COMPARISON OF MISSOURI VALLEY HELL CREEK FORMATION (LATE CRETACEOUS) FOSSIL FLORAS WITH MEGAFLORAL ZONES OF THE WILLISTON BASIN, NORTH DAKOTA

Collection and analysis of 317 fossil plant specimens from six sites in the southeastern portion of the Hell Creek Formation in North Dakota has allowed for the preliminary comparison of Missouri Valley megafloral fossil assemblages in the western Williston Basin. Collections were made under the direction of the North Dakota Geological Survey at the Stumpf Site (Murphy, et al., 1995). Peppe (2003) recorded 18 taxa from this general location ~2 m above the top of the Breien Member of the Hell Creek Formation including taxa such as Nilssoniocladus spp., “Microconium” sp., and “Rhamnus” sp. that defined the zone FHI.

In our study, fossil leaves from the Stumpf Site were collected stratigraphically above Peppe’s samples, between 7 and 13 meters above the top of the Breien Member. Two lithologies, friable, medium-grained sandstone and brown, fissile, “paper” shale were represented, but only the sand facies are discussed herein. Our work to date indicates the presence of multiple Marmarthia spp., as well as ?Fokieniopsis sp., Metasequoia occidentalis, “Vitis” stantoni, Glyptostrobus sp., morphotype HC90 of Johnson (2002), and FH 66 and seed specimen FH 22 of Peppe (2003). An abundance of “Vitis” stantoni was present, but numerous samples of ?Fokieniopsis and Metasequoia were present as well. Poor collecting conditions made quantitative analysis of the flora impossible. Both long- and short-ranging taxa are included, but some species such as “Vitis” stantoni –lobed form --(HCIa—IIb), morphotype HC90 (HCIb), morphotype FH66 (FHI & HC III), and FH22 (FHI & HC III), and a lobed Marmarthia sp. (FH I) are rather restricted in their zonation as indicated above.

Comparisons of flora found in this study with floral zones already established in the region demonstrated no consistency of zone assignment for the eastern-most Hell Creek flora. Within 6 meters of Missouri Valley Hell Creek Fm., flora from HCIa, HCIb, HCIIa, HCIIb, HCIII, FUI (Johnson and Hickey, 1990; Johnson, 1992), and FHI (Peppe and Erickson, 2004) have been found. This supports the suggestion by Peppe and Erickson (2004) that ecological control of Hell Creek plant distribution, mitigated by proximity of the Fox Hills coastal habitat, influenced plant distributions geographically, and therefore stratigraphically as well, in the Late Maastrichtian Williston Basin.