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Magnetostratigraphy of the Late Cretaceous to Eocene Sediments Sampled from the Castle Pines and Kiowa Drill Cores, The Denver Basin, Colorado

HICKS, Jason, Denver Museum of Nature and Science, 2001 Colorado Boulevard, Denver, Colorado, 80205-5798; OBRADOVICH, John, U.S. Geological Survey, Lakewood, Colorado, 80225

Two cored holes have been drilled in the Denver Basin, a Laramide foreland basin that filled with synorogenic sediment shed from the uprising Rocky Mountains at the end of the Cretaceous. The first hole was drilled in 1987 near Castle Pines on the western margin of the Denver Basin, proximal to the Front Range. Penetrating to a depth of 3120 ft, the well encountered D2 synorogenic strata (lower Eocene upper Dawson Arkose), D1 synorogenic strata (lower Paleocene-Upper Cretaceous lower Dawson Arkose, Denver Formation, and Arapahoe Conglomerate), Laramie Formation, the littoral Fox Hills Formation, and the marine Pierre Shale. The K/T boundary was found in the middle of D1. The second hole was drilled in 1999 at Kiowa, near the geographic center of the Denver Basin. Total core length was 2256 ft, and the hole intersected the same stratigraphic sequence that ranges from the Eocene to the Maastrichtian. Both holes were completed at almost exactly the same stratigraphic level in the uppermost Pierre Shale in the ammonite range zone of Baculites clinolobatus. Fifty-nine levels of the Castle Pines core were measured using a combination of AF and thermal demagnetization techniques. The samples responded well to thermal demagnetization at 25oC steps, and no difference could be found between samples demagnetized using AF or thermal. The Kiowa core displayed a very different response. Thermally demagnetized samples displayed essentially random directions, but after an initial thermal step to 125o C, closely spaced AF steps at intervals of 2.5 and 5 mT gave a consistent and repeatable magnetic signal from 64 levels. Analysis of the two cores reveals a sequence of paleomagnetic reversals that can be correlated to the Geomagnetic Polarity Time Scale (GPTS) using ammonite zones, palynology, the age of the K/T boundary interval, and a single radiometric date of 64.13 Ma that was obtained below the paleosol horizon at the D1/D2 contact. The K/T boundary is found palynologically in both cores, but although the K/T appears in its expected position near the middle of C29r at Castle Pines, at Kiowa it lies at the top of C29r indicating that in the distal part of the basin the reversal has been foreshortened by a post-K/T hiatus or erosional event. Correlated to the GPTS, the magnetostratigraphy of the Cretaceous part of the Castle Pines core corresponds well to the interval from C31r to C29r. The sequence also includes the short, 125 Kyr reversal, C30r, which is the first time that this reversal has been recorded in the Western Interior of North America. The Paleocene part of both cores is interpreted to represent the interval from C29r and C29n. The overall sedimentation rate across the Castle Pines core averages about 150 m/Myr (compacted sediment), but there is a measurable increase in the rate of deposition from about 130 m/Myr in the Cretaceous to 210 m/Myr in the Paleogene. The Kiowa core was deposited at an average rate of 110 m/Myr, some 50% less than the western basin margin, and across the K/T boundary there is a much smaller increase in deposition rate from 110 to 115 m/Myr. Above the paleosol, the sedimentary sequence is normal in polarity at Castle Pines, but reversed at Kiowa. This poses a number of questions regarding not only the length of time represented by the paleosol hiatus, but also the style of the Eocene tectonism that governed the resumption of deposition across the basin. The difference in the post-paleosol reversal pattern would strongly argue that one part of the basin began actively subsiding some time before the other.