Late Precambrian Chuar Group, Grand Canyon, Arizona

Abstract

The Chuar Group is exposed in tributary canyons to the Colorado River over an area some 15 mi (24 km) long and 4 mi (6 km) wide. The rocks are faulted against Paleozoic rocks by the Butte fault on the east, and unconformably overlain by Paleozoic rocks to the west. The group is 6,610 ft (2,013 m) thick and has been divided into three formations and seven members. The lower two formations, Galeros (below) and Kwagunt (above) are predominantly argillaceous with subordinate thin limestone beds, while the highest, Sixty Mile Formation, is mostly coarse breccia. Stromatolites are present at three horizons, one of them biohermal. The form-genera Inzeria , Baicalia , and Boxonia indicate an upper Riphean age. The mega-planktonic fossil Chuaria occurs near the top of the Kwagunt Formation. The Chuar rocks are probably younger than any other Precambrian rocks in Arizona. They may be contemporary with rocks below the Cambrian in eastern California, and with the Windermere Formation of the northern Cordillera.