Sedimentología, reconstrucción paleoambiental y significado tectónico de las sucesiones clásticas del Jurásico Medio en el área de Texcalapa, Puebla - Huajuapan de León, Oaxaca: Revisión de las formaciones Ayuquila y Tecomazúchil
Keywords: braided fluvial systems, detrital zircons, tectonic basin, extensional tectonics, Ayuquila Formation, Tecomazúchil Formation, Middle Jurassic, Mixteca region, Mexico
AbstractBased on the recognition of a major angular unconformity, magmatic and detrital zircons dating, as well as facies analysis of pre-Oxfordian clastic succession in the Ayuquila basin, we propose to subdivide the succession into two formations. We documented that the succession accumulated in braided fluvial systems, draining mainly southwards. During Bajocian-Bathonian time, at the beginning of sedimentation, the climate conditions were semiarid and became wet during Callovian time, probably because of the shifting of southern Mexico towards a more equatorial setting. We formally propose the Ayuquila Formation to name the succession below the unconformity, which consists of monomictic conglomerate of metamorphic or sandstone clasts, and lithic and feldspathic greywacke. Ayuquila Formation bears fossil flora reported in the region and includes some lacustrine beds; these accumulated during Bajocian- Bathonian time. We propose as type locality of the Ayuquila Formation, the succession exposed north of Chilixtlahuaca, Oaxaca. We intend to retain the name Tecomazúchil Formation for the Callovian quartzrich succession above the unconformity exposed in the arroyo Tecomazúchil. Tecomazúchil Formation consists of a succession of subarkose, sublitharenite and conglomerate with quartz lenses; the sequence has thinner beds and is finer upwards. Magmatic zircons from the intrusive hosted in the Ayuquila Formation have ages U-Pb of 164.3 ± 3.6 and 156.9 ± 3.5 Ma. Detrital zircons in the Ayuquila and Tecomazúchil formations have ages ranging from1464 Ma to 174 Ma, which are compatible with a provenance from metamorphic and magmatic terranes of southern Mexico. Permian-Triassic detrital zircons might have come from the La Mora formation. The youngest zircon in the Tecomazúchil Formation has an age of 174 Ma; its most possible source might have been the San Miguel intrusives emplaced in the migmatitic Chazumba Lithodem (Magdalena Migmatite).This fact might constraint the Ayú Complex exhumation before Oxfordian time. Dislocation along Petlalcingo-Huajuapan fault conditioned the Ayuquila basin formation, therefore the fault was active during Middle Jurassic time. According to the timing reported for the migmatization process of Ayú Complex, the pre-Oxfordian clastic succession began its accumulation after 171 Ma in the Ayuquila basin. This one is a small, asymmetric, extensional basin formed on a tectonic setting of continental break-up with SSE-NNW strike extension. This extensional process associated with the rupture of Pangaea during Middle Jurassic was simultaneous in the region with the migmatization and exhumation of the Ayú Complex, and probably with the magmatic event of Diquiyú unit.