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Puerto Rico: A Key to the Caribbean Basin

The U.S. Geological Survey's first major program in the Caribbean basin was aimed at providing economic assistance to the Government of Puerto Rico through an assessment of the mineral resources of the island. Shortly after the program began in 1952, FORMENTO, the Puerto Rican Industrial Development Corporation, and scientists of the Geological Survey realized that basic geologic data upon which to make a mineral resource assessment for Puerto Rico were lacking. The Geological Survey scientists began a program of detailed basic geologic mapping of the island to provide the necessary framework for ongoing and future mineral resource studies.

The change in orientation from an economic mineral resource assessment program to a program combining basic geologic mapping with resource assessment resulted in a detailed study of the entire island and involved a wide range of Geological Survey talent. The 62 detailed (1:20,000 scale) geologic maps produced over the 31-year history of the project have transformed Puerto Rico from one of the geologically least known to one of the most thoroughly known terranes in the world. The recently completed Geologic Map of Puerto Rico (1:100,000 scale) combines the essentials of the original 62 detailed geologic maps with modern interpretations made in light of current geologic theory into a convenient single map. The detailed maps and the regional map form a basis for further studies in metallic and nonmetallic mineral resource assessment and for studies of geologic hazards such as unstable ground and earthquakes.

Puerto Rico, like the other islands of the Greater Antilles, Cuba, Hispaniola, and Jamaica, originally formed along a belt of submarine volcanoes that later were raised above the sea in the form of an island arc. The oldest rocks of the island arc are at least 110 million years old, whereas the youngest volcanic rocks are about 38 million years old. The oldest volcanic rocks consist chiefly of lava flows that in surface exposures often resemble masses of irregularly piled pillows. The pillow lavas, along

with glassy brecciated rocks were produced by sudden quenching and fracturing of molten lava by sea water. Evidence that the lavas were formed in part above, or very near, the sea surface comes from the presence of shallow-water clamlike fossils in sandstone and limestone beds interlayered with the volcanic rocks. These older volcanic rocks (fig. 1) characteristically show chemical peculiarities that distinguish them from other island-arc volcanic rocks. They contain relatively little potassium, rubidium, thorium, and uranium and are classified as primitive island arc rocks. The younger volcanic rocks of Puerto Rico formed as lava flows, tuff, and volcanic breccia produced by explosive and quiet eruptions of lava or as debris derived from the erosion of volcanic rocks exposed at the surface. These more evolved island-arc rocks are chemically distinct from those of the primitive island arc because of their greatly increased potassium content in relation to calcium and sodium. Intrusive rocks which gave rise to these younger rocks are known to contain copper deposits, and recent preliminary evidence suggests that they may also contain gold deposits.

Geologists had generally assumed that Puerto Rico and other islands of the Greater Antilles were formed in their present positions or possibly on the western margin of the Caribbean basin near the present position of Panama, Costa Rica, and Nicaragua. Recently completed work on rock magnetism indicates, instead, that Puerto Rico and probably the other islands of the Greater Antilles formed at the latitude of present-day Peru and Ecuador and that they moved into the Caribbean basin at some time in the last 38 million years.

An understanding of the basic geologic framework of Puerto Rico is fundamental to future mineral resource assessment of the island, particularly in regard to predictive resource estimates for copper, gold, and phosphate rock used for fertilizer and to the expected incidence and potential severity of geologic hazards such as earthquakes and landslides.

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Survey from State and local agencies in situations where there is both Federal and State interest in investigation of water resources but where matching Federal funds are either unavailable or are not otherwise applicable to cost sharing.

Budget and Personnel

governments have a mutual interest in evaluating, planning, developing, and managing the Nation's water resources. The immense size of the task of appraising the Nation's water resources precludes accomplishment by Federal efforts only. Similarly, State and local agencies working independently cannot relate to the sizable regional aspects of the hydrologic system. Because many water problems begin at the local level, the Survey has cooperative agreements with all States under which each party funds one-half the cost of financing studies of water resources. Cooperation through this Program provides an economical and comprehensive system for water-resources assessments.

Most projects under the Cooperative Program respond to a recognized problem or define a potential one. In addition to data collection, programs may focus on water use and availability, the impact of man's activities on the hydrologic environment, and energy-related water demands which may strain available water supplies. In emergency situations, such as drought or flood, events are monitored, and the data accumulated under the Cooperative Program prove invaluable.

At the end of fiscal year 1983, the Water Resources Division employed 2,927 full-time personnel. This number included scientists and engineers representing all fields of hydrology and related sciences, technical specialists, and administrative, secretarial, and clerical employees. An additional 1,644 permanent part-time and intermittent employees assisted in the work of the Division.

The $200.8 million obligated in 1983 for water resources investigation activities came from the following sources: 1. Direct Congressional appropriations. 2. Congressional, State, and local ap

propriations for 50-50 funding in
the Federal-State Cooperative

Program. 3. Funds transferred from other Federal

agencies. 4. Funds transferred from State and local


Assistance to Other Federal Agencies


With funds transferred from other Federal agencies, the Geological Survey performs a wide variety of work related to the specific needs of each agency.

In the following sections, highlights from some of the major programs are described.

Non-Federal Reimbursable Program

Non-Federal reimbursable funds are unmatched funds received by the Geological

The Federal-State Cooperative Water
Resources Program-Some Highlights From 1983

The Federal-State Cooperative Program in fiscal year 1983 continued to concentrate on water-resources investigations of highest priority to the Nation. Hydrologic data collection and interpretive studies were proceeding in every State, Puerto Rico, and several of the territories with focus on such current concerns as groundwater contamination, flood analyses, impacts of toxic wastes, acid precipitation, and stream quality.

During the year, this 50-50 matching program was carried out in working partnership with more than 800 State, regional, and local agencies. Joint funding from all sources totaled about $92 million. Details of the program are arranged at State and local levels by representatives of the Survey and the cooperating agencies. This pooling of interests results in a balanced effort that directs combined resources to hydrologic investigations having the most significance to the cooperating parties. A few of the highlights of 1983 are described below.

to simulate flow conditions through the existing bridge opening. The model may also be used to simulate conditions without 1-10 in place, with the effects of alternative bridge designs, or with modifications to the existing bridge.

In the vicinity of Slidell, the Pearl River occupies a flood plain about 5 miles wide that is slightly incised below the surrounding land. Just upstream from 1-10 at Slidell, the river splits into three distributary channels with the main channel running along the west bank of the flood plain. During floods, streamflow, rather than being concentrated along the main channel that parallels the west bank, shifts to the east side of the flood plain. 1-10 embankments and bridge openings cause flow to shift from the west to the east bank farther upstream than it would if the highway were not there. Results of the investigation also show that the l-10 crossing and the resulting shift in flow distribution affect the height as well as the extent of backwater.

Slidell, Louisiana:
Backwater and Flow Distribution of
Pearl River Floods

Pagan Island, Northern Marianas: Effects of Volcanic Activity on Water Quality

Severe flooding on the lower Pearl River in the vicinity of Slidell, Louisiana, occurred in April 1979, 1980, and 1983. Each flood approached or exceeded a 100-year frequency of recurrence. The chance for three such floods happening within a 4-year span is about 1 in 10,000.

Following the 1980 flood, the U.S. Geological Survey, in cooperation with the Louisiana Department of Transportation and Development, Office of Highways, began a study of backwater and flow distribution of the Interstate Highway 1-10 crossing of the Pearl River near Slidell. A finite element model has been developed

The entire population of Pagan Island, in the Commonwealth of the Northern Mariana Islands, was evacuated to Saipan because of Pagan Volcano's eruption in May 1981. The volcano is a potentially explosive one, and eruptions about 1920 caused the evacuation of the population for several years. The people cannot return home until it is determined that their water supply is no longer contaminated. Under a joint-funding agreement, the U.S. Geological Survey is investigating the quality of water in the vicinity of the villages on Pagan Island.

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