A sedimentological evaluation of the devonian-carboniferous boundary stratotype in southern Ireland

Abstract

The stratotype section for the base of the British Dinantian (Devonian-Carboniferous boundary) is currently located at the interface of two marine formations at the Old Head of Kinsale. This corresponds with the boundary of the LN and VI miospore subzone floral assemblages. The succession immediately below the interface records an upward change from inner shelf into transgressive backbarrier sediments, due to the initiation and progressive influence of a storm-dominated migrating microtidal barrier complex. The formation junction is considered to reflect a non-sequence arising from subsequent erosive shoreface retreat and destruction of the barrier superstructure, which was followed by apparently sudden deepening and the deposition of ‘pro-delta’ facies. Comparisons with other sections at this stratigraphical level in southern Ireland indicates that barrier construction with landward migration into backbarrier bays occurred during a destructive phase of deltaic development in latest Devonian times. A rapid transgression associated with a cessation of sand supply then took place, resulting in variable reworking of the underlying sequences and discontinuous shoreline retreat. This was followed by the deposition of storm influenced calcareous shelf and inshore bay sediments over laterally diverse delta platform successions. With continued deepening a ‘pro-delta’ environment was established over the area in the earliest Carboniferous (VI miospore subzone), which thus records the relatively unusual preservation of barrier and backbarrier sequences in a regionally transgressive situation.