Abstract

The Bright Angel and Eminence faults trend northeastward for approximately 60 mi (100 km) through the eastern Grand Canyon region. The Bright Angel fault parallels basement foliation. Activity along the fault dates from Precambrian time. The first record of movement indicates that it was reverse; it coincided with the deposition of the basal Shinumo Quartzite. Additional reverse movement occurred following intrusion of the Unkar Group by diabase sills and dikes resulting in a total of as much as 1,300 ft (400 m) of displacement, east side up. The Precambrian Chuar Group was later broken by a series of northwest-trending normal faults that tilted the section toward the northeast, causing minor adjustments along the Bright Angel fault.

Possible reverse movement along the Bright Angel fault is recorded in a local angular unconformity between the Paleozoic Redwall and Supai Formations. East-dipping Laramide (?) monoclines developed as reverse movement occurred along selected preexisting northwest-trending Precambrian faults. Tensional faulting beginning in Miocene (?) or Pliocene (?) time caused downfaulting, east side down, along the Bright Angel fault. The Eminence fault, west side down, is part of a graben complex.

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