Page 197 - Caldera Volcanism Analysis, Modelling and Response

172 Gerardo J. Aguirre-Dı ´az et al.

Figure 19 Alacra Ł n Ignimbrite type section at the western graben caldera margin and at
another section to the south near San Mart|¤ n de Bolan ì os. (a) Panoramic view to the northeast
of the Alacra Ł n Ignimbrite on the western shoulder of the graben caldera of Bolan ì os, showing
the two main deposits that compose this section, the lower surge deposits and the upper major
ignimbrite with a basal co-ignimbrite lithic-lag breccia.This sequence corresponds to a similar
sequence at the opposite side of the graben shown in Figure19. (b) Surge deposits and minor
ignimbrites forming the lower layered sequence. (c) Closer view of the co-ignimbrite lithic-lag
breccias shown in Figure19b. Note the lense-shape of the deposits that produce a general
cross-bedding. (d) Detail of the co-ignimbrite lithic-lag breccia shown in Figure19c, showing
a lithics-rich base that changes to an upper ash-rich deposit, which is interpreted as the
co-ignimbrite ash fall deposit. (e) Panoramic view to the west of the western wall of the
graben caldera of Bolan ì os, at San Mart|¤ n de Bolan ì os. (f) Lithic-rich surge deposits of the lower
sequence at the western graben caldera wall near San Mart|¤ n de Bolan ì os, showing ballistic
blocks and similar features as those observed at other near vent localities, such as those in
Figure18.