NEW DATA ON THE MAGNETOSTRATIGRAPHY OF THE UPPER CRETACEOUS TO LOWER EOCENE OF THE DENVER BASIN, COLORADO

Two cored holes have been drilled in the Denver Basin; Castle Pines (3120 ft) on the western basin margin and Kiowa (2256 ft), near the basin center. Both cores begin in the Dawson Arkose synorogenic strata (lower Eocene) and pass through the Denver Formation (lower Paleocene and Upper Cretaceous), Arapahoe Conglomerate, Laramie Formation, the littoral Fox Hills Sandstone, and marine Pierre Shale. The Castle Pines core samples responded well to thermal demagnetization at 25^oC steps. The Kiowa core was demagnetized using AF steps at intervals of 2.5 and 5 mT, after initial thermal steps to 125^oC. The sequence of paleomagnetic reversals found in both cores can be correlated to that part of the Geomagnetic Polarity Time Scale that ranges from C31r to C24n. Independent age calibration points were derived from ammonite zones, palynology, the age of the K/T boundary, and a single radiometric date of 64.13 Ma. The K/T boundary was picked palynologically in both cores at the same stratigraphic level within C29r. In the Castle Pines core the mean sedimentation rate on the western basin margin is about 146 m/Myr (compacted sediment), with an increase in the rate of deposition across the KT boundary from about 123 m/Myr in the Cretaceous to 206 m/Myr in the Paleogene (+67%). In the Kiowa core, in the basin center, the average sedimentation rate was 110 m/Myr, and there was a much smaller increase in deposition rate, from 97.5 to 140 m/Myr (+44%), between the Cretaceous and the Paleogene. A large regional paleosol marks an unconformity that separates the Paleocene sequence from the Eocene. The Eocene sequence is dated at about 54 Ma. It is normal in polarity at Castle Pines, but reversed at Kiowa. The difference in the post-paleosol reversal patterns would tentatively indicate that the central part of the basin began actively subsiding before the western basin margin.