Oxygen and hydrogen isotope ratios of chert from the Sixtymile Formation in Grand Canyon National Park, USA: a warm palaeoclimate, freshwater deposit

New oxygen and hydrogen isotope ratios of chert from middle, intraformational breccias, and upper breccia members of the Sixtymile Formation (SMF) in eastern Grand Canyon National Park (AZ) yield palaeoclimate estimates between 27 and 33 °C. The isotopic compositions of cherts define a domain approximately parallel to the meteoric water line when plotted on a δD–δO diagram; these data indicate that meteoric water was involved during formation of the chert. In thin section, the absence of interlocking mega quartz (>35 μm) and silica-filled fractures and veins, along with preserved micromorphological silica fabrics, suggest that the chert has not been permeated by later hydrothermal fluids. Petrographic observations in thin section such as cyclic silica precipitation phases and glaebular micromorphologic fabrics lend support to the interpretation that meteoric waters were involved during chert precipitation. The post 742 Ma SMF has been correlated with diamictite (transition) beds of the Kingston Peak Formation (CA), which in turn have been interpreted to have been deposited during the Sturtian Ice Age (~750–700 Ma). Absence of facetted and striated clasts and other diagnostic glaciogenic features in the SMF, an unconformable contact with the stratigraphically older Chuar Group, coupled with warm palaeotemperature data inferred from stable isotope values of chert, tentatively suggest that deposition of sediment in the SMF likely did not take place during the Sturtian Ice Age.