This study of Cretaceous rocks constitutes a part of the ground-water investigation of the Navajo and Hopi Indian Reservations. The Navajo and Hopi Indian Reservations cover about 25,000 square miles, and about one-third of this area is underlain by Cretaceous rocks. The Navajo country-a more general geographic term that encompasses the Navajo and Hopi Indian Reservations-lies principally in northeastern Arizona, but it also includes adjacent parts of northwestern New Mexico, southeastern Utah, and southwestern Colorado.Cretaceous rocks crop out primarily in two large areas-San Juan Basin and Black Mesa-in the eastern and central parts of the Navajo country, respectively. These rocks are as much as 6,500 feet thick in the San Juan Basin and are about 1,500 feet thick on Black Mesa. At several other places in the mapped area are thin isolated 1•emnants of Cretaceous rock.The Lower Cretaceous Burro Canyon Formation, as much as 200 feet thick, is the oldest Cretaceous unit in the Navajo country. It occurs in a very limited area of extrem~ southeastern Utah and adjacent southwestern Colorado. The Burro Canyon consists of conglomerate, sandstone, and green shale and is conformable and intertongues with the underlying Morrison Formation of Late Jurassic age. In northeastern Arizona and northwestern New Mexico, Lower Cretaceous rocks may have been mapped with the Morrison Formation.Above the Burro Canyon Formation, younger Cretaceous strata are much more widespread and record a detailed history of marine transgression (toward the southwest) and regression (toward the northeast). Two major transgressions a1•e recorded in the San Juan Basin, but only the first of these is recorded on Black Mesa.During the first major marine transgression, the Dakota Sandstone and much of the Mancos Shale were laid down. During deposition of the lower part of the Mancos Shale, the shore of the sea lay to the south and west of the Navajo country, but du1•ing the deposition of the upper part of the Mancos, the shoreline migrated back and forth across the Navajo country, as is recorded in the complex assemblage of intertonguing strata.
El E2HYDROGEOLOGY, NAVAJO AND HOPI INDIAN RESERVATIONS gression, and its withdrawal was more rapid. As the sea retreated to the northeast, the upper part of the marine Lewis Shale, the marginal marine beds of the Pictured Cliffs Sandstone, and the overlying, dominantly continental beds of the Fruitland Formation and the Kirtland Shale were deposited.A comparison with the standard reference sequence for the Western Interior indicates that the Burro Canyon Formation is of middle Early Cretaceous (Aptian) age. In the northeastern part of the Navajo country, the Dakota Sandstone contains the plant Tempskya and, thus, is partly of Early Cretaceous (Albian) age. Elsewhere,• at least part of the Dakota Sandstone is equivalent in age to the middle of the Greenhorn Limestone. The Mancos Shale includes rocks ranging in age from equivalents of early Greenhorn age to equivalents as young as the Telegraph Creek Formation. I...
