Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development

Abstract

The lower part of the St. George Group of western Newfoundland consists of Tremadocian shallow marine platform carbonates of the Boat Harbour (about 180 m thick) and the underlying Watts Bight (about 60 m thick) formations. In the Boat Harbour Formation, dolomitization is pervasive at the top of most shallowing-upward, metre-scale, peritidal hemicycles. Petrographic examination of the Boat Harbour Formation carbonates suggests that the succession has been affected by at least three phases of dolomitization, which influenced the final rock porosity. These phases have crystal-size ranges of about 4 to 40 µm (earliest dolomite D1), 50 to 150 µm (D2), and 300 µm to 20mm (saddle dolomite D3), respectively. They occur as both replacements and cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The occurrence of near-micritic size dolomites (about 4-40 µm) may suggest that dolomitization started at low temperatures during early stages of diagenesis. The lack of evaporite interbeds in the formation and the depleted δ 18 O values (-6.2±0.8‰ VPDB) as well as the low Sr contents (168±45ppm) of the earliest dolomites likely exclude a brine origin from evaporated seawater. The Sr/Ca molar ratios (0.0069 to 0.0017), calculated for the earliest dolomitizing fluid, suggest a mixture of marine and meteoric waters possibly in a mixing zone environment. The petrographic features and geochemical attributes of D2 and D3 phases, such as their depleted δ 18 O values (-6.9±1.5‰ and -8.3±0.9‰ VPDB, respectively) and Sr contents (177±76 and 117±33ppm, respectively), suggest that they were formed under relatively deeper burial conditions and possibly from hydrothermal fluids which is supported by homogenization temperatures (up to 135º C) and estimates of salinities (up to 23 wt. % NaCl) in the latest dolomites (D3). Based on visual estimates from thin sections, the porosity varies from <1% in most of the formation to about 10% in a dolomitized algal lime mudstone bed in the upper part of the formation, a few metres below the Boat Harbour Disconformity. Except for some vugs, the majority of pores are intercrystalline and associated with D2.The dolomitization events recorded by the Boat Harbour Formation carbonates resemble, in their succession and geochemical signatures, those previously documented in the Aguathuna Formation. However, significant differences are noted with the dolomitization events recorded in the Lower Ordovician Romaine Formation on Anticosti Island; these differences are tentatively associated with the very distinct tectonic settings of each case. . Dans la Formation de Boat Harbour, la dolomitisation est envahissante au sommet de la plupart des demi-cercles péritidaux de moindre profondeur vers le haut, à l'échelle métrique. L'analyse pétrographique des carbonates de la Formation de Boat Harbour suggère qu'au moins trois épisodes de dolomitisation ont agit sur la RÉSUMÉ

Biogenic and abiogenic low-Mg calcite (bLMC and aLMC): Evaluation of seawater-REE composition, water masses and carbonate diagenesis

“…The study also investigates the REE composition of altered brachiopods from the Oligocene of the Antarctic, altered lime mudstone coeval with the Silurian brachiopods, and a suite of altered Ordovician lime mudstone and deep burial calcite cements from the St. George Group carbonates of western Newfoundland (Azmy et al, 2008; Appendix 1). The diagenetic state of most brachiopod and whole rock/cement samples (except for the Eocene/Oligocene ones) was petrographically and geochemically investigated by Azmy et al (1998Azmy et al ( , 2008Azmy et al ( , 2009.…”

“…The cement samples were extracted from crystals, which had their temperatures of precipitation (Th, homogenization temperature of their entrapped primary twophase fluid inclusions) established by microthermometric analysis (Appendix 2; Azmy et al, 2009). About 5 mg of sample powder (new material) of each sample was digested in 0.2 M HN0 3 for 70-80 min a nd analyzed by standard addition techniq ues for major (e.g., Ca, Mg), minor (e.g.,Sr, Mn ), and REE (Coleman et al, 1989) using a Perkin Elmer Sciex "Elan DROf' ICP-MS at Memorial University of Newfoundland. Elemental (Ca, Mg, Sr, Na, Mn and Fe) analyses of modern and Eocene/Oligocene brachiopods were conducted at Brock University following well-established procedures (cf.…”

“…Azmy et al, 2008), and burial calcite cements with entrapmenttemperatures (Th) between 68 and 87 CC (cf. Azmy et al, 2008Azmy et al, , 2009) precipitated in an essentially open diagenetic system from burial fluids.…”

“…REEs have been widely utilized as redox indicators in marine carbonates (e.g., Sholkovitz and Shen, 1995;Feng et al, 2008;Azmy et al, 2009) phosphate deposits (e.g., McArthur and Walsh, 1984;Wright et al, 1987;Chen et al, 2003) and hydrothermal carbonate deposits (e.g., Barrat et al, 2000). ceH is oxidized to Ce4+ under oxidizing conditions, resulting in a negative Ce anomaly.…”

“…The recrystallized lime mudstones (diagenetic phase), including the Ordovician and Silurian materials), with documented considerable alteration of their elemental composition (low Sr contents between 182 and 308 ppm, Azmy et al, 1998Azmy et al, , 2008Azmy et al, , 2009, have yielded higher mean tREE content (14.0 ± 8.7 ppm) than those of the modern warm-(0.3 ± 0.2 ppm) and cold-water (0.2 ± 0.2 ppm) shells and fossil shells ( Table 1). The petrographic features of some fabric retention in the lime mudstones and their relatively small crystal size (10-40 ).lffi) suggest that diagenetic alterations occurred at early burial stages most likely in the phreatic zone of the meteoric environment (cf.…”