PALEOCURRENT INDICATORS IN TONGUE RIVER SANDSTONES OF THE BULL MOUNTAIN SYNCLINE, MONTANA

ABSTRACT Paleocurrent indicators in sandstones of the Paleocene Series specify currents flowing to the south in south-central Montana. The sandstones comprise the Tongue River Member of the Fort Union Formation, as it is exposed in the Bull Mountains. The inferred paleoslope should aid in estimating the time of uplift of the central Montana platform relative to the Bull Mountain syncline. The primary empirical basis for the paleoslope is cross bedding. Eight hundred twenty-two measurements yield an arithmetic mean of N. 173° E. A smaller number of measurements on ripple marks and large-scale fabric elements further support this paleocurrent direction. Data on clastic-particle size are inconclusive. The uniform paleocurrent over the study area suggests that the central Montana platform was influencing paleoslope, although probably not contributing sediments, during the Paleocene Epoch. This hypothetical tectonic framework should be tested in stratigraphic studies.