MAGNETIC STRATIGRAPHY OF THE MIOCENE PAWNEE CREEK AND MARTIN CANYON FORMATIONS, NORTHEASTERN COLORADO: IMPLICATIONS FOR THE PROBOSCIDEAN DATUM

The Pawnee Creek Formation consists of about 50 m of volcaniclastic siltstones and sandstones incised into White River Group deposits in northeastern Colorado. Since Matthew's classic 1901 study, it has produced an important assemblage of early and late Barstovian (middle Miocene) mammals, including large collections in the Frick Horse & Mastodon Quarry. Paleomagnetic samples were taken spanning the entire formation at Davis Ranch, and at the type section near Eubanks. The samples showed a single component of remanence, with little overprinting, and most of the remanence is held in magnetite. The normal and reversed mean directions passed a reversal test, showing that the direction is primary and overprints have been successfully removed. Most of the section is reversed in polarity, except for a short normal magnetozone in the top of the lower Pawnee Creek Formation. Based on Ar/Ar dates of 14.5 +/- 0.09 Ma and 14.3 +/- 0.02 Ma on the middle part of the section, we correlate the Pawnee Creek Formation with Chrons C5ACr to C5ADr (14.0-14.9 Ma). This places the Proboscidean Datum (long used to mark the beginning of the late Barstovian) at 14.5 Ma. Unconformably beneath the Pawnee Creek Formation lies the Martin Canyon Formation, which yields an important early Hemingfordian (early Miocene) fauna. A section was taken through the Frick Clay Quarry to the top of the formation. Samples also had a remanence held in magnetite which passed a reversal test. The base of the formation (including Clay Quarry) was reversed in polarity, but the remaining 30 m of section to the top were normal. Based on the early Hemingfordian fauna, we correlate the Martin Canyon Formation with Chron C5Er-C5En (19.0-18.3 Ma).