Dolomite-cemented fractures

Internally complex, ferroan dolomite-cemented fractures at Ballycastle are closely associated with regional normal faulting and are genetically linked with cataclastic textures typical of brittle deformation in porous sandstones. Cemented fractures described in this paper are distinguished from the principal slip planes ( faults ) on the basis of minimal displacement (centimetre scale at most), and the definition encompasses the tectonodiagenetic products of initial cataclasis, cementation of the... 

Fig. 4. (A) Stratigraphy of Carboniferous rocks at Ballycastle, based on outcrop logging and data from Wilson Robbie (1966). Sandstones continue for approximately 200 m below the base of the log but are poorly exposed. (B) Sedimentary log of the study horizon labelled with sampling sites (1) cemented fractures and host sandstone (2) the Main Limestone (3) sandstone of similar facies but lacking dolomite-cemented fractures (4) dolomite beef vein. See text for description of facies associations.

Fig. 6. (A) Location of the mapped foreshore transect (see Fig. 5) the Main Limestone is exposed above in a 15-20 m cliff section 175 m south of the foreshore. (B) Map of dolomite-cemented fractures on a 100 m x 20 m transect some fractures extend landward of the transect. (C) Summary of fracture orientations measured across the transect. (D) Summary of fracture dimensions measured across the transect.

Dolomite-cemented fractures display a unimodel NNW-SSE orientation and variable distribution, with several zones of frequent, closely spaced examples (Fig. 6). Wave action has eroded the relatively friable surrounding sandstones below the high-tide mark, to leave the cemented fractures standing proud (Fig. 7A,B). Above the tidal limit the dolo-... 

Fig. 10. Hand specimen, photograph and simplified drawings of a complex dolomite-cemented fracture, illustrating the adopted terminology and the mutual relationships of CSBs, intergranular dolomite cement (pale grey) and subsequent dolomite-filled dilatational veins (dark grey-black). The graph shows the lateral variation of point-counted intergranular dolomite cement and porosity from the centre to the margin of the fracture.

Triassic (re)activation of NNW-SSE-trending faults at Waterfoot (east Antrim) was accompanied by injection of sandstone neptunian dykes but without associated dolomite-cemented fractures (Kerr, 1987). In contrast, no neptunian dykes are seen at Ballycastle. 

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. 

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