Integrated Sr Isotope Stratigraphy and Relative Sea-Level History of Middle Cambrian Cyclic Carbonates of the Southern Great Basin

I. P. Montanez, J. L. Banner, D. A. Osleger, L. E. Borg

We have integrated sequence stratigraphy, third-order accommodation history, and Sr isotope stratigraphy of Middle Cambrian cyclic carbonates of the southern Great Basin in order to 1) refine the seawater secular Sr isotope curve, and 2) assess a potential link between temporal variations in relative sea-level and the Sr isotopic composition of Middle Cambrian oceans. The Bonanza King Formation is composed of 150 to 250 meter-scale, carbonate. parasequences that systematically stack into four third-order depositional sequences, each spanning ~1-3 m.y. of Middle Cambrian time. The third-order accommodation history of these four depositional sequences can be correlated across the 250 to 300 km wide platform. We have sampled two well-exposed sections, ~50 km apart, on a dm-s ale to delineate the first high temporal resolution Sr isotope stratigraphy for the Middle Cambrian.

Sr isotope analysis was carried out on marine cements, marine-cemented algal carbonates, micrites, trilobites, and early diagenetic dolomite microsamples. Sample selection criteria for delineating secular Sr isotope variations were established in order to identify least diagenetically altered samples and to minimize contamination from noncarbonate minerals. These criteria include: 1) microfabric, 2) luminescence, 3) trace element contents (high Sr and low Mn, Fe, Al contents), 4) ^dgr¹⁸O and ^dgr¹³C values, and 5) Rb concentrations of <0.2 ppm and Rb/Sr ratios typically <0.0005 (corresponding to negligible age corrections on measured ⁸⁷Sr/⁸⁶Sr values). Analysis of multiple sets of contemporaneous samples from individual beds spaced 10 to 0.5 km apart has allowed us to further evaluate whether samples which pass these criteria are reliable recorders of marine ⁸⁷Sr/⁸⁶Sr values.

Our results refine the structure of the Middle Cambrian seawater secular Sr isotope curve by delineating a previously undocumented, long-term increase in ⁸⁷Sr/⁸⁶Sr values of 2 × 10^-4 throughout Bolaspidella to Crepicephalus trilobite zones. Preliminary results also indicate small but measurable Sr isotopic excursions within this long-term trend. These short-term isotopic excursions occur over stratigraphic intervals ranging from 25 m to 275 m, and correlate with the third-order accommodation history of Bonanza King carbonates, whereby apparent decreases in ⁸⁷Sr/⁸⁶Sr values are associated with large magnitude third-order relative sea-level rises, and increases in ⁸⁷Sr/⁸⁶Sr values correla e with significant third-order sea-level falls. These results indicate that refinement of the Cambrian secular Sr isotope curve 1) will require rigorous assessment of sample selection criteria and analytical methods, and 2) has potential for increased chronostratigraphic resolution of Middle Cambrian carbonates significantly greater than that currently provided by trilobite biostratigraphy.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995