A NEW CARBON ISOTOPE CURVE FOR THE EARLY CAMBRIAN SEKWI FORMATION, SELWYN BASIN, NORTHWEST TERRITORIES, CANADA
Three new high-resolution d13C curves for the Early Cambrian Sekwi Formation, Northwest Territories, Canada provide a framework for understanding the local and regional geologic development of the western Laurentian carbonate platform. Each curve contains 250 to 500 d13C measurements from samples collected every 1 to 1.5 m. Two of the three curves, Caribou Pass and Section 4, span the upper Fallotaspis, Nevadella, and Bonnia-Olenellus trilobite zones as well as the Early-Middle Cambrian boundary. The resolution of the Lower Cambrian d13C curve is much finer than trilobite biozones and may provide a way to further subdivide time in the Early Cambrian.
A representative d13C curve for the Early Cambrian Sekwi Formation is from Caribou Pass, which closely replicates the V-IX oscillations of Brasier and Sukhov's (1998) Early Cambrian d13C curve. The base of the Caribou Pass curve has very negative values (-2 to -3 PDB) that rise to values of +3 PDB. These high values are overlain by values between +0.5 to +2 PDB that continue up through the Nevadella zone. A break in the d13C curve occurs at the base of the Bonnia-Olenellus zone due to the formation of a regional sandstone unit that likely formed during the initial part of the Sinsk transgression. Overlying the sandstone the d13C values hover near 0 and then decrease to -3 PDB in a stepwise manner similar to the Brasier and Sukhov (1998) curve. Another gap in the Caribou Pass section corresponds to a thick section of red beds that likely formed during the Hawkes Bay regressive episode near the middle of the Bonnia-Olenellus zone. Above the red beds, d13C values oscillate between +1 and -1 PDB until approaching the Early-Middle Cambrian boundary. The Caribou Pass curve deviates from the Brasier and Sukhov (1998) positive rise across the Early-Middle Cambrian boundary, and is instead marked by two negative spikes (-2 and <-4 PDB). This deviation to markedly negative values was documented elsewhere in western Laurentia (Montanez et al., 2000) and may mark a significant regional or possibly global event.