MAGNETIC STRATIGRAPHY OF THE EOCENE-OLIGOCENE FOSSIL PLANT LOCALITIES

The Cedarville plant fossil localities in the eastern flank of the Warner Range between Cedarville and Alturas, California, provide one of the few areas yielding floras which cross the Eocene-Oligocene transition. However, the precise dating of these floras is problematic. We sampled over 1700 m of the early Tertiary section along Highway 299, as well as other more isolated plant localities, such as Badger's Nose and Granger. Samples were taken evenly spaced every 10-30 m as outcrops allowed, resulting in 44 sites covering the Highway 299 section. After thermal and alternating field demagnetization of every sample, we obtained a remanence held largely in magnetite (based on the low coercivity) but with some hematite (based on the amount of remanence at temperatures above the Curie point of magnetite). Some samples showed overprinting that was removed by thermal demagnetization, but a many showed only a single component of magnetization apparent at NRM. The results passed a reversal test and conglomerate test for stability, showing that a primary or characteristic remanence has been obtained and overprinting removed. The lower 550 m of unfossiliferous lahars and ignimbrites of the McCulley Ranch Formation is almost entirely reversed in polarity. Based on the K/Ar date of 40.0 Ma from this sequence, we correlate the McCulley Ranch Formation with Chron C18r (39.4-40.2 Ma). The 500 m of Badger's Nose Formation that disconformably overlies the McCulley Ranch yields the Badger's Nose and Steamboat floras. Based on a K/Ar date of 33.9 Ma higher in the section, we correlate the fossil-bearing reversed magnetozone with Chron C13r (33.8-34.9 Ma), or latest Eocene. The Granger Canyon flora is also of reversed polarity, and correlates with Chron C12r (31.1-33.4 Ma), based on a K/Ar date of 31.5 Ma just below the locality. This chronostratigraphy provides much better age control for the dramatic climatic change shown by these floras, and suggests that the major phase of cooling occurred during the early Oligocene (between Badger's Nose and Granger floras).