- The processes responsible for transporting and depositing thick
sections of coarse-grained terrigenous clastics on the abyssal floor
and for forming associated sedimentary structures are still conjectural.
Many workers attribute coarse deep-sea sediments and their
probable lithified equivalent, the graywackes of flysch deposits to
some type of density movement.
Deductions concerning the processes operating in a density flow
generally are made from flume studies--in which an artificial situation
may develop, or from lithified units--where the magnitude of
post-depositional change is unknown. Both approaches contribute to
our knowledge, but the unconsolidated elastics themselves should
contain a unique key to understanding the dynamics of abyssal sedimentation.
To test this theory, divisions of parallel lamination, found in
deep-sea sand and silt, were selected for analysis. Since individual
laminae closely approach discrete populations of particles assembled
under contrasting conditions, their use carries environmental sampling
to its practical limits.
Northeast Pacific sediments of late Pleistocene and Holocene
age, from deep-sea channel and abyssal plain environments, and
representing two or three provenances were studied. A total of 115
light-colored and 84 dark-colored laminae were sampled from eight
sequences at five locations. Samples averaged about 0.8 gram and
were quantitatively processed using quarter-phi calibrated sieves and
decantation techniques. Statistical evaluation of the procedure shows
better than 95 percent sample recovery, and indicates that textural
variance between laminae is significantly greater than within-sample
The classic concept of density transport--that coarsest material
is carried by the nose of the current, and that clastic size grades tail-ward
and upward in a uniformly decreasing manner--is not substantiated
by moment measures, sand-silt-clay percentages or factor analysis
of grain-size distributions, at least during deposition of the
coarse division of parallel lamination.
Coarse abyssal lamination develops within a narrow range of
current velocity, the limits of which are defined texturally. Absolute
velocity values for these limits can only be related, at the present
time, to the few flume or in situ bottom current measurements
available. Texture indicates that while the total amount of sand
carried in suspension varies, lamination does not begin to form
until a current is essentially depleted of all material coarser than
fine sand--establishing an upper competency limit. At that time,
coarse suspended material is distributed throughout the flow mostly
in large eddies or vortices whose velocities are estimated on the
order of about one meter/sec. Mean current velocity must be sufficient
to maintain a dispersed traction carpet without deformation of
bedform into ripples. This is postulated at about 50 cm/sec.
A current model, based on textural evidence, is proposed to
account for lamination. It is suggested that the critical stage in the
formation of coarse abyssal lamination occurs while sediment is
being dragged along the bottom as bedload. The flowing clastic traction
carpet acquires kinetic energy as the current bypasses material
lost from suspension. In turn, this energy results in grain shear.
When the concentration of granular material in traction is large, it
dissipates the energy of bottom shear mostly in collision contacts
between gliding grains. The dispersive stresses developed tend to
maintain grain separation and prevent settling. Eventually, turbulence
in seawater entrapped between grains is suppressed and the net
path of grans impelled by repeated collisions becomes quasi-laminar.
Within this quasi-laminar traction system, dispersive pressure
causes some migration of finer sizes toward the base of the carpet
and a concentration of coarser grains in the upper bedload. As new
material is introduced in large quantities from suspension, the zone
of internal shear--the base of the moving carpet--is displaced progressively
upward. As it passes, sediment compacts to a fraction
of its dispersed thickness and a population of grains with a slightly
finer size distribution than the carpet load comes to rest. This is
buried by new deposition and a densely-packed, dark layer continues
to accrete upward as long as a moving traction carpet is sustained
and a dense rain of clastics is contributed from suspension.
When a sand-laden eddy impinges on the bottom, it releases its
coarsest load into traction and the dark layer then accreting increases
significantly in grains larger than 44 microns. Any eddy, whether
laden or not, on striking bottom adds to, or deducts its velocity from
the velocity of the traction carpet and either increases or decreases
bottom shear. Additional impulse given to tractive shear by eddies
merely results in more effective size sorting.
However, an eddy whose velocity of rotation is opposed to current
movement may reduce shear below the critical necessary to
maintain a thick carpet by dispersive pressure, The dispersed carpet
collapses and instantaneously ceases moving. This less-densely
packed layer has a slightly higher sand content than the accreted
material below. When partially dried or weathered, alternate layers
exhibit different moisture retention properties--the less-porous,
accreted layers appearing dark and the more loosely packed layers