Abstract

Laramide arkosic sediments on the Hualapai and Coconino Plateaus in northwestern Arizona fill partially exhumed fluvial channels that are incised into the erosionally beveled margin of the southwestern Colorado Plateau beneath late Oligocene and early Miocene volcanic rocks. A 1200-meter-deep, trunk paleocanyon through Peach Springs Canyon is eroded into Proterozoic bedrock along the downthrown side of the Hurricane fault. The gravel clasts in the late Cretaceous(?)-Paleogene canyon fill preserve a record of Laramide erosional unroofing in which clasts of Proterozoic rocks increase in percentage upward in the stratigraphic sections. Laramide volcanic clasts in similar arkosic gravels east of the Hurricane fault increase to nearly 50 percent of total clasts near the tops of some preserved sections. The volcanic clasts record syntectonic volcanism and sedimentation coincident with Laramide deformation, probably beginning in latest Cretaceous time. Ages of 14 randomly selected volcanic clasts range from 64 to 117 Ma, with the majority falling in the interval from 72 to 84 Ma. The upward increase in volcanic clast percentages records the late Cretaceous migration of volcanic activity toward the modern plateau margin.

South of Grand Canyon near Long Point, a 30-meter-thick freshwater limestone within the arkosic Laramide sediments contains fossil charophytes, stromatolites, and gastropods, including Viviparus, Physa, and Lioplacodes. The gastropods are similar to the suite of genera found in the Paleocene-Eocene rocks of SW Utah, such as the Claron Formation.

The age of the arkosic sediments, along with evidence of synextensional back tilting of the Laramide channels, indicates that the edge of the Colorado Plateau in Arizona was previously higher than the present-day 1200 meters above sea level when channel incision first occurred. The amount of inferred late Mesozoic and Laramide regional northeast tilting is of the same order of magnitude as the paleoslope preserved in structure contours on the Kaibab Formation in northern Arizona. The modern topographic relief along the southwestern border of the Hualapai Plateau near Route 66 is approximately equal to the 305 m (1000 ft) of throw on normal faults that displace the 18.5 Ma Peach Spring Tuff. The throw measured across buried Tertiary volcanic rocks increases to several thousand meters further north near Red Lake. However, there is no compelling evidence for any significant late Miocene or Pliocene uplift of the southwestern margin of the Colorado Plateau in this part of northwestern Arizona. Laramide uplift, modified by subsequent southwesterly back tilting can account for most, if not all, of the modern elevation and topography of the western Grand Canyon region. The Laramide history recorded in the Cretaceous(?)-Paleogene sections in northwestern Arizona is similar to the more detailed tectonostratigraphic record for late Cretaceous-Eocene time in southern Utah.