Subsidence, not erosion: Revisiting the emplacement environment of the Giant's Causeway, Northern Ireland

Abstract For almost a century it has been recognised that the anomalous thickness of columnar-jointed tholeiite basalts at the Giant's Causeway, Northern Ireland, was due to ponding of lava in a topographic depression. For more than 80 years this feature has been ascribed to fluvial incision, yet new field observations have found no evidence for erosion of the underlying Lower Basalt Formation anywhere in the studied area. Basalt weathering and erosion rates also suggest that there was insufficient time for fluvial incision to occur on the scale required. Local dips on the Lower Basalt Formation broadly converge about a low point centred on the Giant's Causeway itself and are discordant with the regional dip of the overlying Causeway Tholeiite Member, observations consistent with a subsidence-based model. The absence of sediment and limited development of hyaloclastite beneath the Causeway Tholeiite Member here indicates that the interval between subsidence of the land surface and emplacement of tholeiite lava was insufficient for sediment, or even a significant depth of water, to accumulate. It implies that subsidence may have been directly related to draining of a relatively shallow magma reservoir from which the tholeiite lavas were erupted. The style of faulting in the Lower Basalt Formation outcrop suggests that a brief period of inflation, perhaps associated with magma injection, may have preceded subsidence caused by magma withdrawal. This study demonstrates that even at sites as well-trodden as the Giant's Causeway, new discoveries can be made from simple observations.