DICING AND SPICING UP THE CAMBRIAN: CHEMOSTRATIGRAPHY OF THE TONTO GROUP, GRAND CANYON AREA, ARIZONA AND NEVADA

Here we present a biostratigraphically and geochronologically constrained δ¹³C chemostratigraphy for the iconic Tonto Group of Arizona and Nevada, based on integrated analysis of seven hingeline-crossing cratonal sections spanning the Grand Canyon (east) to Las Vegas (west) regions. The Tonto succession (<650 m), although condensed and truncated by pre-Late Devonian unconformities eastward, spans more than 13 Ma (Dyeran (Stage 4) to Steptoan (Paibian Stage)), comprising two Sauk sequences. Carbonates in this succession predominantly reflect shallow subtidal to peritidal deposition in mixed carbonate-siliciclastic settings. In the Bright Angel Formation (Lyndon Limestone Member) and Muav Formation (Spencer Canyon Member), regionally extensive negative δ¹³C excursions (the N5 and N6, < -6 ‰) are recognized. Near the top of the Gateway Member of the Muav Formation, a striking offset in δ¹³C trends suggests a cryptic unconformity that is also present in the Banded Mountain Member of the Bonanza King Formation to the west. The lower part of the overlying Frenchman Mountain Dolostone records two negative δ¹³C excursions to -2.7 ‰ that are interpreted as the Drumian Carbon Isotope Excursion (DICE). The upper Frenchman Mountain Dolostone contains a conspicuous positive δ¹³C excursion to 3.5 ‰, which is interpreted as the Steptoan Positive Carbon Isotope Excursion (SPICE).