CARBONATES IN SKELETON-POOR SEAS: NEW INSIGHTS FROM THE MIDDLE AND UPPER CAMBRIAN MARCH POINT AND PETIT JARDIN FORMATIONS, WESTERN NEWFOUNDLAND

Biomineralized skeletons emerged as part of Ediacaran-Cambrian animal diversification. Despite this, skeletons first came to play a dominant role in the marine carbonate cycle with the Ordovician radiation of heavily skeletonized invertebrates and algae. The more or less coeval diversification of skeletal taxa in a number of major groups suggests the influence of extrinsic drivers, either ecological or physical. Cambrian carbonates have the potential to illuminate this issue by offering insights into the biogeochemical nature of oceans after the emergence of skeletal biomineralization but before the Ordovician event. The Middle and Upper Cambrian March Point and Petit Jardin formations, western Newfoundland, accumulated in shallow shelf environments on the southeastern margin of Laurentia. As in coeval deposits elsewhere in Laurentia, skeletal limestones are rare. Carbonate lithologies consist largely of thinly-bedded micrites, oolites, flat-pebble conglomerates, and microbial build-ups. Similar facies occur in Proterozoic carbonate successions, but the older packages differ in the morphology, petrology, and paleoenvironmental distribution of certain microbialites. Within Laurentia, at least, unusual dendritic microbialites occur in later Cambrian and earliest Ordovician rocks (Shapiro and Awramik, 2006). The petrology of Newfoundland examples is distinctive, with Renalcis-like fabrics encrusted by bladed cements. Other microbialites preserve microsparite veneers with penecontemporaneous brittle deformation. Such morphologies and fabrics can be interpreted in terms of locally rapid seafloor carbonate precipitation. Together, facies distribution, morphology and petrology suggest that Cambrian microbialites reflect a world in which animals influenced sediment fabrics and mat distribution, but not yet the carbonate cycle. Microbialite accretion was facilitated by local sources of alkalinity, potentially including upwelling of anoxic waters. Cambrian carbonates, and in particular microbialites, may, thus, record a distinctive state of the global ocean during the transition from a nonskeletal Proterozoic carbonate cycle to a true Phanerozoic carbonate cycle dominated by skeletons.