Page 174 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 149

3. Regional Stratigraphy of the Sierra Madre
Occidental

The SMO volcanic rocks can be divided stratigraphically into three major
sequences, the older one, known as the Lower Volcano-plutonic Complex, the
second (younger), known as the Upper Volcanic Supergroup (McDowell and
Clabaugh, 1979), and the third (youngest), named here the Miocene Mafic Lavas.
The regional stratigraphy of the SMO is shown in Figure 3, which summarises the
stratigraphy reported in several works (Table 2 — Aguirre-Dı ´az and Labarthe-
Herna ´ndez, 2003). In general, the basal portion of the SMO is mainly formed by
rocks of a volcanic arc complex, with a predominance of andesitic–dacitic lavas and
associated volcanoes, but also with important packages of silicic domes and
ignimbrites. In the most eroded portions of the SMO, either in the bottoms of deep
canyons, or at the deeply eroded westernmost margin of the SMO in the northern
states of Sinaloa and Sonora (and in the now separated Baja California Peninsula),
the roots of this volcanic arc are exposed as batholiths, mostly of intermediate
composition. In general these remain little studied (e.g., Henry and Fredrikson,
1987; McDowell et al., 1989, 2001; Albretch and Goldstein, 2000). On the other
hand, at the eastern margin of the SMO, Mezosoic marine sedimentary rocks from
the Mexican folded belt of Laramide age directly underlie ignimbrites of Eocene–
Oligocene age of the Upper Volcanic Supergroup, with a few exposures of the
lower volcanic arc (Aguirre-Dı ´az and McDowell, 1991, McDowell and Mauger,
1994). However, Eocene–Oligocene plutonic bodies in the form of stocks occur
scattered along the central-eastern margin of the SMO (Aguirre-Dı ´az and
McDowell, 1991), some of which could well be interpreted as the roots of silicic
lava domes (porphyry-rhyolite intrusives) that commonly are related to precious
metal ore deposits, such as Mapimı ´, Velarden ˜a, and Guanajuato (Damon et al.,
1981, 1983; Gilmer et al., 1988; Randall et al., 1994).
The Upper Volcanic Supergroup is mostly composed of rhyolitic ignimbrites
and lava domes, but at some sites andesitic–dacitic lavas are also an important
part of the sequence; for instance, at the San Luis Potosı ´ volcanic field and nearby
areas (Labarthe-Herna ´ndez and Trista ´n-Gonza ´lez, 1978, 1980a, 1980b; Labarthe-
Herna ´ndez et al., 1982). Plinian pyroclastic-fall deposits appear to be absent within
the ignimbrite sequence of the SMO. Instead, pyroclastic surge-like, cross-bedded
deposits are relatively common. These deposits and minor non-welded ignimbrites
are generally found beneath the large-volume ignimbrites with no time-breaks
(volcanic hiatus, sedimentary deposits, paleosoils) between them, indicating that this
type of explosive volcanism pre-dated the emplacement of large and catastrophic
pyroclastic flow that formed a major ignimbrite. An example of this situation is
the Alacra ´n Ignimbrite at the Bolan ˜os mining district in Jalisco State (Lyons, 1988;
Gutie ´rrez-Palomares and Aguirre-Dı ´az, 2005), which is described in more detail
below.
In general, the ignimbrites of the upper group are capped by basalts and basaltic
andesite lava flows, that occur scattered throughout the SMO (Figure 3). In some
localities this mafic volcanism consists of alkaline basalts interpreted as intra-plate
magmatism (Aguirre-Dı ´az and McDowell, 1993), but in others they consist of