Stable isotope geochemistry of the lower Cambrian Sekwi Formation, Northwest Territories, Canada: Implications for ocean chemistry and secular curve generation

摘要

Three curves delineating delta C-13 variations across the Early Cambrian Sekwi Formation carbonate ramp, Mackenzie Mountains, Northwest Territories, Canada, are incorporated into a biostratigraphic and sequence stratigraphic framework that provides a higher resolution temporal and spatial framework for the Sekwi Formation. The three delta C-13 curves were correlated across a range of depositional environments, from tidal flat to deep subtidal on the Sekwi carbonate ramp. Eight delta C-13 "cycles" in the curves (A-H) were correlated across the basin, complimenting the sequence stratigraphic framework. The most complete delta C-13 curve from the Sekwi Formation correlates well with a composite Early Cambrian delta C-13 curve from the Siberian Platform. However, the differing magnitude and absolute timing of some of the correlative isotopic excursions records the influence of regional tectonism, sea level changes, and diagenesis on the Sekwi ramp. Additionally, the Sekwi Formation delta C-13 curve has more high-frequency variations than the Siberian curve, due in part to the higher sample density used to construct the Sekwi curve. The Early-Middle Cambrian boundary was not sampled in this study, but a large negative isotopic excursion in the very latest Early Cambrian coincides with a regional drowning that may be correlative throughout western Laurentia, but recognizing this event globally is problematic. (C) 2007 Elsevier B.V. All rights reserved.