The next youngest unit in the GMD is the La Luz andesite (Randall et al., 1994),
but this unit is absent in the vicinity of the city of Guanajuato and along the Veta Madre
that is the focus of this study (Buchanan, 1979). In this area, the Guanajuato Formation
lies unconformably on the Esperanza Formation (Fig. 4). The Guanajuato Fm. is
composed of red, poor- to well-sorted conglomerate, sandstone and shale (Buchanan,
1979). The Guanajuato Fm. ranges from 1500 – 2000 m in thickness and is Eocene to
early Oligocene in age based on vertebrate fossils (Edwards 1955).
The Guanajuato Fm. is conformably overlain bythe Loseros Fm. (Fig. 4), which
is a 10 to 52 m thick, green andesitic volcaniclastic sandstone (Echegoyen-Sanchez,
1964; Buchanan, 1979). While no ages have been determined for the Loseros Fm., it is
assumed to be early Oligocene based on its conformable location between the underlying
Guanajuato Fm. and overlying Oligocene Bufa Fm.
The Bufa Formation conformably overlies the Loseros Fm. The Bufa Fm. (Fig. 4)
represents a 350 m thick distal volcano-sedimentary unit with white, yellow, pink rhyolitc
lapilli air-fall ash that has been dated at 37 +/- 3.0 Ma using K-Ar (Gross, 1975). The
Bufa Fm. is separated from the overlying Calderones Fm. by a disconformity (Buchanan,
1979). The Calderones Formation consists of chloritized, crystal-rich andesitic tuff,
except at the base. Here, andesitic volcanoclastic shale and sandstone lie unconformably
on the Bufa Fm. The Calderones Fm. (Fig. 4) is 200-250 m thick (Gross, 1975; Stewart,
2006) and has been assigned a late Oligocene age based on cross-cutting relationships
Overlying the Calderones Fm. is the Cedros Fm. (Fig. 4) which consists of grey to
black andesitic flows interbedded with grey to green andesitic tuffs (Buchanan, 1979).