Environmental Engineering Reference
In-Depth Information
to 1000 m of reworked and epiclastic deposits of siltstone and sandstone contain-
ing up to 80% volcanic material: altered basaltic glass (tachylite, palagonite),
clinopyroxene crystals, basaltic lava clasts, and pyroclastic lithic fragments,
with minor intercalated tuffs. Correlation of reworked and epiclastic deposits
highlight the development of regional syn-volcanic basins with cumulative thick-
nesses
>
3000 m (Larsen
et al
.,
2003
; Passey and Bell,
2007
).
Overlying this is the main phase of
flood basalt lavas, with a few thin magmatic
tuffs containing Pele
'
s tears and glass shards, concentrated in the lowermost part
of the sequence (Ukstins Peate
et al
.,
2003
). A
ood
lava to highly explosive basaltic phreato-Plinian eruptions occurs in the uppermost
sequence, when active lithospheric rifting and subsidence resulted in
final transition from effusive
flooding
of the nascent North Atlantic Rift proto-ocean basin (Larsen
et al
.,
2003
; Jolley
and Widdowson,
2005
).
1.3.2 Emeishan large igneous province
Research on the Emeishan large igneous province highlights the utility of ma
c
volcaniclastic deposits in addressing questions of large-scale tectonic evolution
during
flood volcanism. A thick and laterally extensive wedge of clastic deposits
(170 m thick, 30 to 80 km wide, 400 km long), emplaced near the base of the
Emeishan lavas, was initially interpreted as an alluvial fan conglomerate, and was
attributed to pre-volcanic, kilometer-scale domal uplift and erosion of underlying
carbonate (He
et al
.,
2003
). The ubiquity of dense to poorly vesicular blocky
sideromelane, pyroclastic textures such as accretionary lapilli, volcanic bombs
with bomb sags, and ductile deformation of ma
c clasts unequivocally identi
es
these rocks as phreatomagmatic lapilli-tuffs and tuff-breccias, and likely repre-
sent near-vent deposits (
Figure 1.1
; Ukstins Peate and Bryan,
2008
,
2009
). The
abundance of marine limestone lithic fragments
-
some containing ma
c clasts
themselves
and the presence of unbound shelly fossil material, strongly suggests
that active carbonate deposition was contemporaneous with volcanism, and that
these units were emplaced near sea level (Ukstins Peate and Bryan,
2008
,
2009
).
Continuing work, focusing on the zone of inferred maximum uplift, has
identi
-
ed a protracted and extensive record of hyaloclastic and phreatomagmatic
volcanism (
Figure 1.1
). Microfossil studies show nascent carbonate platform
collapse immediately prior to initiation of volcanism (
>
200 m: Sun
et al
.,
2010
). The
first phase of volcanism is laterally heterogeneous but dominated by
phreatomagmatic and subaqueous volcanism. Eruptions through shallow-water
carbonates generated thin subaqueous hyaloclastites and subaerial tuff deposits
near Daiquo (Ukstins Peate and Bryan,
2008
), whereas in the Dali area (the core of
inferred maximum uplift), volcanism initiated with a succession (
c
. 750 m) of
