Page 118 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 45 Correlated sections through a Miocene, submarine volcaniclastic mass-flow deposit (Wadaira Tuff D,
Honshu, Japan). The deposit is dominated by juvenile magmatic components (glassy, pumiceous and dense dacite
clasts, crystals, shards) and organized into distinctive doubly graded units. It is interpreted to be syn-eruptive and
generated by a submarine explosive eruption. Modified from Fiske and Matsuda (1964).
Significance debris flows and density-modified grain flows. Mass
flowage involving grain flows, slides, and debris
The pyroclast-rich mass-flow deposits that are common avalanches is due largely to gravity acting on unstable
in ancient, below-wave-base, mixed volcanic- deposits and is especially common in volcanic
sedimentary sequences are, in most cases, volcaniclastic environments. Although none of these flow processes
turbidites or debris-flow deposits. They include syn- is strictly primary, they may be syn-eruptive and leave
eruptive deposits resedimented from subaerial sources, deposits that provide a clear record of an eruption. They
syn-eruptive deposits from shallow subaqueous are the principal means by which clasts formed in
explosive eruptions and post-eruptive resedimented subaerial or shallow marine settings are deposited or
deposits. They give information on the composition, redeposited into deep marine settings (Fig. 48).
eruption style and setting of the source volcanic
terrane, and help discriminate active from inactive Turbidites (29-33)
terranes. Although primary pyroclastic flow deposits
are most common in subaerial settings, there are some Turbidity currents are flows of cohesionless particles, in
well-documented ancient examples of primary deposits which suspended particles are supported largely by an
in shallow submarine settings (Francis and Howells, upward component of interstitial fluid turbulence.
1973; Kano, 1990) and inundated intracaldera Particles that are too dense to be suspended are instead
environments (Busby-Spera, 1984, 1986; Kokelaar and transported by traction (as bedload and by saltation)
Busby, 1992). Explosive eruptions capable of generating at the base of the flow. The sediment suspension is
primary pyroclastic flows are restricted to vents in denser than the enclosing fluid, and flowage is driven
subaerial and relatively shallow subaqueous settings by gravity. Deposits from turbidity currents are termed
(theoretically <1000 m, McBirney, 1963). Deep turbidites (29). Particle concentration and grain size
subaqueous emplacement of primary pyroclastic flow strongly influence the behavior of turbidity currents
deposits, whether sourced subaerially or
subaqueously, has yet to be demonstrated. and the character of their deposits, and are the basis
for recognition of two end-member types, high-
Water-suppported and gravity-driven volcani- density and low-density turbidity currents (Lowe,
1979, 1982).
clastic mass flows and their deposits

Mass flows in which particle support depends on Low-density turbidity currents
interstitial water (or muddy water) are classified These are relatively dilute flows dominated by clay to
according to the dominant particle support mechanism, medium sand-size grains that are supported by fluid
the flow rheology (Fig. 46), and whether laminar or turbulence (Lowe, 1982). Documented low-density
turbulent (Lowe, 1979, 1982) (Fig. 47). The types of turbidity currents are relatively slow moving (10-50
water-supported mass flows that are most important in cm/s), and flow thicknesses range from a few metres to
volcanic terranes are: turbidity currents, cohesive
more than 800 m (Stow 1986).

103