The Hualapai limestone of late Miocene age, herein designated as the upper member of the Muddy Creek Formation, is exposed at the base of Grand Wash Cliffs in the Lake Mead area of southern Nevada and northwestern Arizona. This limestone and associated mudstone, claystone, and siltstone have been considered to be of lacustrine origin and deposited in an environment of interior drainage. However, cherts within the Hualapai Limestone Member that contain cristobalite lepispheres, as well as distinctive diatoms typical of marine or estuarine environments, indicate that deposition took place in a marine to brackish environment, probably an estuary at what was then the north end of the Gulf of California. Carbon isotope data from samples of the Hualapai limestone support either marine or hypersaline conditions. The flourishing paleobiota rules out a hypersaline environment.