Collaborative Research: Interbasinal Applications of Newly- Resolved Middle to Late Cambrian Sr Isotope Stratigraphy: North American Cordilleran Margin

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9628535 Banner The proposed research has one primary objective: to carry out a coordinated sequence stratigraphic and Sr isotope study of Middle to lower Upper Cambrian Carbonates exposed in the southern Canadian Rocky Mountains. This project is a natural extension of related work on coeval strata in the southern Great Basin that integrated a genetic model of deposition with Sr isotope stratigraphy. The Great Basin research has resulted in 1) considerable refinement of a portion of the most poorly-defined interval of the Phanerozoic Sr isotope seawater curve, 2) evidence that the highest seawater 87Sr/86Sr values over the past 2 b.y. likely occurred in latest Middle to earliest Late Cambrian oceans, 3) potential for enhanced chronostratigraphic resolution in this time interval, 4) suggestion of a mechanistic link between eustasy and seawater 87Sr/86Sr during greenhouse times, and 5) potential for placing constraints on the timing and magnitude of Cambrian orogenic events. The ultimate goal of the research laid out in this proposal is to perform an interbasinal test to evaluate the possible connection between eustasy and seawater 87Sr/86Sr during greenhouse times. This collaborative proposal is being submitted in conjunction with a parallel proposal to the VPW Program at NSF that lays out plans for completing and extending the temporal range of the research in the Great Basin. The methods that will be used to carry out the proposed work involved two phases. First, three sections with the most complete exposure and highest degree of biostratigraphic control will be measured in the southern Canadian Rockies, focusing on the Mt. Whyte through lower Lyell Formations. Data from the three sections will be interpreted using the fundamental premises of sequence stratigraphy, with the resulting genetic model used to construct an accommodation history. After all available biostratigraphic information is integrated into the genetic stratigraphy, correlations with the coeval Bonanza King Formati on will be attempted to assess the degree of synchroneity between accommodation events. Of the three measured sections, the one section with the highest petrographic and geochemical analysis of collected samples. All samples will be examined by plane-light, cathodoluminescence, and epifluorscence for evidence of recrystallization, preservation of microfabrics and microstructures, luminescence characteristics, and presence of noncarbonate inclusions which could contribute radiogenic 87Sr during sample dissolution. Samples that meet petrographic criteria will be analyzed for geochemical indicators to assess their state of diagenetic modification as an additional prerequisite to selection of least-altered components for Sr isotope analysis. Finally, samples will be run through pretreatment techniques that have been refined as a result of previous testing on samples from the southern Great Basin. The ultimate goal of the proposed interbasinal project is to evaluate a new model that suggests a mechanistic link between eustasy and seawater 87Sr/86Sr during greenhouse times. Earlier work in the southern Great Basin suggests a correlation between the Sr isotope stratigraphy and long-term transgressions and regressions that affect the amount of continental surface area available for weathering. This model, as well as the secular seawater isotope curve, need to be tested in a completely different basin to assess their scientific validity. If the results from the southern Canadian Rocky Mountains confirm the temporal seawatep 87Sr/86Sr trend and the connection between eustasy and seawater 87Sr/86Sr, then an entirely new level of resolution in the Sr isotope curve for Paleozoic time will have been attained. This may spur further research into searching for trends in the Paleozoicseawater 87Sr/86Sr curve at the 106 year scale that may be related to eustatic variation. On a broader scale, the 87Sr/86Sr data from this integrated study will contribute critical information regarding the widespread Cam brian orogenesis associated with the assembly of Gondwana, Cambrian biotic radiations and extinctions, and perhaps about the extensive development of Cambrian passive margin depositional sequences.