Page 73 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 30 (A) The character and distribution of facies that develop during the emplacement of syn-sedimentary sills,
and the processes involved in peperite formation. (B) Graphic log of a section through the sill and host sediment.

A modern example of this style of activity is provided CASE STUDY: Partly extrusive, submarine,
by the 1953-57 eruptions of Tuluman, a rhyolitic dacite cryptodome, Sock Creek South,
volcano in the Bismarck Sea, Papua New Guinea
(Reynolds and Best, 1976; Reynolds et al., 1980). western Tasmania
Effusion of lava began at a vent in about 130 m water Part of the Mount Read Volcanics intersected in Sock
depth and was followed by explosive eruptions, which Creek South diamond drill holes (SCS-2, SCS-3)
built several small, partly emergent pumice cones. provides an example of the facies and facies geometry
During the initial effusive stage, spectacular explosions associated with a small, submarine, dacitic, partly
were triggered by interaction of water with large extrusive cryptodome. The interpretation is based on the
(several meters across), floating, hot, pumiceous clast shapes and textures that constrain the
rhyolite masses spalled from the sea-floor lava. fragmentation processes of the various clastic facies in
the sequence, the character of contacts between coherent
Tuluman has been used as an analogue for part of the and autoclastic facies, and the emplacement setting
volcaniclastic sequence in the Devonian Bunga Beds, indicated mainly by the associated sedimentary facies.
Australia (Cas et al., 1990) (18). Coherent rhyolite,
monomict in situ rhyolite breccia and rhyolite-sediment A very thick unit of massive to weakly graded, tube
megabreccia (18.2-4) at the base of the sequence record pumice- and lithic-rich breccia is present at the base of
emplacement of a rhyolite dome into wet, both drill holes (Fig. 31; 33.1-2). The section above
unconsolidated sea-floor sediments (18.1) and comprises coherent dacite and dacite breccia,
accompanying quench fragmentation, auto-brecciation interbedded with and overlain by laminated, pyritic,
and resedimentation. Massive, rhyolitic pumice-rich black mudstone and graded, medium to thick beds of
breccia (18.5-6) and stratified, crystal-rich to crystal- volcaniclastic sandstone. On the graphic logs, intervals
poor, pumiceous sandstone and siltstone (18.7) above of dacite and dacite breccia that have distinctive textures
indicate the onset of pyroclastic eruptions. Settling of or structures can be easily identified (Fig. 31A-E) and
the pyroclasts through the water column resulted in provide the framework for genetic interpretation (Fig.
efficient sorting according to density (hydraulic sorting) 32).
during eruption and transport, and stratification
developed in response to rapidly repeated explosions In SCS-2, laminated mudstone is overlain by facies (A)
and/or water turbulence. Pumiceous debris-flow that consists of monomict, jigsaw-fit dacite breccia (in
deposits and turbidites elsewhere in the sequence were situ hyaloclastite), the lowermost part of which locally
produced by downslope slumping and mass-flow has mudstone matrix (intrusive hyaloclastite). At the top
resedimentation events during and after the pyroclastic of facies (A) is a sharp upper contact with massive,
eruptions. Cas et al. (1990) suggested that shoaling of faintly to strongly flow-banded dacite (8.7) and
the dome permitted the change from effusive to monomict, jigsaw-fit dacite breccia (12.6), which are
explosive activity, reflecting the control exerted by the grouped together in facies (B) (coherent dacite and in
confining pressure of die water column on eruption situ hyaloclastite). Above the topmost in situ
style. hyaloclastite interval of facies (B) are massive, matrix-
to clast-supported, monomict dacite breccia and
stratified crystal- and lithic-rich volcaniclastic sandstone

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