The BGS Lexicon of Named Rock Units — Result Details

Seascale Hall Tuff Member

Computer Code: SEHA Preferred Map Code: notEntered
Status Code: Full
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Eutaxitic and parataxitic rhyodacitic lapilli-tuff containing small abundance of pale acid lithic clasts. Interpreted as a welded ignimbrite sheet.
Definition of Lower Boundary: Sharp base within the Brown Bank Tuff Formation at upward change from andesitic lapilli-tuffs to rhyodacitic lapilli-tuff; pyroclastic breccia at base in Sellafield borehole No.10A. Seascale Hall Member oversteps other (unnamed) members of the Brown Bank Tuff Formation and in Sellafield borehole No.5 rests on the Bleawath Tuff Formation.
Definition of Upper Boundary: Overlain by thin, andesitic, lithic rich lapilli-tuff-grade ignimbrite within the Brown Bank Tuff Formation.
Thickness: c.20 - 140m
Geographical Limits: Seen in Sellafield boreholes 2 (NY00SE/28), 4 (NY00SE/29), 5 (NY00SE/30), 10A (NY00SW/38) and RCF1 (NY00SE/35).
Parent Unit: Brown Bank Formation (BNB)
Previous Name(s): Seascale Hall Member (-1812)
Alternative Name(s): none recorded or not applicable
Type Section  Sellafield borehole No.10A (vertical borehole), BGS Registered No NYOOSW/38 
Reference Section  Sellafield borehole No.RCF1 (deviated vertical borehole), BGS Registered No NY00SE/35, 908.71m - 994.09m measured depth beneath Rotary Table Elevation, 902.51m -987.89m measured depth beneath ground level. 
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