chalk bed of early Middle Miocene age, can be seen on the southern 

 portions of all profiles. 



The northern end of the profiles cross the West Caroline Trough, 

 a feature striking ENE for 900 km across the northern portion of the 

 western basin. The eastern end of this feature is definitely trough- 

 like in morphology, as shown in profiles 5-7 (Fig. 4). The eastern 

 trough contains some sediment (0.5 sec), and the sediments on its 

 flanks appear relatively undisturbed. Note also the apparent increase 

 in sediment thickness away from the trough axis, particularly notice- 

 able in profiles 5 and 6. The northern end of these profiles (5 - 7) 

 terminate in features proposed by Bracey and Andrews (1974) to be the 

 remnants of a northward dipping subduction zone flanking the southern 

 margin of the western Caroline Ridge, a possibility to be discussed 

 in a later section. 



In contrast to the trough-like morphology of the eastern end of 

 this feature, the western end (profiles 1-3) is characterized by a 

 sediraent-f ree topographic high with a central valley of 1.0 - 2.0 sec 

 relief, and a width of 20 - 30 km, centered between areas of absent or 

 disturbed sedimentary cover. 



Figure 5 is an enlargement of the northern end of profiles 1-3 

 which more clearly illustrates these phenomena. Profile 1 shows a 

 gradual increase in sediment thickness away from the axial trough 

 (at 5°N) , with a sharp change in sediment thickness across the 1.0 

 sec escarpments at 4.15°N and 6.0°N. There appears to be another 

 increase in sediment thickness to the south of the 0.5 sec 



14 



