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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions Vol. 58 (2008), Pages 461-461

ABSTRACT: Permian Radiolarian Faunal Variations Correlated to Stable Carbon and Oxygen Isotopes in the Lamar Limestone, Delaware Basin, West Texas: Implications for Radiolarian Paleoecology

Yuxi Jin, Paula J. Noble, and Simon R. Poulson

Department of Geological Sciences and Engineering, University of Nevada – Reno, Mail Stop 172, 1664 N. Virginia St., Reno, Nevada 89557

The Lamar Limestone of West Texas is a basinal carbonate that contains a near continuous well-preserved late Guadalupian (Middle Permian) radiolarian record, and reveals eight marked faunal oscillations in a 9-m (~29 ft) interval. Radiolarian assemblages alternate between Albaillelid-dominated and sphaerellarian-dominated faunas. The relative abundance of Follicucullus (Albaillellaria), an important zone fossil, varies from nearly zero to as high as 90%. Geochemical proxies are being used to determine which environmental factors contribute to the faunal changes. Inorganic carbon and oxygen isotope results on 135 samples through the section show d13Ccarb (Vienna Pee Dee Belemnite [VPDB]) values varying from -1.2 to +6.0 per mille (‰), and d18O (VPDB) values varying from -5.9 to -0.3 ‰. Lighter d18O values are correlated with higher concentrations of silt in the samples.

Correlation analysis indicates that the absolute abundance of total radiolarians (AAT, for 38 samples to date) in the dataset is negatively correlated with d18O values of the samples at the 5% significance level. Furthermore, this correlation is mainly due to the significant (at the 2% level) negative correlation between the absolute abundance of sphaerellarians and the d18O values. There is a less well developed correlation (significant at the 10% level) between the AAT and the d13Ccarb values. Since carbonate d 18O values are lighter in freshwater than in seawater, a negative excursion of d18O may represent a decline of paleosalinity through increased riverine input. Hence, these correlations may indicate that sphaerellarians are more sensitive to salinity changes than Follicucullus, and are more prosperous at lower salinities. However, due to the various perturbing factors on oxygen isotope values of old rocks, other potential proxies for paleosalinity such as Sr/Ca, Mg/Ca, and Ba/Ca will be analyzed in relationship to the radiolarian shifts in order to test further this hypothesis.

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