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Summary Full report
Site Type: Coastal section
Site Status: ASSI
District: North Down Borough Council
Grid Reference: J408805, J425816
Rock Age: Carboniferous (Tournasian)
Rock Name: Ballycultra Formation, Craigavad Sandstone Formation, Holywood Group
Rock Type: Algal limestone, Dolomite, Evaporite, Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone
Minerals: Anhydrite, Gypsum
Fossil Groups: Alga, Annelid, Bivalve, Brachiopod, Coral, Crustacea, Fish, Miospore, Plant
Other interest: dessication cracks, ripple marks, Fluvial sediments, Marine sediments

Summary of site:

At the start of the Carboniferous period (around 350 million years ago) Ireland was on the southern shore of the continent of Laurentia. This landmass included parts of what are now northern Canada, Greenland, parts of Europe and a large area of the eastern USA. It straddled the equator and drifted imperceptibly northwards on the jostling continental plates whose edges warped to form natural marine basins that filled with an intermittent supply of sediment throughout the period.

This section at Cultra, on the southern shore of Belfast Lough, shows what the earliest sediments to invade the sea bed in this part of Laurentia looked like. The two formations that make up the Holywood Group sit unconformably on the eroded upturned edges of the Ordovician and Silurian sediments that formed the ancient rocky shore. The lower formation - the Craigavad Sandstone - is a massive, red, coarse-grained sandstone with pebbles of reef quartz and Lower Palaeozoic greywackes from the adjacent desert lowlands. Thin lime-rich beds indicate soil formation. Towards the top there are shaley beds containing a few scattered bivalve molluscs and fish fragments. Taken together, all the evidence indicates that these rocks were formed by fast flowing rivers full of debris washed down from the surrounding hills. The water was shallow and the soft sediment was exposed to the air from time to time, when soils formed. In all, about 140m of Craigavad Sandstone can be seen in the faulted outcrops along the shore. The rocks of the overlying Ballycultra Formation show a marked contrast with the red sandstones below. Whereas the red coloration indicates an environment with plentiful oxygen, the greenish-grey to olive green of the Ballycultra Formation reflects a dramatic change of conditions, with low or no oxygen available. These rocks - a combination of shales, mudstones, siltstones and sandstones - contain finely layered carbonates and the mineral calcite (a replacement of original gypsum or anhydrite). Ripple marks, sun-cracked sediments, evaporite minerals (such as gypsum and anhydrite), incrustations of algae and fine-grained, porcelain-like limestones all indicate an extreme brackish water environment, resulting from a hot dry climate with a high rate of evaporation. Fossils are present - mostly plant fragments, bivalve molluscs, tube worms and microscopic ostracods and algae. This particular combination is a further indication of harsh conditions, with hardy organisms struggling to hold on. Rarely, brachiopods are found, suggesting brief, more amenable interludes, probably due to rainwater flood surges, but evidence for a baking hot and extremely salty tidal flat environment is overwhelming. Open sea conditions were nowhere near at this time. The Ballycultra Formation, also around 140m thick at this locality, shows many of the features of the cementstones of the Midland Valley of Scotland and may be an extension of them. The age of the Ballycultra Formation has been determined from three assemblages of plant spores which place it near the end of the Tournaisian (the earliest series of the Carboniferous). Whether the Craigavad Sandstone is of similar age or significantly earlier is not known. These rocks between Cultra and Craigavad together form the Holywood Group and constitute its ‘type section’ (the standard of reference). Despite their similarity in age to the basal Carboniferous rocks around Armagh, they are completely different. The section also, uniquely, contains the unconformity with the overlying Permian rocks. There are no immediate or obvious threats but leisure activities and developments along this shore (particularly boat slips) need to be controlled.

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