Abstract

The coeval Early Silurian platformal Sayabec and La Vieille formations covered 10,000 and 20,000 km 2 , respectively, in the Gaspe Basin in the Appalachian orogenic belt. Field studies, petrography, and carbon and oxygen stable isotopes have been used to document the diagenetic evolution of the carbonate platforms. Significant diagenetic features included marine cements, shallow burial marine to possible meteoric-phreatic cements, burial cements, pressure-solution stylolites, fractures and minor fracture-fill cements, meteoric dissolution and meteoric-vadose cements. Cementation started in the marine environment (LF1). It was minor (< 1%) and coated the walls of growth cavities in biolithites of the Sayabec Formation. As seen under cathodoluminescence (CL), these cements consist of an isopachous rind of dull to blotchy luminescent, microcrystalline crystals that constitute the basal portion of microdolomite-rich cement layers. These cement layers, including the LF1 crystals, are now neomorphosed to radiaxial-optic calcites. The heaviest isotopic signature of LF1 cement (delta 18 O = -5.3 per thousand ; delta 13 C = +3.8 per thousand ) is taken as the best value for Early Silurian marine composition. The upper part of the radiaxialoptic cement layers is composed of an initial non-luminescent, equant to prismatic spar (LF2) passing into complexly banded luminescent scalenohedral spar (LF3). These two cements constitute the bottom of cement successions in other lithofacies of both the Sayabec and La Vieille formations. They are interpreted as likely shallow marine burial cements, but a possible meteoric-phreatic origin is not ruled out. Both hypotheses rely on CL petrographic similarities with known examples, their depleted delta 18 O signature and wide range of delta 13 C values. Ferroan cements (LF4) fill over 85% of the pore space, occlusion of which was completed at burial depths of around 450 m. LF4 cements are 18 O-depleted (delta 18 O = -8.1 per thousand and -8.7 per thousand for Sayabec and La Vieille formations, respectively) when compared to LF2 (Delta nearly equal 2.3 per thousand ) and LF3 (Delta nearly equal 1.3 per thousand ) cements for both formations. The delta 13 C values for the LF2 to LF4 La Vieille cements are much higher than those of the Sayabec. This is explained by early methanogenesis related to the paleogeographic nature of the platform. Stylolitization, fracturing and minor calcite cementation, minor dolomitization and sulfatization occurred in the burial environment and postdate LF4 cements. Post-initial burial, LF5 (gravitational) and LF6 cements (delta 18 O = -10.7 per thousand and -10.1 per thousand , delta 13 C = +3.7 per thousand and +4.4 per thousand , respectively) are interpreted to be meteoric-vadose cements filling solution vugs of the La Vieille limestones in the western part of the ramp. They are related to Late Silurian uplift in the Gaspe Belt.

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