Correlation and geochronology of middle Eocene strata from the Western United States

Abstract

In this study I integrate marine and continental biostratigraphy, magnetic polarity stratigraphy, and radioisotopic chronology in a synthetic correlation of middle Eocene strata from the western United States. More than 2000 m of section were sampled from volcaniclastic deposits, Aycross and Tepee Trail Formations (northwestern Wyoming); lacustrine and fluviatile deposits, Washakie Formation (southwestern Wyoming); and intertonguing marine and continental strata, La Jolla and Poway Groups (San Diego area, California). Detailed demagnetization studies on numerous pilot samples from all three field areas revealed moderately complex magnetizations. Most sample NRM's are dominated by a strong normal polarity magnetic component; alternating field demagnetization does not consistently isolate the primary magnetization. High blocking temperature hematite is frequently a significant carrier of remanence. Therefore, most samples were subjected to detailed, stepwise alternating field and thermal demagnetization to 600–650°C. The East Fork Basin area (northwestern Wyoming) magnetic polarity sequence consists of five major polarity intervals, A- to E-; the thick Washakie Formation sequence contains four, A+ to D-; and the San Diego area sequence has four polarity intervals, A- to D+. A new biochronologic interval, the Shoshonian Land Mammal Subage (Earliest Uintan), is defined and characterized. In all three field areas Shoshonian (Earliest Uintan) faunas and the Bridgerian/Uintan boundary occur within a single long reversed polarity interval. Correlation of marine biostratigraphy between the San Diego area section and deep sea sections allows precise identification of San Diego polarity interval B+ as Chron C21N. Therefore, the Bridgerian/Uintan boundary and earliest Uintan faunas occur within the reversed interval of Chron C20R. High-temperature, KAr dates bracketing this horizon in northwestern Wyoming provide an age estimate of approximately 49.5 Ma for the top of Chron C21N and 49 Ma for the Bridgerian/Uintan boundary. Berggren et al. (1985) use the age estimate of 49.5 Ma for the top of anomaly 21 (younger boundary of Chron C21N) as one calibration point for the generation of a Paleogene geochronology. The methodology and conclusions of this geochronology are compared to those of other recent geochronologies. Data from independent studies integrating high temperature radioisotopic dates, biochronology, and magnetochronology are used to test the validity of the Berggren et al. (1985) geochronology.