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Geochronology of Lignite Beds in the Fort Union Formation, Williston Basin

Charles Nelson

The terrestrial sediments of the Paleocene Fort Union Formation in the Williston Basin host enormous lignite resources. The lignite beds are temporal stratigraphic markers for the periodic occurrence of climate conditions favorable for precursor peat formation and accumulation. To test whether the temporal stratigraphic architecture of lignite beds in the Fort Union Formation is consistent with global climate forcing, age models were constructed for lignite bearing sediment in several areas of the Williston Basin. Stratigraphic markers used to construct the age models were the Cretaceous-Paleogene (K-Pg) and paleomagnetic boundaries, and volcanic ash origin, sanidine-bearing bentonites. In the Williston Basin, an iridium concentration anomaly occurs at or near the base of the Fort Union Formation. This geochemical anomaly is a global stratigraphic marker for the K-Pg boundary. The K-Pg and paleomagnetic boundary age values (Westerhold et al., 2008) and argon radioisotope sanidine geochronology data for the bentonites (Swisher et al., 1993) were used to construct the sediment age models that were used to interpolate deposition ages for lignite beds. Two key results are that the new sets of geochronology data for both lignite beds and bentonites in the Fort Union Formation obey correct Geomagnetic Polarity Time Scale (GPTS) relationships, and that lignite beds have average temporal spacing of ~80 ky and ~100 ky or whole integer multiples of these periodicities. These periodicities are similar to those of Paleocene lignite beds in the Denver Basin (Nelson 2010) and to the global climate warming and cooling cycle periodicities of paleoclimate data records for Paleocene marine sediments (Zachos et al., 2001). These temporal covariance similarities provide evidence that global climate oscillations during the Paleocene were an important control on the temporal stratigraphic architecture of the lignite beds in the Williston Basin. These results also demonstrate the potential of using the temporal stratigraphic architecture of lignite beds for paleoclimate analysis and reconstruction.


AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012