The BGS Lexicon of Named Rock Units — Result Details

Melby Rhyolite Member

Computer Code: MYRH Preferred Map Code: notEntered
Status Code: Full
Age range: Eifelian Age (DI) — Eifelian Age (DI)
Lithological Description: Silicified rhyolite or ignimbrite, typically brecciated, purple when fresh, pale pink, greenish or brown mottled on weathered surfaces. The rock contains sparse euhedral phenocrysts of feldspar and locally exhibits well developed flow banding. Vesicles lined with quartz and calcite are commonly present in the upper 1.5m. The unit occurs as two lenticular bodies: one interstratified within the lower part of the Melby Sandstone Formation, and the second at the base of the Melby Sandstone Fm, at the NE end of its outcrop.
Definition of Lower Boundary: The lower unit displays a faulted contact along the Melby Fault, against the Sandness Formation of the Lower ORS on the Walls Peninsula. The upper unit displays a highly irregular contact with the 'blocks and tongues' of rhyolite that penetrated the underlying grey, micaceous, ripple laminated sandstone with rhyolite clasts, of the Melby Sandstone Formation, to a distance of about 60 cm whilst it was unconsolidated sediment.
Definition of Upper Boundary: The upper unit shows a disconformable irregular brecciated contact against fine-to medium-grained planar bedded pink sandstone, with partings of flaggy sandstone and siltstone and containing scattered rhyolite clasts of the Melby Sandstone Formation. The contact with the lower unit is not exposed but mapping suggests that it is disconformably overlain by similar sediments of the Melby Sandstone Formation to those overlying the upper unit.
Thickness: The upper unit is at least 70 m thick, whereas mapping suggests that the lower unit is at least up to 150 m thick.
Geographical Limits: The northern portion of the Walls Peninsula, Mainland Shetland, about 32 km NW of Lerwick, BGS Scotland Sheet 127 Western Shetland.
Parent Unit: Papa Stour Volcanic Formation (PSV)
Previous Name(s): Melby Rhyolites (-412)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Ness of Melby, NW portion of Walls Peninsula, Mainland Shetland about 35 km NW of Lerwick. Mykura W and Phemister, 1976, p148. 
Reference Section  Shore exposure about 300 m north of Melby Church. Mykura W and Phemister, 1976, Chapter 10, p148. 
Reference(s):
Mykura W, 1976 British Regional Geology: Orkney and Shetland (HMSO, Edinburgh).] 
Mykura W and Phemister J. 1976. Geology of Western Shetland Memoirs of the Geological Survey of Great Britain, Scotland [Explanation of One-inch Geological Sheet 127 and parts of 125, 126 and 128] 
Stephenson D. 1999. Late Silurian and Devonian volcanic rocks of Scotland: Introduction. 481-488. In: Caledonian igneous rocks of Great Britain. Stephenson D, Bevins R E, Millward D, Highton A J, Parsons I, Stone P and Wadsworth W J (Editors and compilers) Geological Conservation Review Series, 17 (Joint Nature Conservation Committee, Peterborough). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
S127