A Paleocene Timescale for the Rocky Mountains: Status and Potential

The uppermost Cretaceous through Paleocene is characterized by major biotic events including the K-T extinction, the radiation of mammals and emergence of modern floras. This period also saw dramatic climatic variations, which provide invaluable context for understanding the relationships between marine and terrestrial records and the dynamics and evolution of greenhouse climate systems. High-precision sequencing and calibration of geologic history is critical for determining the timescales of these events. The Paleocene timescale, however, relies on scant ⁴⁰Ar/³⁹Ar age constraints on the Geomagnetic Polarity Time Scale (GPTS) with interpolation. Large uncertainties exist due to relatively imprecise geochronology and lack of dates at key magnetic/fossil intervals that limit robustness of interpolation between tie-points; major improvements are possible.

The Laramide basins of the Rocky Mountain region contain a rich record of climate change, biotic diversity, and extinction including an exceptional record of the K-T boundary and recovery through the Paleocene to the PETM. The Denver Basin, including the Kiowa core, provides a nearly continuous record from Maastrichtian through early Paleocene, ideal for combined paleontological, geochronological and magnetostratigraphic studies. Our initial work has successfully produced high-precision U-Pb dates from small drill-core samples of volcanic ash linking paleomagnetics to the timescale; two samples within Chron C28r are 64.7 Ma. Combined with outcrop studies throughout Colorado, Wyoming, Montana, and North Dakota that include new dates from C31n (68.7 Ma) to the K-T boundary (66.1 Ma) and to C26r (59.3 Ma) at Polecat Bench, we are developing a new high-precision calibration of Paleocene time. This allows the linking of terrestrial sections with the high fidelity isotopic/climatic records from ocean basins which will lead to tests of cyclostratigraphic models and a better understanding of greenhouse climate dynamics and related terrestrial events.