FINDING THE PRE-GRAND CANYON COLORADO RIVER: PETROLOGY OF THE MUDDY CREEK FORMATION NORTH OF LAKE MEAD

The path of the pre-Grand Canyon (Miocene) Colorado River, and even its existence, remains one of the major unanswered questions about late-Cenozoic landscape evolution of the Colorado Plateau. This research uses sedimentary petrography to test the hypothesis that the river exited the Colorado Plateau to the north of its present path, accounting for much of the siliciclastic detritus of the Muddy Creek Formation north of Lake Mead.

Sediment samples from the Muddy Creek Formation were collected along a west-to-east transect as well as at different stratigraphic levels at select localities. Petrographic provenance studies were performed on sand grain mounts and heavy-mineral separates of these samples, and from localities chosen to represent key sediment sources. Approximately 400 grains were counted on each grain mount, and heavy-mineral counts proceeded until 400 non-opaque grains were catalogued.

Results indicate a predominately volcanic source for the siliciclastic component of the Muddy Creek Formation, with volcanic detritus from the Caliente and Kane Springs Wash caldera complexes entering basins through gaps to the north. It was hypothesized that the Muddy Creek Formation should become more "Colorado River-like" in a west-to-east direction towards a paleo-delta, but instead samples appear to follow a random path. This is because volcanic sources skew the provenance signal and volcanic contribution increases upwards in the stratigraphy as well. However, if the volcanic component is removed, a paleo-Virgin or paleo-Colorado River source is discernable in the petrologic data. Continued provenance research is needed to determine which of these potential paleo-rivers is responsible.