has
been the dominant weathering process in these two members. Karstic features observed on the east Devon and west Dorset
outcrops include widespread pervasive dissolution that has locally reduced the in situ thickness of the Whitecliff Chert and Bindon
Sandstone members to less than half their original thickness, along with deep solution pipes, and at one locality, caves. These
discrete solution features occur beneath a thick capping of Chalk that is not markedly affected by dissolution. Over much of east
Devon and west Dorset, the residual loose sands and chert blocks derived from the dissolution of the Upper Greensand were
remobilised during the late Pleistocene to form extensive Head deposits.
The mid to late Triassic Mercia Mudstone Group exposed on the east Devon coast between Sidmouth and Seaton consists of
c. 450 m of predominantly red mudstones that were deposited in low-relief sabkha environments in hot deserts. In marked
contrast to this, the Dunscombe Mudstone Formation in the middle part of the group consists of 35-40 m of interbedded and
interlaminated green, purple and grey mudstones, breccias, muddy limestones and lenticular siltstones/sandstones that were
deposited in a wetter, possibly cooler climate. The formation is poorly exposed inland, but the striking colour difference from that
of the adjacent formations enables its outcrop to be traced more or less continuously from the Devon to Somerset coasts. A detailed
study of the sedimentology and ichnology of the formation, in particular that of laterally impersistent arenaceous members in its
lower part, has shown that it was deposited in a succession of shallow, freshwater lakes in a low-relief topography that was at
times crossed by broad shallow distributary channels. A combination of palynology and magnetostratigraphy is interpreted to
represent that the Dunscombe Mudstone Formation is a condensed succession that occupied most of the late Triassic Carnian Stage,
about 11.5 million years. It was deposited at a time of active tectonic subsidence, as a result of which it is laterally highly
variable in thickness. In its extensive subcrop in the Wessex and Bristol Channel basins it locally reaches over 150 m in thickness
in the more rapidly subsiding graben areas by the addition of halite. The northern boundary of the depositional area roughly
followed the Variscan Front, along the line of the present-day Mendip Hills and South Wales coast.
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