USING HIGH-RESOLUTION CHRONOSTRATIGRAPHY TO EVALUATE THE POSITION OF THE CRETACEOUS-PALEOCENE [KP] BOUNDARY, GARFIELD COUNTY, NORTH-CENTRAL MONTANA

The lithostratigraphic KP boundary separating the underlying Upper Cretaceous (Maastrichtian) Hell Creek Formation from overlying strata of the Paleocene Tullock Formation in north-central Montana is delineated at the lowermost coal (Z coal) at the base of the Tullock. Since meandering stream floodplain coals, such as the Z coal, record deposition in areally-restricted, lacustrine environments, we hypothesize that the KP lithostratigraphic boundary (Z coal) is diachronous. To test this assertion, we utilized a chronostratigraphic framework previously developed using multiple radiometrically-dated (⁴⁰Ar-³⁹Ar) ash horizons and a laterally continuous Ir-rich clay that defines the chronostratigraphic KP boundary to examine the physical stratigraphic relations between strata of the uppermost Hell Creek and lowermost Tullock Formations.

The uppermost (3 m) Hell Creek Formation consists of massive mudstone and siltstone with sporadic coal stringers. The lowermost (3 m) Tullock Formation consists of massive mudstone and siltstone, with the discontinuous Z coal at its base. No sedimentary structures are present. A coarsening upward relationship in these sections, with siltstones becoming more common, combined with significant coal horizons in the Tullock Formation suggest base level lowering as an important near KP driver. Three ash horizons, distinguished in the field by color and mineralogy, were used to establish correlations. The lower two ashes contain sanidine and biotite and include: 1) a lowermost gray ash dated at 65.16 ± 0.04 Ma, and 2) an overlying cream-colored middle ash dated at 64.95 ± 0.05 Ma (Swisher et al., 1993). The upper pink ash contains sanidine with no biotite; no date is available for this ash.

Four sites including, from east to west, Lerbekmo, Pearl, Bone Hollow, and Nirvana have been studied. Each site was trenched 3 meters above and below the Z coal and lithofacies described on a centimeter scale. The lithostratigraphic boundary at Lerbekmo coincides with the chronostratigraphic boundary, is 258 cm below the chronostratigraphic boundary at Nirvana, 9 cm above at Pearl, and 58 cm below at Bone Hollow. These observations demonstrate that the lithostratigraphic boundary is diachronous implying portions of the Hell Creek are Paleocene in age and portions of the Tullock are Cretaceous in age.