Abstract

To determine the geomagnetic polarity stratigraphy and the duration and age of the Ludlow Member of the Fort Union Formation (Lower Paleocene), we constructed a 325 m composite lithostratigraphic section of the Upper Cretaceous Hell Creek Formation and the Lower Paleocene Ludlow and Tongue River Members of the Fort Union Formation in the Little Missouri River valley of North Dakota, USA. We analyzed paleomagnetic samples from nine of the logged sections. The principal magnetic carrier in the Ludlow Member sediments is likely titanomaghemite, as indicated by predominantly irreversible thermomagnetic curves measured from sandstone, siltstone, and carbonaceous shale samples. The analyzed paleomagnetic samples document a series of polarity zones that can be correlated from C29n to C27r on the geomagnetic polarity time scale (GPTS). We infer that the magnetization of the samples is primary because the characteristic directions are consistent with those of the Paleocene of North America, and the reversal stratigraphy from this section corresponds to the GPTS with reasonable sediment accumulation rates. By extrapolating the measured sediment accumulation rate from the Cretaceous-Tertiary (K-T) boundary to the top of C28n and then to the top of the Ludlow Member, we estimate the duration of the member to range from 2.31 to 2.61 m.y. This is the first estimate for the duration and age of the Ludlow Member, and it can be used as an important tool for interpreting rates of biotic recovery after the K-T extinction.

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