DISCUSSION 



Sediment Transport and Deposition 



Spatial variations in gross texture and grain 

 size parameters of sediments are powerful tools to 

 a sedimentologist in the inference of the sediment 

 source, direction of transport, and deposition, as 

 well as in deducing the physical competency and 

 fluctuation of sedimentation over a depositional 

 area. Research on the deltaic sediments of north 

 arctic Alaska is incomplete and, therefore, some 

 of the following conclusions should be considered 

 tentative. 



An interesting observation in the north arctic 

 delta under study is that the sediments of the 

 lagoon, bay and adjacent shallow marine facies 

 (Table 1) lack gravel-sized materials, or, at most 

 contain only insignificant amounts. A similar ob- 

 servation was made by Tucker (1972). He 

 analyzed some 100 sediments from the Simpson 

 Lagoon and found a significant amount of gravel 

 only in about 6 samples, and most of these sam- 

 ples were located near the gravelly coastal or 

 barrier beaches. This paucity of gravel in the 

 deltaic sediments is contrary to expectation, be- 

 cause in the barriers and coastal beaches there is 

 a ready source of gravel. It was expected that the 

 shore fast ice of these areas during spring breakup 

 would pick up gravels, ice raft them offshore, and 

 deposit most of these gravels in the lagoon and 

 shallow marine facies of the delta subsequent to 

 melting of the ice. The aforementioned dearth of 

 gravel in the lagoon, bay and adjacent shallow 

 marine area may be attributed to one or a combi- 

 nation of the following factors, (i) Contemporary 

 tansport of gravel from the coastal and barrier 

 beaches to the lagoon, bav and open marine envi- 

 ronments of the delta, by ice-rafting and/or cur- 

 rents may be insignificant, (ii) The rate and 

 amount of sand, silt, and clav deposition in the 

 deltaic area may be relatively much higher than 

 that of gravel and, therefore, the amount of gravel 

 would be quantitatively greatly "diluted." (iii) 

 There is a possibility of error arising from the 

 sample collection and analytical methods used; 

 generally about 0.5 kg of a sediment sample was 

 collected from each location, and from this about 

 100 gm were taken for size analysis. To consist- 

 ently detect and measure small amounts of gravel. 



it might well be deemed necessary' to take and 

 utilize larger amounts of sample materials. 



Comparison of the size analysis data in Table 1 

 and that presented by Burrell et al. (1970) and 

 Naidu (1973) clearly shows that, on the basis of 

 gravel contents offshore the deltaic and the 

 contiguous nondeltaic shelf sediments of north 

 arctic Alaska have different lithologies. Unlike 

 the deltaic deposits, the shelf sediments fre- 

 quently (72% of the samples) do have gravels. 

 These lithological differences naturally lead to 

 the question of origin of the gravel on the nondel- 

 taic shelf, which has been discussed at length by 

 Naidu (1973). It is concluded that the bulk of the 

 exposed shelf gravel is a relict sediment. The 

 relict origin for most of the gravel on the shelf is 

 primarily ascribed on the basis of the following 

 premises, (i) Observations to date show that con- 

 temporary transport of gravel by ice-rafting to the 

 Beaufort Sea shelf is insignificant (Barnes, 1972; 

 Reimnitz, 1972; and Naidu, 1973). (ii) There is 

 no coarse to fine sediment gradation from the 

 coast to the outer shelf. This fact may be consi- 

 dered, as suggested by Emery ( 1968), and Swift et 

 al. (1971), as well as by several others, a reliable 

 criterion in establishing the relict nature of 

 marine sediments, (iii) The shelf gravel appears 

 to be in disequilibrium with present hyd- 

 rodynamic conditions. Although no long-term 

 data on water currents is available, good reasons 

 exist to believe that at present there are no bottom 

 currents of sufficient strength to transport gravel 

 on the shelf. This is inferred indirectly from the 

 presence of ferrimanganic coatings and of growths 

 of encrusting Bryozoa and tube-forming 

 polychaetous annelids only on the gravel surfaces 

 facing the sediment top. Naidu (1973) has inter- 

 preted, based on sediment interstitial water 

 studies, that the ferrimanganic coatings are con- 

 temporary precipitates. If at present there were 

 strong currents on the shelf to transport these 

 gravels intermittently, it would be expected that 

 the ferrimanganic and biogenic encrustations 

 would not be restricted solely to the gravel tops. 

 The lack of strong currents at the present time is 

 also substantiated by heavy mineral studies, re- 

 sults of which will be discussed in detail later in 

 this report. 



The processes by which the shelf gravels were 

 transported and deposited in the past remains a 



245 



