Sedimentary Environments 



Preliminary analyses suggest that the shelf sed- 

 iments can be grouped into three types on the 

 basis of the agent responsible for the sediment's 

 primary character: 1) wave and current con- 

 trolled, 2) ice-rafted and ice gouged, and 3) ice- 

 rafted. The first type occurs inside the fast ice- 

 pack ice boundary, which occurs between the 10 

 to 20 meter contour. Here the influence of drifting 

 ice on the bottom is least felt, owing to the ab- 

 sence of ice movement during winter (Weeks and 

 others, 1971). Here, also, the influence of waves 

 and currents during the open season is most pro- 

 nounced. It is also in this zone that ice-rafting is 

 least effective as: (a) little or no ice moves in this 

 zone during the winter, and (b) it is the zone most 

 likely to be free of ice during the summer. It is in 

 this zone that the better sorted and layered sandy 

 sediments are developed under the influence of 

 the open season waves and currents (fig. 12). 



The second type occurs at mid-shelf depths 

 and is characterized by unstructured, gravelly 

 muds. The muds exclusive of the gravels are 

 believed to be derived from local rivers, as their 

 abundance, expressed as a decrease in overall 

 sediment grain size, is greater off the western part 

 of the study area. Such a gradation would exist 

 assuming westerly transport and a system closed 

 to the introduction of new sediments in the east- 

 ern end. However, the scatter plot of mean diame- 

 ter and sorting (fig. 10) shows that the well-sorted 

 fine-grained material collected from sea ice may 

 be an end member source of the fine-grained 

 sediment. 



The lack of bedding structures in box cores 

 from this area reflects intense gouging (Reimnitz 

 and others, 1972a; Reimnitz and others, 1973). 

 The repetitive scouring and plowing of the bottom 

 by ice homogenizes the sediments and weakens or 

 obliterates any structures which may have been 

 previously developed. A core taken from the side 

 of an ice gouge shows a pronounced slope to the 

 sediment water interface and contains a non- 

 horizontal block with layering relict of a previous 

 episode of sedimentation (fig. 16). The absence of 

 significant bioturbation suggests that these sedi- 

 ments are presently being reworked. Further 

 offshore where gouging is apparently less intense 

 or older bioturbation is better developed, bottom 



photographs of the central shelf show a patcliiness 

 of animal distribution which correlates with ice 

 gouging. Areas with a well-developed fauna show 

 little evidence of bottom disruption, and the mic- 

 rorelief is usually much subdued from additional 

 sedimentation (fig. 17). Bottom photos of areas 

 showing fresh breaks and scarps reveal only a 

 sparse benthic fauna (fig. 18). Presumably the 

 areas with abundant fauna have not been scoured 

 by ice as recently as areas with fresh breaks and 

 fewer animals. 



The third sediment type occurs along the shelf 

 edge, where ice-rafting appears to dominate the 

 surficial sedimentary processes. While gouging is 

 present (Reimnitz and Barnes, 1972), there is 

 nothing to indicate positively whether or not the 

 gouges are modern or relict features. As men- 

 tioned above there is much more evidence of 

 bioturbation in this zone (fig. 13). This, coupled 

 with the presence of horizontal layering, would 

 suggest an older age for the ice-bottom interac- 

 tion. Along the central and western shelf break 

 only the surficial deposits are gravels. The under- 

 lying materials are laminated muds, strongly sug- 

 gesting that the gravels have been rafted. From 

 the Canning River eastward, box cores have not 

 been able to penetrate through the gravels, pre- 

 sumably because they thicken in this direction. 



Determining the transport mechanism for the 

 shelf gravels presents problems. The lithologies, 

 shapes, and surface striations indicate that the 

 gravels are ice rafted from source areas in the 

 Southern Canadian archipelago (Craig Rodeick, 

 oral communication, 1973). Modern ice rafting is 

 suggested by the occurrence of surface gravels at 

 the shelf edge; however, we see no modern evi- 

 dence for this rafting. The ferro-manganic coat- 

 ings on some gravels (Naidu and Sharma, 1972) 

 also suggest an older age for the gravels. 



The eastward increase noted in the abundance 

 of gravel indicates an eastward source; however, 

 the gradient of increase is perhaps too steep for a 

 source as distant as the Canadian islands. With 

 such a distant source, the concentrations of gravel 

 should be more equitable on the Alaskan shelf. 

 Furthermore, gravels should occur on the slope. 

 The most plausible explanation is that relict 

 rafted gravels have been "diluted" by subsequent 

 finer grained sedimentation. Toward the east 



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