Relocate the base of the Arundian?: a re-evaluation from south Cumbrian sections and implications for British and Irish Lower Carboniferous successions

Abstract

The largely ice-free world of the ‘Chadian’-Arundian (early Visean) is investigated in two successions in south Cumbria, in order to help better understand the relationships between transgression, hiatus and the position of the basal Arundian. A detailed foraminiferal biostratigraphy is used to age constrain the transition from the late ‘Chadian’ to early Arundian. This demonstrates a good synchronous relationship between the sequence stratigraphy of the two successions and the first occurrence of key species (mostly primitive archaediscids, rapidly followed by more evolved forms of the same family), suggesting there is no significant hiatus, and a closely comparable record of faunal changes in key successions. A synthesis of other ‘Chadian’-Arundian boundary successions in Britain and Ireland indicates some key failings of the current Arundian boundary stratotype, in practical usage. We propose a stratigraphically higher position for the Arundian boundary in the stratotype, which is easier correlated to the Bobrikian and Moliniacian substages of eastern Europe and Belgium, respectively, and tied to foraminifera first occurrences, allowing more robust age assessments, free from the confounding factors of inferred lithological or sequence stratigraphic changes.