Neomorphism and cementation in ancient deep-water limestones, Cow Head Group (Cambro-Ordovician), western Newfoundland, Canada

Abstract

Lithification of thinly bedded lime mudstones and grainstones in the Cow Head Group resulted mainly from precipitation of calcite cement and neomorphic spar at shallow burial depths below the sea floor. These crystals are predominantly non-ferroan although the iron content and the size of neomorphic spar commonly increase near the margins of nodular and continuous beds of lime mudstone. Displacive fringes of ferroan fibrous calcite represent the last stage of carbonate precipitation in some limestones. Most calcite δ 13C values range between 0 and −3% (PDB) but δ 13C values as negative as −15% indicate that bacterial degradation of orgnaic matter variably influenced pore-water composition. Serial analyses from the centres to the margins of nodular and continuously bedded lime mudstones suggest that many of the limestones in this sequence were lithified prior to the precipitation of calcite having the prominent negative δ 13C values typical of concretionary carbonate. Calcite δ 18O values range from −5 to −8ℵ (PDB), indicating precipitation from 18O-depleted pore-waters of uncertian origin. Cathodoluminescence shows that aggrading neomorphism in the lime mudstones did not take place by porphyroid or coalescive consumption of a more finely crystalline mosaic to yield a more coarsely crystalline mosaic. Instead, scattered crystals enlarged by precipitation of progressively younger and eventually ferroan calcite, especially at the margins of nodular and continuous limestone beds. The cathodoluminescence zoning in calcite crystals is similar to that observed in meteorically altered limestones where pore-waters were initially oxidizing but became progressively more reducing with time. The preservation of trace element zoning, crystal growth microfabrics, and δ 13C values in authigenic calcite suggest collectively that recrystallization did not occur during subsequent burial.