Demmitts Farm

Crediton, United Kingdom

Demmitts Farm

Crediton, United Kingdom
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Cornford C.,IGI Ltd | Jones M.,Geo optics | Scrivener R.C.,Demmitts Farm
Geoscience in South-West England | Year: 2011

Bideford Black is found in two forms - sheared 'coal seams' and organic rich shales or carbargillites. Both have been exploited commercially in the 19th and first half of the 20th Century, with the coal worked for fuel, and the carbargillites as mineral pigments - 'Bideford Black' sensu stricto. A collection of nine samples of coal and carbargillite were assembled from the Bideford area forming an east-west section running from the East-the-Water Mine, via Bideford High Street to the coast at Greencliff and Abbotsham. Treated as coals these samples showed mean vitrinite reflectance values between 1.66 and 2.58%Ro, with 3-81%Ro being found at Greencliff. This compares with the coeval strata to the north and south in the range 4.4-5.0%Ro. The Bideford area is thus equivalent to low volatile bituminous to anthracite coal rank, compared with meta-anthracites as the norm elsewhere in the Crackington and Bude formations of North Devon and North Cornwall. A 'thrust-back thrust' popup would typically explain anomalously low maturity within a thrust belt, which suggests that the Bideford Formation comprises an allochthonous nappe. Reflectance measurements of the nine Bideford Black samples was made with linearly polarised light allowing the determination of maximum and minimum reflectance which adds precision to estimates of maximum burial. By analogy with reflectance gradients of other Upper Carboniferous sequences, a maximum palaeo-burial of between 5,700-8,100 m is indicated for the Bideford Black levels of the Bideford Formation. Organic petrographie examination shows all the Bideford Black samples to be fairly mono-maceralic, comprising uniform vitrinite with minor inertinite and locally exinites (cutinite). This macerai association has been attributed to palaeo-'log jams' formed within river channels during flood events, and agrees with the local association of the Bideford Black 'seams' with the channel sands of the formation. Knowledge that the Bideford Black accumulations were initially log jams could assist any future exploration for new deposits - should they make a surprising return to commercial exploitation.

Bridgland D.R.,Durham University | Bennett J.A.,4 Crockernwell Court | McVicar-Wright S.E.,University of Exeter | Scrivener R.C.,Demmitts Farm
Proceedings of the Geologists' Association | Year: 2014

Fluvial rocks and sediments form an important part of the geological record from the terrestrial environment, from the Archaean to the recent. Precambrian fluvial archives record the change in Earth's atmosphere from anoxic to oxygen-rich, while the absence of land plants led to significant differences between Precambrian and Palaeozoic fluvial regimes and those from later in the geological record. In the Mesozoic and Cenozoic, river valleys were populated by land animals and their deposits are repositories for the fossils that record these biomes: dinosaurs and mammals being the prominent groups, respectively, in these eras. By the Cenozoic some of the drainage systems that exist at present had been initiated; their evolution during this era, and especially in the Quaternary, is evident from fluvial archives worldwide. The record of Quaternary rivers reveals the increasing influence of global cooling, with the severity of climate a potential driver of erosional isostatic uplift, bringing about incision of landscapes and the formation of river terraces in all but the most stable areas and subsiding basins. In the Holocene the fluvial environment experienced increasing influences from early humans as catchments and slopes were transformed by deforestation and farming, and later by industries such as mineral mining. These themes are represented in contributions to this special issue, arising from a Geologists' Association conference and field meeting convened in Exeter in October 2012. © 2014 The Geologists' Association.

Barr M.W.C.,Alvington Churchill Axminster | Scrivener R.C.,Demmitts Farm
Geoscience in South-West England | Year: 2014

Bayes' theorem of conditional probabilities is used to evaluate the potential for tungsten mineralisation in the granites and surrounding country rocks of the southwest peninsula of England. The approach uses the distribution of known tungsten mineralisation in relation to geological controls that can be mapped, to calculate the probability by unit area of as yet undiscovered mineralisation. It permits the evaluation of the efficiency of the chosen exploration criteria one by one and in combination. The method indicates areas of up to 20 km2 where the posterior probability of a square kilometre containing tungsten mineralisation approaches unity, located in a discontinuous zone extending from the east side of the Godolphin Granite in the south, through the eastern margins of the Cam Marth Granite to St Agnes and Cligga Head with small outliers elsewhere on or close to the outcrop of the southwest granites. '.

McVicar Wright S.E.,University of Exeter | Scrivener R.C.,Demmitts Farm
Geoscience in South-West England | Year: 2012

The purpose of the field excursion was to examine rocks of Permian age cropping out in the Crediton Trough. The trip included visits to five locations where a range of sedimentary strata and igneous rocks was exposed.

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