Depositional system and plant ecosystem responses to long-term low tempo volcanism, the Interbasaltic Formation, Antrim Lava Group.

The Antrim Lava Group, which crops out in northeast Ireland, comprises a volcanic sequence dominated by basaltic lava flows. Including subsidiary sedimentary interlayers and some evolved lavas and intrusions, the overall sequence reaches a cumulative thickness of ~800 m. However, the tempo of eruption of the Antrim Lava Group is poorly constrained, but can be evaluated via weathering patterns and environmental reconstructions derived from lava-flow interbeds. In this contribution we present palynology from a newly-identified and well-developed 2-2.5 m thick sedimentary sequence (interbed) at Ross’s Quarry, Ballycastle, Co. Antrim, that helps elucidate the contemporary development of environments in a setting subject to periodic basaltic volcanism. The interbed is subdivided into geologically distinct subunits of cross-bedded sandstone, bedded sandstones and sandy siltstones, all rich in visible organic remains such as rootlets and fragments of wood and bark. A total of 19 samples were collected from the sequence and subsequently analysed for palynological content. The palynomorph data point toward a diversity of inputs ranging from estuaries, chalky soils, dry soils, swamps, lakes, floodplains, sand bars, wet soils, established bogs and fenlands. In contrast to current understanding, the palynological data and their inferred environments collectively reveal the presence of flora that favour a temperate climate rather than the subtropical climate that has previously been inferred from the lateritic interbeds of the Antrim Lava Group. By combining the Ross’s Quarry observations with palynological data from other quarry sites and boreholes in Antrim, we provide new insights into the climate, weathering systems and eruptive history of the Antrim Lava Group.