|Newtownbutler Stream, between Mockbeggar and Mullynagowan townlands, Co. Fermanagh.|
|Site Type: ||Stream section|
|Site Status: ||PASSI|
|District: ||Fermanagh District Council|
|Grid Reference: ||H4226|
|Rock Age: ||Carboniferous (Visean)|
|Rock Name: ||Ballyshannon Limestone Formation, Bundoran Shale Formation, Tyrone Group|
|Rock Type: ||Limestone, Shale|
|Fossil Groups: ||Brachiopod, Coral, Foraminifera, Microfossil, Microfossils, Trace fossil, Trace fossils|
|Other interest: ||No data, Marine sediments|
Summary of site:
Pinning down geological events to periods of time is a key skill in geology and enables comparisons of rocks of similar age to be made. Two prominent and important formations in the Carboniferous period are the Ballyshannon Limestone Formation and the succeeding Bundoran Shale Formation. The boundary between them dates to 344 million years ago and at this locality there are two strong marker features that aid in its recognition.
The rocks on either side of this boundary are variable and in the area east of Newtownbutler good exposures in the Newtownbutler stream show important variations across the junction.
The topmost beds of the Ballyshannon Limestone Formation are highly characteristic upstream and downstream of the bridge on the farm lane. Around 8 m of limestone can be seen here including the junction with the Bundoran Shale but unfortunately the actual plane of the junction is not exposed. The section commences with a bed, about 50 cm thick, sandy in its lower part but consisting of carbonate grains packed together with a clear calcite matrix for the upper 25 cm. Above this bed the rest of the section continues with this pale grey to fawn, grainy limestone in clear cement, usually in thick beds. Recognisable large fossil fragments are rare but a single brachiopod species, Composita ambigua, a smooth-shelled form resistant to abrasion appears to have survived with both valves intact. A few microscopic fossils (a simple plant and a couple of general of foraminifera) are also present. This entire upper portion of the Ballyshannon Limestone is recognisably different to its equivalents elsewhere and has been designated the Newtownbutler Limestone Member.
At the junction between the Ballyshannon Limestone and the Bundoran Shale there is an abrupt change of rock type from the clean limestones of the Newtownbutler Limestone Member to a sandstone about 2 m thick. It rests directly on top of the Newtownbutler Limestone and although there is no direct evidence, it seems likely that there was a time interval between them. This thin sandstone, the Mullinagowan Sandstone Member, commences with a 12 cm bed of finely laminated quartz sand with some feldspar grains. There is also a hint of siltyness and a small lime content. The bed was disturbed and churned by the burrowing of invertebrate animals. Medium to dark grey silty sandstones follow with more animal disturbance and three burrow forms, Chondrites (small diameter burrows branching from a larger central one) predominates with Rhizocorallium (a U-shaped feeding burrow) and Teichichnus (a horizontal burrow with inclined backfilled layering) also present. Fragmentary fossils, particularly crinoid plates, become more common towards the top with the appearance of more fine laminations. No single outcrop shows all these features but they can be seen in the immediate area. Above this member, the usual Bundoran Shale lithologies of muddy and silty shales with very thick dark grey limestones appear, containing crushed coral colonies of two species of Siphonodendron and the less common genus Diphyphyllum.
Both these members suggest a measure of shallowing, the first creating the Newtownbutler Limestone Member in clear water free of fine sediment, the second, the Mullinagown Sandstone, reflecting an inshore environment with periodic exposure of the sands carried in off an adjacent landmass, presumably by a local river. The burrowing strongly suggests that the sands were rich in food particles.
The Newtownbutler stream is the stratotype for both these members. Stratotypes are the rock exposures from which new rock divisions are named and ideally they define the base of each unit and exhibit its typical features. By definition, they are the standard reference sections for comparison with all rocks of similar age and are therefore vital to the interpretation of events and relationships in these remote times. For these reasons the Newtownbutler stream section should be protected from any form of adverse development.