3-2. Single Channel Seismic Reflection Interpretation 　[G. Moore]

During Leg 1A, we collected single channel seismic reflection data along two lines across the distal Nu`uanu landslide and one line across the distal Waianae slump (Figure SCS1). Initial on-board processing consisted of bandpass filtering (20-100 Hz) and scaling. Additional processing, including migration, will be applied at the University of Hawaii after the cruise.

The Nu`uanu seismic lines demonstrate the thinning of the sedimentary units as the lines cross from the Hawaiian moat (deepest areas) to the north Hawaiian arch (shallowest areas) (Figures SCS 1,2,3). We crossed several landslides blocks that are about 1-3 km wide and up to 200m high (Figure SCS 4). Recent hemipelagics and distal turbidites overlie the chaotic unit interpreted as the Nu`uanu debris avalanche, which in turn overlie laterally continuous turbidite units. The base of the landslide blocks appears to lie within the Nu`uanu avalanche unit. The avalanche deposit is about 50-75 m thick. Line 2-1 crosses a block that is inferred to be a Cretaceous seamount based on lack of continuity of sedimentary reflections beneath it (Figure SCS 5).

The Waianae lines show much less landslide debris than the Nu`uanu lines (Figure SCS 7). The flexural moat is unfilled with sediment, so the seismic lines show a general deepening of the moat toward Oahu. At least 500 msec (~500-750 m) of stratified sediment is imaged in the moat, with the deepest layers being tilted toward Oahu. There is a series of blocks that become wider and shallower toward Oahu. At least 350 msec (350-500 m) of stratified sediment is ponded behind each block. The deepest strata are tilted toward Oahu. The shallowest 30-60 msec of sediment in the moat and in the basins behind the blocks is transparent and is interpreted as recent hemipelagic sediment deposited after movement of the landslide blocks.

Figures
a b C
Figure SCS 1a,b,c = Seismic shot point location map

Figure SCS2.  Single channel seismic reflection line 1-1

Figure SCS3.  Single channel seismic reflection line 2-1 and 2-2.  Note that, because of increase in speed, horizontal scale of line 2-2 is twice that of 2-1.

Figure SCS4.  Comparison of approximate intersection of single channel seismic lines 2-1 and 1-1.  Because the end of line 1-1  is about 10 km north of line 2-1, there is a difference in sediment thickness between the two lines.  A) Join of lines at the seafloor showing that the upper sediment section matches very well; B) Join of lines at the base of sedimentary section showing good match of reflections.

Figure SCS5.  Section of seismic line 1-1 showing seismic stratigraphic interpretation. 

Figure SCS6.  Sections of seismic lines 1-1 (left) and 2-1 (right) showing the differences in character of the sedimentary layers around the landslide blocks and inferred Cretaceous seamount.

Figure SCS7.  Seismic line 3-1 over the distal portion of the Waianae slump.

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