GEOLOGIC HISTORY OF LAKE MEAD NATIONAL RECREATION AREA PORTRAYED IN NEW GEOLOGIC MAP DATABASE

The US Geological Survey just completed a new geologic map database of the Lake Mead National Recreation Area for the National Park Service. The Recreation Area is bisected by the lower Colorado River, from westernmost Grand Canyon to north of the Nevada-California-Arizona border. The river, upon leaving the Grand Canyon, crosses multiple broad basins and bedrock canyons of the Basin and Range on its course to the Gulf of California. The map area includes key information about regional geology, including the origin and evolution of the lower Colorado River.

The area is divided, both physiographically and geologically, into a northern part that includes Lake Mead and a southern part straddling Lake Mohave. The geology of the two parts is distinctly different (although intricately linked) because of late Cretaceous to Pliocene geologic events. As a result of these events, fault blocks in the Lake Mead area form diversely oriented ranges that expose colorful Paleozoic to Mesozoic rocks overlying Proterozoic crystalline basement, whereas in the Lake Mohave area northerly striking ranges expose mostly dark colored volcanic rocks overlying a crystalline basement of Miocene and Late Cretaceous plutons and Proterozoic crystalline rocks. The events start with Late Cretaceous Sevier thrusting that telescoped Paleozoic and Mesozoic sedimentary rocks in the northwest part of the Mead area, while the Mohave area remained in the foreland. Next, in the Mohave area Laramide uplift by basement cored folds and pluton intrusion was accompanied by erosional stripping of Paleozoic and Mesozoic strata, while the Mead area remained low. Extensive calc-alkaline-type volcanism in the Mohave area began ca 20 Ma but did not reach the Mead area until ca 13 Ma. Extension began at ca 15 Ma throughout the recreation area, dominated by N-S striking normal faults in the Mohave area and NW-NE striking strike-slip faults in the Mead area. Strata deposited in fault-formed basins were dominantly volcanic in the Mohave area and sedimentary in the Mead area. Volcanism shifted to mostly basalt after about 11 Ma and waned, continuing only locally until ca 4 Ma. Extension waned ca 12 Ma, but faulting continued into the Quaternary locally in the Mead area. Integration of the Colorado River system beginning ca 6 Ma incised the ranges and basins.