Dolomite cement intergranular

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.

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... 

In contrast, if the intergranular dolomite precipitated during pore fluid flow associated with cataclasis (Hippier, 1993 Fowles Burley, 1994), it becomes difficult to account for the medial position of the subsequent veins. The greatest rheological contrast would be expected at the margins of the dolomite cementation rather than within the ce-... 

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. 

Despite initial dolomite cementation, the central fractures remained planes of relative weakness and were subsequently reopened a number of times. Because of the surrounding intergranular cement, each dilation episode was followed by passive infill by finely crystalline dolomite, producing a complex symmetrically banded pattern. Precipitation is ascribed to the rapid diminution of pore pressure as dilation took place, and the resultant vein morphologies are typical of hydraulic fracture mechanisms. 

The typical value of porosity for a clean, consolidated, and reasonably uniform sand is 20%. The carbonate rocks (limestone and dolomite) normally exhibit lower values, e.g., 6-8%. These are approximate values and do not fit all situations. The principal factors that complicate intergranular porosity magnitudes are uniformity of grain size, degree of cementation, packing of the grains, and particle shape. 

The intergranular carbonate cement is always iron-rich dolomite (or ankerite). Electron probe microanalysis gives between 7 and 18 mol% FeCOj (and 0.2-0.8 mol% MnCOj) (Fig. 11). The calcium content of the dolomites is relatively stoichiometric, with an average of 51 mol% CaCOj (range 50.0-53.3 mol%). The cement is generally dull to very dull red-brown in CL, with subtle concentric zonation. It is relatively homogeneous when examined in BSEM, suggesting that intracrystalline chemistry is reasonably consistent within and between individual fractures. The dolomite crystals have a variable fluid inclusion density, with a vast preponderance of monophase aqueous inclusions. Any two-phase inclusions identified in thin section... 

Influence of "Early" Cement. The presence of calcite, dolomite and anhydrite in the form of early cement strongly hinders pressure solution and thereby contributes to the preservation of intergranular space. This observation may also be made for early quartzose cement. The latter contributes to the formation of a solid framework in the sandstones which obstructs their later compaction by equalizing tension along the boundaries of the quartz grains of the framework. 

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