ABSTRACT

Carbon and oxygen stable isotopic and trace-elemental variations in diagenetic calcite and dolomite from slope and platform-margin carbonates of the Cambro-Ordovician Cow Head Group, western Newfoundland, are explained by calculations that consider variations in the water: rock ratio during burial history of a reconstructed cross-section of the continental margin.

Platform-margin deposits are preserved as blocks of limestone with calcified algae in slope strata. The blocks contain neomorphic, radial (¹⁸O = -9.3 to -5.8 PDB; ¹³C = 0.7 to + 1.4 PDB) and blocky spar (¹⁸O = -8.3 to -6.7 PDB; ¹³C = 0.6 to + 1.2 PDB) calcite, sparry dolomite (x¹⁸O = +5.8 PDB; x¹³C = +0.6 PDB), and lutecite (length-slow chalcedony; ¹⁸O = +27.9 to +29.7 SMOW). Reaction of these almost isotopically homogeneous rocks with sea water, meteoric water, and upward-migrated basinal water is suggested by their distinctive fabrics and isotopic compositions.

The beds of slope limestone contain abundant calcite microspar and pseudospar (¹⁸O = -9.2 to -6.6 PDB; ¹³C = -5.6 to +0.6 PDB), dolomite (¹⁸O = -9.3 to -2.8 PDB; ¹³C = -3.0 to +1.0 PDB), and minor chert (¹⁸O = +23.4 to +29.1 SMOW). The three dolomite fabrics are fine-crystalline rhombohedra that grew in clay-rich carbonate mudstone, medium-crystalline replacive mosaics that are nonfabric selective, and coarse, white, sparry rhombohedra associated with stylolites and fractures. The microspar and pseudospar grew by widespread neomorphism of 1) fine-grained, detrital calcite or aragonite in lime mudstone, and 2) calcite cement in calcarenite. Cathode-luminescent patterns of calcites and dolomites suggest a prolonged history of recrystallizations (dissolution-precipitation on a fine-scale) during burial diagenesis.

With increasing distance basinward from the platform margin, diagenetic calcite and dolomite are systematically depleted in the heavy isotopes of oxygen and carbon, and diagenetic calcite is enriched in magnesium, suggesting crystal growth and recrystallizations in evolved disgenetic fluids. These data, along with the burial history, indicate that the limestones of the Cow Head Group were systematically altered as greater amounts of burial fluids were either focused along preferred permeability pathways during compactive expulsion or recirculated during possible convective flow through strata farther from the platform margin.