Sequence stratigraphy and chemostratigraphy of the Lower Cambrian Sekwi Formation, Northwest Territories, Canada.

摘要

Early Cambrian is an important time period in Earth's history, recording a widespread radiation of skeletal organisms as well as a mass extinction, the Botomian-Toyonian Crisis. Lower Cambrian carbonate rocks should preserve the geochemical record of seawater during both the radiation and mass extinction. There is a paucity of detailed geochemical studies of trilobitic Early Cambrian strata and this study aims to refine both a delta13C and 87Sr/86Sr global secular curve.; The well-exposed Early Cambrian Sekwi Formation in the Mackenzie Mountains, Northwest Territories, Canada records a relatively intact record of both carbonate platform and basinal sediments. Three trilobite zones, Fallotaspis, Nevadella, and Bonnia-Olenellus, indicate that the approximate age range of the formation is ∼525-509 Ma. Sequence stratigraphic analysis, in conjunction with new and previous biostratigraphic collections, indicates that the Sekwi Formation is composed of seven unconformity-bounded sequences that can be grouped into the lower Sekwi (S0-S1) and the upper Sekwi (S2-S6).; Chapter 2 describes the well-exposed, shallow and deep-water facies of the lower Sekwi Formation and constructs a sequence stratigraphic framework for this unit. The deposits of the lower Sekwi Formation are unique to western North America, recording deep-water deposition on a tectonically active, mixed carbonate-siliciclastic ramp. Understanding the sequence stratigraphic position, composition, and local geometry of sediment gravity flow deposits is the key to deciphering the depositional record of this unit, especially tying the shallow- and deep-water facies across this sometimes tectonically active ramp.; Chapter three presents an integrated, carbon isotope chemostratigraphic, biostratigraphic, and sequence stratigraphic framework for the Sekwi Formation. The new integrated framework provides a higher temporal resolution framework compared to the traditional trilobite biozones. delta13C provides a context in which to better understand both local and global oceanic processes.; Chapter four presents Sr-isotope data from the Sekwi Formation and 87Sr/86Sr ratios were determined by two different analytical techniques. The advantages and disadvantages of each technique are discussed in detail. The 87Sr/86Sr isotopic record provides insights into the climatic, eustatic, tectonic, and weathering processes during the Early Cambrian.