GEOCHRONOLOGY AND DEFORMATION OF THE AHUICHILA FORMATION IN COAHUILA, DURANGO AND ZACATECAS STATES, MEXICO

Ahuichila is a continental deposit of north-central Mexico; its name has been wrongly applied to successions near the Sierra Madre Occidental and elsewhere in Mexico. Ahuichila is >800 m thick and crops-out between Bajío Ahuichila and the Nazareno valley. It is composed by orthoconglomerate, ash-fall tuff, sandstone, evaporite, and fresh-water limestone. The unit accumulated in a closed, continuously subsiding basin, which included piedmont, fluvial, and shallow lacustrine sub-environments. Tuffs in Ahuichila indicate extra-basin volcanism. U-Pb zircon ages of a sandstone from the middle part of Ahuichila were analyzed by LA-ICPMS. The sandstone gave a weighted mean maximum depositional age of 28.1±0.5 Ma. Additionally, three tuffs also from the middle part of Ahuichila were dated to better constraint the age of that part of the unit. A sample yielded a TuffZirc age of 26.4+1.1/-0.4 Ma consistent with its youngest single grain (YSG) age of ~24±2 Ma. Another tuff yielded is 26.2+0.6/-0.6 Ma, and the last tuff is 27.8+0.4/-0.6 Ma. YSG's ages in both samples agree with their TuffZirc ages. No evidence of intra-basin volcanism exists in the study area. This contrast with other Paleogene red beds in Mexico, where intra-basin volcanics are common. Ahuichila rests on an erosional surface, and the upper stratigraphic contact is eroded. There are localities where Ahuichila strata have a disconformity contact and share a similar attitude (up to 80° dip) with underlying marine strata. Ahuichila rests atop clear angular unconformity with the marine rocks in nearby sites. The disconformity and structural attitude indicate that both successions were deformed together. The angular unconformity seen elsewhere is evidence that deformation began prior to the accumulation of Ahuichila. Our new isotopic ages of Ahuichila and structural relationships suggest a protracted period of crustal shortening, which ended at least 14 Ma later than previous estimates for the end of the Laramide orogeny in the Parras Transversal Sector. We also found evidence that crustal extension, probably associated with Basin and Range activity, is present in the study area. We see the current relation among the Frontón and Bajío de Ahuichila as an inverted topography. Normal faulting in the region must be younger than ~24 Ma. Financial support: Papiit IN106820