Rathlin basin

The Rathlin basin has undergone a complex tectonosedimentary evolution, and this is reflected... 

Fig. 1. Tectonic map of northeast Ireland-southwest Scotland, showing the Rathlin basin and the onshore area investigated near Ballycastle (arrowed). Carboniferous outcrops in black. Cross-section X-Y is shown in Fig. 2.

Fig. 2. Simplified subsurface section across the southern Rathlin basin to the Ballycastle study area (arrowed) see Fig. 1 for location. Adapted from McCann (1988).

Fig. 3. Reconstructed burial histories for the Carboniferous succession onshore (top) and in the southern Rathlin basin (bottom). The dark line in the top diagram represents the approximate position of the studied sand body, assuming 400 m post-Carboniferous erosion estimated from regional geology. Rathlin basin data from Parnell (1992). Burial temperatures at positions A-C are discussed in the text.

The facies relationships, spatial distribution and petrographic characteristics of dolomite cement in the Ballycastle foreshore are incompatible with a dolocrete origin. Formation from evaporite-related fluids can be discounted if the dolomites are pre-Late Permian in age (and no Permian evaporites have been proven in the Rathlin basin e.g. Mc-... 

Internally complex, ferroan dolomite-cemented fractures are a prominent feature within a fluvial sand body at Ballycastle, on the margins of the Rathlin basin in northeast Ireland. The cemented fractures display a tight modal orientation that is coincident with the dominant local normal faulting trend, and are interpreted to have formed in the same tectonic regime. However, there is no clear trend in the spatial distribution or width of the fractures with respect to a fault plane situated at one end of the outcrop. 

Dolomite cement components were all sourced from outside the sand body, most probably from local or basinal mudrocks. Stable isotope data indicate a mixed organogenic-marine carbonate source, and precipitation at relatively low temperatures (s70°C, if pore fluids were sourced from clay mineral dehydration reactions during deep burial of Carboniferous mudrocks in the Rathlin basin 55°C if they were locally sourced). Thermobaric mass transfer was enhanced by tectonic pulsing and dolomite precipitation was driven by CO2 degassing. 

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