THE ROLE OF SKELETONS IN THE EARLY PALEOZOIC CARBONATE FACTORY: EVIDENCE FROM WESTERN NEWFOUNDLAND AND WESTERN UTAH

The evolution of mineralized skeletons near the Proterozoic-Cambrian boundary set the stage for a major shift in the processes and patterns of carbonate deposition in the world's oceans. Quantitative analysis of petrofabrics in the Middle to Upper Cambrian Port au Port Group, western Newfoundland, and Ordovician strata of the Ibex Area, western Utah, indicates, however, that despite the multi-clade radiation of carbonate skeletons earlier in the Cambrian Period, the skeletal sink for carbonate did not develop fully until Ordovician time.

Carbonate facies in the Port au Port Group consist of oolites, flat-pebble conglomerates, microbial buildups, and micrite -- much like Neoproterozoic ramps and platforms. Skeletons make up only a small proportion of carbonate observed in outcrop. In contrast, Lower and Middle Ordovician carbonates of the Ibex area consist predominantly of grainstones, packstones and wackestones that are typically fossiliferous. To complement field observations, a petrographic study was conducted to quantify the carbonate components preserved in thin section. Common facies were analyzed to determine if the apparent contribution of carbonate skeletons to total carbonate was influenced by the size of fossil material. Both field and thin section studies show that carbonate fossil material comprises only a small proportion of the total carbonate that accumulated during later Cambrian time. A few individual beds contain relatively abundant fossil material (18.5% of points counted). Mean abundance of skeletal components in the Cambrian carbonates examined, however, is 4+/-6%, with a median and mode value of 0. Preliminary observations of Ordovician facies from the Ibex Area, western Utah, suggest there is an increase in fossil content in thin section relative to facies examined from the Cambrian.

Quantification transformed paleobiological studies of the fossil record years ago, and it promises to facilitate geobiological comparisons, as well. Despite the appearance of skeletonized taxa near the Proterozoic-Cambrian boundary, carbonate skeletons account for only a small proportion of total carbonate production until the Ordovician, when a marked diversification of heavily calcified animals and algae solidified the global importance of skeletons in limestone deposition.