A NEW EARLY ORELLAN FAUNA FROM THE BLOOM BASIN AT BADLANDS NATIONAL PARK AND THE IMPLICATIONS FOR THE CHADRON-BRULE CONTACT IN SOUTH DAKOTA

A new fossil locality, here termed the Bloom Basin Local Fauna, discovered within the Bloom Basin at Badlands National Park is providing important insights into the nature of the Chadron-Brule contact in this area of South Dakota. The fossil-bearing horizon consists of a brown mudstone bed situated within three meters of the base of the Scenic Member (Brule Formation), and is positioned on the north flank of a paleotopographic hill that was once an erosional remnant of the Chadron Formation. At this location, the base of the Scenic Member is deposited within an erosional surface cut into the underlying Chadron Formation. As a result, the fossil-bearing horizon is situated within one of the lowest deposits of the Scenic Member in the local area, 28 meters below a widespread, originally horizontal marker layer: the Hay Butte marker. The early Orellan age of this fauna is confirmed by the presence of the characteristic Orellan artiodactyls Hypertragulus calcaratus and Leptomeryx evansi. A second Leptomeryx morphotype is also present that displays both the diagnostic ‘Palaeomeryx fold’ of L. evansi and the ectostylid that diagnoses the older Late Chadronian species Leptomeryx speciosus. This latter form may represent a transitional morphotype intermediate between L. speciosus and L. evansi, and its presence may indicate that the Bloom Basin Local Fauna is positioned close to the Chadronian-Orellan transition. This hypothesis is supported by fact that the rodent fauna at this locality includes two taxa, Eutypomys parvus and Adjidaumo lophatus,which are otherwise only reported from the Chadronian of Saskatchewan, Montana, Wyoming, and North Dakota.

The contact between the Chadron and Brule Formations within the Bloom Basin was previously placed coincident with a series of four freshwater limestone beds collectively referred to as the Bloom Basin Limestone Bed. However, the Bloom Basin Limestone Bed directly overlies this new locality, indicating that the localized limestone is positioned stratigraphically higher within the Scenic Member than previously proposed. Thus, a re-evaluation of the inferred age and depositional history of the Bloom Basin Limestone Bed is needed.