Age and origin of the continental Teocaltiche paleo-basin, Mexico
An extensive lake existed during part of the Oligocene and almost all the Miocene in the region located east of the southern end of the Sierra Madre Occidental (SMOc). The lake was formed in a tectonic depression associated with the Aguascalientes graben and the Teocaltiche half-graben. Sediments accumulated inside this continental basin are epiclastic, mainly derived from volcanic rocks of the SMOc, and from the volcanic cover of the Mesa Central, chemical and biochemical precipitates (fresh water limestone), and mixed sediments, as well as far less voluminous, but ubiquitous, volcanic ash-fall layers, which were used to obtain U-Pb ages from zircons. The age range of the basin-fill sediments is broad, varying from Rupelian (~28.6 Ma) to Tortonian (~7.6 Ma). Previously, on the basis of fossil fauna, the succession was considered only as late Miocene in age.
The southern end of the SMOc includes several NNE-trending basins of tectonic origin that contained lakes. Based on the characteristics of the individual sedimentary successions, and on their fossil faunas, it is known that three of these basins (Tlaltenango, Juchipila and Teocaltiche) had lakes at the same time. The fourth basin, the Bolaños graben, located west and parallel to the Tlaltenango basin, probably also had a lake during the Neogene, but almost all its sediments have been eroded, so that little is known about them. In order to explain the simultaneous partial closure of all the drainage systems a common, regional, cause must be invoked. All the tectonic basins in the southern part of the SMOc end close to the tectonic depressions related to either the Tepic-Zacoalco or Tula-Chapala rifts. Isostatic uplift of the footwall blocks of these ~E-W-trending rifts may have provoked partial closure of hydrologic systems within the NNE-trending grabens. It is not known when the basins were opened and ceased to act as aggrading systems, but the apparent absence of Blancan age fossils within them may indicate that they could have been opened prior to ~4.75 Ma, which is the numerical value of the beginning of this faunal stage. A probable common cause for the simultaneous opening could be a marine invasion in the region now occupied by the Gulf of California. The new base level of erosion probably increased the stream gradients in the fluvial systems, providing enough energy to the axial rivers within the grabens to overcome the uplift associated with ~E-W rift activity.