STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS, AND TECTONIC SIGNIFICANCE OF THE TARAY FORMATION, NORTHERN ZACATECAS STATE, MEXICO

The Taray Formation is located in the Solitario de Teyra Range, in northern Zacatecas State. It has been considered either part of the Sierra Madre terrane or the Oaxaquia block. It is the oldest exposed unit of the region. Although there are no exposures of the basement, other authors have considered it to be Grenvillian. The Taray Formation is made of highly disrupted rhythmic succession of quartz-rich sandstone and shale. It has interbedded thin layers of black chert, channel-filled conglomerates, and lenses of detrital limestone that contains fragments of crinoids, gastropods, corals, bivalves, and bryozoa. Where the bedding is not disrupted, laminations, inverse grading in conglomerate lenses, sedimentary folding and convolute bedding can be observed, indicating turbiditic flows deposited in a marine setting. This sequence forms a matrix within which blocks of several compositions can be found. The blocks are strongly deformed and in sheared contact with the matrix, are from 1 to 300 m in length, and are all made of the quartz-rich sandstone, black and green chert, pillowed basalts, serpentinite, and scarce crystallized limestone. The matrix shows locally low green schist metamorphic facies, mostly around the blocks. Within the blocks there are in echelon quartz veins that indicate a SW-NE direction of tectonic transport. It is covered unconformably by the Middle Jurassic Nazas Formation. The age of the Taray Formation remains undetermined. There is a report of fusulinids found in a limestone block of Late Paleozoic age. One sandstone sample from the turbidites, analyzed for detrital zircon provenance, yielded ages that cluster around 300 Ma, 600 Ma, and 1 Ga. The youngest zircon ages (around 270 Ma) suggest that deposition of the turbidites had occurred sometime between Late Permian and Middle Jurassic. The Taray Formation has been interpreted as an accretionary prism. We considered that this unit is recording the collision of the Central terrane, of probable oceanic affinity, to the western margin of the North American Plate (Oaxaquia).The Taray Formation has structures originated by two phases of deformation. The first are typical of accretionary prisms. The second set of structures has been observed in the Jurassic Formations as well, and is interpreted to be Laramide in origin.