Summary of site:
Fossils serve many roles in geology but they are particularly valuable in the dating of rocks. The time span from the earliest appearance of a particular fossil species in the geological record to its final demise is described as its range. Some species have very long age ranges (over 10 million years) and are only valuable for dating in the most general of terms but others exist only fleetingly and give precise age information. Ideal zonal fossils (defining a time zone) should be plentiful, wide ranging (not confined to a specialised habitat), visible to the naked eye and rapidly evolving so that individual species have only a brief life span. For significant parts of the last 570 million years (from the time when recognisable fossils first appear in rocks) all these criteria are not met and in these circumstances geologists may resort to microfossils (microscopic remains such as spores, foraminifera, conodonts etc) to provide a chronological framework. There is one other serious limitation of fossils; they cannot give an exact numerical age i.e. in millions of years, described as an absolute age. They can only give an age relative to other fossils. Absolute ages are the preserve of radiometric dating, based on the decay rates of radioactive minerals. Correlations of the two methods are now refined and present geologists with very powerful tools.
The Carboniferous rocks of the Ballycastle area have posed a long-standing dating problem. Along the coast from Doon, east of the golf course extending east around Fair Head into Murlough Bay, there is a series of poor quality coal measures that were exploited from mediaeval times until the second half of the 20th century. It has always been known that they were much earlier than the productive measures in Britain and that they spanned the boundary of two major epochs of the Carboniferous period, the Visean and the succeeding Namurian. The problem was exactly where to draw the line.
The rocks in the key part of the section are sandstones, mudstones and shales with coal seams and occasional marine bands. Limestones are few and thin but the Main Limestone Member, mostly mudstone but with three thin limestones, is around 20 m thick and is of particular importance because it contains a large brachiopod, Gigantoproductus giganteus with a late Visean age, probably restricted to its last two age stages, the Asbian and the Brigantian, but most common in the Brigantian. The next fossil of importance occurs about 80 m above the top of the Main Limestone in McGildowney’s Marine Band and is another brachiopod, Schellwienella rotundata, definitely of Namurian age. With no other age indicators the boundary between the Visean and the Namurian was arbitrarily drawn at the base of the Main Coal, about 30 m below McGildowney’s Marine Band.
In 1973, in the course of routine fieldwork, goniatites (early relatives of the ammonites) were found in a lime-rich sandstone immediately above the Main Limestone Member. Goniatites match almost all the criteria of good zonal fossils and have very limited age ranges. All 15 specimens collected belonged to the genus Sudeticeras and one was sufficiently well preserved to be identified to species, namely Sudeticeras adeps, confined to the P2b goniatite zone which is limited to the Brigantian stage, definitely Visean and its very last part, giving an unambiguous age.
The last 1.25 m of shales and limestones above the goniatite bed mark the transition from marine to deltaic conditions but the boundary between the Visean and the Namurian has remained at the base of the Main Coal.
This site demonstrates the significance of new fossils finds in rocks previously considered to have been thoroughly studied and illustrates the importance of accumulating dating evidence from good zonal fossils in age determination.