Editing Cretaceous-Palaeogene (K-T) Boundary, Northern Ireland

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The Clay-with-Flints is the only deposit in Northern Ireland that represents part of the 10Ma gap between the Late Cretaceous chalk (late-early Maastrichtian) and earliest Palaeogene basalt lavas (c. 59Ma). It is exposed at the margins of the Antrim Plateau and consists of multicoloured clay with clasts of flint derived from the Cretaceous chalk. The deposit was traditionally believed to have formed as a palaeosol [[Media:P948015.jpg| (P948015)]], combining the weathering products of chalk and flint, an aeolian component and illuvium from the overlying basalt lavas.<ref>Smith, B J, and McAllister, J J. 1995.  Mineralogy, chemistry and palaeoenvironmental significance of an Early Tertiary Terra Rossa from Northern Ireland: A preliminary review.  Geomorphology, 12, 63&nbsp;73.</ref> It is now recognised that contemporaneous volcanism contributed much of the clay fraction.<ref>Mitchell, W I, Cooper, M R, Hards, V L, and Meighan, I G. 1999.  An occurrence of silicic volcanic rocks in the early Palaeogene Antrim Lava Group of Northern Ireland.  Scottish Journal of Geology, 35, 179–85.</ref>
 
The Clay-with-Flints is the only deposit in Northern Ireland that represents part of the 10Ma gap between the Late Cretaceous chalk (late-early Maastrichtian) and earliest Palaeogene basalt lavas (c. 59Ma). It is exposed at the margins of the Antrim Plateau and consists of multicoloured clay with clasts of flint derived from the Cretaceous chalk. The deposit was traditionally believed to have formed as a palaeosol [[Media:P948015.jpg| (P948015)]], combining the weathering products of chalk and flint, an aeolian component and illuvium from the overlying basalt lavas.<ref>Smith, B J, and McAllister, J J. 1995.  Mineralogy, chemistry and palaeoenvironmental significance of an Early Tertiary Terra Rossa from Northern Ireland: A preliminary review.  Geomorphology, 12, 63&nbsp;73.</ref> It is now recognised that contemporaneous volcanism contributed much of the clay fraction.<ref>Mitchell, W I, Cooper, M R, Hards, V L, and Meighan, I G. 1999.  An occurrence of silicic volcanic rocks in the early Palaeogene Antrim Lava Group of Northern Ireland.  Scottish Journal of Geology, 35, 179–85.</ref>
  
The present thickness of Cretaceous chalk is a reflection of tectonism and the severity of weathering prior to the accumulation of the Clay-with-Flints. Some indication of the thickness of eroded chalk is provided by the occurrence of paramoudra [[Media:P948013.jpg| (P948013)]] in the Clay-with-Flints at Cloghfin [J 484 942]. There, the deposit rests on the Glenarm Chalk Member, but since paramoudra are restricted to the uppermost Ballymagarry Chalk Member [[Media:P947944.jpg| (P947944)]] at least 38&nbsp;m of post-Glenarm Chalk Member, and probably much more, has been removed to release them for incorporation into the Clay-with-Flints. Despite the erosion of the chalk, the Clay-with-Flints probably accumulated on a gently undulose land surface. At many exposures, the presence of grikes up to 5&nbsp;m deep illustrate the effects of solution on the chalk and while usually infilled by the Clay-with-Flints, the intervening clints may also be buried by 1–2&nbsp;m of the deposit.
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The present thickness of Cretaceous chalk is a reflection of tectonism and the severity of weathering prior to the accumulation of the Clay-with-Flints. Some indication of the thickness of eroded chalk is provided by the occurrence of paramoudra [[File:P948013.jpg| (P948013)]] in the Clay-with-Flints at Cloghfin [J 484 942]. There, the deposit rests on the Glenarm Chalk Member, but since paramoudra are restricted to the uppermost Ballymagarry Chalk Member [[Media:P947944.jpg| (P947944)]] at least 38&nbsp;m of post-Glenarm Chalk Member, and probably much more, has been removed to release them for incorporation into the Clay-with-Flints. Despite the erosion of the chalk, the Clay-with-Flints probably accumulated on a gently undulose land surface. At many exposures, the presence of grikes up to 5&nbsp;m deep illustrate the effects of solution on the chalk and while usually infilled by the Clay-with-Flints, the intervening clints may also be buried by 1–2&nbsp;m of the deposit.
  
 
In detail the Clay-with-Flints comprises a soft, highly weathered, clay matrix consisting of an intergrowth of fine-grained quartz, clay minerals, disseminated opaque oxides and haematite. While many of the flint clasts are broken into sharp fragments, some still retain their original shape. This is evident at a shoreface exposure [D 119 416] 250&nbsp;m northwest of Ballycastle harbour, where there is no clay matrix and the 0.4&nbsp;m thick deposit consists of self-supporting, whole and fragmented, flints and a matrix of coarse flint chips. The typical Clay-with-Flints deposit is exposed in the cliff face, 100&nbsp;m to the northwest. The colour of the Clay-with-Flints reflects those of the component clay minerals and ranges from grey and dark bluish grey to white, pale fawn, purplish to deep reddish brown. Flints usually retain the dull grey and greyish brown hues, with a white weathering crust, which predominate in the chalk. At some localities the flints have a superficial red colour which is merely surface staining. However, at others, such as Donald’s Hill, the flints show a pervasive concentric banding in shades of brown, pink, purple and red probably due to baking. The flint is reddest where it cooled most slowly and in the centre of the clasts is intensely coloured jasper due to haematite production.
 
In detail the Clay-with-Flints comprises a soft, highly weathered, clay matrix consisting of an intergrowth of fine-grained quartz, clay minerals, disseminated opaque oxides and haematite. While many of the flint clasts are broken into sharp fragments, some still retain their original shape. This is evident at a shoreface exposure [D 119 416] 250&nbsp;m northwest of Ballycastle harbour, where there is no clay matrix and the 0.4&nbsp;m thick deposit consists of self-supporting, whole and fragmented, flints and a matrix of coarse flint chips. The typical Clay-with-Flints deposit is exposed in the cliff face, 100&nbsp;m to the northwest. The colour of the Clay-with-Flints reflects those of the component clay minerals and ranges from grey and dark bluish grey to white, pale fawn, purplish to deep reddish brown. Flints usually retain the dull grey and greyish brown hues, with a white weathering crust, which predominate in the chalk. At some localities the flints have a superficial red colour which is merely surface staining. However, at others, such as Donald’s Hill, the flints show a pervasive concentric banding in shades of brown, pink, purple and red probably due to baking. The flint is reddest where it cooled most slowly and in the centre of the clasts is intensely coloured jasper due to haematite production.
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[[Category:The geology of Northern Ireland]]
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[[Category:Northern Ireland]]

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