GEOCHEMICAL ANALYSIS OF THE CAMBRO-ORDOVICIAN COW HEAD GROUP, WESTERN NEWFOUNDLAND

The Cambrian Explosion, approximately 530 million years ago, marked the first appearance of most major phyla, including carbonate-secreting organisms. The diversity and abundance of these skeletonized taxa did not increase substantially until the Great Ordovician Biodiversification Event (GOBE), approximately 475 million years ago. Following the GOBE, the nature of marine carbonate deposition changed fundamentally to favor skeletal groups as one of the dominant carbonate producers. The Cambro-Ordovician Cow Head Group, western Newfoundland, Canada, is constrained biostratigraphically and is correlated with an established Global Boundary Stratotype Section and Point (GSSP) for the Cambro-Ordovician boundary. The appearance of complex skeletal life in the Upper Cambrian to Middle Ordovician interval is recorded at Cow Head in approximately 255m of deep-water slope deposits consisting of shale, limestone, and conglomerates. Shale and limestone are interbedded, and conglomerates range from massive (clasts >5 meters in diameter) to smaller lenticular units consisting of cm to dm-sized intraclasts. A pilot carbon isotope study was conducted on fine-grained limestones and the matrix of conglomerates to gain a better understanding of the environmental conditions surrounding the biological changes during Cambro-Ordovician time. This preliminary ¹³C_carb profile revealed a negative isotope excursion (from 0‰ to -5‰) in a Furongian Cambrian—Lower Ordovician massive conglomerate (20-m thick) and overlying fine-grained dolomitic carbonate (5-m thick). This excursion, though sampled in coarse resolution, may be related to a large negative isotope excursion (HERB event) reported in the latest Cambrian in other places. Future high-resolution sampling will investigate the nature of this excursion and will determine how the carbon cycle functioned prior to and during the GOBE.