Magnetostratigraphy of interfingering upper Cretaceous–Paleocene marine and continental strata of the Williston Basin, North Dakota and Montana

Abstract

We have carried out a magnetostratigraphic study of uppermost Cretaceous and lower Paleocene strata of the Williston Basin that are exposed in central North Dakota and easternmost Montana. Five overlapping sections were sampled that extend from the marine Fox Hills Formation through the predominantly continental Hell Creek Formation and Ludlow Member of the Fort Union Formation into the overlying marine Cannonball Member of the Fort Union Formation. Rock magnetic study indicated that the detrital magnetic mineral carrying the sediment's natural remanent magnetization is an ilmeno-hematite with low Curie temperature (<225 °C) and that secondary viscous and chemical overprints are routinely present. The base of magnetic polarity Chron 29r, which contains the K-T boundary, is located at the Hell Creek-Fort Union formational contact in central North Dakota but lies within the uppermost Hell Creek Formation in eastern Montana. This is consistent with palynological placement of the K-T boundary within the Ludlow Member in central North Dakota and at the Hell Creek-Fort Union formational contact in eastern Montana. The paleomagnetic data suggest that the Hell Creek Formation in central North Dakota terminated at least 90000 yr before the palynological K-T boundary, while the Hell Creek Formation in eastern Montana terminated synchronously with the palynological K-T boundary. This resulted from a time-transgressive change in paleoenvironmental conditions related to westward transgression of the Cannonball Sea, which was present in central North Dakota about 160000 yr after the K-T boundary. The earlier Upper Cretaceous marine Fox Hills regression and overlying marine Breien Member (Hell Creek Formation) incursion occurred during normal magnetic polarity Chron 30n.