Boletín de la Sociedad Geológica Mexicana Volumen 64, núm. 3, 2012, p. 411-425 http://dx.doi.org/10.18268/BSGM2012v64n3a10

Geología de la región de Salamanca, Guanajuato, México

Ángel Francisco Nieto-Samaniego^1,*, Ángel Catarino Ojeda-García¹, Susana A. Alaniz-Álvarez¹, Shunshan Xu¹

¹ Universidad Nacional Autónoma de México, Campus Juriquilla, Centro de Geociencias, Boulevard Juriquilla 3001, Juriquilla, Qro., Querétaro, CP 76230, México.

^* This email address is being protected from spambots. You need JavaScript enabled to view it.

 Abstract

Geological cartography of the Salamanca-Aldama region in the state of Guanajuato, Mexico, was carried out at a scale of 1:50000. The study area is located between the Transmexican Volcanic Belt and the Mesa Central physiographic provinces. During field work we identified 16 stratigraphic units. The oldest unit at a regional level is formed of sedimentary marine rocks, which belong to the Late Jurassic - Early Cretaceous (Tithonian-Hauterivian) Sierra de Guanajuato volcanosedimentary complex. This unit does not crop out within the study area. The oldest outcropping unit in the mapped area is a continental polymictic conglomerate, which is correlated with the “Guanajuato Red Conglomerate”. An andesitic unit covers the conglomerate, and rhyolite domes and lava flows overlie the andesite. Although no topaz was observed during fieldwork, we correlate those rhyolites with the “topaz bearing-rhyolites” of the Oligocene volcanism of the Mesa Central. In addition, we identified three ignimbritic units of late Oligocene – early Miocene age. These units correspond to the ignimbritic cover of the Mesa Centra, and include an interlayered andesite unit. Two units of andesitic-basaltic composition cover the ignimbrites. One of them is formed of rocks associated with La Ordeña caldera and the other unit constitutes lava plateaus observed throughout the study area. Also, monogenetic volcanoes and some lava flows were mapped as volcanic rocks of the Transmexican Volcanic Belt.

The oldest faults mapped trend NW-SE and are located in the northeastern part of the study area. These faults cut the Oligocene rhyolitic lava flows and domes. The second group of faults have a NE to ENE trend and cut the ignimbritic cover. The most important structure is the El Bajío fault, trending NW with ~500 m of offset. The youngest movements affected late Miocene rocks and formed the El Bajío graben, which is the boundary between the Transmexican Volcanic Belt and the Mesa Central. In the southern part of the study area N-S normal faults cut Pliocene-Quaternary rocks of the Transmexican Volcanic Belt.

Keywords: El Bajío fault, geologic map of Salamanca, volcanic stratigraphy, Cenozoic volcanism.