|Site Type: ||Stream section|
|Site Status: ||PASSI|
|District: ||Armagh District Council|
|Grid Reference: ||H882463, H88494596|
|Rock Age: ||Carboniferous (Tournasian)|
|Rock Name: ||Annaclare Group, Armagh Group, Ballynahone Micrite Formation, Drumman More Sandstone Formation|
|Rock Type: ||Limestone, Sand, Sandstone|
|Fossil Groups: ||Bivalve, Brachiopod, Crustacea, Foraminifera, Gastropod|
|Other interest: ||No data, Marine sediments|
Summary of site:
The geology exposed in the bed of the Ballynahone River shows the only clear exposures of rocks at the top of the Annaclare Group (the oldest Carboniferous rocks of the area) and the basal beds of the overlying Armagh Group. Such exposures are invaluable in this area where superficial deposits mask almost all of the solid (underlying) geology.
The lowest beds of the Annaclare Group, the Killuney Conglomerate Formation and the Retreat Siltstone Formation, are described from the Killuney stream section. The topmost beds of the Retreat Siltstones can be seen in the Ballynahone River bed about 50m north of Deanís Bridge. There is then a break of about 50m to limestones of the Ballynahone Micrite Formation. These dolomitic limestones are pale brown-grey in colour, very fine grained and almost porcelain-like in texture; they contain a limited fauna consisting of bivalves, gastropods (sea snails), brachiopods and ostracods (microscopic water fleas). There are frequent thin microbial laminations and oncoids (subspherical pellets that are the fossilized remains of algal and bacterial colonies). Limestones can be glimpsed intermittently in the bed of the river for the next 100m, at which point an accessible outcrop of 4m of mudstone, varying in colour from grey-green to pale brown and incorporating a few beds of sparsely fossiliferous, dolomitized, fine-grained limestone is clearly exposed. Again the limestones contain algal laminations. Further beds of the Ballynahone Micrite appear in the river bed roughly 100m beyond this outcrop but nowhere is the top of the formation seen.
The Ballynahone Micrite Formation represents the first incursion of the Carboniferous sea into the area just north of Armagh. The Killuney Conglomerate at the base of the Annaclare Group was deposited on an arid land surface which was then inundated by the river and lake deposits of the Retreat Siltsone Formation; the Ballynahone Micrite was deposited on top of these, in lagoons and inlets in advance of a northerly encroaching sea. At this time (around 350 million years ago), the Armagh area was equatorial and the shallow sea, combined with tropical temperatures and evaporation, created high water temperatures and high salinity. These hot brines could be tolerated by only a restricted fauna but appear to have been an ideal environment for the algae and bacteria which formed the laminations and oncoids common in the limestones.
Neither the base nor top of the Ballynahone Micrite Formation is seen in this, its type section, so it is not known whether true open-water marine conditions became established here before the end of the Tournaisian epoch.
The second section exposed appears about 40m beyond the last occurrence of the Ballynahone Micrite Formation at a marked bend in the river. Here about 30m of medium to dark-grey limestones occur with a sparse fauna of larger fossils but with a rich fauna of foraminifera (as revealed under the microscope). The fourteen genera identified indicate a middle Arundian age for these rocks and imply a departure from the hot brine lagoonal conditions to a more open water environment. These limestones and their shaley siltstone partings form the lowest beds of the Armagh Group and are seen intermittently for the next 130m.
The Arundian fossils that follow the marine advance raise a problem in dating the Ballynahone Micrite, which was supposed to be late Tournaisian in age. The Chadian stage (the first Visean stage after the Tournaisian) appears to be missing at Ballynahone, indicating a possible break in the sequence, or that it could be part of the micrites which have no diagnostic fossils or, just possibly, that the rocks have been concealed by the complex faulting just north-east of Armagh. Clearer exposure of the river bed in dry summers may assist in resolving this problem.
The final outcrop in the river section reveals a soft sandstone glinting with mica, carbon stained and with a slight sparkle from its quartz grains. It varies between light brown and grey in colour. It is defined by faults in the river bed but is known (from borehole cores drilled just to the north) to occur on top of the earliest limestones of the Armagh Group. This Drumman More Sandstone Formation is seen in only one other small outcrop in the area. It is believed to be Holkerian in age and to indicate inshore sedimentation, probably fed by a river draining land somewhere to the north or north-west. It was quarried in the nineteenth century and provided stone for the refacing of what is now the Protestant Cathedral in Armagh. Nothing of the quarry survives, except for a scatter of loose blocks.