Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA

Abstract

We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The formation consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meter-scale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.