The ‘Clay-with-Flints’ deposit in Northern Ireland: reassessment of the evidence for an early Paleocene ignimbrite

• Published in: Geological Magazine
• Main Authors: Cooper, Mark R., Troll, Valentin R., Lemon, Kirstin
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: North Atlantic
• Online Access: http://nora.nerc.ac.uk/id/eprint/518826/; https://nora.nerc.ac.uk/id/eprint/518826/1/Cooper_et_al_Clay_With_Flints_2017.pdf; https://doi.org/10.1017/S0016756817000760

Reassessment of key geological sections, field relationships and petrographical characteristics of the Northern Ireland ‘Clay-with-Flints’ and ‘Donald's Hill Ignimbrite Formation’ show they formed dominantly by sedimentary processes. The involvement of a previously postulated pyroclastic flow during early Paleocene time is not recognized and, as such, the Donald's Hill Ignimbrite Formation stratigraphic term is discounted. Instead a multistage model of formation by sedimentary accumulation and remobilization is presented and the term Clay-with-Flints is retained. Regionally, two dominant facies are recognized in most Clay-with-Flints sections. Facies 1 was formed by an initial accumulation of flints on a chalk landscape undergoing karstification, and involved deposition of a clay matrix derived predominantly from contemporaneous erosion of subtropical soil horizons formed mainly on basalt. In Facies 2, evidence is observed for widespread remobilization of Facies 1 deposits by high-density mudflows driven by the advancement of the Antrim Lava Group, which caused the blockage of subsurface and marginalization of surface drainage. A stratigraphical constraint imposed by the presence of a supposed ignimbrite in this part of the North Atlantic Igneous Province has been problematic, but this is resolved by its identification as a diachronous, sedimentary deposit that formed until buried by either the lower or upper formations of the Antrim Lava Group.