pre-Holocene (?) stiff silty clay have also been 

 found. 



Two other generalities can be deduced from the 

 mean diameter and sorting diagrams. First, there 

 is an east-west change in sediments that occurs 

 north of Cross Island. Sediments on the shelf to 

 the east are generallv more coarse than those to 

 the west (fig. 7. 8). The eastward coarsening is 

 most pronounced on the outer and central shelf. 

 Secondly, northeast of Cross Island a band of 

 coarser sediments including gravels extends ac- 

 ross the shelf (figs. 7, 8, & 9). 



Radiographs of shelf sediments from 18-140 m 

 show three distinct types of sedimentary facies. 

 Inshore (at 15-20 m water depth), the cores are 

 characterized by laminated sands and clays (fig. 

 12). On the central part of the shelf, between 20 

 and 50 to 70 m of water depth, muds intersperse 

 with sand and gravel. These sediments exhibit 

 virtually no structures, and little bioturbation can 

 be seen in the samples (fig. 1.3), which charac- 

 teristically resemble thoroughly mixed chocolate 

 chip cookie batter. 



Along the shelf break from 50-140 m a se- 

 quence exists capped by a bioturbated, muddy 

 gravel averaging 5-20 cm in thickness and often 

 containing abundant fauna. This gravel layer 

 tends to become more pronounced toward the 

 eastern part of the study area. Underlying the 

 gravel and mud is a silty mud often showing fine, 

 faint laminations. Pebbles and bioturbation are 

 uncommon in this zone. 



SecHinents from the base of the continental 

 slope show numerous poorly developed lamina- 

 tions in a matrix of fine muds (fig. 15). There 

 appears to be a complete absence of sand and 

 gravel material in this core, although a grab sam- 

 ple at this same station (Sta. 19b, Appendix A) 

 recovered coarse sands. 



Heavy-Mineral Studies 



Heavy-mineral concentrates have been ob- 

 tained from sand-size fractions (2.0-0.062 mm) of 

 129 samples, using tetrabromoethane (S. G. = 

 2.96) as the separation medium. The heavy- 

 mineral concentrate ranged from 0.1 to 4.3 per- 

 cent of the samples' original dry weight. The 

 concentrations of heavv minerals in the sediments 

 reveal crude trends, parallel to the shelf break. 



Percentage values appear to be consistently 

 below 1% within 40 km of the coast. The highest 

 percentages (2-3%) appear in a linear trend along 

 the slope break. 



Grain mounts were made of the very-fine-sand 

 fraction (0. 125 - 0.062 mm) using Lakeside 70 as 

 a mounting medium on glass slides. Where there 

 was insufficient material, fine sand (0.250 - .125 

 mm) was also included. In a point-count, 300 

 grains are counted (150 in a N-S sweep and 150 in 

 an E-W sweep). Counting is complicated by the 

 moderate to severe staining and diversity of the 

 mineral suite. 



Based in a cursory examination of the samples 

 and a selected count of a few slides, the entire 

 sampled area has been divided into regions. Most 

 of the heavy mineral grains examined are opaque 

 minerals, rock fragments, and mineral aggre- 

 gates. Minerals of the ortho-and clinopyroxene 

 and epidote groups rank second in abundance. 

 Other minerals counted are garnet, tourmaline, 

 apatite, zircon, sphene, rutile, monazite, 

 hornblende, and chloritoid. The large variety of 

 unstable minerals attests to the immaturity of the 

 sediments and suggests little reworking by water. 



There appears to be a significant variation in 

 the distribution of only one mineral: garnet. Off 

 Point Barrow, garnet comprises 10 percent of the 

 mineral suite. North of Cape Halkett, the percen- 

 tage decreases to 6, and near Barter Island, the 

 percentage is about 1. More grain mounts must be 

 examined before this distribution can be defi- 

 nitely confirmed. 



Geochemical Studies 



The organic content of 13 sediment samples 

 from 11 locations averaged 1.5 percent of the dry 

 sediment weight (Appendix B). The alkaline- 

 solution part of the organic matter is dominantly 

 of the humic type (from plants) and averaged 

 about 0.2% of the total sediment weight. The 

 bitumen or petroleum-like fraction constitutes an 

 average of less than 0.0004% of the sediment dry 

 weight — much lower than bitumen values found 

 at lower latitudes (Vernon Swanson, written 

 commun., 1972). These results suggest: 1) very 

 low background values of a petroleum-like sub- 

 stance in the sediments, and 2) the contribution of 

 terrestrial organic fraction (humus) masks the 



189 



