Magnetostratigraphy of the White River Group and its implications for oligocene geochronology

Abstract

Magnetostratigraphic studies of the Oligocene White River Group in Wyoming, Colorodo, Nebraska, and the Dakotas have yielded a radiometrically-dated polarity stratigraphy. They provided a mid-Tertiary calibration point for the marine magnetic polarity timescale. An unusually long interval of reversed polarity in the Flagstaff Rim section, Natrona Co., Wyoming, is bracketed by K/Ar dates (biotite) of 32.4 Ma slightly above its top and 34.6 Ma at its base (corrected for new decay-constants). It probably corresponds to the long reversed interval between marine magnetic anomalies 12 and 13. Also, the magnetostratigraphies of 17 other fossiliferous sections of Chadronian-Whitneyan (Oligocene) rocks have been correlated with anomalies 9–12. On this basis, the “boundaries” of the Oligocene North American land mammal “ages” are: Chadronian-Orellan — mid-anomaly 11–12 reversal (about 32.4 Ma); Orellan-Whitneyan — mid-anomaly 10–111 reversal (about 30.7 Ma); Whitneyan-Arikareenan — base anomaly 9 (about 29.0 Ma). These dates are in good agreement with recent estimates based on mammalian biochronology and with a corrected radiometric date of 28.7 ± 0.7 Ma at the base of the Arikareean.