Description
Metamorphosed and complexly deformed sedimentary and plutonic rocks are recognized in the hills south of Ejido Aquilles Serdan, northwest Sonora, Mexico. These rocks are subdivided into nine field units of which the metasedimentary sequence is more than 1000 meters thick. Fossil and lithostratigraphic evidence indicates that part of the metasedimentary section is consistent with Permian-Triassic Grand Canyon stratigraphy. Representative Grand Canyon units include the Lower Permian Supai Formation, Coconino Sandstone, and Toroweap/Kaibab Formation, the Lower Triassic Moenkopi Formation, and the Upper Triassic to Lower Jurassic Aztec Sandstone. The protoliths of Grand Canyon equivalent strata provide some clues to the wide diversity of Late Paleozoic to Early Mesozoic depositional patterns along the southwestern margin of cratonal North America. Significant is the predominance of marine units produced by clastic/carbonate facies mixing as recorded in the Supai, Toroweap/Kaibab, Moenkopi, and Jurassic sedimentary units. In sharp contrast, the Coconino and Aztec Sandstones represent eolian desert environments. Positioned midway between the Permian-Triassic cratonal Grand Canyon strata and miogeoclinal strata of the same age at El Antimonio, Sonora, Mexico, Ejido Serdan metasediments represent a critical link between these two environments. Ejido Serdan metasediments are located southwest of the current Silver-Anderson, Mojave-Sonora megashear and between strands of the Dickinson megashear. Thus, if the Mojave-Sonora megashear exists, it must be located west or southwest of Ejido Aquilles Serdan.
