Dolomitization of Deep-Water Sediments, Cow Head Group (Cambro-Ordovician), Western Newfoundland

Abstract

ABSTRACT Dolomite in the Cow Head Group occurs as 1) dolomitic siltstone interbedded with dolomitized or undolomitized shale; 2) prominent tan weathering, argillaceous partings between bedded limestones; 3) disseminated crystals in limestones; and 4) as a component of the interparticle matrix in conglomerates. Dolomite crystals are present as detrital cores, cement, or replacement of limestone and shale. Crystals are commonly characterized by inclusion-rich cores, inclusion-poor rims, and late iron enrichment. Many inclusion-rich cores in disseminated dolomite crystals and dolomitic siltstones are detrital. Most inclusion-rich cores, however, probably originated from replacement of peloids, and possibly other allochems. These detrital and authigenic cores are overgrown by inclusion-poor, authi enic rims. Other authigenic crystals lack an obvious core region and consist entirely of inclusion-poor dolomite. The authigenic dolomite originated in uncompacted or partially compacted sediments during shallow burial below the sediment-seawater interface, as indicated by 1) the occurrence of dolomitic siltstone and shale clasts in polymictic limestone conglomerates; 2) sharp, erosive contacts between dolomitic siltstone and shale, and overlying undolomitized shale or conglomerate; and 3) spatial association with columnar calcite cement, interpreted to be a shallow-burial precipitate, in clast-supported conglomerates. The dolomite crystals continued to enlarge, rather than recrystallize, during deeper burial, as indicated by variable and complex cathodoluminescence zoning. An organic signature is not apparent in their carbon isotopic compositions. The oxygen isotopic composition of these crystal reflects the deeper burial conditions of higher temperatures, more evolved pore waters, or both. The Cow Head Group also contains minor, late diagenetic dolomites which occur as cement and a replacement of limestone, shale, and earlier dolomite. This dolomite postdates tectonic stylolites and is spatially associated with joints and faults.