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Tircrevan Burn (palaeontology)Londonderry
Summary Full report
Site Type: Stream section
Site Status: PASSI
Grid Reference: C701326
Rock Age: Cretaceous (Cenomanian)
Rock Name: Cretaceous Basal Conglomerate, Hibernian Greensands Formation
Rock Type: Bone-bed, Conglomerate, Glauconitic sandstone, Sand, Sandstone
Fossil Groups: Bivalve, Crinoid, Fish
Other interest: No data, No Data, sea-floor 'high'

Summary of site:

The complete sequence of rocks seen in Tircreven Burn is described under that name as record 1206. This account deals specifically with a thin bed of rolled material, a conglomerate, at the very base of the Cretaceous rocks in the upper part of the valley.

Tircreven Burn is the deeply incised stream flowing northwards, draining the eastern slopes of Binevenagh. It originates in the Binevenagh Forest and in its descent to the Big Drain it cuts through the Tertiary, Upper Basalt Formation, the Ulster White Limestone Formation and the Hibernian Greensands Formation both of the Cretaceous period and Lower Lias strata from the first major epoch of the Jurassic period. The depth and steepness of the valley render it unstable with frequent rock falls and debris slides and consequently the appearance of the valley can change dramatically over relatively short periods. The sites described by 19th century geologists are no longer recognisable. There are no rocks in the stream section between the last Liassic bed and the Cretaceous Basal Conglomerate so here there is a time gap of over 100 million years. We know from other parts of Northern Ireland that Jurassic marine conditions extended into Middle Liassic times because ammonites of that age have been found in local glacial drift and could not have travelled far. Even allowing for that, the time gap remains at about 100 million years, more than half the Jurassic period and all the first half of the Cretaceous. It could be that rocks were deposited at this time and completely eroded away but it is much more likely that the area was dry land for most of this time, all the more intriguing since this was the heyday of the dinosaurs. The basal conglomerate seen over the last half century is a brown bone bed no more than 8 cm thick but attracting disproportionate interest. This bed consists of reworked Lias, by which is meant that it is composed of shore debris washed from the Liassic rocks exposed along the Cretaceous shoreline, with some quartz and glauconite, the latter a common mineral of the Cretaceous Greensands. The “bones” of the bed are, in fact, chiefly small fish teeth. The common Liassic oyster Gryphaea (better know as the Devil’s Toenail) and some Liassic crinoids are also present. Only Joseph Portlock in the mid-19th century attempted to identify the teeth and suggested that some were from a shark, Oxyrhina. In Cenomanian times in the middle of the Cretaceous period, around 100 million years ago, a shallow sea encroached onto land flanking the Rathlin Basin to the east. Because the area was stable, the water never achieved great depth resulting in a much condensed rock sequence compared with that of, say, the Ballycastle area. The Cretaceous Basal Conglomerate at Tircreven is the first unit in this sequence and is dominated by fossils derived from the Lias immediately beneath. The high concentration of fish teeth, generally uncommon though highly resistant to erosion, suggests that they could be an accumulation from a long period of erosion without sediment deposition but, on the other hand, all the larger fossils are similar to those in the top few metres of the local Lias so the variety seen in the conglomerate may simply reflect the fossil diversity in the immediate area. Much more research is needed to test these competing hypotheses. Tircreven Burn is a nationally important site, the only one on the western margin of the Antrim Plateau to expose the basal beds of the Cretaceous succession, with their problematic vertebrate fauna. The contact between the Jurassic and Cretaceous is poorly known in Northern Ireland and there is scope for major research on the basal Cretaceous beds everywhere. This thin but enigmatic bed will be a vital component of such investigations. The site is very difficult to access and its instability makes exposure unpredictable but it should be designated for its unique interest.

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