Regional deposition and diagenesis of Lower Ordovician epeiric, platform carbonates: The Romaine Formation, Mingan Archipelago and Subsurface Anticosti Island, eastern Quebec.

Abstract

This study integrates Mingan Islands outcrop and Anticosti subcrop facies analysis with petrography and stable isotope geochemistry to establish a current framework for the Lower Ordovician Romaine Formation in the western Anticosti Basin. Inner to midshelf strata form a wedge, thinning from midshelf to inner shelf and from west to east. A basal transgressive unit onlaps Precambrian basement; it is overlain by muddy, bioclastic limestones and dolostones arranged in two depositional sequences. In the lower sequence the transgressive lag is overlain by a peritidal unit (not developed in the inner shelf), followed by a subtidal unit and a second peritidal unit; the sequence boundary is nonerosive. In the upper sequence, peritidai facies are overlain in the west by a second subtidal unit, which is truncated from west to east by the regional unconformity. In the inner shelf, carbonates are pervasively dolomitized; in the midshelf, peritidal units are pervasively dolomitized but subtidal units are partially dolomitized. Petrography shows two stages of dolomitization, an early, synsedimentary to shallow burial stage characterized by planar replacement dolomites and a late stage characterized by nonplanar replacement dolomite, and pore and fracture filling dolomite cements. Stable Isotope data suggest two phases of late stage dolomitization, the first resulting from intraformational waters warmed by burial, and the second from warm basin derived brines.