The geology of the McCoy Mountains Formation, southeastern California and southwestern Arizona

Abstract

The McCoy Mountains Formation is a 7.3-km-thick metasedimentary sequence exposed in at least 6 mountain ranges in southeastern California and southwestern Arizona. The siliciclastic McCoy Mountains Formation is deposited on, and interbedded at its base with, a Jurassic volcanic terrane and is intruded by undeformed Upper Cretaceous(?) plutons. It has been assigned a Cretaceous-Paleocene age on the basis of fossil angiosperm wood found within the upper third of the sequence. Presently known exposures of McCoy Mountains Formation define a west-northwest-trending basin 140 km long by 25 km wide which was filled from the north by predominantly alluvial processes. The McCoy Mountains Formation is divided into six members, each containing distinctive stratigraphy and sedimentary petrology. Sandstone consists of quartz, feldspar, and lithics, and each member has a different composition. The source evolved from volcanic-sedimentary to plutonic-sedimentary composition with time. Much thinner, laterally equivalent clastic sequences are now known on the craton adjacent to the basin's northern margin. The McCoy Mountains Formation is homoclinal and south-dipping, and its uppermost strata are deformed into a north-verging, overturned syncline with sympathetic south-dipping cleavage. Northern exposures of McCoy Mountains Formation and underlying Jurassic volcanic terrane contain a north-dipping cleavage and south-verging, overturned folds. Sympathetic to the cleavages and folding are north- and south-bounding faults which limit McCoy Mountains Formation exposures and place the North American craton (north side) and the Mojave-Sonora Complex or Tujunga terrane (south side) onto McCoy Basin rocks. Structural analysis and paleomagnetic data strongly suggest that the thrusting over and folding of the McCoy Basin rocks took place in Late Jurassic time as a single event. The fault along the southwestern margin of the McCoy Basin is thus apparently a Jurassic terrane boundary and may be a northern extension of the Jurassic Mojave-Sonora megashear. If true, proposed early Tertiary accretion of the Tujunga terrane must have occurred farther to the southwest.