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White HeadAntrim
General view of the lowest members of the Ulster White Limestone Formation at White Head, Co.Antrim.
Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Carrickfergus Borough Council
Grid Reference: J475913
Rock Age: Cretaceous (Santonian)
Rock Name: Cloghfin Sponge Beds Member, Ulster White Limestone Formation
Rock Type: Limestone
Fossil Groups: Belemnite, Sponge
Other interest: No data, Marine sediments

Summary of site:

This site is celebrated for the great quantity and variety of fossilized sponges that occur in the Cloghfin Sponge Beds Member of the Ulster White Limestone Formation. The member is Cretaceous in age, around 85 million years old.

White Head is the prominent point on the northern shore of Belfast Lough where it opens into the North Channel. The rocks of the headland range from chalky white limestone on the foreshore to the capping, black Antrim Basalts. 8 members of the Ulster White Limestone are present in this section but it is the lowest, the Cloghfin Sponge Beds that draws particular attention because here it develops its maximum thickness of just over 1.7 m and its richest fauna. It is divided by well-defined bedding planes into 3 units, beds A to C, all containing fossil sponges. Bed A is called the cobble bed, a reference to its weathered appearance, and consists of chalk dotted with glauconite grains that give a green tinge. Bed B is less glauconitic and contains orangey pseudomorphs, particularly near its base. Bed C is particularly sponge-rich, has fewer glauconite grains and has markedly wavy bedding planes. The sponges are dominated by a class called the hexactinellida characterised by meshes of minute opaline silica spicules with rays at 90 degrees to each other. 7 genera are common but there are considerably more species. The sponges appear to have been washed into position from elsewhere but the member can be divided into two based on two species of the sponge genus Rhizopoterion, the lower typified by Rhizopoterion tubiforme, the upper by Rhizopoterion cribosum. The division between the two is in bed C. The member is dated from more ubiquitous fossils, particularly sea urchins, belemnites and crinoids. These indicate the Uintacrinus socialis zone, a subdivision of the Santonian stage, about 85 million years old. This site gives the member its name because it is here that it achieves maximum thickness and diversity. It is the standard for the member and is therefore its stratotype (type section). The Cretaceous sea bed of the time was sub-tropical and divided into a number of basins separated by shallow ridges. This site lies in the Hillsport Basin. The range of fossil sponges at the type section and the limited distribution of the member, mark this site out as of national importance, and a priority for conservation.

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