Burial dolomitization and porosity development in a mixed carbonate‐clastic sequence: an example from the Bowland Basin, northern England

Abstract

Widespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock‐dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature. All stages of calcite cement pre‐date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late‐stage diagenetic origin is also indicated within the sandstones, with dolomite post‐dating the development of quartz overgrowths. Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space‐filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO3. Generally the dolomites have isotopic compositions depleted in δ18O compared to the host limestone, with similar or lighter δ13C values. Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ18O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements. Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity.

Geochemistry of a high-T hydrothermal dolostone from the Emirli (Ödemiş, western Turkey) Sb-Au deposit

“…Dolomite crystals in the layer are closely packed with no intercrystalline matrix, and according to the classifications of Gregg and Sibley (1984), Gawthorpe (1987), and Sibley and Gregg (1987), show an overall planar-s (subhedral) texture characterized by subhedral to anhedral dolomite crystals with straight, compro mise boundaries and crystal-face junctions (Fig. 3a,b).…”

Disseminated ‘jigsaw piece’ dolomite in Upper Jurassic shelf sandstones, Central North Sea: an example of cement growth during bioturbation?

“…Burley, 1993), or from dewatering mudrocks surrounding the reservoir (e.g. Gawthorpe, 1987;Macaulay et al, 1993). Likewise, dolomite formed in coastal aquifers tends to show complex zonation, re¯ecting the dynamic chemistry of mixing zones (e.g.…”

Karstified and fractured Lower Carboniferous (Mississippian) limestones of the UK ‒ A cryptic geothermal reservoir

“…At core plug (small core cut into main core for reservoir quality measurement) scale (2 × 8 cm), porosities up to about 30% have been recorded but more commonly lie in the range 1-5% (Bouch et al 2004;Total 2007). The highest porosities typically occur in dolomitised rock (Gawthorpe 1987) as roughly biomouldic pores. Commonly such porosity is vuggy and poorly interconnected yielding permeabilities of typically less than 10 mD and often less than 1 mD (approx.…”