The BGS Lexicon of Named Rock Units — Result Details

Brown Bank Tuff Formation

Computer Code: BBA Preferred Map Code: notEntered
Status Code: Full
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Sheets of massive, unbedded, poorly sorted, generally lithic-rich welded lapilli-tuff of andesitic and rhyolitic composition; interbedded volcaniclastic sandstone, siltstone, pumice lapilli-tuff and accretionary lapilli-tuff. Contains the rhyolitic welded lapilli-tuff (Seascale Hall Member) and, in BH RCF1, a rhyolitic lava flow or lava-like tuff.
Definition of Lower Boundary: At the change from coarse, eutaxitic, fiamme-rich lapilli tuff at the top of the Bleawath Formation to generally lithic-rich sequence above. Various members of the Formation overlap on to the top of the Bleawath Formation.
Definition of Upper Boundary: At the upward change from the generally lithic-rich sequence to homogeneous andesitic welded tuff of the Fleming Hall Formation.
Thickness: 255.55m in Type Section (355.5m in core because of andesite sill); 188m in Sellafield BH No.2; 127m in Sellafield BH No. 4; 45m in Sellafield BH No.5.
Geographical Limits: Seen in Sellafield boreholes 2 (NY00/28), 4 (NY00SE/29), 5 (NY00SE/30), 10A (NY00SW/38), RCF1 (NY00SE/35), RCF3 (NY00SE/37), RCM1 (NY00SE/41) and RCM2 (NY00SE/40).
Parent Unit: Borrowdale Volcanic Group (BVG)
Previous Name(s): Brown Bank Ignimbrite Formation [Obsolete Name and Code: Use BBA] (-1967)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Nirex Sellafield borehole No 10A, BGS Registered No NY00SW/38, 1053.0-1408.5m below Rotary Table Elevation, 1012.5m - 1368.0m below OD. 
Reference Section  Nirex Sellafield borehole No RCF1, BGS Registered No NY00SE/35, 908.71m - 1150m (section incomplete) belo Rotary Table Elevation. This is a deviated vertical borehole; true depths and thicknesses are as yet unknown. 
Reference(s):
Millward, D, Beddoe-Stephens, B, Williamson, I T, Young, S R, and Petterson, M G. 1994. Lithostratigraphy of a concealed caldera-related ignimbrite sequence within the Borrowdale Volcanic Group of west Cumbria. Proceedings of the Yorkshire Geological Society, Vol.50, part 1, 25-36. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable