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

Published online by Cambridge University Press:  27 October 2017

MARK R. COOPER ,
VALENTIN R. TROLL  and
KIRSTIN LEMON
MARK R. COOPER*
Affiliation:
Geological Survey of Northern Ireland, Dundonald House, Upper Newtownards Road, Belfast, BT4 3SB, UK
VALENTIN R. TROLL
Affiliation:
Department of Earth Sciences, Section for Mineralogy, Petrology and Tectonics, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden
KIRSTIN LEMON
Affiliation:
Geological Survey of Northern Ireland, Dundonald House, Upper Newtownards Road, Belfast, BT4 3SB, UK
*
†Author for correspondence: Mark.Cooper@economy-ni.gov.uk
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Abstract

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.

Keywords

‘Clay-with-Flints’ignimbritePaleocenestratigraphy
Type
Original Article
Information
Geological Magazine , Volume 155 , Issue 8 , November 2018 , pp. 1811 - 1820
Copyright
Copyright © Crown Copyright. Published by Cambridge University Press 2017 

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