To meet overall PANASH project objectives, three cruise legs of RV Melville targeted six study areas for mapping and coring (Figs. 3.1, 3.2, 3.3). The legs are referred to as Leg 1 (Cairns to Port Moresby), Leg 2 (Port Moresby to Port Moresby), and Leg 3 (Port Moresby to Cairns) in this report; they have formal SIO designations of MV VANC 24, MV VANC 25 and MV VANC 26. The six areas targeted were:
(1) Southern Ashmore Trough. The southern part of Ashmore Trough extends ~40 km between the GBR to the west and Ashmore Reef to the east (Fig. 1.1). With reefs bounding both sides, this region should receive large amounts of neritic carbonate, at least during sea level high stands. However, siliciclastic material might arrive from the GOP shelf to the north and along the trough, particularly during sea level low stands or transgressions. Overall, sedimentation in this area is expected to be somewhat similar to locations immediately east of the GBR off the North Queensland Margin.
5.1 Operations
A ~ 300 km2 area of southern Ashmore Trough was mapped between the GBR and Ashmore Reef to the east (Fig. 1.1). Most of the area was covered by Seabeam imaging, and ~210 line nautical miles of 3.5 kHz were collected (Fig. 3.1).
Three stations were cored (Table 5.2; Figs. 3.1; 5.1). At the first two stations, 18 and 15 cm of calcareous sand were recovered in box cores the upper few cm. At station 3, on the Ides of March, King Neptune got hungry and the box corer remained on the sea floor, never to return to the ocean surface. However, a 3.70 m gravity core and 11.28 m piston core were recovered at this third station.
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Table
5.2. Cores from Southern Ashmore Trough, MV24 03/04 |
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Time |
Water |
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Leg |
Date |
Station |
Core ID |
Latitude |
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Longitude |
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Bottom |
Depth |
Recovery |
Remarks |
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(Deg) |
(Min) |
(Deg) |
(Min) |
(hr:min) |
(m) |
(m) |
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1 |
14-Mar-04 |
1 |
mv24-0403-01BC |
-10 |
-18.550 |
143 |
59.997 |
16:35 |
342 |
0.18 |
Well
sorted calcareous
sand. |
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1 |
14-Mar-04 |
2 |
mv24-0403-02BC |
-10 |
-18.549 |
144 |
9.789 |
19:43 |
661 |
0.15 |
Fine sand |
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1 |
14-Mar-04 |
2 |
mv24-0403-03GC |
-10 |
-18.549 |
144 |
9.791 |
21:00 |
661 |
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Empty |
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1 |
14-Mar-04 |
3 |
mv24-0403-04BC |
-10 |
-26.400 |
144 |
12.300 |
0:07 |
775 |
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Empty |
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1 |
15-Mar-04 |
3 |
mv24-0403-05BC |
-10 |
-26.400 |
144 |
12.300 |
1:10 |
775 |
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Lost box
corer! |
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1 |
15-Mar-04 |
3 |
mv24-0403-06GC |
-10 |
-26.399 |
144 |
12.301 |
6:01 |
775 |
3.70 |
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1 |
15-Mar-04 |
3 |
mv24-0403-07PC |
-10 |
-26.399 |
144 |
12.301 |
10:19 |
773 |
11.28 |
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Water depths in the trough increase to the south, and away from the shelf and Ashmore Reef (Fig. 5.2). However, bathymetry is skewed such that the deepest part on E-W transects lies much closer to Ashmore Reef. These deepest parts appear to define a generally southward flowing channel with several tributaries. Slopes steepen significantly adjacent to reefs on both sides. On the western slope of Ashmore, the slope angle distinctly steepens from the 500 m contour.
The modern seafloor within southern Ashmore Trough is relatively smooth. South of Ashmore Trough proper, however, the seabed is ’Äúhummocky’Äù with numerous depressions (Fig. 5.2).
Except in a few places, a 20 to 60 m thick, relatively transparent sediment package lies between the seafloor and a bright subsurface reflector. Along some lines, and particularly in deeper parts of the basin, the shallow sequence becomes less transparent and contains minor reflectors (Fig. 5.3). The sequence is particularly transparent on slopes of reefs (Fig. 5.4). In general, the shallow sequence is thickest along the slopes reefs.
| Figure 5.6: Sub-bottom SE-NW profile across hummocky seafloor of south of Ashmore Trough. Note that scours cut through sub-bottom reflectors. Location on line X-Y at nominal position 10 32¬ƒ S, 144 05¬ƒE, and 555 to 585 m water depth. |  |
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Figure 5.7: Sub-bottom S-N profile between Ashmore Trough and the Hummocky seafloor. Note previous scour filled with recent sediment. . Location on line X-Y at nominal position 10 26¬ƒ S, 144 02¬ƒE, and 525 to 530 m water depth. |
Surface sediment is red-orange but becomes creamy gray in the upper few cm, reflecting a dominance of carbonate. Proximal to the reef, the carbonate-rich sediment is composed of coralgal sand, including abundant Halimeda flakes. Farther out in the basin, it contains more foraminifera and pteropods (Fig. 5.8a). At depth, however, sediment becomes dark gray-green where siliciclastic clay is dominant. (Fig. 5.8b).
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Southern Ashmore Trough is a tropical mixed siliciclastic/carbonate system, receiving siliciclastic material from the shelf (presumably from the north) and carbonate from surrounding reefs. The relative proportion of sediment arriving from these two sources definitely changes over time. The clay-rich interval at the bottom of Core 07PC may represent lowstand, although this supposition is premature without appropriate age control.
The hummocky seafloor south of Ashmore Trough appears to reflect modern day erosion. This may be the result of bottom currents, and it is interesting to note that the hummocky bathymetry only occurs beyond the lee of Ashmore Reef. That buried scours can be found filled suggests that this process has operated in the past.
The channel leaving Ashmore Trough suggests that some amount of material leaves Ashmore Trough, although perhaps in the past more than at present.