Geoscience Reference
In-Depth Information
Process Characteristics Deposit
Rockfall
Creep
Avalanche deposit
Creep deposit
Slide
Slump
Slide
Slump
Debrite
Debris flow
Grain flow
(fluidized flow)
Grain/fluidized
flow deposit
Turbidite
Fig. 1.11
Processes and deposits associated with
rock and sediment movement along a continuum of
decreased concentration and increased internal
disaggregation. (Adapted from Stow 1986.)
Turbidity
current
transport are recognized: (i) rockfalls, (ii) slides
and slumps, and (iii) mass flows. These are re-
cognized to occur along a continuum whereby
there is an increase in the degree of internal dis-
aggregation and a reduction in the concentration
of the sedimentary material (Fig. 1.11). Rock falls
are defined as the collapse of rock or sediment
primarily along a vertical plane. They may be
caused by tectonic movement or by weathering
in upland settings and typically produce scree
deposits. Slides and slumps occur over lower
angled slopes and involve transport along an
inclined shear plane. They are thus characterized
by movement over both vertical and horizontal
displacement planes. In slides, the sediment gen-
erally remains undisturbed, whereas in slumps
the original sedimentary structures are normally
disrupted or destroyed. The presence of water
along a shear plane acts as a medium to initiate
both slumping and sliding. At higher water
contents the process of slumping grades into
that of mass flows, a term used to encompass a
spectrum of transport processes including debris
flows and grain flows. Debris flows involve the
transport of rock and fine sediment that 'flows'
downslope as a chaotic mass and these occur in
a range of environments from deserts to contin-
ental slopes. They typically require the presence
of unconsolidated sediment and steep slopes
and, on land, low vegetation cover and heavy
rainfall to initiate movement (Chapter 5). Grain
flows occur within finer sediments and require
steep slopes and a confined channel margin. They
occur most commonly on the continental slopes
and form graded deposits.
POST
-
DEPOSITIONAL PROCESSES
1.5
Processes acting internally and externally upon
a sediment after deposition can be physical,
chemical or biological. Physical processes include
compaction, resuspension, erosion or dredging
of sediment. Chemical and biological processes
include the series of early diagenetic, bacterially
mediated redox reactions, which result in the
oxidation of carbon species (organic matter) and
the reduction of an oxidized species. Although
post-depositional processes acting upon sedi-
ments are varied and have a range of impacts,
of most importance in the context of environ-
mental sedimentology is the chemical remobil-
ization of nutrients and contaminants during
early diagenesis, and the release of contaminants
from floodplains.
1.5.1 Early diagenesis in aquatic sediments
Upon the consumption of O
2
, a series of anaerobic
bacterial reactions are favoured, utilizing oxygen
in species such as nitrate (NO
2−
), iron oxide
(FeOOH), manganese oxide (MnO
2
) and sulphate
(SO
2−
). These anaerobic early diagenetic reac-
tions are many and complex. The most significant
