SECT. 2] ABYSSAL PLAINS 347 



Vema Gap is a large magnetic anomaly suggesting a prominent buried ridge 

 beneath the sill. In contrast, the anomalies over the Theta Gap are quite small 

 and can be attributed to the visible topographic features of the area. 



The Cascadia Channel (Fig. 27) mentioned previously is an example of a sub- 

 marine canyon deep-sea channel system which directly connects through a 

 tectonically controlled abyssal gap. The Cascadia Channel is both an extension 

 of a continental slope canyon and an interplain channel. The tectonic trough 

 which the Cascadia Channel follows through the ridge and valley province 

 connects both to the north and to the south to a series of deeper trenches, 

 leaving little doubt of the tectonic character of this feature. 



An abyssal gap connects the Colombia and Venezuela plains in the Caribbean. 

 Others will undoubtedly be discovered. 



6. Sediments on the Abyssal Plains 



The abyssal jjlains of the abyssal floor generally lie at such depths and 

 distances from shore that the normal pelagic sediment covering them is deep- 

 sea red clay. However, in the abyssal plains of the northeast Atlantic and 

 Mediterranean, the normal pelagic sediment is Globigeriyia ooze and in the 

 small abyssal plains of the continental margin basins lying off Southern Cali- 

 fornia, the normal sediment is gray terrigenous clay. There is a sharp contrast 

 in sediment type between the abyssal plains and adjacent, slightly elevated 

 areas of nearly the same depth (Fig. 28). 



On the rises only pelagic ooze is found. These sediments consist essentially 

 of the shells of planktonic Foraminifera mixed in varying proportions with 

 red clay or brown lutite. Because of the solution of calcium carbonate, the 

 shells of Foraminifera decrease in number with increasing depth of water. 

 Below about 5000 m, shells of Foraminifera are almost absent and the sediment 

 consists essentially of brown lutite, which is the classical "red clay". This 

 sediment accumulates through a constant rain of particles from above. The 

 particles themselves are either wind-blown or transported by oceanic currents. 

 Much of the calcium carbonate is precipitated originally in the upper layers of 

 water in the form of the shells of planktonic plants and animals. In any case, 

 the route of transportation of the particles is not along the ocean floor. Core A 

 160-1, at 5100 m def)th on the Bermuda Rise, contains typical normal abyssal 

 sediment, free from turbidity-current deposits. Throughout its length of 458 cm 

 it is dark brown lutite. Shells of Foraminifera make up less than 1% by weight 

 of the sediment. Micronodules of manganese oxide are common. Sand and silt 

 layers are absent and there are no abrupt changes in lithology. 



On the abyssal plains the normal pelagic sediments are interbedded with silt, 

 sands, gravels and anomalous pelagic sediments normally found in shallower 

 environments. Such detritus is absent from the rises and abyssal-hills area. For 

 example, of 12 sediment-core stations, taken before 1954 in the Sohm Abyssal 

 Plain, northwest of the Bermuda Rise, cores were obtained from only seven 

 and at only tlii-ee of these stations did the cores exceed 1 m in length. At five 



