GEOPHYSICAL OBSERVATIONS 



Seismic Reflection Profiles 



East-west seismic reflection profiles across the Ayu Trough are 

 shown in Figure 3. This trough, together with the contiguous Palau 

 Trench, forms the western margin of the Caroline Basin. The trough is 

 sediment-free, and there is an increase in sediment thickness in either 

 direction from the trough. This is particularly apparent in profile 

 L-D 2. Note also the dramatic increase in sediment thickness east of 

 the Mapia Trough and west of the Ajoe-Asia Island Ridge shown on pro- 

 file A. 



Another feature of interest is the sediment-free trough west of 

 the Ajoe-Asia Island Ridge at 130. 5°E shown on profile A. The bathy- 

 metry of Figure 2 indicates that this feature may extend for at least 

 150 km to the northeast, parallel to the Ayu Trough, but without sub- 

 stantiating bathjrmetric and seismic reflection evidence the possi- 

 bility of a northward extension of this sediment-free feature cannot be 

 confirmed. 



Figure 4 shows seismic reflection profiles in the West Caroline 

 Basin. The western profiles (1-4) exhibit a particularly rough base- 

 ment surface. There is a broad, low basement arch fronting the New 

 Guinea Trench, and the basement reflector can be seen to dip southward 

 and disappear beneath the trench at about the 8.0 sec level. Sediment 

 thickness increases to the south in all profiles. 



An internal reflector, believed by Bracey (1975) to be identical 

 to "horizon X" identified by Den and others (1971) as a nannofossil 



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