TOWARDS AN ORIGIN FOR BIDEFORD BLACK

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 petrographic examination shows all the Bideford Black samples to be fairly mono-maceralic, comprising uniform vitrinite with minor inertinite and locally exinites (cutinite). This maceral 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.