The BGS Lexicon of Named Rock Units — Result Details

Papa Stour Volcanic Formation

Computer Code: PSV Preferred Map Code: notEntered
Status Code: Full
Age range: Eifelian Age (DI) — Eifelian Age (DI)
Lithological Description: Comprises principally rhyolite lava, rhyolitic tuff and agglomerate and basalt. In its type area on Papa Stour, the lava is largely devitrified. It occurs in two thick flows with intercalations of tuff, basalt and sandstone. The rhyolite is strongly banded. The basalt, which is intensly weathered locally, occurs in up to four flows. These have thick scouriaceous upper zones with chaledony-, calcite-barytes- and zeolite-filled vesicles. Similar rhyolite and pyroclastic rocks , such as the Melby Rhyolite Member, occur on the Walls Peninsula on Mainland Shetland.
Definition of Lower Boundary: In the type area, there is an unconformable contact upon sandstone and tuffaceous sandstone of the Melby Sandstone Formation. In the Melby area, it is in faulted contact along the Melby Fault against the Sandness Formation (Lower ORS] on the Walls Peninsula, or along a highly irregular contact with 'blocks and tongues' of rhyolite, that penetrated the underlying grey, micaceous, ripple laminated sandstone with rhyolite clasts of the Melby Sandstone Formation, to a depth of about 60 cm whilst it was an unconsolidated sediment.
Definition of Upper Boundary: In the type area, the rock at the top of the Papa Stour volcanic sequence is the youngest exposed bedrock. In the Melby area, the top of the upper unit of the Melby Rhyolite Member is a disconformable, irregular, brecciated contact against fine-to-medium-grained, planer bedded pink sandstone, with partings of flaggy sandstone and siltstone, and containing scattered rhyolite clasts of the Melby Sandstone Fm. The upper boundary of the lower unit is not exposed, but mapping suggests that it is disconformably overlain by similar sediments of the Malby Sandstone Fm to those overlying the upper unit.
Thickness: Up to at least 200 m in the type area.
Geographical Limits: The island of Papa Stour is about 2 km off the western coast of Mainland Shetland, about 36 km NW of Lerwick and the northern portion of the Walls Peninsula, BGS Scotland Sheet 127 Western Shetland.
Parent Unit: St Magnus Bay Group (STMA)
Previous Name(s): Middle Old Red Sandstone Volcanic Rocks And Sediments Of Papa Stour (-471)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Shore exposures, about 300m north of Melby Church. Mykura, W and Phemister, 1976, Chapter 10, p148. 
Type Area  The whole island of Papa Stour, about 2km of the western coast of Mainland Shetland, about 40 km NW of Lerwick. Mykura, W, and Phemister,1976, Chapter 11. 
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