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Kilcoan, Island MageeAntrim
Summary Full report
Site Type: Cutting
Site Status: PASSI
District: Larne Borough Council
Grid Reference: J461986
Rock Age: Cretaceous (Cenomanian, Coniacian, Santonian)
Rock Name: Belfast Marls Member, Cloghfin Sponge Beds Member, Hibernian Greensands Formation, Island Magee Silts Member, Kilcoan Sands Member, Ulster White Limestone Formation
Rock Type: Sand, Sandstone, Siltstone
Fossil Groups: Belemnite, Brachiopod, Echinoderm
Other interest: No data, Marine sediments

Summary of site:

This site is important because it is the stratotype (see glossary) of the Island Magee Siltstone Member and the Kilcoan Sands Member of the Hibernian Greensand Formation. The Hibernian Greensand is the lower of two major divisions of the late Cretaceous rocks of Northern Ireland; the other being the Ulster White Limestone. The Island Magee Siltstone Member is newly named and combines two previous members, the Grey Marls and the Yellow Sandstone. The succession is a monotonous sequence of green to khaki coloured sandstones, some with silt or clay, totalling about 4.6 m thickness in all. Bed thickness varies from a few centimetres up to a metre.

It rests on the Belfast Marls Member, previously known as the Glauconitic Sands, again recently renamed. Here it consists of about 1.2 m of deep green, glauconitic, muddy sands containing the remains of two ammonite genera, Schloenbachia and Acanthoceras, both useful in determining the age of the beds. A break in time (an unconformity) separates the basal beds of the Kilcoan Sands Member (previously known as the Chloritic Sands and Chloritic Sandstone) from those below including the Island Magee Siltstone. Its total thickness is about 7.5 metres commencing with a thick basal conglomerate whose pebbles and small cobbles are derived from an earlier Cretaceous greensand. The following coarse, soft sandstones contain many fossils, chiefly brachiopods of a major group called rhynchonellids and oysters. Not quite two metres from the base, broken, thick shell fragments of a bivalve, Inoceramus, become common and are joined by two brachiopod genera for the next metre and a half at which point fossils become rare, only reappearing near the top of the member where two genera of sea urchins appear. The member ends at the unconformity with the Cloghfin Sponge Bed Member, the first appearance of the Ulster White Limestone. At the time these beds were being deposited, about 90 million years ago, an extensive marine basin, the Larne Lough Neagh Basin, covered the area. The local succession seen at Kilcoan formed in a smaller sub-basin, the Hillsport Basin, which was strongly affected by earth movements that raised the sea bed and eroded the exposed sediments for long periods (the unconformities). The basal conglomerates of the Kilcoan Sands contain pebbles and cobbles of the earlier Devonian and Triassic conglomerates, transported from the north. An interesting feature of these basal conglomerates is a small group of early Jurassic, middle Liassic, fossils that have been found from time to time. Middle Liassic rocks are not found in Northern Ireland at the present time. It is believed that they were present once, probably slightly north west of this site but, with many other Jurassic and Cretaceous sediments, were eroded soon after their formation providing material, including fossils, for the Kilcoan Sands conglomerates. Such derived fossils, surviving erosion of the parent rock to become incorporated into a later one are said to be reworked or remanie. This site, with two stratotype sections, incorporates much sedimentary and tectonic information of events in the late Cretaceous, Hibernian Greensand times and is vital to our understanding of this period of Northern Ireland's geological history. Conservation is essential and a management plan to clean the site and clear the sections should be put in place.

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