About 96,000 cartographic features such as rocky reefs, kelp beds, rocks, islets and others were proofed, edited and digitized from the smooth sheets and charts. The most common feature was kelp beds (it should be noted that kelp beds are seasonal and their size and location are variable from year to year). The second-most common feature was rocks, at just less than 29,000, with the majority occurring in the Kodiak, Kenai and Sitka areas. Rocky reefs were third in occurrence, and there were 15,000 islets almost equally split between western and eastern sides of the CGOA.
Multibeam Data of Central Gulf of Alaska.
Multibeam Data of Central Gulf of Alaska
Geological Features of Central Gulf of Alaska.
Geological Features of Central Gulf of Alaska
Our bathymetry editing resulted in the first detailed imaging of several noteworthy geological features including banks, earthquake faults and probable glacial moraines. Interesting examples of these geological features include the Kayak Trough depressions, Fairweather Fault Zone, relic marine terraces around Middleton Island, and faults off Kodiak Island.
Kayak Trough
The depressions within Kayak Trough, a glacial feature composed of a flat floor bordered by steep edges along its inland margins, are about 70 m deep on the eastern side and about 20 m deep on the western side. These depressions are remnants of a deeper Kayak Trough, the center of which has been filled with sediment (Sean Gulick, Research Assoc. Prof., Institute for Geophysics, Jackson School of Geosciences, Univ. Texas at Austin, personal communication, 2012; Worthington et al. 2008) from the Copper River (Jeager et al., 1998). Currents may play an important role in forming (scouring) and maintaining these depressions (Sean Gulick, personal communication, 2012).
Geological Features of Central Gulf of Alaska
A trace of the Fairweather Fault Zone was found in the soundings from Survey H04529, a 1925 small-scale (1:100,000) smooth sheet (A), off of Yakobi and Chichagof Islands, just south of Cross Sound. The fault zone is imaged as an east facing scarp (cliff face) and a western uplifted structural block (plateau or ridge) about 25 km long. Shallower soundings in the north (B), center (C) and south (D) show the rough outline of the uplifted structural block.
The explanation of the Fairweather Fault Zone's presence in the area, provided by Peter Haeussler Research Geologist, US Geological Survey, (personal communication, 2011), is corroborated by a single-beam echosounder pass across the structure (E) collected during the 2005 GOA trawl survey (Raring 2007).
Middleton Island Submerged Marine Terraces
Left is (A) unedited and right is (B) edited bathymetry of the Middleton Island Submerged Marine Terraces.
The discovery of the Middleton Island submerged marine terraces, which were not previously imaged, proves the benefit of carefully editing and plotting the bathymetry data. A) The unedited bathymetry, uncorrected for digitization and datum errors, is a mixture of pre- and post-1964 earthquake soundings, which produces numerous confusing artifacts. B) The edited, post-quake bathymetry produces a much cleaner surface, even though it uses fewer soundings, revealing submerged marine terraces, which generally lie parallel to the island's coastline. These are the relic marine terraces (George Plafker, Scientist Emeritus, US Geological Survey, personal communication, 2012), perhaps 20 of them, similar in size and orientation to those on the island described by Plafker and Rubin (1978).
Kodiak Fault Zone
Narrow Cape Fault and the Kodiak Fault Zone (KFZ) on Southern and Middle Albatross Banks, south of Kodiak Island, as depicted by slope (A), or depth change (von Huene et al. 1980; Carver et al. 2008, see Plate 1). The KFZ appears to consist of an elevated platform and a steep south-facing scarp in the slope data. The inset of a single-beam echogram (B) shows a depth change from 62 to 50 m over a distance of about 200 m as the KFZ is approached from the south (indicated with black arrow), south of Sitkinak Island, which images a south-facing scarp and associated uplifted northern platform.
Seafloor Changes of Central Gulf of Alaska
Great Alaska Earthquake of 1964
Great Alaska Earthquake of 1964.
An added difficulty in describing bathymetry across this vast area is that it is changing faster than it is being surveyed. The best known example of seafloor change is the great Alaska earthquake of 1964 (magnitude 9.2), centered near Valdez, which abruptly altered the seascape across a large expanse of the CGOA (National Research Council 1972). A comparison of smooth sheet surveys conducted before and after the 1964 earthquake showed sinking of 0.2 to 9.8 m in Resurrection Bay, rising of 6.1 m at Cape Clear, and rising of 1.6 to 4.2 m at Middleton Island.
Katalla Bay Shoreline
Other significant bathymetry changes are more localized. For example, a shoreline accretion of about 600-800 m in Katalla Bay, near the Copper River, occurred between the 1905 (H02768) to 1971 (H09207) surveys, perhaps as a result of heavy sediment deposition in this area (Jeager et al., 1998).
Taylor Bay Shoreline
There was approximately 4500 m horizontal change in the location of the shoreline of Taylor Bay between the 1901 survey of H02558 (green) and the 1992 survey of H10425 (red) due to isostatic uplift of about 20mm/year (Freymueller et al. 2008) and significant possible sedimentation (Jeff Freymueller, Professor of Geophysics, Geophysical Institute and Dept. of Geology and Geophysics, Univ. of Alaska Fairbanks, personal communication 2014). This figure was created by plotting the partially transparent smooth sheet of the older survey on top of the smooth sheet of the newer survey, resulting in some imagery faintness. The shallowest modern soundings occur on top of 26 fathom soundings from the old survey.
Bathymetry of Norton Sound
As a continuation of work in Alaskan waters, scientists with the AFSC’s Groundfish Assessment Program (GAP) have published smooth sheet bathymetry for Norton Sound, Alaska. This work is part of a project using smooth sheets to provide better seafloor information for fisheries research.
The Norton Sound project includes smooth sheet bathymetry editing, the digitizing of sediments, inshore features, and shoreline, as well as incorporating higher resolution multibeam bathymetry data, where available, to supersede some areas of older, lower resolution smooth sheet bathymetry.
Over 230,000 National Ocean Service (NOS) bathymetric soundings from 39 smooth sheet surveys in Norton Sound were corrected, digitized, and assembled, as well as over 6000 soundings from a GAP research cruise, and three NOS multibeam surveys. The bathymetry compilation ranged geographically from the eastern point of St. Lawrence Island, southeast to the Yukon River delta and north along the Seward Peninsula and around the point of Cape Prince of Wales.
Our Norton Sound coverage is very shallow, with a maximum depth of 63 meters in the outer waters along the Bering Sea, while the sound itself, bounded by the westernmost point on the Yukon River delta along the south and Nome on the North, has an average depth of just 13 meters. The original, uncorrected smooth sheet bathymetry data sets are available from the National Geophysical Data Center (NGDC), which archives and distributes data that were originally collected by the NOS and others. These data are not to be used for navigational purposes.
Alaska Regional Office's Essential Fish Habitat (EFH) and Habitat and Ecological Processes Research (HEPR) funding made this work possible. This Norton Sound bathymetry and sediment work was done in response to a NOAA Fisheries AKRO (Alaska Regional Office) request to provide information for a new predictive modeling effort examining Norton Sound red king crab and potential effects of offshore marine mining activities on their habitat. The Alaska Regional Office will also investigate use of the bathymetry and sediment information to oversee sustainable fisheries, conduct Essential Fish Habitat (EFH) reviews, and manage protected species. This Norton Sound bathymetry compilation is part of a GAP (Groundfish Assessment Program) effort to create more detailed bathymetry and sediment maps in order to provide a better understanding of how studied animals interact with their environment.
The most common sediment categories, those having more than 25 occurrences are depicted; listed in decreasing prevalence; all other categories are grouped together. Not to be used for navigational purposes.
Sediments of Norton Sound
We digitized 4,305 verbal sediment descriptions present in 32 of the 39 surveys. There were 120 unique verbal descriptions, with over half of the categories having only a single occurrence. Of the sediment descriptions which occurred more than once, Hard (n=1623), Soft (n=1289), and Sticky (n=591) were the most prevalent.
Smooth sheet features are shown; Rocks in red, Islets in black, and all other features grouped together in teal. Not to be used for navigational purposes.
Features of Norton Sound
A total of 312 features were digitized from the smooth sheets. Only 14 of the 39 smooth sheet surveys contained features. They were predominantly rocks and islets.
The MHW is shown, in meters. The highest shoreline, in red, is located in the inner sound while low shoreline is depicted in blue, near Sledge Island and the Yukon River delta. Not to be used for navigational purposes.
Shoreline of Norton Sound
A total of 107,112 individual shoreline points were digitized, describing 2,142 km of mainland and 837 km of island shoreline. The shoreline is defined on the smooth sheets as MHW (Mean High Water), the same vertical tidal datum as the bathymetry, which typically ranges only as shallow as MLLW (Mean Lower Low Water), defined as zero meters depth. The MHW shoreline ranged from -1.28 m in the Norton Bay area to -0.18 m north of Sledge Island.
By adding the digitized shoreline to the digitized bathymetry, a complete bathymetry map for Norton Sound was assembled without the typical gaps between the shallowest soundings and the shoreline. Thus, researchers were able to determine that at high tide (MHW) the total volume of Norton Sound is 435.8 km3, while the surface area of the sound is 31,379.3 km2.
With the addition of the shoreline, a comparison with similar work in Cook Inlet shows how shallow Norton Sound is. While Norton Sound has a larger surface area, 31,379.3 km2 to Cook Inlet’s 20,540 km2, it has less than half the MHW volume, 435.8 km3 compared to 1,024.1 km3.
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