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):
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). 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E279