FAUNAL AND CARBON-ISOTOPIC CHANGES ACROSS THE BASE OF THE LOWER ORDOVICIAN STAIRSIAN STAGE IN WEST TEXAS AND SOUTHERN NEW MEXICO

Trilobite collections from the highest strata of the Skullrockian Stage and basal beds of the Stairsian Stage in the Hitt Canyon Formation in Texas and New Mexico tightly constrain the position of the stadial boundary (base of the Leiostegium-Kainella Zone) to less than a meter in some sections. In the Caballo Mountains of New Mexico, the faunal turnover through the boundary interval is accompanied by a change from cliff-forming fine grainstone with the pre-extinction fauna of the Bellefontia Zone to a 1.7m package of coarser bioclastic grainstone that includes centimeter- to decimeter-scale skeletal lags of brachiopod and trilobite debris reminiscent of coquinas that characterize biomere boundaries in the Upper Cambrian. The lowest post-extinction trilobite lag, less than 1.5m above the highest Bellefontia collection, yielded a nearly monogeneric fauna dominated by the hystricurid Parapethopeltis, suggesting that it might represent the Paraplethopeltis Zone at the top of the Skullrockian Stage. However, the collection also includes two specimens of Kainella and therefore is assigned to the base of the Stairsian. Leiostegium was recovered less than 0.5m higher from the top of the grainstone package. Hematitic staining of surfaces within the highest meter of the Bellefontia-bearing grainstones suggests the possibility of a stratigraphic break. However, the highest Bellefontia Zone collection contains the hystricurid Politicurus, which occurs only high in the Bellefontia Zone. Additionally, the coarse Paraplethopeltis-bearing grainstone yielded conodonts characteristic of the Rossodus manitouensis Zone, which ranges upward only a very short distance into the Stairsian Stage. Thus it appears that the thin Paraplethopeltis Zone is absent, but little (if any) of the upper Bellefontia Zone or lowest Leiostegium-Kainella Zone are missing. Carbon isotopic data revealed a sharp, short-lived positive excursion of 1-1.5 per mil just a few meters above the base of the Stairsian Stage. This prominent peak in the isotope curve, which is the only level within the Stairsian where positive values are reached, facilitates recognition of the age-equivalent level in non-fossiliferous sections in the southwestern USA and is also recognizable in the Lower Ordovician of the Precordillera of Argentina.