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43382-G8
Integrated Sequence and Chemostratigraphic Study of the Cambrian Succession in the Southern Great Basin
Global sea-level change has been interpreted as the major controlling mechanism for both depositional sequences and carbon isotopic excursions. Yet the exact relationship between sea-level changes and variations in seawater geochemistry remains at issue, with considerable debate about the inconsistency between carbon isotope maxima/minima and sea level revealed by sequence stratigraphic studies. The proposed research is aimed at a detailed evaluation of the relationship between sea-level changes, sequence development, and carbon isotope excursions by sampling multiple sections across a platform-to-basin transect of the Cambrian platform in the southern Great Basin. The focus of this PRF-funded research was to characterize the following: (1) carbon isotope variations across sequence boundaries (and equivalent surfaces), (2) relationships between high- and low-frequency sea-level changes and temporal carbon isotope variations, and (3) local carbon isotope departure from average seawater signal of origin to facies variation and diagenesis.
During the 2006-2007 grant-supporting period, Robyn Howley (PhD student), Lael Vetter (MS student), and Shereena Dyer (Summer undergraduate scholar) and I have conducted field trips in central, eastern, and southern Nevada and western Utah. We collected more than 1,200 samples spanning the late Early Cambrian to Late Cambrian from three key sections in Wah Wah Range (Utah, shallow-water facies), House Range (Utah, deep-water facies) and Frenchman Mountains (Nevada, shallow-water facies) to test (1) the lateral isotope variations between the shallow-water platform and the deep-water House Range Embayment, (2) the relationship between isotope excursions and sea-level regressions, and (3) the potential global carbon isotope excursion(s) in the Middle Cambrian (e.g., the Drumian Carbon Isotope Excursion – DICE). In combination with the results from the funding period of 2005-2006, a total of 686 analyses have been completed and another 500 sampling are being analyzed. Existing results indicate that (1) sequence boundaries in shallow-water facies correspond to condensed deposits in deep basinal facies due to decreased platform production; (2) within a sequence stratigraphic framework, carbon isotope values of the regressive facies in basinal section show a 1-2 per mil negative shift compared to equivalent regressive facies in platform sections. This may reflect enhanced sulfate reduction in the anoxic basin associated with increased sulfate supply during regression; and (3) the Middle Cambrian carbon isotope excursion (DICE) is correlatable across the platform to basin and potentially represents a regional to global isotope excursion. The results are being prepared for submission to high-quality peer-reviewed journals.
To reach the goal of testing the relationship between sea-level change and isotope excursions, Paul Kosmidis (MS student), Heather Proa (undergraduate student), and Rachael Johnson (undergraduate student) conducted isotope analyses for the Late Ordovician strata and the late Neoproterozoic strata where similar relationship between sea-level changes and isotope excursions may be produced. These students are partially supported by this grant. To compensate the cost, the PI did not get summer salary from this grant during the supporting period.
Two papers have been published during the 2006-2007 funding period, with acknowledgement to ACS-PRF support (Howley et al., 2006, Palaeoworld, v. 15, p. 360-366; Jiang et al., 2007, Earth and Planetary Science Letters, v. 261, p. 303-320), and at least another two will be submitted to journals in the next academic year. Because there are still some analyses not yet completed, I have asked the no-cost extension of the project to 08/31/2008.
The students and I are satisfied with the progresses that have been made and feel grateful to the support from ACS-PRF. The results from this project, when fully published, will provide valuable contributions towards the understanding the interactions between sea-level changes, sequence development and carbon isotope excursions.
