LONG-DISTANCE LATERAL VARIABILITY IN BIOTAS AND LITHOLOGIES OF THE CAMBRIAN BRIGHT ANGEL FORMATION (TONTO GROUP), GRAND CANYON, USA

The Bright Angel Formation of the Grand Canyon of the southwestern United States contains significant biotas of arthropods and other fossils that were collected primarily by Charles Walcott and E. D. McKee in the late 19th and early 20th centuries. New collections, the first major ones in 70 years, add to this picture. Biotas include diverse trilobites, hyoliths, brachiopods, eocrinoids, and bradoriid arthropods; trilobites and brachiopods are relatively abundant throughout the formation. Samples from large sites with 100-500+ specimens are diverse but are often dominated by dolichometopid trilobites, primarily “Anoria” tontoensis, Glossopleura mckeei, and G. walcotti; ptychopariid trilobites are rare (0-15% of samples). Bradoriid arthropods (Dielymella and Walcottella) occur at just two historical localities plus at least two new (2019) localities, all occurring at sites stretching from Bass Trail east to Marble Canyon; these bivalved arthropods in some cases are the most abundant fossils at their respective localities but also occur with dolichometopid trilobites and can constitute a substantial percentage of the sample at each locality. Olenellus Zone species are known near the base of the section from Frenchman Mountain east to Granite Park; Glossopleura Zone trilobites occur high in the section in the west and low in the east and most of the Bright Angel Formation appears to be within the Glossopleura Zone. Regional changes, from western outcrops at Frenchman Mountain and Grand Wash Cliffs to the eastern-most exposures in Marble Canyon, demonstrate several patterns in the distribution of fossils and facies. Shales dominate western outcrops and the unit transitions to increasing abundance of sandstone eastwards, with far eastern sections (e.g., Marble Canyon) dominated by thin beds of sandstone and sandy siltstone with abundant detrital mica and rare thin shale interbeds suggesting proximity to the paleoshoreline. Detrital zircons in the underlying Tapeats Sandstone show the Bright Angel Formation to be < 508 Ma across the entire transect. Global calibration of the Glossopleura Zone suggests most of the Bright Angel Formation was deposited between 506 and 504 Ma. The long-proposed time-transgressive nature of the formation is supported by the Olenellus Zone being only in the west, but the timeframe for time-transgression likely involved only 1-2 million years.