COMPARING THE SEDIMENTOLOGY OF THE WHITE RIVER GROUP IN LUSK, WYOMING TO SECTIONS IN TOADSTOOL GEOLOGIC PARK, NEBRASKA AND BADLANDS NATIONAL PARK, SOUTH DAKOTA

A new section of the White River Formation/Group was examined in the region north and west of Lusk, Wyoming. This section was compared to previously published sections of the White River Group in Toadstool Geologic Park, Nebraska, and Badlands National Park, South Dakota. Sections were measured in detail to include bed thicknesses as well as grain size information and sedimentary structures. A generalized composite section was created using high quality GPS data to tie together multiple sections measured at different elevations.

The composite section showed considerable variation in grain size, ranging from coarse to very coarse sandstones with pebble and boulder sized clasts found mainly near the base of the section to fine silt and mudstones closer to the top. The White River Group overlies a Cretaceous shale and the contact was found and documented in two locations. Several large channel fill deposits were found approximately half way up the section containing cobble and small boulder sized grains. These channels were cut into medium to coarse sandstone that showed some evidence of crossbedding. Other areas, however, showed what appeared to be laterally continuous alternating beds of fine and medium grained sandstone. Above those beds the section consists of primarily fine grained material of possible aeolian origin.

In contrast to the sections measured at Lusk, the stratigraphy around Toadstool Geologic Park, located ~60 km to the east of Lusk, consists of silt at the base with sheety sand intervals and finer grained material above such as aeolian silt and a large amount of volcanic ash. Only one major channel system has been documented. The White River Group sections at Badlands National Park, located 110 km northeast of Toadstool Geologic Park, contain yet fewer channel deposits and an even higher proportion of fine grained material such as silt, silty sandstone and mudstone. These differences are consistent with the deposition of these White River outcrops according to the Distributive Fluvial System model and suggest the possible existence of a large fan with its apex west of Lusk and toe east of Badlands National Park.