EVOLUTION OF THE WESTERN MARGIN OF LAURENTIA DURING THE EARLY PALEOZOIC: TECTONIC AND EUSTATIC EVENTS INTERPRETED FROM SEQUENCE STRATIGRAPHY AND CONODONT COMMUNITY PATTERNS IN THE CANADIAN CORDILLERA

Several field seasons in the Canadian Cordillera resulted in the measurement, description and sampling of over 20 000 meters of Lower Paleozoic strata from 26 stratigraphic sections across four platform-to-basin transects, with the recovery of over 100 000 conodonts, representing over 200 species, from more than 1200 4-5kg samples. This work was part of the LITHOPROBE/SNORCLE project, but is also being extended through a Pan-LITHOPROBE project to understand the tectonic and eustatic response of much of the northern Laurentian plate through the Early Paleozoic. This part of Laurentia did not act as a classic passive margin during the Early Paleozoic, but rather underwent a series of tectonic events complicated by eustatic changes. The field data allowed a sequence stratigraphic interpretation for the complex stratigraphic framework, from which a relative sea level curve was derived. Following detailed conodont taxonomic and biostratigraphic studies, the conodont distributional data were subjected to cluster analysis that has recognized an evolving series of conodont communities through space and time. The communities were partitioned across the platform-to-basin gradient and provide a sensitive indicator of relative sea level change. These two independent approaches generated comparable eustatic curves for much of the Early Paleozoic of this Laurentian margin; some global eustatic events noted by earlier workers were identified. Some of the ancient water-mass interactions along this margin have been tracked using neodymium isotopes from the conodont bioapatite. Four main tectonic phases and six principal eustatic events are recognized. This detailed work yielded success at filtering the global and regional eustatic/tectonic effects and in proposing causes for some of the events that shaped the overall evolution of the western Laurentian margin through time.