350 HEEZEN AND LAUGHTON [CHAP. 14 



stations only a few grains of sand were taken from a bent coring tube. At 

 Stations J) and K (Table I) only the up])er layer of red clay, 10-20 cm thick, 

 remained in the pipe after the lower portions of the core had run out between 

 the leaves of the core retainer. The entire sample ran out of the pipe at Stations 

 A, B, I, J and K. Therefore, the only cores of significant length obtained from 

 the abyssal plain were F, G, H and L-. All these cores contained sand and silt, 

 so it is concluded that the reason for the lack of recovery on the other core 

 stations is that the material was sand or silt which, unlike clay or ooze, could 

 run out tlu'ough the core retainer. In each case, a few grains of sand were 

 imbedded in the grease around the core retainer. Cores G, H, I, J, K and L JV^^ 



' Mere taken for the specific i)urpose of testing the prediction of Heezen and -'■'^'■- 

 Ewing (1952) that there would be in this area an uppermost layer of graded 



' sand and silt. This prediction was based on the interpretation of the cable 

 breaks following the Grand Banks earthquake as the work of turbidity currents 

 triggered by slips, which in turn resulted from an earthquake shock. An upper 

 layer of graded silt in G and H, and indications of a similar layer in I, J and K, 

 confirmed this prediction. 



In each core from an abyssal plain there is evidence of turbidity-cm-rent 

 deposition, i.e. shallow-water Foraminifera, graded quartz sand, gray clays and 

 bed of silt, all interbedded with lutite of abyssal facies. 



Core A 153-141, at 5350 m depth on the Sohm Abyssal Plain, contains 

 typical normal abyssal sediment of red clay facies to a depth of 30 cm. Below 

 this is a series of alternating layers of red clay, gray clay and quartz silt to 

 400 cm. From 400 to 600 cm there is a micaceous glauconitic sand. At 460 cm 

 the sand has a median diameter of 110 [x and is well sorted, having a quartile 

 deviation of 0.75 (p. Between 600 and 718 cm there is a series of brown clay 

 layers alternating with silt and sand. From 718 cm to the bottom of the core, 

 there is again glauconitic sand, which has at 790 cm a median diameter of 

 103 [x and is also well sorted, having a quartile deviation of 0.85 cp. 



The lower sand layer shows a gradual increase in particle size downward. 

 Both sand layers contain a few Forminifera, among which are the shallow-water 

 benthic forms Elphidium incertmn var. clavatum, Eljjhidiella arctica and 

 N onion labradoricum. 



This contrast in sediment type between the abyssal plains and the neigh- 

 boring oceanic rises has been found in all oceans and seas containing abyssal 

 plains. In several cases, more than 6 m of graded sand and silt have been found 

 under a few centimeters of lutite in cores taken from abyssal plains. A further 

 example is offered by the abyssal plain of the western equatorial Atlantic. 

 Locher (1954) reports graded beds of terrigenous sand and silt in all cores taken 

 from this plain. The shallow water origin of the material is strongly supported 

 by Phleger (1954), who found shallow-water benthonic Foraminifera in the 

 sands of fom- of these cores that he studied. Without excejDtion, every abyssal 

 plain from which data are available has yielded cores exhibiting the alternations 

 in sedimentary type characteristic of turbidity-current deposits. 



In Fig. 28 all deep cores taken in the North Atlantic are shown on a chart of 



