Paleoenvironmental and paleoecological implications of Permian (Guadalupian) radiolarian and geochemical variations in the Lamar Limestone, Delaware Basin, West Texas (USA)

Geochemical proxies integrated with well-preserved radiolarian data from the Lamar Limestone and lower part of the Reef Trail Member of the Bell Canyon Formation provide insights into paleoceanographically and climatically driven controls on radiolarian distribution in the northern part of the Delaware Basin of west Texas, during late Guadalupian time. Data from a section of the Lamar Limestone (~ 8.9 m; ~ 500 kyr interval), indicate that major variations between sphaerellarian-dominated and Follicucullus-dominated faunas appear to be controlled by fluctuations in nutrient supply and salinity. Geochemical parameters of bulk carbonate carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb), organic carbon isotope (δ13Corg), and total organic carbon (TOC) were analyzed and show variations on two scales. On a fine‐scale, sphaerellarian-dominated beds are associated with a relatively siltier lithology, higher TOC, and higher radiolarian richness but lower diversity, and are interpreted as the result of a rise in productivity stimulated from increased terrestrial input. The radiolarian variations and geochemical evidence imply that the Permian spongiose spumellarians are more opportunistic than Follicucullus (albaillellids) in response to eutrophication. On a broad-scale, δ18Ocarb variations in limestone samples are interpreted mainly as a proxy for paleosalinity in response to changes in basin circulation. Mg/Ca ratios from the carbonate fraction co-vary with δ18Ocarb and are used as an additional paleosalinity proxy in this basin. The limestone beds in the middle part of the section have lighter δ18Ocarb values compared to limestone beds above and below, and likely represent the interval when the Delaware Basin was less restricted and had relatively normal marine salinity. Impacts of diagenesis are mild but are likely the cause for systematically light δ18Ocarb values in silty limestone beds in the middle interval. The Mg/Ca ratios and δ18Ocarb values in the limestone samples both increase from the middle to the top of the section, implying a gradually enhanced evaporitic environment coupled with increased salinity towards the top section. Collectively, these data paint a picture of intermittent restriction in the Delaware Basin, prior to the change to a dominantly evaporative regime in the late Permian, and on a finer scale, show pronounced and frequent ecological fluctuations that appear to be driven by fluxes in terrestrial input and, to some extent, paleoproductivity.