PALEOGENE EMERGENCE OF THE SOUTHWESTERN MARGIN OF THE COLORADO PLATEAU AS A TOPOGRAPHIC BARRIER: EVIDENCE FOR PRE-BASIN AND RANGE, LATE EOCENE(?)-EARLY OLIGOCENE TECTONISM

Pre-Colorado River, Laramide drainage incision along the west edge of the Colorado Plateau south of Grand Canyon (Hualapai Plateau) was accomplished prior to Eocene time as previously demonstrated by thermochronometry and by paleontological dating of the arkosic Music Mountain Formation. Basin and Range extension began in the Lake Mead area at 16 Ma. However, older basaltic volcanism in the Aquarius Mountains, 50 km to the south, began circa 24.9 ±0.9 Ma. Although some maps arbitrarily place a Plateau-Transition Zone boundary at the Kaibab scarp on the southern Shivwits Plateau, this omits western Grand Canyon, the Hualapai Plateau and areas further south from the Plateau province. This entire region is characterized by nearly flat-lying Paleozoic strata with minimal faulting southward to Trout Creek, and should be considered structurally as part of the Colorado Plateau. The western Plateau margin south of Grand Canyon is strongly embayed by erosion associated with the Laramide uplift and clearly differs from the younger, less dissected fault boundary at the Grand Wash Trough. Northeast-flowing Laramide drainage was abruptly terminated in early Oligocene or Eocene time, as demonstrated by an Oligocene ash (23.97±0.03 Ma) located near the top of the locally derived Buck and Doe Conglomerate at Peach Springs. The abrupt transition from regional, through-flowing Laramide drainage with exotic crystalline clasts to infilling of the same incised paleocanyons with locally derived, Paleozoic-clast gravels implies that an early (pre Basin and Range) topographic boundary existed between the two provinces a minimum of 9 million years prior to documented extensional faulting in the Lake Mead region. The severed Laramide canyons and abrupt change in the post-Laramide sedimentary record at the Plateau margin requires significant down-faulting (or uplift) at the boundary. Younger Miocene extension is typically indicated by rotational fault displacements and half-graben sedimentary basins. Thus, it is reasonable to assume that unspecified physiographic differentiation along the Plateau-Basin and Range boundary well south of Grand Canyon began with a much earlier structural disruption that was distinct from the onset of middle Miocene Basin and Range extension as generally recognized in the Lake Mead region.