history and evolution of the west-central part of the Pacific Basin, the detailed structure and evolution of a coral atoll, the rise and fall of sea level in response to global climatic changes and the measurement of the magnitude of these events, speciation and migration of microscopic marine life, and chemical and physical changes in atoll carbonate rocks. In addition, data from this program are applicable to such practical issues as the potential use of an atoll as a safe repository for high-level nuclear

waste.

Two submarine craters, KOA and OAK (fig. 12), that were formed in 1958 near the northern perimeter of Enewetak Atoll by 1.4- and 8.9-megaton bursts, respectively, were selected for the USGS to investigate. A basic problem in the investigation was that data from the PPG craters were sparse. The PPG high-yield craters appeared to differ in a number of

strategically critical ways from craters produced by both non-nuclear and low-yield nuclear bursts in dry, continental test sites, such as the Nevada Test Site. The high-yield craters appeared to have very broad, shallow saucer-shaped profiles and consequent large volumes of excavated materials. In contrast, the continental test craters, far easier to directly measure and sample, had smaller bowl-shaped profiles and small volumes of excavated materials. For many years, the conclusion generally was accepted that the high-yield devices were more efficient in the excavational process than the low-yield ones that formed the continental craters. However, research undertaken by the DNA in the 1970's suggested that physical processes operating on a scale greater than hours and gravity effects in "special" geologic environments such as a coral atoll might play a more important role in the final crater size and

morphology than in dry continental sites. By the end of the last decade, computational models were capable of simulating accurately an array of phenomena associated with high-explosive and low-yield nuclear shots. However, the PPG craters (as observed and measured) formed by the high-yield events and scaled to their continental and meteor-impact counterparts were anomalous and could not be modeled confidently. The marked difference between these simulations and existing observations from the PPG were of considerable concern to the DOD. In fact, the lack of confidence in these data cast doubt on the DOD's ability to predict how effective nuclear weapons would be against hardened targets and how well strategic defense systems would survive in the event of nuclear attack.

Participation of the USGS in fieldwork on Enewetak was divided into two major parts. The first, the marine phase, began in April of 1984 with a pilot borehole-gravity study by the USGS and DNA in a deep borehole drilled in 1952 on Medren Island (fig. 12). Most of the marine phase was conducted during the summer and early fall of 1984 by USGS personnel, but scientific advisors from DNA and logistic support from the Pacific Area Support Office of the Department of Energy (DOE) also participated in the marine phase. The second phase, the drilling phase, was conducted

from late winter through the summer of 1985 and was conducted jointly by personnel from the USGS, DNA, and DOE, which contracted the 245-foot drilling vessel, the Knut Constructor (fig. 13). The DOE also obtained necessary cooperation of the officials of the Republic of the Marshall Islands to conduct the fieldwork on Enewetak and to provide extensive logistic support.

The marine phase concentrated on mapping seafloor features and profiling subbottom characteristics of KOA and OAK using shipboard geophysical techniques and scuba and submersible surveys (figs. 14, 15). Most of these analyses are reported in USGS Bulletin 1678. The geophysical studies incorporated data from sidescan-sonar images, single-channel and multichannel seismic surveys, and refraction surveys; the geologic investigations included seafloor observations, collection of bottom (benthic) samples, and shallow boreholes drilled by scuba teams.

Thirty-two deep boreholes were completed in the vicinity of KOA and OAK during the drilling phase. These provided information on the stratigraphic framework of the upper 1,200 feet of the carbonate cap of the atoll and ground-truth for the geophysical profiles and other marine phase data. The deepest hole was drilled about 1,800 feet below sea level in roughly 200 feet of water near OAK ground zero. Samples of rock and sediment from the

Figure 14. Airbrush-enhanced sidescan-sonar image of waterfilled OAK crater (lighting from southeast) showing major crater and natural features. Lagoonward edge of reef plate is darker gray area to northwest; natural lagoon floor (see arrow on southeast side of image) is about 150 feet deep with patch (pinnacle) reefs rising to nearly sea level; ground zero (GZ) is in almost 200 feet of water. (The imagery techniques and airbrush enhancement employed by USGS laboratories at Flagstaff were basically those developed by the USGS for the National Aeronautics and Space Administration to process images from Mars.)

162°6'30"E

boreholes were used for lithostratigraphic and biostratigraphic (geologic) analyses for strontium-isotope dating by the USGS, for stable carbon- and oxygen-isotope analyses (Brown University), for shockmetamorphic studies (California Institute of Technology), for material-properties (engineering) studies (DNA), and for radiochemical analyses (DNA). Geophysical logs were run and shipboard gravity measurements were made in selected boreholes on one of the OAK transects by DNA personnel cooperating with the USGS and Los Alamos National Laboratory.

The collaborating teams from the USGS, DNA, DOE, and McClelland Engineers, Inc. (the drilling contractor), produced a number of major technologic and scientific "firsts":

The first boreholes successfully drilled from shipboard through the highly dis

materials

turbed within a high-yield nuclear crater. This was done with a high degree of recovery of sample and core. • The first detailed biostratigraphic analyses using microfossils (ostracodes and foraminifers) of near-reef facies of an atoll, which provided the first demonstration that microfossils could be used to determine the depth of origin (provenance) and extent of mixing in the crater-fill materials of layers excavated by the nuclear device. This provided critical information about the chronology of the crater formation and its evolution since the nuclear tests.

The first use of data from strontiumisotope geochemistry as a high-precision stratigraphic tool for atoll carbonates and its application to determine the source depths of crater debris and ejecta.

• The first sidescan sonar maps of a water-filled nuclear crater.

The first borehole gravity measurements made on a coral atoll and the first successful application of a slimline borehole gravity tool from aboard ship. These measurements provided essential bulk density and porosity data for the computational modeling.

The multidisciplinary approach confirmed that the original excavational craters of KOA and OAK were transient features significantly smaller than originally thought and far more compatible with DOD predictions. Processes operating on a scale of hours to years greatly enlarged the size and modified the configuration of the initial craters in the water-saturated test beds. These processes included major failure of the sidewalls of the excavational crater, shock-induced liquefaction, consolidation, upward piping of material (partly from beds far below the excavational crater), and subsidence of the materials below and adjacent to the initial crater.

Federal/State

Cooperative Geologic Mapping 1985-1987

By Wayne L. Newell

COGEOMAP, the Federal/State Cooperative Geologic Mapping program, is proving to be an effective program for providing geologic maps of high-priority areas in a timely manner that meet the varied needs of geologic map users. In its third year of activity in fiscal year 1987, COGEOMAP has grown to encompass 30 Federal/State cooperative projects (fig. 16). When the program began in fiscal year 1985, 18 State-proposed cooperative projects were accepted and funding was at the $1 million level. Congress increased funding to the $1.5 million level in fiscal year 1986 and fiscal year 1987.

During fiscal year 1987, major projects under the program included continuing work on new State geologic maps for Arizona, Hawaii, Montana, New Hampshire, New Jersey, New Mexico, Virginia, and Washington. Detailed mapping projects were carried out in Alabama, Alaska, Arkansas, Idaho, Illinois, Indiana, Maine, Minnesota, Missouri, Nebraska, Nevada, North Carolina, North Dakota, Oklahoma, Oregon, Rhode Island, Tennessee, Texas, Utah, Vermont, Wisconsin, and Wyoming to identify mineral, energy, and water resources and delineate geological hazards. Significant results from the overall program include

• Discovery of new high-BTU coal seams and fluorspar exploration targets in Illinois.

• Development of land-use plans for the southern coast of Maine that will protect ground water in this urbanized area.

• Completion of detailed geologic mapping of four mountain ranges in the Phoenix, Arizona, region, which has stimulated mineral exploration and aided in the planning for construction of a major earth-fill dam.

Stimulation of mineral exploration in the Wind River Mountains area of Wyoming and of petroleum exploration in the Ouachita Mountains of Oklahoma and Arkansas.

Beginning in fiscal year 1988, the COGEOMAP program becomes a major component of the new National Geologic Mapping (NGM) program, which will seek to accelerate geologic mapping to meet the continuing strong demand for modern geologic maps from the public and private sectors. Other major goals of the NGM program are

• Identifying, on a province-by-province basis, critical earth-science data needs that require new or additional intermediate- to large-scale geologic mapping.

• Establishing national geologic mapping priorities by province in order to focus future mapping on critical areas.

• Increasing the coverage of the United States by intermediate- and large-scale geologic maps in provinces or portions of provinces of highest national priority. • Coordinating and integrating subsurface studies, particularly geophysical, geochemical, and hydrologic investigations, with surface geologic mapping.

• Preparing and maintaining a system for an annual nationwide inventory of current geologic mapping and published map

coverage.

• Encouraging greater production and public availability of geologic maps. • Cooperating with the State geological surveys and the National Academy of Sciences to set standards for future geologic maps.

• Evaluating and implementing new technologies and methodologies for geologic map compilation.

Within the broad program goals of NGM, COGEOMAP will continue to link USGS geologists possessing regional experience and technical expertise with the staffs of State geological surveys who possess detailed local information.