Paleohydrographic influences on permian radiolarians in the Lamar Limestone, Guadalupe Mountains, west Texas, elucidated by organic biomarker and stable isotope geochemistry

The late Guadalupian Lamar Limestone Member of the Bell Canyon Formation, Delaware Basin, west Texas, records dramatic fluctuations in radiolarian faunal composition that are interpreted to record basin-scale changes in hydrography. Samples taken across one of these intervals of fluctuation show a change from a sparse, nearly monospecific fauna dominated by Follicucullus ventricosus to a more radiolarian-rich interval dominated by spongy spumellarians of the Copicyntrinae and Spongodiscidae. Carbonate stable isotope and organic biomarker geochemical data from this same interval support a strong terrestrial- freshwater influence on the spumellarian-dominated beds. Specifically, biomarker data show lower C ₂₂/C ₂₇ ratios and n-alkane distributions indicative of terrestrial organic matter sources in the spumellarian-dominated beds. In the whole-rock carbonate fraction, δ ¹⁸O and δ ¹³C show a shift to more negative values, consistent with increased meteoric water influence in the spumellarian-dominated interval. Lithologically, the spumellarian-dominated beds are siltier; however, they lack any increase in shallow water allochems, such as sponge spicules, that might indicate the spumellarians and associated terrestrial signature are a function of transporting a shallower radiolarian fauna into the basin via turbidity flows. Faunal changes more likely represent a stimulatory response to increased runoff by in situ spumellarians in the surface waters of the basin.