Dolomite-cemented fractures cementation

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... 

Fracture-related dolomite cements in porous sandstones... 

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.

Table 1. Modal analysis data for cemented fractures and porous sandstones examined during this study (a) dolomite-cemented sandstones adjacent to fractures (b) sandstones immediately surrounding the cemented fractures and (c) sandstones above the Main Limestone without cemented fractures. Quartz overgrowths (OG) could only be clearly distinguished in one sample, although they have been observed more widely in SEM...

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-... 

The overall width of the cemented fractures (defined as the extent of complete dolomite cementation) ranges from 0.5 cm to 1 m (Fig. 6D), and shows no relationship to position on the transect (Fig. 8A). The thickest examples are termed composite, consisting of two or more closely spaced. 

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.

Fig. 12. cross-plot of stable isotope data from the dolomite cement types discussed in the text. Gastropod-fill and beeF samples are not from the fractured sand body (see Fig. 4). 

Stable isotope analysis of intergranular, vein-fill and poikilotopic dolomite cements produced very similar results (Fig. 12). Overall, dolomite 5 0 values fall between -3.8 and -0.9%o pdb and 5 C values between -4.2 and -9.5%o pdb. The bulk of the data display a crude positive covariance, albeit with several outlying points. However, no consistent trends were detected across individual cemented fractures. Three samples of ferroan dolomite pseudomorphing gastropods in the overlying... 

The mass transfer problem may be resolved if eogenetic pore fluids were transmitted locally across the GGF in response to fault-related changes in pore pressure and near-surface hydrology (e.g. Knipe, 1993). The cemented fractures at Ballycastle show clear evidence for elevated pore fluid pressures and hydraulic fracturing, and the pulsed nature of dolomite cementation implies a tectonic drive. Kerr (1987) commented on the likelihood that the GGF was active in the Late Carboniferous/ Permian. Argillaceous coastal plain sediments asso-... 

Regional uplift accompanying faulting in the late Carboniferous would have favoured hydraulic fracturing, and the sandstones would have been at relatively shallow burial depths (Fig. 3). This concurs both with the petrographic characteristics of intergranular dolomite cementation and with the isotopic data in terms of potential fluid sources. 

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. 

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