High-Resolution Seismic Survey of an Offshore Area Near Nome, Alaska By A. R. TAGG and H. G. GREENE STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA GEOLOGICAL SURVEY PROFESSIONAL PAPER 759-A Description and interpretation of data from seismic records and bathymetric charts UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress cardlog-card No. 72-600167 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 2401-00288 CONTENTS Page Page Abstract ..................................................................................... A1 Interpretation .Continued Introduction .............................................................................. 1 Buried topography -- subsurface structures.............. Geologic setting........................................................................ 2 Buried channels........................................................ Equipment, procedures, and methods ................................... 4 Acoustical sinks........................................................ Interpretation........................................................................... 5 Glacial deposits ......................................................... Surface (sea-floor) topography .......... .. ...................... 5 Faults......................................................................... Beach ridges .............................................................. 5 Bedrock ...................................................................... Fans or deltas........................................................... 9 Summary and conclusions ....................................................... Surface channels or depressions ............................ 14 Selected references ................................................................... ILLUSTRATIONS Page Index map of Alaska showing location of study area......................................................................................... A1 Generalized geologic map and cross section of the coastal plain at Nome ................................................... 3 Track chart of Nome nearshore seismic survey....................................................................................................... 5 Maps showing: 4 . Surface and shallow subsurface features of the Nome nearshore area............................................... 5. Deeper subsurface features of the Nome nearshore area..................................................................... 6. Shallower subsurface features of the Nome nearshore area................................................................... Line drawings and geologic interpretation of seismic profiles: 7 . Profile A-A' across a buried stream channel, beach ridge, and acoqstical sinks............................. 8. Profile across a fan......................................................................................................................................... Map showing bottom topographic features off Nome ......................................................................................... Line drawings and geologic interpretations of seismic profiles: 10. Profile B-B' across buried channels............................................................................................................. 11. Profhe C-C' across an ancestral channel of Snake River ..................................................................... 12. Profile D-D' across a terminal and a recessional moraine ...................................................................... 13. Profile across the large bedrock fault zone east of Penny River ........................................................... Map showing six offshore areas of possible concentration of placer gold and other heavy detrital min- HIGH-RESOLUTION SEISMIC SURVEY OF AN OFFSHORE AREA NEAR NOME, ALASKA By A. R. TAGCand H. G. GREENE ABSTRACT 150 feet below the sea bottom. Information from drill A high-resolution seismic survey was made south of Seward logs and bottom samples taken in 1967 and 1968 was Peninsula in Norton Sound, Alaska, during the summer of used in the interpretation of the seismic records. 1967, to identify sites of possible offshore gold concentrations. These data provide the basis for extending the coastal- Interpretation of the seismic records was confined primarily to the parts above the first sea-bottom multiple, generally plain geology at Nome (Hopkins, 1967) into the less than 150 feet below the sea floor. Sediments within this off shore area. interval consist mostly of unconsolidated Holocene sand and The study area is in the northwestern part of gravel covering Pleistocene glacial and marine sediments. Norton Sound, an embayment of the northern Bering Paleozoic metamorphic rocks are near or at the sea floor in Sea (fig. 1). The area extends 10 miles east and west the northwestern part of the area. Interpretation of bottom topography and subbottom struc- of Nome, from Cape Nome to Rodney Creek, and up tures disclosed many features that may be important in gold to 10 miles offshore. exploration. Bottom-surface features include at least five out- wash fans, several beach ridges, and discontinuous stream channels. Subbottom features include many buried stream channels, areas of glacial drift, and at least three shallow faults. Also noted were acoustical sinks that have been inter- preted as gravels associated with buried stream channels and beach ridges. In addition, six areas were delineated where placer gold may have been concentrated. INTRODUCTION This report describes and interprets features seen on high-resolution seismic-profiling records and bathymetric charts of an offshore area near Nome, Alaska, and discusses the possible significance of these features as placer-gold exploration sites. It is based upon a continuous seismic-reflection survey conducted in the summer of 1967 by the U.S. Depart- ment of the Interior. The investigation was conducted jointly by the Geological Survey and the Bureau of Mines. The 1967 studies included offshore drilling at FIGURE1. - Index map of Alaska showing location of study sites chosen on the basis of this seismic survey, Sam- area. pling of the sea bottom and modern beach, seismic- refraction surveys of part of the modern beach, and Several reconnaissance seismic-reflection studies, onshore geologic studies. both shallow and deep, have been made of the Bering The continuous seismic profiles were interpreted Sea. Moore (1964) discussed the results of a high- jointly by the Geological Survey and the Bureau of resolution shallow seismic-reflection survey of parts Mines. Interpretation of the deeper reflectors, mostly of the Bering and Chukchi Seas. Scholl, Buffington, of Tertiary age, was undertaken at the Marine Min- and Hopkins (1968) have made deep seismic-reflec- erals Technology Center of the Bureau of Mines. This tion surveys of the outer shelf areas of the Bering Sea, report describes the shallower reflectors, probably all and Scholl and Hopkins (1969) extended this work to of Quaternary age, that extend to depths of less than the northern Bering Sea. The results of a shallow A2 STUDIES ON THE MARINE GEOLOGY OF THE BERING SEA seismic-profiling survey of the northern Bering Sea The next younger shoreline deposits, Inner and have been reported by Grim and McManus (1970). Outer Submarine Beach, lie at depths of -35 and Private groups have undertaken seismic surveys of -20 feet onshore at Nome, in the area between the nearshore waters along parts of the south coast of lower course of the Snake River and the present Seward Peninsula, but these studies have not been shore. The Submarine Beach complex rests on bed- published. rock in some places and on Pliocene (?) sediments in We thank Warren Woodward and Arthur Daily others. Submarine Beach has yielded pollen and of Shell Oil Co., Ralph Saleski, John Fagerstrom, mollusks of a late Pliocene or early Pleistocene age and Ben Herman of the U.S. Army Corps of Engi- and is a product of the Beringian transgression neers, and Burton Barnes, Gary Leo, Adolph Poston, (Hopkins, 1967, p. 53). and Dale Stevenson of the U.S. Bureau of Mines for Submarine Beach is overlain by four lithologic assistance in various parts of the fieldwork. units: drift of the Iron Creek Glaciation of pre- Illinoian age, a layer of fossiliferous marine sand GEOLOGIC SETTING and gravel, till and outwash of the Nome River The Nome coastal plain and adjacent offshore Glaciation of Illinoian age, and marine sediments of areas are underlain by Pliocene and Pleistocene Second Beach of Sangamon age (fig. 2). Mollusks marine and glacial sand and gravel. A geologic map and pollen in the marine layer between the two drift and a cross section of the Nome coastal plain are sheets suggest that the marine layer is correlative shown in figure 2. Separate deposits are recognized with Intermediate Beach, a shoreline deposit on onshore and are locally known as Submarine Beach, bedrock beneath Nome River drift at an approxi- Fourth Beach, Third Beach, Monroeville Beach, mate altitude of 20 feet (Hopkins, written commun., Intermediate Beach, and Second Beach. They were 1969). Inland from Submarine Beach are three other produced during six different sea-level stands in late shoreline deposits: Monroeville Beach at an altitude Pliocene and Pleistocene time (Hopkins,