A NEWLY RECOGNIZED WIDESPREAD BED CONTAINING BOTH FRESH AND BRACKISH ORGANISMS, LOWER TONGUE RIVER MEMBER (UPPER PALEOCENE), S.W. NORTH DAKOTA

Bivalves from both freshwater (Unionidae, Plesielliptio) and brackish (Corbulidae, Pachydon? mactriformis) families occur together in a 45-cm-thick, unconsolidated, weakly layered mud, the Van Daele shell bed (VDsb). The Plesielliptio shells are all disarticulated and broken, and most but not all of the Pachydon? shells are disarticulated. None of the shells are in life position. Broken gastropods of at least two species and an ostracode (Candona), typically interpreted as freshwater in origin, are also found in the VDsb. This bed occurs on the Hanson Ranch (103° 31.035’ W, 46° 36.533” N), Slope County, as well as 10 km to the west on the Van Daele Ranch (103° 42’ 35.24” W, 46° 38’ 27.24” N), Golden Valley County.

Although Pachydon? mactriformis belongs to a family containing marine and brackish taxa, its faunule associations in the Fort Union Formation indicate euryhalinity. In the Tongue River Member, it had complete tolerance of fresh water; in the Cannonball Member, it existed under fully marine conditions. In specific regions where the two facies intertongue, there may be a mixing of marine and freshwater shells. The reported assignment of corbulids to P.? mactriformis from the marine Cannonball Member is, as of now, unprovable because of poor shell preservation and conservation, a matter currently being addressed.

Tongue River Member sediments in North Dakota were deposited along a low-lying coastal plain west of the Cannonball Sea. Most Tongue River strata are freshwater, although numerous thin units of fine sand show marine trace fossils (Tibert et al., 2001). In the Van Daele shell bed, the association of P.? mactriformis with freshwater organisms may have resulted from a short-term sea-storm event accompanied by a land-flooding event. The degree of disarticulation of the bivalves and the weak layering in the VDsb are consistent with a mixed source of shells.

The Van Daele shell bed is traced 10 km to the west of the Hanson Ranch to the Van Daele Ranch, where Pachydon? mactriformis is associated with freshwater ostracodes (numerous Candona and more rare Bisulcocypridea). This bed provides east to west correlation for the top of the underlying Harmon-Hanson coal zone and thus for the correlation over 10 km of the Tiffanian-2 mammal site (Hunter, 1999) that lies 11 m below that zone at the W.K. Johnson Ranch, which lies at the western end of the study area.