Neomorphic spar versus cement in some Jurassic grainstones: significance for evaluation of porosity evolution and compaction

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Abstract
Thinly-bedded Upper Bathonian grainstones with shales, in Wiltshire, UK, have intergranular spar of 2 kinds, a primary calcite cement and a neomorphic spar that has replaced an earlier fibrous cement. The earlier cemented grainstones have been less compacted than those cemented by a late calcite spar (fewer broken grains, fewer grain contacts, more intergranular spar). The crystals in the neomorphic intergranular spar are mostly subequant, have irregular intercrystalline boundaries, patchy extinction, some dusty relics of earlier cement fringes and, with cathodoluminescence, show bright orange, needle-like relics of earlier cement fabric. Most of this subequant neomorphic spar is indistinguishable from the neomorphic spar that has replaced the aragonite of molluscan shells. It is concluded that calcitization of an early cement and of the aragonite shells, along with the precipitation of the true cement spar, were delayed until after the carbonate sands had undergone some compactive fracture and grain-grain pressure-solution.