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Research in the Marine Frontier
On March 3, 1879, the U.S. Congress established the U.S. Geological Survey and charged it with the responsibility for the classification of public lands and the examination of the geologic structure, mineral resources and products of the national domain. In 1879, much of this Nation's land area was frontier. Today, the Nation's geographic frontiers primarily lie beneath its surrounding oceans. By Presidential proclamation in 1983, the U.S. Exclusive Economic Zone was established. The proclamation claimed the seabed resources in this underwater frontier to 200 nautical miles offshore (fig. 1). This zone, which includes the waters surrounding the continental United States, Hawaii, Alaska, Puerto Rico, and U.S. territories and possessions, constitutes an area about 1 70 percent larger than the size of the onshore area. Within this vast underwater domain lie resources of immense importance to the Nation: an estimated 35 percent of the economically recoverable oil and gas yet to be found in the United States; strategic metals like cobalt, manganese, and nickel in sea-floor crusts and nodules; massive sulfide deposits actively forming today on ridge crests; and major concentrations of heavy minerals in nearshore sand bodies.
Along with the immense potential for increasing our domestic resource base, exploration of the seabed provides an ex
citing scientific frontier. Most of the geologic framework of the Nation's continental margins (shelf, slope, and rise) is known from a relatively few and widely spaced deep seismic reflection profiles and a few deep drillholes that recovered rocks from within the margin. The frontier character of the offshore domain is emphasized when one considers that major sedimentary basins with petroleum potential have been discovered within the last 8 years, sulfide deposits at sea-floor spreading centers have been known only since 1978, cobalt-rich manganese crusts on Pacific seamounts (underwater extinct volcanoes) were recognized as important potential resources in 1982, and additional sea-floor features, such as submarine canyons and seamounts, are discovered virtually every year. Through marine geologic surveys, the dynamic aspects of plate tectonics can be studied: the continual creation of the Earth's new crust at the midocean spreading centers; the formation of volcanic arcs, basins, and trenches where the crustal plates collide; and the migration of the plates over hotspots in the Earth's mantle, creating chains of volcanic islands. Through geologic time, this plate tectonic process has caused rocks formed on the sea floor (even whole marine basins) to be accreted to the continents where erosion may expose them for study today. Because these rocks originated in the oceans,
the marine realm remains the natural laboratory where the fundamental tectonic and volcanic processes responsible for their formation and resource potential may be studied best.
The marine program has three major elements: Regional Geologic Framework, Sedimentary Dynamics, and Marine Energy and Mineral Deposits. Scientific studies in these program elements provide answers to major geologic questions that bear on the fundamental geologic makeup of the Earth and concerns about the future availability of resources.
The Regional Geologic Framework element includes investigations in regional stratigraphy, paleoenvironmental conditions, and structural settings of the continental margins, and marginal sea and deep ocean basins. These investigations focus on processes that influence the formation of petroleum and mineral deposits. Other studies of tectonism, seismicity, magmatism, and volcanism provide information on the settings of marine deposits, the process of ocean-floor spreading, the development of oceanic trenches, the evolution of landmasses or islands, and the geologic hazards that could impede future use of offshore lands. In the context of geologic history and crustal framework, these activities lead to the analysis and interpretation of the evolution of continental margins, spreading centers, and island masses.
The Sedimentary Dynamics element includes studies of active geologic processes that are modifying the sea floor and coastal zone today, with awareness that similar processes acted to form sedimentary deposits throughout geologic time. Topics of concern are the shape and sedimentologic character of major marine sedimentary deposits and the mechanics of how they formed. Sedimentary dynamics research defines the geologic conditions which control large-scale sediment failures (subsea landslides), the erosional processes forming scarps and submarine canyons on the continental margin, the dynamics of transport of sediments and pollutants on the sea floor, and the geologic processes operating in and shaping estuaries and coastal zones. Because many basins on land today formed in
marine environments, there is much transfer of information and insight from marine studies to land studies and vice versa.
As we seek to understand the nature of the zone where the ocean and continental crusts meet and the dynamic processes that are involved, we are becoming increasingly aware of significant processes of erosion on the continental shelf and slope. The first evidence was seen in geophysical profiles that revealed buried ancient continental shelves as much as 113 miles seaward of the present shelf edge. More recently, high-resolution acoustic (sonar) imaging devices have revealed sea-floor features never before seen. Many more canyons on the continental slope have been discovered than were previously suspected, suggesting that submarine erosion is a geologic agent modifying the morphology of the slope at a higher rate than previously considered. Geologists on deep submersible dives east of Florida have observed a near vertical wall of limestone about 2.5 miles high that may have eroded back as much as 9 miles in the past 37 million years. The evidence for erosion on such a scale in the past has significant implications for the preservation of oil and gas trapped within the continental margin and for resource exploration and development in deep water.
Finds of new mineral deposits in the oceans are high on the list of exciting geologic discoveries of the past decade. The Marine Energy and Mineral Deposits element supports investigations of these deposits and the processes that formed them. For several years, geologists had speculated that seawater and rock might react in areas of volcanic heating to form mineral deposits. In 1 979, massive sulfide deposits were discovered accumulating around a hydrothermal vent on the crest of the midocean ridge in the Pacific Ocean. The midocean ridge is a world-encircling mountain range, the crest of which is a rift zone and spreading center where new seafloor crust is formed. As the sea floor is pulled apart along the rift zone, molten rock rises to the sea floor. Cold sea water percolates downward along faults or cracks in the sea floor and is heated by and reacts with the hot rock. Then it is convected vigorously to the surface where it discharges in geyser like plumes of hot water (350° C.) laden with sulfides of zinc, copper, and silver, some of which precipitate on the sea floor (figs. 2, 3).
Since the initial discovery, such sulfide deposits have been found at five other locations along the ridge. For the first time, geologists can witness a massivesulf ide type of deposit actually being formed on the sea floor. Research into this process provides important insights into exploration techniques for new onshore deposits.
In addition to these rapidly forming sulfide deposits, manganese crusts recovered from seamounts in the Pacific Ocean contain cobalt, the concentration of which varies with water depth. The most favorable water depth range, where the cobalt content exceeds that of commercial deposits onshore, is about 3,280 to 8,530 feet. These deposits lie on the flanks of islands and seamounts, some of which are located within 200 nautical miles of U.S. territory.
The program element for marine energy and minerals is concerned with developing geologic concepts from which settings and conditions of the origin and concentration of marine minerals and hydrocarbons can be defined. The United States is dependent on foreign sources for a large percentage of its petroleum and for several strategic minerals, such as cobalt. This dependence underscores the need to perform research on the distribution and origins of these and other resources in the marine realm.
All three program elements converge in the study of marine basins and sediment fans or wedges for hydrocarbons. It is necessary to understand the setting and character of the sediment accumulation, the source of the sediments, the amount of organic material present, and whether the thermal maturity is sufficient to generate hydrocarbons that might migrate and become trapped. During the past few years, research by the Geological Survey has revealed previously unknown Arctic offshore basins with good petroleum resource potential. In addition, program scientists identified the possible occurrence of gas in the deep Aleutian Basin of the Bering Sea, through inferences from seismic records, and they confirmed the presence of gas hydrates (frozen gas) in sediments lying in deep water off the Atlantic coast. Gas hydrates may also form an impermeable seal under which recoverable gas and oil may collect. As technology advances, the gas hydrate itself may become an exploitable resource.
The life blood of a marine geology program is its marine operations, and the Geological Survey operations are based in Menlo Park, California, and Woods Hole, Massachusetts. Two large research vessels
(R/V) capable of open-ocean surveys are operated by the Survey and are in steady use. The R/V Samuel P. Lee (208 feet) is equipped for deep-penetration seismic reflection profiling and other geophysical surveys. It is ice strengthened and is used for research operations at high latitudes where ice may be a danger. The R/V Polaris II (160 feet) is equipped for highresolution seismic reflection profiling, light geophysical surveys, and sea-floor sampling. In addition, the Survey maintains four smaller vessels for nearshore marine surveys. Each year, other vessels from oceanographic institutions and industry are used as needed to accomplish the Geological Survey mission to acquire knowledge about the Nation's underwater domain. In 1982, the R/V Samuel P. Lee conducted research in waters off Alaska to add information on the geologic framework and potential for petroleum resources of large sedimentary basins on the continental shelf and to obtain new data on the plate tectonic setting of the Aleutian Island chain. Other important cruises added to our understanding of the Atlantic continental margin, especially concerning the basic framework and processes of sedimentation and erosion but also concerning mineral resources of the large underwater Blake Plateau off the coast of the Carolinas. Also in 1982, the R/V Samuel P. Lee con
ducted geophysical surveys, sponsored by the United States, Australia, New Zealand, and the United Nations, around the Pacific island areas of Tonga, Vanuatu, and the Solomon Islands. During a cooperative investigation in the central Pacific, Geological Survey and German Government scientists recognized the potential for cobalt resources in manganese crusts on the sea floor.
In August 1983, the longest, most elaborate marine survey ever planned by the Geological Survey began. The R/V Samuel P. Lee left San Francisco on a scheduled 40,000-mile cruise from pole to pole. Planned activities included sea-floor photography of sulfide deposits on Juan de Fuca Ridge, seismic surveys in the Chukchi Sea north of the Bering Straits in the Arctic, sampling in the central and southern Pacific for cobalt-manganese crusts, seismic surveys in the south Pacific, and framework/resource surveys in the Ross Sea-Wilkes Land continental shelf area off Antarctica. In addition, a joint Survey-Geological Survey of Canada expedition used a Canadian-developed seafloor mounted drill to obtain samples from sulfide deposits on Juan de Fuca Ridge. As 1983 ended, prospects appeared bright for a proposed expansion of the marine program to cover the vast new national frontier of the Exclusive Economic Zone.