The Gran Tesoro and Nazas formations: evolution of Upper Triassic-Lower Jurassic sequences in Mexico and their tectonogenetic meaning

This article is an integration and interpretation of data that has the objectives to formally redefine the Gran Tesoro Formation and to revise the stratigraphy of the Nazas Formation, to correlate both deposits with other stratigraphic sequences of similar age range, and link these deposits to a regional geodynamic event. The Gran Tesoro Formation is a succession of slate and phyllite, which preserves textures of the protolith formed by igneous, clastic and carbonate sedimentary rocks with olistolites containing Paleozoic fossils, which is the reason this formation was considered to have the same age. The presence of Classopollis cf. C. classoides Pflug, Classopollis cf. C. kieseri Reyre y Micrhystridium lymensis var. lymensis Wall reveals that the Gran Tesoro Formation is a Mesozoic deposit, younger than previously thought. Originally, the Gran Tesoro Formation included a schist facies, but this paper rejects the schist from the formation due the following reasons: 1) for being of different ages, the green schist with an isotopic age of 326 +/- 26 Ma (Mississippian) and the slate and phyllite of Mesozoic age; 2) because their degrees of metamorphism reveal different geologic histories, and 3) the contact relationships between schist and slate and phyllite are tectonic. This article formally appropriates the mnemonic Pescadito Formation and the Gran Tesoro Formation under the Code of Stratigraphic Nomenclature. The Nazas Formation, in the area of Villa Juarez, Durango, has two members, both deposited in a continental environment. The lower member is dominated by volcanic flows, with siltstones and hypabyssal igneous rocks. The upper member is composed of sandstones and siltstones that rest in angular unconformity on the lower member. The top of the Nazas Formation underlies the La Joya Formation in angular unconformity, and this in turn is transitional to the La Gloria Formation (Oxfordian age). The Gran Tesoro and Nazas formations are part of a sedimentary sequence (sensus strictus), with an angular unconformity at the top, similar to other stratigraphic sections of Upper Triassic - Lower Jurassic age observed in Mexico, sections that have been deformed by a poorly studied tectonic event. However, the regional analysis of the stratigraphic relationships of these sedimentary sections in this geological interval leads us to propose the name American Tectogeny, for a post-Paleozoic and pre-Oxfordian deformation, independent of the time lapse of the Nevadan and Laramide orogenies, both younger in age.