The Taray Formation: Jurassic (?) mélange in northern Mexico—Tectonic implications
Thomas H. Anderson, Norris W. Jones, James W. McKee, 2005. "The Taray Formation: Jurassic (?) mélange in northern Mexico—Tectonic implications", The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, Thomas H. Anderson, Jonathan A. Nourse, James W. McKee, Maureen B. Steiner
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The Taray Formation is a mudstone-rich body of rock characterized by dismembered beds of sandstone and shale and fragments of a great range of sizes in a finer matrix. The formation is known only from exposures in northern Zacatecas, in the southern part of the San Julian uplift. Structurally, it underlies a thick Jurassic sequence that includes the volcanogenic Caopas, Rodeo, and Nazas formations and younger rocks of La Joya (volcaniclastic) and Zuloaga (limestone) formations. The base of the Taray is unknown. The texture and structures of Taray and its tectonostratigraphic association with Jurassic volcanic rocks suggest that it is formed of mélange of probable Jurassic age.
Sections of interbedded sandstone and mudstone commonly show progressive dismemberment. Initial subtabular beds commonly pinch and swell. Further extension was accommodated by boudinage or by faults, which cut gently across bedding and dissect sandy layers into lenses and slivers. Progressive stratal disruption formed isolated, deformed inclusions of sandstone, commonly mixed with volcanic and cherty debris, encased in dark-gray, fine-grained, foliated, crenulated matrix. Some large boulders of quartzose sand that slid downslope, crumpling sediments at their bows, were subsequently encased by matrix and both were distinctly extended so that the boulders were fractured, whereas the weaker, pliant matrix responded ductilely.
Taray also contains blocks (as much as hundreds of meters in maximum dimension) of diverse composition including: laminated and massively bedded, light-colored chert; fragmented volcanic flow rock; fine, quartzose sandstone; and carbonate beds, some of which contain fusulinids and crinoid debris. Some of these rocks show tight folds that formed, in some cases, while they were semiconsolidated; later they were incorporated in muddy matrix as lithified fragments.
The unexposed contact between mudstone-rich Taray Formation and the volcanic and volcaniclastic beds of the Nazas Formation also marks changes in the attitude of planar structures. We interpret Taray as part of a Jurassic(?) accretionary prism. The tectonostratigraphic sequence at this latitude (west to east) of accretionary prism (Taray), arc (Nazas, Rodeo, Caopas Formations), and overlying clastic cover (La Joya Formation), which developed against and upon a margin of Paleozoic and Precam-brian crust, cannot be extended northward and must either turn abruptly westward along the southern edge of the Mesozoic Coahuila Island or be truncated and offset, probably along the Mojave-Sonora megashear.