Geoscience Reference
In-Depth Information
Sand ribbon
Ephemeral
sand
Sand sheet
A'
B'
Licence area
Dredging lane
Sand
patch
A
B
Infill
B'
A
B
A'
A period immediately after dredging
A period of months after dredging
A period of years after dredging
Fig. 10.11
Cartoon of main sedimentary consequences of dredging. Hypothetical distribution of major sedimentary bedforms around
an individual dredging lane. Net bed sediment transport is towards the left. A-A
′
and B-B
′
are sections across the dredging lane.
of marine aggregate dredging on the sea bed have
focused on the benthic biological communities,
although the primary sedimentary factors are
increasingly being recognized. In the UK, various
'biotopes' are defined to help assess the nature
of impacts, 'recovery' and mediation, these being
(i) shallow-water mobile sands, (ii) shallow-water
stable gravels with transient sands and (iii)
stable gravels (EMU 2004). Biological 'recovery'
requires the sediments to revert to pre-dredge
or otherwise stable conditions, and tends to be
more rapid in mobile sediments such as sands in
shallow waters (few months to 4 years). In con-
trast, in stable areas (usually gravelly and deep),
'recovery' of the fauna is much slower (up to
>
quantities of valuable minerals. Those minerals
containing gold, diamonds, platinum and tin are
the most important. Such minerals tend to be
highly resistant to water and abrasion, and of
high density (
2.9). Physical concentration of
such minerals usually takes place as a result
of transport, and on continental shelves these
minerals tend to be associated with deposits
of high-energy environments that operated for
extended periods.
Diamonds occur in sediments of the western
shelf of southern Africa. Exploration licences have
been granted and active prospecting is taking
place in shelf areas off Namibia and South Africa,
at depths down to 200 -500 m. Off southern
Namibia, diamond-bearing deposits are associ-
ated with Pleistocene gravels that developed
at the LGM coastline, near the modern 130 m
depth contour (Rogers & Li 2002). These beach
gravels are now buried beneath a thin Holocene
sequence of sediments (
>
15 yr) because of the presence of long-lived
species (EMU 2004). There is great variation in
the nature and rates of faunal change 'recovery',
however, and meaningful assessments of recovery
really can be made only on a site-specific basis.
30 cm thick) associated
with the Holocene progradation of the Orange
River delta. Overall, the thin Holocene sedi-
ment package thus has a characteristic fining
upward sequence representing the post-glacial
landward migration of sedimentary facies, over-
lain by a coarsening upward progradational
marine deltaic sequence, although bioturbation
<
10.3.2.4 Marine mining
Marine mining differs from aggregate extrac-
tion because mining exploits marine sediments
specifically for their mineral content rather than
their texture. Such deposits fall into the category
of 'placer' deposits, which contain economical
