The Neoproterozoic Noonday Formation, Death Valley region, California

Abstract

The Neoproterozoic–Cambrian succession in the Death Valley region of the southwestern United States is among the best exposed and easily accessible in the world. The largest single exposure of these strata occurs in the Panamint Range on the west side of Death Valley, and it, although variably metamorphosed, contains the most complete sections of the Noonday Formation cap carbonate sequence. New geological mapping, measured sections, and high-resolution carbon isotope data for the Noonday Formation in this range enable establishment of a unified stratigraphy across the Death Valley region that consists of three units. From the base upward, they include: the Sentinel Peak Member of light-gray, massive to laminated fine dolostone locally containing vugs and tubes variably filled with micrite, spar, or quartz (and rarely galena), and varying from 2 to 200 m in thickness; the Radcliff Member, composed of feldspathic shale and sandstone and thin-bedded limestone, 0–200 m thick; and the newly defined Mahogany Flats Member of gray, commonly stromatolitic, thin- to medium-bedded fine dolostone that is ~200 m thick at its type locality. Carbon isotopic trends in the Panamint Range match to within 1‰–2‰ reproducibility those known for the equivalent nonmetamorphosed strata in the eastern Death Valley sections. A composite section of the Noonday Formation displays a chemostratigraphic profile with values near −3‰ through the cap dolostone of the Sentinel Peak Member, a decline to −6‰ in the lower part of the Radcliff Member, followed by a recovery to near 0‰ and subsequent decline toward −2‰ in the remainder of the Radcliff units and lower Mahogany Flats Member, and then a return to positive values (4‰) through the remainder of the Mahogany Flats Member. This pattern matches the Ediacaran cap carbonate in Namibia remarkably well, and, assuming our carbon isotopic correlations are correct, it indicates that Noonday deposition occurred at the beginning of the Ediacaran Period, and that the immediately underlying Wildrose Diamictite of the Kingston Peak Formation probably represents the younger Cryogenian (Marinoan) glacial episode.

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