STRATIGRAPHY AND PALEOENVIRONMENTS OF THE BASAL LUDLOW FORMATION (FORT UNION GROUP) IN SOUTHWESTERN NORTH DAKOTA

Recent fieldwork in the basal part (~lowermost 8-12 meters) of the Ludlow Formation, (Fort Union Group), between the formation contact and the Marmarth Ash, (a regional marker horizon), in southwestern North Dakota, has resulted in the discovery of 41 vertebrate localities documenting 15 taxa belonging to 15 families in close proximity to the Cretaceous/Tertiary boundary. These sites range over a 6 kilometer transect along the eastern margin of the Little Missouri River drainage. The basal part of the Ludlow Formation in this study area, which consists of yellow and brown siltstones and mudstones typical of the Fort Union Group, has been determined by pollenomorphs to be Cretaceous in age. The occurrence of a change in lithology below the Cretaceous/Tertiary boundary implies that environmental changes had occurred prior to the Cretaceous/Tertiary boundary and the associated vertebrate extinctions in this study area. All lithologial facies: variegated beds (ponded water), sandstones (channel and near-channel), and siltstones (floodplains), produce vertebrate specimens. Most of the specimens found within these lithologies are comparable to late Cretaceous, non-dinosaurian, vertebrate taxa found in the Hell Creek Formation rather than new forms that appear later in the Paleocene. This stratigraphic interval of the lowermost Ludlow Formation is constrained above and below by fossil mammals (Lancian mammals below, and Puercan mammals [near the Pu1/Pu2 boundary] above). This constraint implies that the lowermost Ludlow Formation correlates to late Lancian through Puercan 1, implying that little or no time was lost in the deposition of sediments following the terminal Cretaceous events in the study area. The collected evidence indicates that the local environments, following the terminal Cretaceous events, contained a viable non-dinosaurian vertebrate fauna similar to that of previous Cretaceous deposits. No hiatus in time following the Cretaceous/Tertiary boundary seems to have been necessary before the local environment was repopulated.