Page 126 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

stratification with the foresets dipping downstream. to those generated by subaqueous, high-density
An additional increment of current velocity has the turbidity currents (Lowe, 1982; Smith, 1986; Smith
effect of "smearing out" the dune bedforms and and Lowe, 1991).
generates horizontal, planar bedforms, that are internally
represented by horizontal, planar laminations. Plane Hummocky cross stratification is a tractional sedimentary
beds constitute a transition from a relatively low- structure characteristic of fine to medium sand
energy, low-flow regime to a higher energy, high-flow deposited in shallow subaqueous or near-shore settings
regime where antidunes develop. The underlying (Walker, 1979; Dott and Bourgeouis, 1982; Duke et al.,
structure of antidunes is low-angle (<20°), upstream- 1991). The structure occurs both within thick
dipping cross stratification. sandstone units and within sandstone beds in
interbedded sandstone-mudstone sequences. Sets
At peak flow velocities, the sediment load may be too comprise broadly convex-up (hummocks) and concave-
high for true tractional transport to operate. Under up (swales) layers, with dips less than 12°, and are
such conditions, the system becomes a separated by low-angle truncations (Fig. 55). Layers may
hyperconcentrated flow, in which other particle support thicken into swales and thin over hummocks, so the
processes (dispersive pressure, hindered settling, structure tends to die out upward. Hummock-swale
buoyancy) are important. Deposition from wavelengths are 1-5 m, and sets are up to about 25 cm
hyperconcentrated flows occurs very rapidly when the thick. Hummocky cross stratification is thought to
flows decelerate, and involves progressive base- develop in response to vigorous and complex wave and
upwards aggradation. Sand- and granule-dominated current activity during storms and is only preserved in
deposits show planar, thin, diffuse stratification, in sediment at water depths between fair-weather and
which separate layers are massive or graded. storm wave base. The structure is, thus, a very useful
Hyperconcentrated flows that carry coarser grains water depth indicator.
(pebble-size) produce normally graded, clast-supported
units. Deposits from hyperconcentrated flows are similar

Fig. 54 Tractional sedimentary structures and bedforms formed in cohesionless sediments in response to increasing
current velocity. Modified from Friedman and Sanders (1978).

especially coarse particles, rapidly become rounded.
Because particles are carried and deposited
independently, aggregates are generally well sorted.
Thus, traction current deposits are characterized by the
presence of tractional sedimentary structures, particles
with appreciable rounding and relatively good sorting
(38). However, particles rounded during traction transport
may be redeposited by mass-flow processes, and are
not restricted to traction current deposits.

Significance
Fig. 55 Main features of hummocky cross stratification.
Upward-convex hummocks alternate with upward- Wind and pyroclastic surges are restricted to subaerial
concave swales. Low-angle truncations between sets settings, and their deposits are, therefore, very important
and shallow dips of component layers characterize the in constraining ancient depositional environments.
internal structure. Modified from Allen (1985). Water currents capable of generating fractional
sedimentary structures operate in a wide spectrum of
Characteristics
settings but are abundant in fluvial, shoreline and
Traction transport involves repeated collisions of above-wave-base subaqueous environments (38.1-4,
particles until they are finally deposited, and particles, 40.5-8). Hummocky cross stratification is generally

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