Low-magnesian calcite marine cement in Middle Ordovician hardgrounds from Kirkfield, Ontario

Middle Ordovician (Trentonian) biosparites from the Bobcaygeon and Verulam formations near Lake Simcoe, Ontario, exhibit numerous features demonstrating lithification near the sediment/water interface. These include planar erosion surfaces which truncate shells and intraclasts, undercut hummocks on upper surfaces, vertical borings which cross-cut fossil debris and early cement, and a diverse fauna of encrusting organisms which grew attached to lithified surfaces. A marine origin for early cement is indicated by the development of hardgrounds within a transgressire regionally onlapping sequence, the occurrence of dozens of such units within normal marine sediments, and a lack of features recording early oxidation and/or partial dissolution during exposure to meteoric water. Marine cement from four of these units, examined in thin section, by SEM, and by cathodoluminescence, consists of clear zoned syntaxial overgrowths on echinoderm debris and equant spar on trilobite, bryozoan, and brachiopod fragments. As such, this cement: 1) is morphologically identical to meteoric phreatic low-magnesian calcite cement from calcitized Pleistocene sequences, 2) is interpreted as having been precipitated as low-magnesian calcite, 3) is anomalous in habit and inferred composition in comparison with Holocene marine cement, and 4) demonstrates that generalizations which relate cement morphology and composition to environmental water chemistry in modern diagenetic systems may not be valid when applied to more ancient carbonate sequences.