FOSSIL FLORAS FROM THE SENTINEL BUTTE FORMATION OF NORTH DAKOTA: A NEW COMPRESSION FLORA AND NEW INSIGHTS INTO MODES OF PLANT PRESERVATION

The Late Paleocene Sentinel Butte Formation is over 1,000 ft thick in McKenzie County, North Dakota. A zone of petrified logs, some of which are preserved in an upright position, marks the base of the unit in the Beicegel Creek area. The Beicegel Creek floras of central and western North Dakota occur 90-100 feet above the Sentinel Butte Bentonite marker bed. These floras, which occur in siliceous shales, are unusual since the plants are preserved both morphologically and anatomically and can be studied with the cellulose acetate peel technique. These floras are not associated with molluscan faunas, although they occasionally contain several types of fish scales. The diagenetic processes responsible for forming these discontinuous, fossiliferous, siliceous units are unknown, however, it is evident that several generations of silica are present. We propose to use cathodeluminescence microscopy to distinguish different generations of silica enrichment. During the 2005 field a season, a new compression flora stratigraphically up section from the siliceous shale floras was also studied. This flora includes well articulated branches of Zizyphoides and Nordenskioldia and fruits and leaves of a cercidiphyllaceous plant. Leaf types assignable to Aesculus and platanceous leaves and fruits are also present. This compression flora occurs in association with a molluscan assemblage that allows for assignment to a middle Tiffanian (Ti3) age. Associated units below have yielded fragments of Plastomenus (soft-shelled turtle) and a single crocodilian tooth. The depositional environment is interpreted as an oxbow lake-type environment on a floodplain based on the fine grain size, high fossil content, the presence of lignite and lignitic clays, and the thick channel sands. The new compression locality will provide some significant clues regarding plant assemblages in a lower energy environment vs. plant assemblages found in the siliceous unit representing a higher energy environment.