Page 192 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 167

Figure 15 Co-ignimbrite lithic-lag breccias and intra-graben caldera collapse breccias of
Malpaso graben caldera and El Ocote ignimbrite. (a) Co-ignimbrite lithic-lag breccia on the
northern margin of the graben caldera showing angular lithics up to 50 cm in diameter in a
pumiceous brown matrix. (b) Large lithics in the co-ignimbrite lithic-lag breccia on same site
as (a); lithic is 1.5 m in diameter. (c) Intra-graben caldera collapse breccia at the southern fault
scarp, next to Sierra El Laurel. (d) Closer view of the collapse-breccia deposit shown in (c).
(e) Pre-El Ocote ignimbrite layered sequence at the northern fault scarp composed of surge
deposits and thin, non-welded ignimbrite units. (f) Surge deposits at the northern fault scarp
showing pumice-rich, lithic-poor pyroclastic £ow deposits overlain by a lithic-rich, lithic-lag
breccia-like pyroclastic £ow deposit (note pen at top of lithic-poor surge deposit).

851 E, following the main trend of the graben of Juchipila. It is 10–20 m wide,
intrudes welded ignimbrites, and is partly covered by white Miocene–Pliocene
lacustrine deposits that filled the graben after collapse that include index vertebrate
fossils of 6–3 Ma (Aguirre-Dı ´az and Carranza-Castan ˜eda, 2000; Carranza-Castan ˜eda