High-resolution definition and correlation of the Asbian–Brigantian boundary in northern England and the Scottish borders, using foraminiferal diversity and richness

Abstract

Foraminiferal diversity and taxa richness from beds transitional between the Asbian and Brigantian substages (Middle Mississippian) show patterns of secular change which allow detailed inter-regional correlations to be established. Foraminifera from the Askrigg Block, Stainmore Trough, Alston Block, South Cumbria Shelf and Solway Basin show similar secular changes (foraminiferal trends, FTs), allowing correlation to be made with the basal Brigantian Stratotype at Janny Wood. Despite the absence of consistent microfossil first occurrence markers for the recognition of the base of the Brigantian, this horizon can be confidently recognized by means of foraminiferal trends. The FTs allow the precise location of the base of the correlated Brigantian in sections where this boundary was questioned or controversial in previous studies, as well as to amend the position of the foraminiferal zones and subzones during the late Asbian and basal Brigantian. This type of analysis when used in combination with foraminiferal zonations, emergent surfaces and lithological cyclicity, together, provide a robust means for high-resolution correlation. This methodology, provides the least uncertainty in sections that have been most densely sampled, whereas for less intensely sampled sections there is more correlation uncertainty.