GEOLOGIC MAP OF THE TUBA CITY 1:100,000 QUADRANGLE, SOUTHEASTERN PORTION, COCONINO COUNTY, ARIZONA: A PRELIMINARY MAP

Preliminary 1:24,000 scale geologic mapping has been partially completed for the Navajo Nation and Moenkopi Hopi lands around Tuba City, Arizona. Mapping encompasses the northern end of the Moenkopi Plateau, which includes portions of Coal Mine Mesa, Tohnali Mesa, and Ward Terrace. The mapping area also includes the valley of Moenkopi Wash, the southern end of the Echo Cliffs, the northern end of the Adeii Eechii Cliffs, and the northwestern end of the Painted Desert. Gently eastward-dipping strata of Triassic (Chinle Formation), Jurassic (Moenave, Kayenta, Navajo, Carmel, and Entrada Formations), and Cretaceous (Dakota Formation and Mancos Shale) rocks crop out in an east-stepping succession of west-facing escarpments. Mesa tops are capped with remnants of river gravel deposits of probable Pliocene to early Quaternary age. The gravels are overlain by northeast-trending linear dunes that include both actively forming dunes and ancient, stabilized sand bodies. Some linear dunes extend along the ridges between parallel canyons on the northwestern side of Coal Mine Mesa. These dune-capped ridges demonstrate that some of the linear dunes predate the formation of the northeast-trending tributary valleys of Coal Mine Canyon. The map area also encompasses the Hollow Place, a 30 km² and 100 meter deep topographic basin on Coal Mine Mesa, which has no external drainage. The margins of the Hollow Place are capped by ancestral river gravels that may have protected upland portions of Coal Mine Mesa from wind erosion that carved the basin over a period possibly spanning millions of years. Research on dune deposits and ponded sediments in the Hollow Place and other natural topographic basins in the Tuba City area may provide detail about regional climate history.