Earth Science Conservation Review
| Introduction to Clay-with-Flints of Northern Ireland | |
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| Rocks | |
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| Rock Age: | Cretaceous, Tertiary |
| Rock Type: | Chalk, Clay, Flint, Basalt |
| Interest | |
Summary of site:
The Clay-with-Flints in Northern Ireland is a unique deposit whose distribution is mostly confined to the margins of the Antrim Plateau. Stratigraphically, at most localities, the deposit occurs between white chalk of the Late Cretaceous Ulster White Limestone Formation below and lava flows of black basalt of the Early Palaeogene Antrim Lava Group above. However, at a few locations on the west side of the Antrim Plateau the Cretaceous chalk is absent and the Clay-with-Flints rests on Precambrian, Upper Palaeozoic and Mesozoic rocks. The period of time within which the Clay-with-Flints formed spans about 10 million years of the Late Cretaceous and Early Palaeogene, but the deposit demonstrably post-dates the K-T (Cretaceous-Tertiary) boundary.
The origin of the Clay-with-Flints has been a subject of debate for generations of geologists both in Ireland and with regard to the contemporaneous deposit in England. With the notable exception of Portlock (1843) virtually all other commentators on this deposit concluded that its origins and its two main components, clay and flint nodules, were derived mainly by the weathering and dissolution of the underlying chalk. With respect to the flint content of the deposit, that conclusion is clearly correct as the Cretaceous chalk is the only known source of those characteristic clasts in Ireland and Great Britain. It is with regards to the clay matrix of the Clay-with-Flints that new evidence, which is discussed in this report, demonstrates conclusively that its origins lay, not as an autochthonous by-product of weathering of the chalk but from an external volcanic source whose eruption was linked to the opening of the Atlantic Ocean, but preceded the main period of basaltic volcanism in the north of Ireland. In the relatively short period of time between the deposition of the Clay-with-Flints and the outpouring of the first basalt lava flows, the clay matrix was subjected to intense weathering which developed the multiple colour patterns that are characteristic of the deposit and contrast so markedly with the white chalk below and the black basalt above. Intense weathering has also affected the chemical composition and physical integrity of the deposit. Nevertheless, at many locations, it has been possible through geochemical and petrographical analyses to ascertain the affinity of the parental volcanism and to define the sedimentological history of the deposit.
This report describes in detail the Clay-with-Flints deposit at 11 separate locations around the margins of the Antrim Plateau. All of these locations were visited recently to ascertain the accessibility of the critical section. At only one, Soldierstown Quarry near Moira, was the deposit now concealed but because of its importance to our understanding of the evolution of the Clay-with-Flints this locality is retained in the report and its features illustrated by photographs. A photographic record of the deposit at all the remaining sites is also included in the report.
The origin of the Clay-with-Flints has been a subject of debate for generations of geologists both in Ireland and with regard to the contemporaneous deposit in England. With the notable exception of Portlock (1843) virtually all other commentators on this deposit concluded that its origins and its two main components, clay and flint nodules, were derived mainly by the weathering and dissolution of the underlying chalk. With respect to the flint content of the deposit, that conclusion is clearly correct as the Cretaceous chalk is the only known source of those characteristic clasts in Ireland and Great Britain. It is with regards to the clay matrix of the Clay-with-Flints that new evidence, which is discussed in this report, demonstrates conclusively that its origins lay, not as an autochthonous by-product of weathering of the chalk but from an external volcanic source whose eruption was linked to the opening of the Atlantic Ocean, but preceded the main period of basaltic volcanism in the north of Ireland. In the relatively short period of time between the deposition of the Clay-with-Flints and the outpouring of the first basalt lava flows, the clay matrix was subjected to intense weathering which developed the multiple colour patterns that are characteristic of the deposit and contrast so markedly with the white chalk below and the black basalt above. Intense weathering has also affected the chemical composition and physical integrity of the deposit. Nevertheless, at many locations, it has been possible through geochemical and petrographical analyses to ascertain the affinity of the parental volcanism and to define the sedimentological history of the deposit.
This report describes in detail the Clay-with-Flints deposit at 11 separate locations around the margins of the Antrim Plateau. All of these locations were visited recently to ascertain the accessibility of the critical section. At only one, Soldierstown Quarry near Moira, was the deposit now concealed but because of its importance to our understanding of the evolution of the Clay-with-Flints this locality is retained in the report and its features illustrated by photographs. A photographic record of the deposit at all the remaining sites is also included in the report.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Introduction to Clay-with-Flints of Northern Ireland, County , site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1300. Accessed on 2024-12-26 |
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