United States

Department of the Interior 

Geological Survey

MAP OF POTENTIAL GEOLOGIC HAZARDS ON THE 

NORTH ALEUTIAN SHELF (LEASE SALE 92), BERING SEA

by

Bruce F. Molnia, William C. Schwab, and William A. Austin

OPEN-FILE REPORT 

83-247

This report is preliminary and has not been reviewed 

for conformity with U. S. Geological Survey editorial 

standards and stratigraphic nomenclature.

Any use of trade names is for descriptive purposes only 

and does not imply endorsement by the U. S. Geological 

Survey.

Menlo Park, California 

1983
MAP OF POTENTIAL GEOLOGIC HAZARDS ON THE 

NORTH ALEUTIAN SHELF (LEASE SALE 92), BERING SEA

by 

Bruce F. Molnia, William C. Schwab, and William A. Austin

INTRODUCTION

Between July and October 1981, Marine Technical Services (MTS) of Houston, 

Texas, under contract to the Conservation Division of the U. S. Geological 

Survey (now the Minerals Management Service), collected approximately 4,000-km 

of geophysical data in the North Aleutian Shelf area (Bristol Bay) (Data Set 

AK 19891, National Geophysical Data Center, Boulder, CO.). The purpose of the 

survey was to provide marine geophysical information for use in support of Outer 

Continental Shelf (OCS) Lease Sale 75. The information would be evaluated to 

determine the type and extent of marine geohazards that exist within the sale 

area. However, Lease Sale 75 was deleted from the current five-year OCS oil 

and gas leasing schedule and the next scheduled lease sale for this area is 

Sale 92, scheduled for April 1985. 

DESCRIPTION OF THE MTS DATA SET

Geohazards depicted on this map are plotted from a combination of geophysical

3 systems. The systems used were: an array of 1 to 4 15-in water guns; an 800-

joule 40-tip sparker; a 3.5-kHz piezoelectric profiler; and side-scan sonar. 

Data quality varied significantly. Many records suffered from cross-talk and 

60-cycle noise. Multiples were present on all medium-penetration seismic profiles, including processed data. The multiple problem made interpretations 

below the first multiple difficult. The most useful data obtained was from the 

water gun analog and side-scan sonar systems. 

DESCRIPTION OF NORTH ALEUTIAN SHELF GEOHAZARDS

Analysis of the data yielded only three geological conditions that may in 

some way be hazardous to future petroleum development. These are: 1) numerous 

scours, which suggest active erosion; 2) faults which appear to have a surficial 

expression, suggesting present-day activity; and 3) megaripples and sandwaves, 

suggesting active sediment transport. 

Scours

Thousands of scours, many of which are parallel and linear, some of which 

are incised as deep as 5 meters into sandy seafloor, were identified. Frequently, 

the scours occur in groups or sets, some sets with more than-;500 distinct linear
scours. Many individual scours have minimum lengths of 300 meters, the limit 

of side-scan sonar coverage. Widths of individual scours range from only a few 

meters to more than 250 meters.

Orientation of scour sets varies significantly, from shore parallel to shore 

perpendicular. Some areas contain two discrete sets of overlapping scours with 

each set having its own orientation. Other areas show a more complex scour 

system characterized by irregular scours, many having sinuous or free-form shapes.

Current measurements and video observations made on a September 1981 cruise 

aboard the U. S. Geological Survey Research Vessel S. P. Lee suggest that scouring and sediment transport are both actively occurring. The floors of many 

scours are covered by a rippled coarse sand or gravel lag. Scours are generally 

asymmetric in cross section; their shape suggests a concentration of flow due 

to Coriolis effect, which is accentuated at high latitudes. Possible causes 

of the active erosion are: tidal currents; topographic acceleration of the 

water mass caused by constriction over transverse longitudinal ridges which 

cover much of the shelf (Austin and others, 1982); currents exhibiting helical 

flow; storm surge effects; storm waves; and shelf geometry.

In over 50 of the scours, marked with "R" on the map, sediment transport 

was almost always parallel to linear scour orientation but was quite variable 

when compared to shoreline orientation and bathymetry.

In addition, the dynamic nature of the bottom is further supported by the 

fact that while the entire region is heavily fished, in only one area, near 

latitude 56°20' longitude 161°45' (marked with "T" on the map), were we able to 

recognize gouges left by deep-sea-bottom trawlers. 

Faults

Faults that appear to reach the seafloor or that extend upward into the 

bubble pulse were observed at nine locations, with the greatest concentration 

west of Amak Island. No seafloor scarps were noted in association with any 

fault. Rather, many of the faults appeared to be associated with broad surficial sags. Active sediment transport observed in association with the scours 

may mask the existence of distinct linear surficial scarps.

Bedforms

In the northeasternmost corner of the map area almost all lines show submarine dunes or megaripples. Generally, the bedforms were either megaripples with
parallel or subparallel crests (catenary swept or straight swept in the nomenclature of Alien (1968), or catenary/lunate, both in and out of phase). Spacing 

of parallel raegaripple crests ranges from about 25 to 40 meters. 

Other Hazards

Examination of the geophysical data set showed no evidence for the existence of submarine sliding or slumping, shallow gas-charged sediment, surface 

pockmarks or craters, ice-related erosional features, or any conditions other 

than those identified earlier that could present problems to future petroleum 

operations.

REFERENCES

Alien, J. R. L., 1968, Current ripples: Their relation to patterns of water

and sediment motion: North Holland Publishing Company, Amsterdam, 433 p.

Austin, W. A., Molnia, B. F., and Schwab, W. C., 1982, Bathymetric map of the 

North Aleutian Shelf, Bristol Bay, Alaska: U. S. Geological Survey OpenFile Report 82-1035, 1 sheet, scale 1:250,000.