COMPARISONS OF MEGAFLORA ASSEMBLAGES BETWEEN THE LATE CRETACEOUS (MAASTRICHTIAN) HELL CREEK AND TERTIARY (PALEOCENE) LUDLOW FORMATIONS AT THE STUMPF SITE NATURAL AREA, MORTON COUNTY, NORTH DAKOTA
The Stumpf Site in the Missouri River Valley area of south-central North Dakota, has yielded 8 megaflora localities in the Late Cretaceous (Maastrichtian) Hell Creek and Paleocene Ludlow Formations. Forty-four meters of Hell Creek Fm. and 9 m of Ludlow Fm. are exposed. The interval contains the K-T boundary. During 2004 and '05, seven megaflora sites were located at ascending stratigraphic levels (Smrecak, et al., 2005) between 7 and 13 m above the marine Breien Member. The 8th site was located in the Ludlow Fm., ~ 7 m above the K-T boundary. Missouri River Valley floras appear less diverse than those in western North Dakota (Johnson, 2002) although this may be a collecting artifact. The Hell Creek flora becomes less diverse vertically to the top of the formation, followed by an evident change of flora in the Ludlow Fm.
Sites ND05-70, ND04-34, ND04-33, ND04-36, ND04-35, ND04-32, and ND04-37 in the Hell Creek Fm., have yielded over 30 morphotypes, including ?Fokieniopsis sp., Metasequoia sp., Vitis sp., Vitis stantoni, Glyptostrobus sp., and ?Cinnamomum affine, and others similar to morphotypes of Hell Creek workers such as HC90 (Johnson, 2002), and FH66 and FH22 (Peppe, 2003). Stratigraphically the lowest of these sites lies 7 m above the Breien Mbr. and 5 m above site ND05-10 described by Peppe and Erickson (2002) which yielded 18 morphotypes, 2 of which equate to ours. The floral site in the Ludlow Fm., ND05-71, has 7 morphotypes; two are similar to morphotypes HC2 and HC9. Many Ludlow Fm. specimens have oblong to ovate leaf shapes.
Sedimentation rates of the Hell Creek Fm. (73.3m/my) and Fort Union Fm. (95.6m/my) (Hicks et al., 2002) have been used to determine site ages ranging between 65.97 Ma and 65.44 Ma, with the K-T boundary datum placed at 65.51 Ma. When 2 sites containing the Fox Hills I flora (Peppe et al., 2002) are included, a floral record from 10 m below the top of the Fox Hills Fm. (ND03-1) at 66.46 Ma, and including Hell Creek site ND05-10, located 43 m beneath the K-T boundary at 66.09 Ma, to 7 m above the K-T boundary (65.44 Ma) is defined in the Missouri River Valley region of North Dakota. This is the easternmost K-T megafloral stratigraphic record in the Williston Basin. These new data will allow improved chronostratigraphic assessment of floral changes within the Wodehouseia spinata pollen assemblage zone of the Late Maastrichtian.