the hyaloclastite. Deposits of LDCs may include a range of broadly autoclastic fragments including
glassy curved splinter shards and blocky shards, parapillows Walker, 1992, and pillow fragments
Table 3, which may be segregated into coarse debris-fall deposits and finer-grained turbidites
and lava-marginal fall deposits. Fine ash is not an important component because few fine-ash grade
fragments are formed by autoclasis McPhie et al., 1993. Fluidal sideromelane fragments are uncom-
mon, and where present indicate weakly explosive interaction of fluidal lava flows with ambient or
sediment hosted water.
5. Related deposits: column-margin fall
Associated with eruption-fed subaqueous den- sity current deposits beyond the vent area are a
‘normal’ sedimentary deposits, either background ones e.g. pelagic ooze or those from remobiliza-
tion and redeposition of tephra from eruption-fed deposits; b aqueous fallout deposits, which may
be derived from subaerial or subaqueous eruption plumes, and in some cases; c lava flows or
shallow intrusions. At the margins of a sub- aqueous vent, an additional type of deposition
may take place out of eruption columns from which water is excluded by magmatic volatiles.
These deposits form in subaqueous settings within vents or on their margins, yet show no evidence of
interaction with water. Kokelaar 1983 termed such water-exclusion zones ‘cupolas’, and de-
scribed evidence from Surtla for clast agglutina- tion on impact which he interpreted to support
the existence of a water-excluded zone during that entirely subaqueous eruption. One facies at Pah-
vant Butte has similarly been interpreted to have formed in a zone of water exclusion on the basis
of position near the base of the volcano, local distribution, and presence of impact sags, ar-
moured lapilli and accretionary lapilli White, 1996. In both cases, the phenomenon of water
exclusion is inferred to be associated with continuous eruption. Rheomorphically welded ig-
nimbrite,
similarly inferred
to have
formed in a water-excluded zone in a rhyolitic vent is
discussed by Kano et al. 1997, Schmincke and
Bednarz 1990
identify subaqueously
formed spatter
agglutinate in
the deep-
water Troodos Ophiolite, and Gill et al. 1990 describe tack-welded scoria from the modern
seafloor of the Sumisu Rift. All of these units are inferred to have formed by fallout or very local-
ized flow at high depositional rates and particle concentrations, in or at the margins of vents.
Although not themselves generally products of subaqueous density currents, study of such de-
posits can provide important ancillary informa- tion to reconstruct the evolution of an eruption
responsible for a suite of density current deposits. Cas and Wright 1991 infer that some rocks
interpreted as subaqueously welded ignimbrite are the result of this sort of process, and that they
should not be considered pyroclastic flow de- posits.
Fig. 12. Schematic illustration of processes resulting in limu- bearing sheet hyaloclastite at c. 2 km water depth. Four
aspects of note are a spalling and thermal granulation of advancing lava form blocky, poorly or nonvesicular siderome-
lane shards; b water entrapped beneath the front of rapidly advancing thin, glassy lava flow expands to form bubbles
which burst, yielding limu fragments; c steam forms and flows along lava-flow margins, and hot water rises vigorously
from flow, lofting fragments above the seafloor to form a particulate dispersion; d excess density of the dispersion
results from particle accumulation or heat loss, resulting in dilute vertical sediment-gravity flows which encounter the
seafloor and runout downslope.
Table 4 Summary information for column-margin fall deposits formed in water-excluded zones
Feature Interpretation
Inference Diffuse or parallel bedding contacts; local Absence of current during deposition
Suspensionfall deposition no traction features
normal grading Ballistic transport in gas rather than
Ballistic impact sags Local bomb or block sags
liquid medium Tachylite or microcrystalline clasts; little
Pyroclasts not in contact with water Heat retention features, indicative of
or no sideromelane; welded clasts or emplacement at high temperature
prior to deposition matrix with uniform paleomagnetic
signature Local armoured lapilli andor core-type
Accretion of damp ash to form Transport as dispersed particles in
droplet-laden gas; pyroclasts not aggregate grains
accretionary lapilli? enclosed in water during particle
accretion
5
.
1
. Recognition of column-margin fall deposits Key indicators of column-margin water-exclu-
sion zones are heat-retention features, such as welding and clast agglutination, and accretionary
textures such as armoured lapilli Table 4. Evi- dence of heat retention by small clasts is especially
important — larger clasts may evolve their own steam carapaces or exclusion zones because of
their relatively great heat content, but small clasts are rapidly chilled and cannot retain significant
heat if dispersed in water. Agglutination of clasts or welding of vitric ash are diagnostic, though
McPhie and Hunns 1995 offer a cautionary dis- cussion of a tuff welded subaqueously by later
hypabyssal intrusion of basalt. Accretionary tex- tures typically form where water vapor facilitates
adhesion of small grains, and do not form in the presence of excessive water Schumacher and
Schmincke, 1995, such as where tephra grains are entrained in water. Similarly, well developed
block and bomb sags require high impact veloc- ities resulting from ballistic transport, and do not
occur where clasts fall to the depositional site through water.
6. Concluding comments
