Geoscience Reference
In-Depth Information
Taieri
River
Tokomairiro
River
Clutha
River
800 m
Shelf processes
Direction of inner shelf transport
(wave and storm dominated)
Direction of mid- to outer-shelf transport
(Southland Current and storm dominated)
Tide-dominated shelf
Sand wave field
Sand ribbons
Terrigenous sediments
Modern sand
Modern mud
Relict gravel
Relict/palimpest sand
Biogenic
Relict/palimpest sand and gravel
with local modern deposits
Bryozoan meadows
Fig. 10.6
The
sedimentary facies and
transport regimes of the
South Otago shelf.
(Adapted from Carter
et al. 1985.)
dispersal on this highly dynamic shelf is influenced
by the inertia of the river flow, the along-shelf
Guiana Current, which flows at 35-75 cm s
−1
to the north-west, waves driven by trade-winds
and by locally strong tidal currents (see also
Case Study 9.1). The sediments are sands at
river mouths, and further along the transport
path are clayey silts, then interbedded silts and
sands, and sandy silts, which form the top of the
delta and occupy the zone of highest oceano-
graphic energy. Immediately to seawards, in
depths of 40 - 60 m, is a zone of faintly laminated
mud, where the highest sediment accumulation
rates are located, and the most abundant benthic
biology. Over time-scales of 100 years or so,
these accumulation rates may reach 10 mm yr
−1
(Fig. 10.7). Down-core sulphur concentrations
and
210
Pb profiles indicate catastrophic accumu-
lation events of up to 5 m yr
−1
, as well as epi-
sodes of high rates of reworking and sediment
removal (Aller & Blair 1996; Allison et al. 1996;
Sommerfield et al. 1996; see also Kineke et al.
1996). At their seaward edge, these Amazon-
derived sediments become mottled and biotur-
bated, and beyond is a zone of relict fine sands.
Organic-rich laminae may occur widely across
the shelf, derived from either seasonal river
input and/or plankton blooms. A large area of
the shelf sediments (31,000 km
2
) is gas-enriched
(Figueiredo et al. 1996), with the gas formed
from breakdown of organic matter derived from
marine and terrestrial sources.
