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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Tempestite Deposition
Paul M. Myrow (1), John B. Southard (2)
ABSTRACT
Most recent authors have considered excess-weight (density-induced)
forces to have been relatively unimportant in ancient storm deposition.
This view results from a major leap in understanding of modern storm processes,
particularly the dynamics of combined-flow bottom boundary layers, during
the last 15 years. It also comes from the unsubstantiated view that because
the bottom slopes and measured storm-generated near-bottom sediment concentrations
of modern shelves are presumably too low for autosuspension, such forces
are unimportant. Experiments on the interaction of waves and density flows
define the conditions under which mixing forces destroy density stratification,
and also raise the possibility that with high Richardson numbers, wave-generated
shear stresses may enhance tur ulence just enough to raise sediment concentrations
in the boundary layer and thus facilitate transport by excess-weight forces.
We also believe that excess-weight forces are potentially important for
the following reasons: (1) sediment concentrations during peak storms conditions
exceed 1000 mg/l on inner shelves, and may therefore be nonnegligible and
important for cross-shelf transport with or without currents and waves;
(2) one cannot rule out catastrophic introduction of sediment by river
floods, earthquakes, or other events that caused liquefaction during ancient
storm events, particularly given the significant difference in maximum
thickness between ancient and modern storm-generated beds; (3) the slopes
of modern continental shelves may be anomalously low as a result of Holocene
ea-level rise, and therefore poor analogs for many ancient storm-influenced
settings. Higher slopes may have been the norm in a wide variety of ancient
tectonic settings, thus providing greater offshore-directed driving force
for sediment-rich, storm-generated suspensions.
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