Strontium Isotope Stratigraphy of the Permian-Triassic Boundary Interval in the Great Basin, USA: How Much of a Record Is Preserved?

Events surrounding the Permian-Triassic boundary are poorly understood because few continuous sections are preserved from this interval. In western North America, a major Permian-Triassic hiatus resulting from subaerial erosion has been inferred based on a disconformable contact and missing biozones. However, petrographic work on the Gerster-Thaynes formational contact in the Great Basin region has suggested that evidence for subaerial exposure is equivocal. Furthermore, low abundance and diversity of fossils within upper Permian strata limit biostratigraphic correlation. Here we attempt to utilize Sr-isotope chemostratigraphy to constrain the timing and duration of the Permian-Triassic hiatus at the Gerster-Thaynes contact.

A pilot Sr-isotope study of the Gerster and Thaynes formations in the Confusion Range in Utah reveals a trend similar to the Middle Permian through Early Triassic global composite of Korte and others. Values in the lower Gerster are as high as 0.7072 and fall to a low of 0.7070 in the middle Gerster before rising again to 0.7072 in the upper Gerster. This is consistent with the Guadalupian age for the uppermost Gerster Limestone but does not preclude a Lopingian age. A steady increase from ~0.7072 to 0.7078 occurs in the lower ~20 meters of the Thaynes formation and suggests a highly condensed but relatively continuous record of the Early Triassic. Previous conodont studies of the basal Thaynes have indicated a Smithian (lower Olenekian) age based on the presence of the diagnostic species triserratus. Hindeodus parvus, the conodont species that defines the basal Triassic, has not been reported in the Thaynes. In the composite of Korte and others, the lowermost Triassic sample containing H. parvus has a value of 0.7074; our lowermost Thaynes sample has a value of .7074. Therefore, the presence or absence of Induan (Griesbachian and Dienerian) strata will require a more detailed, integrated bio-, chemo- and sequence stratigraphic study.