Jökull - 01.01.2001, Blaðsíða 57

Caldera formation at Cerro las Cumbres, Mexico
Figure 7. First plinian pumice–fall phase showing crude bedding of the pumice. The ash–flow deposit forms
the floor of the quarry. – Snið í gegnum fyrsta fasa loftbornu gjóskunnar. Takið eftir grófri lagskiptingu hennar.
Botninn í námunni er myndaður úr gjóskuflóðasetinu.
magma indicated by its fine grained characteristics.
Several shard types were recognized and in order of
abundance are: plate–like and angular blocky shards,
rod and Y shaped bubble–wall fragments, and vesic-
ular pumices (Figure 4). Rare crystals include sani-
dine, plagioclase, occasional biotite and hornblende.
Adherence of dust–size particles onto the surfaces of
larger grains is common. Vesicles are abundant, even
in the smallest grains (Figure 4).
The pyroclastic–fall deposits
The isopach map indicates a west-trending axis for the
main pyroclastic–fall units, with a total thickness of
15 m at 10 km distance from the caldera rim (Fig-
ure 2a). Closest to the caldera the tephra fall deposits
consist of two crudely bedded, plinian pumice layers,
separated by planar to crossbedded vulcanian ash de-
posits (Figure 3). In distal outcrops the pyroclastic–
fall deposit occurs as a single unit (e.g. section Cd Ser-
dan, Figure 3).
The pyroclastic–fall deposit consists of two dis-
tinctive lithofacies; plinian and vulcanian (Fisher and
Schmincke, 1984). The more voluminous and
widespread lithofacies is comprised of two massive
and crudely stratified plinian layers (Figure 3) that
proximally contain lapilli pumice deposits with minor
ash and lithic fragments (Figure 7). At 9 km from
the vent the maximum clast diameter is about 10 cm,
decreasing to about 5 cm at a distance of 15 km. In
proximal outcrops ( 9 km) the two pumice–fall lay-
ers are separated by low–angle cross–stratified base-
surge deposits on the cm scale interbedded with rhyth-
mically layered vulcanian ash–fall units (Figure 3).
The rhythmically layered units are reverse graded and
are often deformed by ballistic obsidian and lithic
blocks forming well defined bomb–sags within the
units (Figure 8).
Volume estimates of for the plinian fall deposits
were obtained using the method of Fierstein and Na-
thenson (1992), indicating a bulk volume of 11 km


for the deposits which amounts to 3 km


when calcu-
lated as dense rock equivalent, DRE (Table I).
In thin section pumices show porphyritic fluidal
texture with sanidine (0.5–2 mm), quartz ( 0.5–1 mm),
biotite (0.1–2 mm) and rare orthopyroxene and am-
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