FOX HILLS I, A NEW LATE MAASTRICHTIAN MEGAFLORAL ZONE FROM THE MISSOURI VALLEY REGION, DEMONSTRATING EASTWARD DIACHRONEITY OF THE HELL CREEK FORMATION IN NORTH DAKOTA

Discovery of a large, well-preserved flora at the top of the Late Maastrichtian Fox Hills Fm. in central North Dakota permits chrono-stratigraphic comparison with well-documented floral zones established in the Hell Creek Fm. by Johnson and Hickey (1990) 240 km to the west.  More than 2770 specimens were examined in a siltstone 20 m below the Fox Hills-Hell Creek contact and roughly 70 m below the local K-T boundary.

Analysis of this remarkable leaf assemblage allows recognition of Fox Hills I, a new megafloral bio-zone in the Williston Basin.  Applying the techniques of Johnson and Hickey (1990), FH I is defined by Nilssonia yukonensis (11.5%, M1), N. compulta (5.0%, M59), Rhamnus salicifolius (4.4%, M10), Thuja n. sp. (2.1%, M24), N. serotina (1.0%, M62), and Macclintockia cf. electilis (.09%, M28).  Dominance by the cycad genus Nilssonia (23.2% of total) is striking.  The FH I assemblage is made up of 57 morphotypes, which include 5 cycads, 4 conifers, 1 aquatic macrophyte, 1 fern, and 46 angiosperms.   N.  yukonensis, R. salicifolius, and Thuja n. sp. were also found in a collection from the lower Hell Creek Fm. in Morton Co., approximately 30 km NW from the Emmons Co. site.  These indicate the continuation of FH I upward into the Hell Creek Fm. to a position at least 45 m below the K-T boundary.

During rapid deltaic progradation, the Hell Creek III megaflora expanded eastward, as is demonstrated by occurrence of 10 HC III plant morphotypes within FH I; however, the eastward-shifting flora was also changing composition, as indicated by 47 additional morphotypes, by dominance of cycads, and by the predominance of high latitude taxa within FH I.

Near the K-T boundary, the rate of deposition slowed in western N. Dak., reducing the likelihood of fossilization of both vertebrates and plants; deposition rate increased markedly in central N. Dak. Preserving FH I, a floral zone younger than HC III in a stratigraphic unit lying below the Hell Creek Fm.  Progradation rate and geomorphology of the Sheridan Delta created a significant, diachroneity in both Fox Hills and Hell Creek strata west to east.  The 50+ m of Hell Creek strata in the Missouri Valley are represented by less than 5 m of sediment in the Marmarth area.  We expect to identify additional zones thereby clarifying floral conditions chronologically closer to the K-T boundary than previously known.