Abstract

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 C22/C27 ratios and n-alkane distributions indicative of terrestrial organic matter sources in the spumellarian-dominated beds. In the whole-rock carbonate fraction, δ18O and δ13C 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.

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