SEDIMENTOLOGY AND TECTONIC SIGNIFICANCE OF PERMIAN FLYSCH IN THE MINA MÉXICO FOREDEEP, CENTRAL AND EAST-CENTRAL SONORA, MEXICO

The Mina México Formation (Lower and Middle Permian) comprises >3,000 m of interbedded siliciclastic turbidites and subordinate calciclastic turbidites deposited in a west-trending foredeep that formed on the subjacent Early Permian carbonate shelf of Laurentia. We infer that the foredeep formed by crustal flexing and thrust-sheet loading in front of the advancing northwest-directed accretionary wedge (Sonora allochthon) created by continent-volcanic arc (Laurentia-Gondwana) collision during the Permian. The allochthon is considered part of the Sonora segment of the Ouachita–Marathon–Sonora orogenic margin of southern Laurentia.

The contact between carbonate-shelf and foredeep deposits grades and intertongues through a transitional interval as thick as 50 m. This interval comprises carbonate sediments and shallow-water fossils derived from the Laurentian shelf to the north, and siliceous sediments derived from the evolving accretionary wedge to the south. In the Sierra Santa Teresa, ~15 km SE of Hermosillo, Sonora, transported land plants associated with marine orbiculoids and fusulinids occur 1–2 m above the base of the Mina México Formation (unit 7 of Stewart et al., 1997).

In the vicinity of Mina México-Cañon Río Bacanora (east-central Sonora) and Cerro San Francisco–Sierra Santa Teresa–Sierra La Flojera (central Sonora), the siliciclastic turbidite facies consists of medium- to thin-bedded (<0.5 m) quartzites and siltites and subordinate thin- to thick-bedded (<2 m) calciclastic turbidites. Interturbiditic siltstones contain deep-water ichnofossils, including Lophoctenium, Cosmorhaphe, and other Nereites-facies trace fossils. The siliciclastics are composed primarily of angular to rounded fine grains of quartz and chert. The calciclastics are bioclastic coarse lime grainstones composed of detrital crinoids and fusulinids, and fragments of other shallow-water invertebrates. Common rounded quartz grains and scattered angular chert grains are present in the lime grainstones. The rounded quartz grains were derived primarily from the shelf, whereas the angular chert grains possibly came from the accretionary wedge.