to the northwest of the ridge and by the change in sediment characteristics. The 

 influence of bottom turbidity currents appears quite obvious from topographic con- 

 siderations alone. The size, shape, and areal extent of the Indus cone, as well as 

 the presence of apparent distributaries and natural levees, point to a primary tur- 

 bidity current origin of this feature . 



The close similarity of ridge and adjacent deep sediments is probable evidence 

 of the importance of wind and ocean currents in sediment distribution . Dust is un- 

 doubtedly an important source of sediment over the entire northern Arabian Sea and 

 particularly in the Gulf of Oman Basin. Sugden (1963) estimates that one-thirdof 

 the sediments being deposited in the adjacent Persian Gulf are wind-derived. In 

 the northern Arabian Sea area, the occurrence of wide-spread dust storms during the 

 monsoons is well documented . The extreme aridity of the bordering land masses and 

 the paucity of perennial streams, particularly around the Gulf of Oman Basin, in- 

 creases the relative importance of aeolian material . Frosted quartz grains noted in 

 ridge sediment are proof of the importance of wind in this area . Aeolian quartz is 

 very likely derived from desert sands south of Ra's Al Hadd and from the Makran 

 coast in the vicinity of Gwadar. Indirect evidence of the primary sediment source 

 wind direction is the increase in biotite with distance from the Arabian shore, which 

 may reflect wind sorting effects on the flat mica grains. Judging from available 

 water current data, this is not likely to be an aqueous sorting effect. 



The few coarse fraction heavy and light mineral suites that were quantitatively 

 analyzed were highly unstable. This may reflect two factors: (1) pre-existing sed- 

 iments are not major contributions to the coarse fraction and (2) the regional aridity 

 inhibits extensive chemical weathering . Pre-existing sediments do contribute some 

 coarse material to the northern Arabian Sea, notably the rounded dolomite and cal- 

 cite grains and frosted quartz grains . 



The clay mineral distribution strongly reflects source rocks. The high montmo- 

 rillonite province in the southeastern corner of the study area is adjacent to and 

 undoubtedly derived from the abundant basic rocks in the area (Deccan basalts). 

 With increasing distance from these source rocks, the montmorillonite content of 

 the marine sediment decreases . 



The Indian continental shelf exhibits the often observed sediment distribution 

 pattern of fine sediments near shore and coarser material on the central and outer 

 shelves. Very likely the neorshore material is recent while the remainder of the 

 shelf sediment is relict, having formed during lower sea level stands of the Pleisto- 

 cene . The area of sand-size sediment off the mouth of the Indus River reflects a 

 relatively high rate of sedimentation associated with the river . On the other hand, 

 the relatively coarse patches of sediment on the outer shelf south of the present 

 Indus River mouth af largely calcareous in composition and reflect very low rates 

 of present day terrigenous sedimentation . 



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