|Site Type: ||Quarry (disused)|
|Site Status: ||PASSI|
|District: ||Coleraine Borough Council|
|Grid Reference: ||C883389|
|Rock Age: ||Tertiary (Eocene, Palaeocene)|
|Rock Name: ||Lower Basalt Formation, Port Na Spaniagh Member, Tholeiitic Middle Basalt Member|
|Rock Type: ||Basalt, Lignite, Palagonite, Tholeiite|
|Minerals: ||Analcite, Chabazite, Chalcedony, Phillipsite|
|Other interest: ||amygdales, columnar jointing, pipe vesicle, Intrusion, buried topography, laterite, river valley|
Summary of site:
At the commencement of the Tertiary, 65 million years ago, the Laurasian continental plate began to split and the widening North Atlantic started to separate Europe from Greenland and North America. This event triggered extensive volcanic activity along major fractures and the extensive lava fields of the Antrim basalts were one result.
The disused quarry at Craigahulliar documents the events following the first prolonged volcanic episode that created the Lower Basalt when the volcanic and fissure eruptions suddenly ceased and the entire region became dormant. There was a warm temperate climate with a marked seasonal rainfall that carved the lava fields into a landscape of river valleys separated by broadly domed hills. Craigahulliar was situated in one such valley with a north-south trend. Protracted deep weathering of the lava surface, to a depth of 5m, created red lateritic soils by a process of oxidation and hydration (the Port na Spaniagh Member). It was on these grey and red soils that forests of cedar, pine, hazel, spruce and alder thrived, creating a landscape that would look familiar to us today.
This quiescent period, lasting tens of thousands of years (perhaps over a hundred thousand) abruptly ended when new fissures opened and volcanoes punched through the crust, catastrophically flooding the landscape with very mobile lavas (the Causeway Tholeiite Member). The first flow blocked the river, ploughing on through the forest and into any small bodies of standing water in its way. The masses of water trapped in the soil and vegetation beneath the lavas vaporized and vented to the surface through elongated bubbles eventually linking into open tubes. Zeolite minerals (largely chabazite and phillipsite) and later chalcedony crystallized in these tubes and on the walls of the many bubbles trapped in the cooling and thickening lava. The flow crusted over, its surface inflated by blisters creating a pattern of small domes.
In the short period after this eruption, rainfall drained into small lakes linked by muddy and silty torrents. These sediments settled in the stiller waters, depositing leaves, stems and other plant debris. A second flow followed, in turn burying this surface and entirely covering the first flow and reaching further up the valley wall. They were buried by later flows.
Each flow cooled slowly and, as it solidified and contracted, cooling stresses eventually fractured the rock into regular columns similar to those at the Giant’s Causeway.
This is the interpretation of the extensive outcrops in Craigahulliar Quarry which expose the red interbasaltic Port na Spaniagh Member at the base; extensive laterites and lithomarges are draped with much plant debris and lignite on its upper surface. The first flows of the Causeway Tholeiite Member show developments of pillow lavas and hyoclastite deposits in their lowest levels - both indications of lava quenched in water. Crude ‘colonnades’ of the first flows are topped by the fine ‘wormy’ columns of the ‘entablature’ formed by rapid surface cooling. Finer colonnades can be seen elsewhere in the quarry.
The concentration of interest at this site, predominantly of the lateratization process of basalt weathering and the details of volcanic action make it an outstanding study and research area. It is also one of the few sites yielding evidence of the nature of early Tertiary landforms.
The quarry is likely to become a landfill site but discussions on its geological importance have resulted in a management plan that will retain most, if not all, of the principal features while also offering public access.