Bovey Basin Research Articles

About a basin—The Palaeogene pull‐apart fluvial Bovey Basin, South Devon, UK

The Bovey Basin in south Devon, UK, provides an instructive example of a Palaeogene pull‐apart basin with a complex fluvial sedimentary fill. The basin, one of three that lie along the NW‐SE‐trending Sticklepath Fault Zone, is filled by about 1200 m of sediment, of which about the lower 700 m (mainly lower Bovey Formation) is concealed. The main Bovey Basin is separated from a smaller sub‐basin (the Decoy Basin) south of Newton Abbot. Two main lithofacies are present in the Bovey Formation: lignite‐clay; and clay‐sand, with subordinate lithofacies of red‐mottled clay and sand; and gravel and conglomerate. The upper 500 m or so of the sedimentary infill (middle and upper Bovey Formation) is divided into 10 members. Deposition in the basin was by a variety of fluvial processes, notably floodplain clays and sands deposited by meandering rivers; forest swamps with associated short‐lived shallow lakes; and (mainly in the upper Bovey Formation) braided stream sands and gravels that formed mainly on alluvial fans. Lignites in the basin formed from plants that grew in place in forest swamps, a revision of the long‐held view that the lignitic material was transported into the basin from forests of the conifer Sequoia couttsiae growing outside the basin on adjacent uplands. Newly discovered ichnofabrics occur in some lignitic sequences and probably represent root traces (rhizoliths).

Terrestrial carbon isotope stratigraphy of the Eocene–Oligocene transition, Petrockstowe and Bovey basins, Devon, UK

The terrestrial sediments of the Petrockstowe and Bovey basins in Devon, UK were examined. Their age is considered to be Eocene and Oligocene. The sediments (kaolinitic clays, silts, sands, gravels, and lignites) from both basins were analysed for carbon isotopes of organic material, in conjunction with total organic carbon and palynological analyses used to unravel the type of and provenance of organic matter present. Within the Petrockstowe Basin, the lowermost interval examined shows a palynological distribution dominated by phytoclasts, whilst the upper part of the core is dominated by higher concentrations of palynomorphs (up to 90 %) and an increase in amorphous organic matter consistent (up to 37 %) with a change from sand-filled fluvial channels followed by an ephemeral lake or lake margin setting. Our palynological data from the South John Acres Lane Quarry section, Bovey Basin, show that within the lignites palynomorphs are high again (up to 95 %) consistent with them representing more ephemeral lakes or lake margins periodically exposed with mires. Our palynological data set further allows us to determine that isotope trends are not overly determined by the source of carbon in the basins. Our study suggests that the observed patterns were primarily produced by variations of the isotope ratios of terrestrial atmospheric carbon reservoirs. Even with our less than well constrained biostratigraphical control, the data indicate that the carbon isotope excursions seen in the Eocene and Oligocene could be associated with several transient carbon isotopic shifts (associated with the Paleocene–Eocene Thermal Maximum). Our findings therefore appear to lend support to the surface ocean and atmosphere behaving as coupled reservoirs at this time.

The North Sea Silverpit Crater: impact structure or pull-apart basin?

The Silverpit Crater consists of a set of concentric faults up to 20 km in diameter in the southern North Sea. Although formerly described as the first impact structure to be recognized in the UK, its spatial association with local tectonic and stratigraphic features suggests that it is unlikely to have been caused by a random extra-terrestrial event. This paper proposes a terrestrial origin for the Silverpit Crater as a Palaeogene pull-apart basin linked to strike-slip faulting in the Carboniferous basement. Faulting and thinning of Permian evaporites underlying the crater acted as a buffer to basement extension and caused both a flexural collapse of the overlying Triassic strata and circumferential listric faulting in the Cretaceous succession. Reactive diapirism in Lower Jurassic shales may have contributed to the formation of a central uplift at base Cretaceous level. The paper compares the tectonic setting of two contemporaneous strike-slip-related structures in southern England (Bovey Basin in Devon and the Compton Valence structure in Dorset) with that of the Silverpit Crater, and concludes by briefly considering whether a pull-apart model can help to explain the formation of Upheaval Dome, a similar putative impact structure in the Canyonlands National Park of Utah.

Strike-slip relay ramps

Areas of reorientated bedding at contractional oversteps between strike-slip faults are here called strike-slip relay ramps. Metre-scale examples are described from the Jurassic sediments at East Quantoxhead, Somerset U.K. Larger strike-slip relay ramps occur in the Rio de Peixe Basin, NE Brazil, along the Newport-Inglewood Trend, California, and in the Bovey Basin, SW England. Although the geometry and development of strike-slip relay ramps are similar to those of relay ramps in normal fault systems, there are differences in the structures which accommodate the transfer of displacement between the overstepping faults. Whereas strike-slip relay ramps are typically transpressional, with pressure solution often occurring, relay ramps in normal fault systems are dominated by extension or transtension. Care needs to be taken when interpreting areas of reorientated bedding between overstepping faults, particularly when displacement directions are unknown, for example when using seismic data. This is because relay ramps can occur in both strike-slip and normal fault zones.

95/00001 Along-strike variation in geochemistry and mineralogy of Oligocene lignite from the Bovey Basin, UK

95/00001 Along-strike variation in geochemistry and mineralogy of Oligocene lignite from the Bovey Basin, UK

Along-strike variation in geochemistry and mineralogy of Oligocene lignite from the Bovey Basin, UK

Mineralogical and geochemical data are presented for samples taken over a 200 m lateral interval from a lignite seam in the Tertiary Bovey Basin, Devon. The proportions of the minerals present vary erratically along the strike. The coefficients of variability of the elements vary markedly. In order from the most to the least variable, these elements are Cu, Zn, Sc, Rb, Ti, Zr, Ba, Nb, Y, Ni, Cr, V, Fe, Mn and Sr. Most of the elements studied are strongly correlated with ash content, Rb, Nb, Ti, Zr, Y, Sr, Ni, V, Ba, Sc and Cr positively, but Cu, Zn, Mn and Fe negatively. Interpolation of geochemical results between boreholes should be done with extreme care.

A Tertiary basin at Dutson, near Launceston, Cornwall, England

A previously undescribed, partly fault-bounded basin contains Palaeogene fluvial and ?fluvio-lacustrine kaolinitic clays, lignitic clays and clayey sands comparable to the Bovey Formation in nearby Tertiary basins in SW England. One sample from a 9.35 m borehole yielded a good Upper Eocene pollen and spore assemblage corresponding in age to part of the mainly concealed lower Bovey Formation in the Bovey Basin, to shelf and lower axial-trough sequences in the Petrockstow Basin, and possibly to part of the lower sequence in the Stanley Banks Basin. The clay mineralogy of 28 samples suggests derivation from immature tropical weathering profiles developed on Upper Palaeozoic sediments. The sediments are more closely comparable with the Petrockstow Basin deposits than with the known Bovey Basin sequence.

The nature and origin of sesquiterpenoids in some tertiary fossil resins

Analyses of two Tertiary fossil resins and a Tertiary montan wax have revealed the presence of a number of sesquiterpenoids. Using gas chromatographic retention data obtained on both polar and non-polar stationary phases and mass spectral comparisons with authentic compounds, the sesquiterpenoids α-cedrene, cedrane, cuparene, cedren-10-one, 8βH cedran-9-one and cuparenic acid have been identified in the fossil resin retinellite and a montain wax (both Oligocene) from lignite beds in the Bovey basin of Devon, England, and in the Pliocene fossil resin ionite from the lone valley of California. U.S.A. The sesquiterpenoids α-cedrene, cuparene and cuparenic acid are characteristic constituents of the essential oils of the Cupressaceae, a family of trees and shrubs of the order Coniferales, and it is argued that these fossil resins, and the montan wax, were derived from older forms of this family of plants. The ketones cedren-10-one and 8βH cedran-9-one, and the saturated hydrocarbon cedrane, are not reported to be constituents of modern genera of Cupressaceae and may have been formed during diagenesis. The cedrane occurring in these deposits appears to be a chemical hydrogenation product of α-cedrene. The ketones, on the other hand, may have resulted from the oxidation of α-cedrene although the mechanism of these reactions is not clear. If the ketones are not undetected constituents of these plants, then the presence in these deposits of both oxidised and fully-reduced α-cedrene products infers that separate phases of oxidation and reduction occurred during the diagenesis of the fossil resins and the montan wax.

Tertiary and Cretaceous sediments in solution pipes in the Devonian Limestone of South Devon, England

SummaryLarge sediment-filled solution pipes in Middle Devonian Limestone near Newton Abbot are described. A succession of gravels, sands and clays is identified in the pipes, and the presence of umber, ochre and residual clays is noted. The unusually large size of the pipes is discussed and the infilling sediments are related to the Cretaceous and Tertiary formations of the nearby Bovey Basin and Haldon Hills.

Tertiary sediments and structure of the Bovey Basin, south Devon

The Bovey Basin is filled by a thick (about 1200 m) sequence of Tertiary kaolinitic clays, lignites and sands, the Bovey Formation. This paper presents for the first time a detailed stratigraphy of the upper part of the formation (the lower 900 m being unexposed), and suggests the environments of deposition of its component members. Three major and four lesser subdivisions of the upper Bovey Formation are recognised. The oldest of the major units exposed in pits, the Abbrook Clay and Sand, comprises siliceous clays and sands, and is overlain by a brown clay and lignite sequence the Southacre Clay and Lignite. The Blatchford Sand, which rests on the Southacre Clay and Lignite, includes thick beds of coarse to gravelly sands. Marginal facies in the basin are represented by the Ringslade Clay, which includes a mottled-clay facies containing Devonian slate fragments. The stratigraphical position of the Chudleigh Knighton Clay in the north-east of the main basin is uncertain, but it is thought that it may occupy a position intermediate between the Abbrook Clay and Sand and the Southacre Clay and Lignite. The results of the present work indicate that sedimentation in the Bovey Basin is likely to have occurred largely in flood-plain conditions, with probable lacustrine and alluvial fan components, taking place in an enclosed, subsiding, partly fault-bounded basin. It is thought unlikely that the Bovey Basin was ever the site of a large ‘permanent’ lake. Only the upper 350 m or so of the Bovey Formation sequence is at present known from surface exposures and boreholes; the unknown sediments beneath are considered from indirect evidence to have been deposited in a wholly fault-bounded subsiding trough, while the later sediments (Abbrook Clay and Sand and younger units) are thought to have resulted from a phase when sedimentation spread out from the initial fault-bounded basin to overlie older rocks with unfaulted contacts, except in the west and south of the main basin.

The haldon gravels of South Devon

The Haldon Gravels comprise four components, three Eocene and one Pleistocene. The Tower Wood Gravel, of early Eocene age, contains weathered but unabraded flints in a clay matrix of well-ordered kaolinite with little illite, and was formed from in situ solution of the Chalk. The overlying Buller's Hill Gravel contains abraded flints and exotic pebbles in a matrix of kaolin clay and sand; it was formed largely by fluvial reworking of the Tower Wood Gravel on a wide flat plain during approximately Reading Beds to Bagshot Beds times. Bodies of ‘ball-clay’ kaolin now found within the Buller's Hill Gravel but differing from it in their clay mineralogy originated as a sheet of clay overlying the gravel, deposited on a river flood-plain or in a coastal lagoon and tentatively correlated with the Bracklesham and Barton Beds. Downwarping of the Bovey Basin prevented further, sedimentation in the area. During the Pleistocene the Haldon Gravels were affected by periglacial processes; the flints were frost-shattered, the gravels mixed by cryoturbation and solifluction to form Head Gravel, and the overlying sheet of clay broken up and incorporated as masses of clay in the Buller's Hill Gravel.

A Tertiary thrust fault on the southern margin of the Bovey Basin

SummaryGeological investigations and borehole drilling on the southern margin of the Bovey basin around Mainbow ball clay mine and Ringslade open pit have shown that Devonian slate up to 30 m thick is superimposed on Tertiary sediments. A drag fold beneath the slate indicates that the slate has moved eastwards over the Tertiary material. As the slate is relatively intact and not disaggregated, hillcreep cannot be responsible for the superimposition. A large landslip involving a rotational shear cannot be responsible, as the movement plane dips into the hill over too large an area. The southern margin of the Bovey basin is therefore interpreted as a thrust fault of Tertiary age, with thrust plane dipping 10° to 30° S. It is suggested that this is a rejuvenation of the Variscan thrust recognized in the Bickington area and further west in the Holne area and that the movement on the Tertiary thrust is related to the dextral wrench movement on the Sticklepath fault.

The Upper Devonian Rocks of The Chudleigh Area, South Devon

I. Introduction The Upper Devonian rocks under consideration occupy a small portion of the area of the 1-inch geological map (Sheet 339). They occur as small faulted inliers on the north-eastern side of the Bovey basin in a region otherwise occupied by Culm overlain by Permian and Cretaceous strata. The main (Chudleigh) outcrop, which is a mile across at its widest part, extends from the neighbourhood of Harcombe to Chudleigh Rocks—a distance of about 3½ miles. East of this outcrop, and on the same latitude as Chudleigh, is the small triangular patch of Lower Dunscombe, and farther south lies the much larger but partially concealed Ugbrooke-Well inlier, which appears on the map as three detached outcrops. The nearest edge of the Dartmoor granite lies about 2½ miles to the west. The Devonian inliers include the upper part of the Massive Limestone of South Devon thrust over thinly bedded limestones and shales. The Massive Limestone is here shown to be Lower Frasnian and the thinly bedded limestones and shales Upper Frasnian and Famennian respectively. The Kate Brook, the main stream in the area, has cut its valley along the main outcrop of the Famennian shales. The lowest part of the valley lies below the 100-foot contour. The Massive Limestone flanks the depression on the east, rising to 300 or 400 feet; the surrounding Culm forms undulating ground 400 to 500 feet high. The Permian with its Cretaceous capping rises to 700 feet in the north-east. My thanks are due to Lord

V.—The Boring at Heathfield, Devon

The geological importance of this boring lies in the fact that it is the deepest which has yet been made in the Bovey Basin, that the site is nearly in the middle of that basin, and that it is evidence of the great depth of the syncline or trough in which the Bovey deposits lie. Although it did not reach the base of these deposits it is highly desirable that we should know the exact depth to which it was carried, as it is the only means of estimating the minimum depth of the basin.

III.—The Depth and Succession of the Bovey Deposits

In the report of an excursion organized by the Geologists' Association to Newton Abbot in 1900, Mr. H. B. Woodward stated that a boring had been made by Messrs. Candy & Co. at Heathfield, near the middle of the Bovey Basin, and that it “had been carried to a depth of 520 feet from the surface through clays, sands, and lignites without reaching the base”.

The Depth and Succession of the Bovey Deposits (Devon)

The total thickness of the Tertiary Beds in the Bovey Basin has never yet been ascertained, because no boring has yet reached the bottom of the basin in which they lie; and no one has yet attempted to make out a stratigraphical succession from the sections exposed in the numerous clay-pits. Some years ago, however, Messrs. Candy & Co., of the Heathfield Potteries, put down a boring which reached a depth of 526 feet from the surface. Having obtained some particulars concerning the beds traversed by this boring, the Author is able to discuss the succession of the Bovey deposits, so far as they hate been explored. The following is a generalized description of the strata seen in the Heathfield pit, and penetrated by the boring from the bottom of that excavation :— The Author confirms the conclusion arrived at by Pengelly in 1861, with regard to the relative age of the beds exposed in the ‘ old coal-pit’ south-east of Bovey Tracey and those proved in a boring to the east of it. From all the data mentioned, and assuming the actual base of the Tertiary deposits to be not more than 30 feet below the bottom of the Heathfield boring, he estimates the total thickness of the ‘ :Eocene’ beds to be about 613 feet. The Bovey Basin itself is regarded as a tectonic basin or post-Eocene centrocline, and not as a lake-basin; although, during the deposition of the higher part of the series, it may have formed

The Eocene Deposits of Devon

In a paper read before this Society in 1896 I gave a short description of the Eocene Deposits of Dorset, and remarked that ‘It is noteworthy that the new evidence discovered in the western end of the Hampshire Basin strongly supports the idea that the pipeclays of the Bagshot Series are derived from the weathering of the Dartmoor Granite, and that the Bovey Tracey outlier, so like the deposits around Bournemouth, is, as maintained by Mr. Starkie Gardner, of the same age and deposited in the same basin, though in Devon Eocene rest directly on Palæozoic rocks.’ Since these words were written I have had occasion to re-examine the Bovey basin on behalf of the Geological Survey, with the result that light can now be thrown on several obscure points in the geology. As my notes on the Bovey area will complete the general description of the western end of the Hampshire Basin, it will be useful to lay before the Society the results, especially as much detailed work remains to be done and it will be at least 3 years before any general memoir can be published. In the year 1875, when Mr. H. B. Woodward was revising the geology of the Bovey basin, and I was engaged in mapping the Greensand and plateau-gravel of Haldon, we were much exercised as to the age and relationship of the deposits, but then had no clear evidence to permit us to go contrary to the received ideas, though liar. Woodward published

Notes on the Gravels, Sands, and other Superficial Deposits in the Neighbourhood of Newton Abbot, Devonshire>

T he visitor to Newton Abbot cannot fail to be struck with the number of sand- and gravel-pits, some of great size, which are scattered about the neighbourhood, not only in the valleys, but high up the slopes of the hills, and in several instances on their summits. On giving some attention to the character and distribution of these deposits of sand and gravel, it will be found:— first , that two main divisions may be made in them, namely those of an evidently alluvial character, and those which occupy positions quite unconnected with the present systems of drainage; and, secondly , that these latter deposits are nevertheless confined to the Bovey Basin and bordering hills, not descending into any of the other lateral or outlying valleys. A reference to the geological maps of the country will not serve to throw much light on the correlation or identification of the beds when they are examined in detail. But since the days when De la Beche and Godwin-Austen recorded their observations, many new sections have been opened up. The rapid growth of Torquay and Newton Abbot has been attended by a call for sand, which is largely used in the preparation of “cobb,” a kind of plaster, unfortunately necessary to envelop and keep the damp out of houses for the most part built of marble. The examination of these sections, together with those dug for the foundations of new villas on Woolborough Hill, has led me to differ from the opinions and eonclusi,ons which have

On the Bovey Deposit

Introduction .—Singularly enough, as geologists approach our own era the difficulty of determining the relative age of a particular stratum generally increases; and it is in the more modern tertiaries, or deposits succeeding to these, that the greatest amount of difficulty occurs. Among the strata not yet referred to any certain epoch, but broadly designated “tertiary,” are the clay-, sand- and lignite-beds, known to geologists as the “Bovey deposit.” Having been for the last ten years engaged in working and boring the various beds of clay, I may have become possessed of facts not generally known to geologists, bearing on the origin and nature of the deposit, and which may assist in some degree to fix its relative age. The Thysical features of the basin .—The Bovey basin is a depression beneath the level of the surrounding country; its length, from Bovey-Tracey to about two miles south of Kingskerswell, is about 10 miles; its breadth at the upper end about 2½ miles, becoming miles, becoming much narrower towards its southern extremity. Two rivers, the Teign and the Bovey, both having their sources in the granite of Dartmoor, run into this basin, meet above Stover, and fall into the sea at Teignmouth. The Teign, the larger and more circuitous, for about 13 or 14 miles before entering the Bovey basin, flows through the slate ; and the Bovey River, rising near the centre of the moor, crosses for a short distance the slate, and rims into the