First Records of Late Triassic Conodont Fauna and δ13Ccarb from the Dengdengqiao Section, Dangchang County, Gansu Province, Northwestern China

Based on a study of 49 conodont and 57 geochemical samples from the Upper Triassic, carbonate-dominated Dengdengqiao Formation, Qinling Basin, China, the Carnian conodonts and carbon isotope records are first reported. Two genera and four species have been identified amongst 87 conodont elements: Mosherella praebudaensis, Mo. longnanensis sp. nov., Mo. sp., and "Misikella" longidentata. The presence of Mo. praebudaensis indicates that the lower part(bed 2) of the formation is attributable to the Julian(lower Carnian) substage. A radiolarian fauna identified in a previous study belongs to the upper Carnian, but the sampling horizon is unclear. The δ13Ccarbcurve shows a ～1.8‰ negative excursion beginning from upper part of bed 3, but its stratigraphic location is uncertain. The Dengdengqiao Formation is clearly at least partly of Carnian age but could include younger strata. The abundant calcareous algae at the section is probably due to some transport rather than preserved in site. The unusual ecosystem with rare marine organisms may reflect long-term stressful and unfavorable conditions at Dengdengqiao.