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 to 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 that there is no significant hiatus and that the record of faunal changes is closely comparable in the key successions. A synthesis of other ‘Chadian’–Arundian boundary successions in Britain and Ireland indicates some key problems with the current Arundian boundary stratotype in practical usage. We propose a stratigraphically higher position for the Arundian boundary in the stratotype, which is more easily correlated to the Bobrikian and Moliniacian substages of eastern Europe and Belgium, respectively, and tied to the first occurrences of foraminifera species, allowing more robust age assessments, free from the confounding factors of inferred lithological or sequence stratigraphic changes.