INTEGRATED CARBON ISOTOPE CURVE, SEA LEVEL HISTORY, AND BIOSTRATIGRAPHY FOR THE EARLY CAMBRIAN SEKWI FORMATION, SELWYN BASIN, NORTHWEST TERRITORIES, CANADA

Three new high-resolution d¹³C curves of the Early Cambrian Sekwi Formation, Northwest Territories, Canada are correlated within a new sequence stratigraphic framework that is constrained by biostratigraphy including: trilobites, hyolithids, chancellorids, archeocyathans, Tabulaconus, and a variety of trace fossils. The depositional profile of the Sekwi Formation changes upsection from a steep, faulted margin to a gently sloping ramp. The Sekwi d¹³C curves have similar trends and shapes, but the amplitude of isotopic excursions is greatest toward the open ocean and is dampened toward the shoreline. Eight distinct isotopic cycles (labelled A-H) are delineated in the two most basinward sections. Five of these cycles (A, B, and D-F) are correlated to the most shoreward section. The peaks, or highest value of the excursions, in cycles A and D correspond with the Fallotaspis-Nevadella and Nevadella-Bonnia-Olenellus zone boundaries, respectively. A pronounced negative excursion (up to 4 permil) at the top of cycle H corresponds with a regional drowning event at the Early-Middle Cambrian boundary.

The Sekwi Formation is a single 2nd-order megasequence containing six 3rd-order sequences (1-5 Ma duration) and the associated relative sea-level history appears to be tied to the d¹³C record. Minor variations in d¹³C may be related to environmental factors, in general, deep-water facies correspond with lower d¹³C values, whereas shallow-water facies correspond with higher d¹³C values.

Our d¹³C curves for the Early Cambrian are very similar to the curve from Siberia (Brasier and Sukhov, 1998) except for a negative excursion at the Early-Middle Cambrian boundary. Additionally, our curves provide more structure where the Siberian curve is monotonous. The negative excursion at the Early-Middle Cambrian boundary also was documented elsewhere in western Laurentia (Montanez et al., 2000) and may mark a significant regional or possibly global event. The resolution of the Sekwi d¹³C curves are much finer than trilobite biozones and may provide a way to further subdivide time in the Early Cambrian and enable us to use isotope curves for regional and global correlations during this period of rapid biologic evolution.