Crockanaver
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Geology & landscape
Geological Feature: Rock Outcrop · Mining / Industrial: Quarry · Geological Period: Carboniferous · Rock Type: Limestone, Shale
Description
Crockanaver Carboniferous stratigraphy Crockanaver is important because of its geology. In particular the stratotype section in Crockanaver Quarry exposes a thick sequence of the Carboniferous Crockanaver Member lying above limestones and thin shales that are typical of the Ballyshannon Limestone Formation of which this Member is a part. The base of the member is exposed and the sequence also preserves an abundance of fossils. The Ballyshannon Limestone Formation crops out over the largest part of this area and is well exposed in many quarries. However, at the disused quarry at Crockanver the base of the Crockanaver Limestone Member can be observed. In the quarry the lowest 12m of strata are part of the Ballyshannon Limestone Formation. They are thin to very thick dark grey medium-grained crinoidal packstones, variably argillaceous. Fossils occur sporadically throughout these strata, in particular brachiopods, including rare specimens of Delepinea carinata, indicating a mid-Arundian age, and occasional colonies of the coral Siphonodendron martini. The contact between the base of the Crockanaver Member and the underlying Ballyshannon Limestone is very sharp. The Crockanaver Limestone Member is approximately 10m thick and consists of thin to thick beds of medium fawn to pale grey crinoidal grainstones. Fossils are invariably fragmented and all fragments are usually micritised. The one exception to this is the brachiopod Composita ambigua which occurs commonly as unbroken specimens with both valves conjoined. Corals do not occur in the member. Coarse, shale-free, grainstones such as the Crockanaver Limestone Member are believed to indicate deposition of lime sand in shallow, agitated water. The presence of strong currents is responsible for winnowing-out all fine-grained lime mud or terriginous muds, leaving behind only the coarsest particles of carbonate debris and, more rarely quartz grains. The carbonate sand is loosely packed and individual grains are cemented together by coarse calcite spar. The lenticular shape of individual outcrops of the Crockanaver Member, and its discontinuous outcrop pattern, suggests that shallowing of the sea bed was not a universal feature of the basin at this time. The grainstones probably developed as shallow water, submarine, carbonate sand banks on what was still a very shallow widespread platform at that time in much of northwest Ireland. https://www.daera-ni.gov.uk/publications/crockanaver-assi