Geoscience Reference
In-Depth Information
Case study 10.2 The south-east Australian and south-east African continental margins: shelf
sedimentation and dispersal influenced by intruding oceanic currents
Off southern Queensland, Australia, shelf sedimentary facies include inner-shelf quartz sands
(0 -50 m depth), mid-shelf mixed carbonate-quartz sands (40 - 80 m depth) and, at a range of
depths, but mostly on the outer shelf (depths
40 m), pebble- to cobble-sized rhodoliths (Harris
et al. 1996). Rhodoliths are a type of coated grain, with a laminated internal structure related to
episodes of growth of coralline algae. With repeated movements, the laminations formed are con-
centric, and the threshold for movement of 50% of the rhodoliths in this area is 45 - 80 cm s
−1
,
depending on their size. Current-meter data indicates that the southward-flowing East Australian
Current (EAC) episodically intrudes onto the shelf, moderating the tidal flows and inducing
near-bed current speeds of up to 130 cm s
−1
, even at depths of over 70 m (Case Fig. 10.2A). For
depths of
c
. 80 m, only the largest storm waves can create oscillatory currents at the bed of
over 60 cm s
−1
, and tidal currents alone are relatively weak (
>
<
20 cm s
−1
). The EAC adds up to
153
o
E
(a)
40
140
60
20
5
7
25
o
S
3
4
2
Study
area
1
120
Fraser
Island
Australia
20
26
o
Current meter
site
1
0 (km) 20
(b)
Current meter 2
Case Fig. 10.2A
(a) The outer part
of the southern Queensland shelf and
associated currents, and current-meter
moorings located off northern Fraser
Island. (b) Hourly average current
speeds and direction on the outer
part of the southern Queensland shelf.
Note the residual currents to the
south-south-east, indicating the
presence of the East Australian Current.
(Adapted from Harris et al. 1996.)
N
Current meter 4
50 cm s
−
1
0
5
10
Days
20
25
30