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Submarine Topography in the Gulf of Alaska

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Submarine Topography in the Gulf of Alaska BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 71, PP. 1087-1108. 12 FIGS.. 1 PL. JULY 1960 SUBMARINE TOPOGRAPHY IN THE GULF OF ALASKA By WILLIAM M. GIBSON ABSTRACT A bathymetric chart of the Gulf of Alaska and approaches covering an area of about 800,000 square nautical miles has been prepared from Coast and Geodetic Survey hydrography, 1925-1957, with a view to defining regional physiographic provinces. The basic data comprise 90 sounding lines across the gulf, 42,000 miles of graphically recorded profiles obtained during the last 5 years, and detailed surveys of 60 seamounts, seaknolls, and ridges. Tentative names are assigned principal features of the sea floor to facilitate discussion. The bathymetry and illustrated profiles reveal clearly the form of the Aleutian Trench, two submarine mountains, several seamount chains and groups, a ridge and trough prov- ince, a 200-mile trench west of Vancouver Island, a great trough paralleling the West Coast, and an inferred fracture zone extending in several wide echelon bands across the gulf. The submarine topography is discussed in relation to existing theories of earth science and correlated with features previously mapped on the mainland and in the Central Pacific Ocean. TEXT ILLUSTRATIONS Page Figure Page Introduction 1087 1. Great Circle map of structural lineations. 1091 Acknowledgments 1088 2. Transverse profiles of the Aleutian Trench, Surveys 1088 Cape St. Elias to Unimak Pass 1092 Submarine terminology 1089 3. Axial profile, floor of the Aleutian Trench 1093 Description of the gulf coastal area 1089 4. Transverse profiles adjusted normal to General description of the gulf floor 1090 axis, Surveyor Deep-sea Channel 1094 Western and northern margin 1090 5. Transverse profiles of continental slope, General description 1090 eastern margin of Gulf of Alaska 1095 Continental shelf 1091 6. Transverse profiles, Baranof Sea Valley. 1096 Continental slope 1091 7. Transverse profiles, Moresby Sea Valley. 1097 Aleutian Trench 1092 8. Transverse profiles, Explorer Trench 1098 Western floor of the gulf 1093 9. Transverse profiles, Ridge and Trough Eastern margin 1094 province 1099 Great Trough 1096 10. Transverse profiles, Mid-Gulf zone 1102 Ridge and Trough province 1097 11. Profiles along the Patton-Faris Seamount Kodiak Seamount Chain 1100 Group 1103 Mid-Gulf zone 1101 12. Transverse profiles across Parker-Bean and Parker-Bean and Patton-Faris seamount Patton-Faris seamount groups 1104 groups 1102 Unimak-Sirius Seamount Chain 1104 P ate Facing page Columbia fracture zone 1105 1. Bathymetric chart showing submarine to- Interpretation 1105 pography in the Gulf of Alaska and loca- References cited 1107 tions of illustrated profiles 1108 INTRODUCTION the Coast and Geodetic Survey initiated a pro- gram of running deep-sea traverses in 1925. This study of submarine topography in the For several years sounding lines were obtained Gulf of Alaska and approaches embraces an only on the return trip of survey ships after the area of about 800,000 square nautical miles. season's work in waters of western Alaska. In Although the whole area is referred to as the more recent years, because of modern naviga- Gulf of Alaska in this report, the name applies tion requirements and increased interest of to a more limited northern area. marine biologists, oceanographers, seismolo- Knowledge of this region was sketchy until gists, and geologists, sounding lines were run on 1087 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/71/7/1087/3427260/i0016-7606-71-7-1087.pdf by guest on 26 September 2021 1088 W. M. GIBSON—SUBMARINE TOPOGRAPHY, GULF OF ALASKA each crossing. Limited-area surveys of sea- by the Office of Naval Research and Bureau of mounts and other features were also made en Ships will add materially to existing knowledge route. It is a continuing program. of the northeastern Pacific area. The Gulf of Alaska was formerly included on Coast and Geodetic Survey Chart 9000 at very ACKNOWLEDGMENTS small scale, 1:5,000,000; but in 1952 the first edition of Gulf of Alaska Chart 8500 was pub- The officers and men of the ships and the lished at scale 1:2,100,000. The depth-contour cartographers in the Washington Office of the interval of 500 fathoms on this larger-scale Coast and Geodetic Survey have made this chart does not, however, reveal the secondary work possible, but special credit should accrue features so important to a study of submarine to those who set the initial and continuing topography. First details of the gulf topography policy of providing the hydrographic coverage were presented by Murray in 1941, followed in of this great area at little or no extra cost to the 1945 by his report on the Aleutian Trench. In Government. Rear Admiral H. A. Karo par- 1951 Menard and Dietz presented a compre- ticipated in both the field and office work, en- hensive geological study of the area. This study couraged preliminary study, and lent direction was based krgely on the Coast and Geodetic and assistance in the presentation. Survey deep-sea hydrography, supplemented Inasmuch as the study was made away from by sounding lines obtained by Navy ships. The the depository of survey records in the Washing- results of a joint exploration by the Navy ton Office, valuable assistance at that office was Electronics Laboratory and Scripps Institution given by G. F. Jordan in the preparation and of Oceanography in 1951, including the dredg- review of illustrative material. ing of tops of seamounts and coring the sea floor, were presented by Menard in 1953. In 1955 Menard reported further studies of the SURVEYS Gulf of Alaska. A considerable accumulation of original ship The hydrographic surveys from which the observations, including graphically recorded bathymetry on Plate 1 was developed are com- profiles of the floor and seamounts, was avail- posed of a continuing accumulation of deep-sea able for this study. The fathogram profiles are sounding lines obtained en route to and from reconstructed in several illustrations presented coastal areas of western Alaska. These recon- here, because various scales and vertical exag- naissance surveys have greatly improved in gerations resulted from the use of several types quality in recent years. of echo sounders and also from the effect of The early traverses relied solely on astro- different ships' speeds. nomic observations for positioning. Overcast Although bathymetric details of this report and storms were often experienced on return are considered to be almost complete in certain trips in September, and occasional dead-reckon- areas, considerable surveying is still needed to ing courses for long stretches resulted. Depths delineate all features in detail. Many sections were ascertained at intervals by sonic reflections of the depth contours could rightly be repre- read at an instant on a calibrated dial or timed sented by broken lines; solid-line bathymetry to hydrophone reception. Early improvements is presented with this qualification. An indica- in echo sounders were soon reflected in nearly tion of the density of deep-sea sounding lines continuous and more reliable soundings. An on which this study is based is given in the in- eventual adjustment of these traverses was con- dex-map inset on Plate 1. Numerous peak templated at a later date when the framework of depths of seamounts were deleted from copies well-controlled lines became established. The of the survey sheets used in this study. In very large amount of work required in the analysis of control data in the proposed adjust- recent years, survey information of this nature ment is now considered unwarranted. Inade- has not been available for unrestricted publica- quacies in the early lines do not materially tion. A discussion of flat-topped seamounts affect small-scale deep-sea charting. versus seapeaks is therefore not included in this Most traverses and all the limited-area sur- report. Reference is made to unpublished re- veys were made in more recent years with sults of the Chinook and Mukluk expeditions benefit of loran positioning and modern re- made by scientists of Scripps Institution of cording-type ultrasonic sounders. A large ac- Oceanography in 1956 and 1957. Geophysical cumulation of well-controlled graphic profiles data collected on these expeditions sponsored has resulted. On both outbound and inbound Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/71/7/1087/3427260/i0016-7606-71-7-1087.pdf by guest on 26 September 2021 SURVEYS 1089 trips the ships survey seamounts and other and less regular topography than a rise; sea- features which lie on or adjacent to their trav- mount is defined as an isolated elevation of the erses. The traverses and limited surveys are deep-sea floor of about 3000 feet or more and planned each year to fill voids in untraversed circular or elliptical in form. The term "ridge" areas. Lines necessarily converge at the end of has been applied previously to immense struc- the Alaska Peninsula, Kodiak, and Strait of tures like the Mid-Atlantic Ridge. Throughout Juan de Fuca. Also, traverses have been con- the area of this study are weltlike forms 20-90 centrated in seamount areas to permit surveys miles in length and about 10 miles in width. of the features discovered on earlier crossings. These structures do not differ greatly from the These detached surveys have now covered 60 welts forming the Ridge and Trough Province seamounts, ridges, and knolls. Total crossings off the West Coast (Menard and Dietz, 1951, amount to 90, including the 1957 lines. In the p. 1274). They are termed ridges in this study. past 5 years 42,000 nautical miles of graphically The International Committee favors geo- recorded profiles were obtained in this area by graphic names over personal names (Wiseman ships of the Coast and Geodetic Survey.
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