Proterozoic ministromatolites with radial-fibrous fabric

Small digitate stromatolites with diameters in the range of 0·2–20 mm (ministromatolites) are common in Early and Middle Proterozoic carbonate sequences, and extend stratigraphically from the Archaean to the Holocene. An occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial-fibrous fabric and microscopically crinkled (microcrenate) lamination is described from the ? 1·9 Gyr old Belcher Supergroup (McLeary Formation) in southeastern Hudson Bay, Canada. The structures, which can be considered to be a variety of tufa, are unusually well preserved because of early diagenetic silicification. Columnar types are referable to Pseudogymnosolen (Asperia), and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern Canada, where the fabric is normally preserved by a secondary mosaic of dolomite. The textural evidence of angulate cross-sections and rectilinear divergent patterns indicates that the radial-fibrous fabric represents primary or very early diagenetic precipitation, and that pseudogymnosolenids with mosaic dolomite originally also had radial-fibrous structure. The precipitation may have been within, or on, microbial mats.