Page 122 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Cohesive debris flows, volcaniclastic debris In general, debris-flow deposits are very poorly sorted,
flows and their deposits (34) consisting of particles ranging from clay to cobble or
boulder size (Fig. 50). Both clast-supported and matrix-
Cohesive debris flows involve high-concentration, poorly supported fabrics occur, and most examples are non-
sorted, sediment-water mixtures. Fine (clay-size) graded, with sharp but rarely erosive lower contacts.
particles totally suspended in the water create a muddy Deposits display tabular geometry and range widely in
water or watery mud cohesive slurry that has sufficient thickness from less than 1 m to greater than 100 m. The
strength to support very large clasts during flowage lowermost part of some deposits directly above the basal
(Lowe, 1979, 1982). Buoyancy, hindered settling and contact comprises a thin layer lacking in coarse clasts. In
dispersive pressure probably also contribute to clast volcaniclastic debris-flow deposits, clast types reflect the
support. However, a proportion of the larger clasts may source volcanic edifice and, therefore, can be markedly
not be fully supported, and instead may roll or slide polymict or else show a dominance of one clast type.
along as bedload at the flow base. Cohesive sediment- Deposits from volcaniclastic debris flows triggered by
water mixtures have a yield strength which must be phreatic explosions lack juvenile magmatic clasts.
exceeded for flowage to occur. Once that condition is Juvenile magmatic clasts occur in other syn-eruptive
met, cohesive flows behave as viscous fluids, and flowage volcaniclastic debris-flow deposits but can be both
can be a combination of laminar and plug flow or difficult to recognize and present in only very minor
turbulent. Deposition begins if the applied shear stress amounts.
decreases below the yield strength, commonly as
flows decelerate on encountering gentle slopes. Flows Volcaniclastic debris flows are generated in both
"freeze" en masse, or else in a piecemeal fashion subaerial and subaqueous environments (34, 35).
progressively from free surfaces inward and downward. As Volcaniclastic debris flows from subaerial sources that
a result, the deposit is reasonably similar in thickness reach the coast can feed subaqueous debris flows or else
and internal fabric to that of the parent flow (Smith and be transformed into other subaqueous mass-flow types.
Lowe, 1991). The term mud flow is used both for clay- The characteristic internal textures of volcaniclastic
rich, cohesive debris flows with low proportions of debris-flow deposits are very similar to those of some
coarse clasts, and as a synonym for cohesive debris primary pyroclastic flow deposits, especially non-welded
flow. ignimbrite and block and ash flow deposits. In ancient
sequences, establishing a primary origin for poorly
Volcaniclastic debris flows are dominated by volcaniclastic sorted, ungraded, pyroclast-rich, monomict mass-
particles and are generally poorer in clay than non- flow deposits that lack evidence of hot emplacement
volcanic cohesive debris flows. As a result, particle can thus be very difficult. Even establishing a genuine
support probably depends on both grain collisions and clastic fabric can be difficult in ancient sequences,
cohesion between silt-size ash particles, and deposition especially altered sequences. Hydrothermal alteration of
may involve progressive aggradation rather than en coherent lava and autoclastic breccia can produce
masse freezing (Smith and Lowe, 1991). Volcaniclastic apparent matrix-supported, poorly sorted fabrics and
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debris flows have volumes ranging up to 10 m and very apparent polymict clast populations that are typical of
long outflow distances (up to 100 km), reflecting the debris-flow deposits (Part 5).
coincidence of abundant particles, steep unstable
slopes, frequent triggering mechanisms, and water or
snow in volcanic areas.

Fig. 50 Cohesive debris-flow deposits: (A) Massive, matrix-supported pebbly mudstone deposited from a debris flow in
which the clasts were suspended in and supported by the matrix. (B) Massive, clast-supported muddy conglomerate
deposited from a debris flow in which the clasts were not fully supported by or suspended in the matrix. (C) Matrix-
supported, clast-rich muddy conglomerate that shows dewatering structures and overlies strongly deformed (middle)
and sheared (basal) clast-poor zones. Modified from Lowe (1982) and Stow (1986).
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