The BGS Lexicon of Named Rock Units — Result Details

Middle Hope Volcanic Member

Computer Code: MHVO Preferred Map Code: notEntered
Status Code: Full
Age range: Tournaisian Stage (CT) — Tournaisian Stage (CT)
Lithological Description: The lower part of the member comprises upward-coarsening multi-coloured mainly red-brown and green tuffs, with bioclastic material. Lapilli-rich beds, 3 to 5 cm thick, within this unit also increase in grain size upwards. Associated with the tuffs are thin-bedded limestones, some planer stratified or showing symmetrical ripples. The multi-coloured tuffs are overlain by green, graded and ungraded lapilli-tuffs, with clasts of devitrified amygdaloidal basalt and bioclastic material. Calcite vein networks are prominent locally. Within the upgraded lapilli-tuffs there are matrix- and clast-supported conglomerates, with clasts of chert nodules and limestones. Also associated with the lapilli-tuffs are beds of bioclastic limestones and both cross-stratified and laminated sandstones. Within the lapilli-tuffs is a prominent, laterally impersistent basaltic pillow lava [3.5 to 4.3 m thick], very weathered with abundant calcite-filled amygdales up to 10 cm across. The upper surface of the basalt is very irregular and highly amygdaloidal.
Definition of Lower Boundary: The boundary is placed at a sharp conformable boundary where bioturbated wackestones and packstones of the Black Rock Limestone Subgroup, are overlain by multi-coloured tuffs
Definition of Upper Boundary: The upper boundary is taken at a sharp, erosive boundary where red nodular limestones with green tuff laminae and beds are overlain by cross-bedded, bioclastic grainstones of the Black Rock Limestone Subgroup.
Thickness: From 37 m thick at Swallow Cliff in the west, to 4 m in the east of the site, at ST 348 669.
Geographical Limits: Middle Hope at ST 322 659, north Somerset.
Parent Unit: Black Rock Limestone Subgroup (BRL)
Previous Name(s): Woodspring Lava And Tuff (-1697)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Swallow Cliff at Middle Hope, north Somerset. Whittaker and Green, 1983. 
Reference(s):
Whittaker A and Green G W. 1983. Geology of the country around Weston-super-Mare. Memoir of the Geological Survey, England and Wales, Sheet 279 with parts of sheets 263 and 295. 
Geikie A, and Strahan A. 1899. Summary of progress of the Geological Survey for 1898. Memoir of the Geological Survey of Great Britain, 104-111. 
Faulkner T J. 1989. The Early Carboniferous (Courceyan) Middle Hope Volcanics, of Weston-Super-Mare: Development and demise of an offshore volcanic high. Proceedings of the Geologists' Association, Vol.100, 93-106. 
Jeffreys, D H, 1979. Sedimentology of the Dinantian Volcaniclastic carbonate rocks of Middle Hope and Spring Cove, Weston-Super-Mare, University of Reading, Unpublished M.Sc Thesis. 
Matthews, S, Butler, M and Sadler, P. 1973. Lower Carboniferous successions in North Somerset. Report by Directors of Field Meeting. Proceedings of the Geologists' Association, Vol.84, 175-179. 
Morgan C, and Reynolds S. 1904. The igneous rocks associated with the Carboniferous Limestone of the Bristol District. Quarterly Journal of the Geological Society of London, 60, 137-157. 
Reynolds S. 1917. Further work on the igneous rocks associated with the Carboniferous Limestone of the Bristol District. Quarterly Journal of the Geological Society of London, 72, 23-42. 
Speedyman D L. 1977. Volcanic rocks of the Bristol Region. In: Geological Excursions in the Bristol district (Editor: Savage R J G.), pp. 47-54, University of Bristol. 
Waters, C N. 2003. Carboniferous and Permian igneous rocks of central England and the Welsh Borderland. In: Carboniferous and Permian Igneous Rocks of Great Britain North of the Variscan Front. (editors Stephenson D, Loughlin S C, Millward D, Waters C N, and Williamson I T.), pp. 279-316, Geological Conservation Review Series, No. 27 (JNCC). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E279