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observations are also essential to document and evaluate the importance of infrequent or catastrophic events, such as floods or strong storms, on sediment transport and coastal circulation.

Figure 4. USGS bottom tripod system for long-term studies of currents and sediment transport. The instrument measures current, temperature, light transmission, and pressure, and it also photographs the sea bottom at selected intervals. Sediment traps obtain samples of sediment suspended in the water column. (Photograph by Bradford Butman.)

Currents and sediments that had been resuspended were measured in three locations in western Massachusetts Bay (depth range 72–216 feet) during the winter and spring of 1987. The nearbottom observations were made by means of an instrumented tripod (fig. 4). Currents, light transmission, and temperature were also measured at middepth and near the sea surface using moorings next to the tripods. Sediment traps were used to collect material suspended in the water column.

The mean current flow at station A was toward shore at all levels of the water column (fig. 5). This onshore flow suggests local recirculation in the region south of Nahant and that discharge of treated effluent in this location would be unwise because it would be transported toward the shore. The near-bottom observations showed resuspension of the surficial sediments during winter storms. The most severe storms resuspended sediments at each of the mooring locations, but the storms of less intensity resuspended material at only the two shallower sites (A and B, fig. 5). These data suggest that accumulation of finegrained sediment may occur during the summer months when storms are infrequent but that the same sediments may then be resuspended and transported toward deeper water or protected inshore areas during winter storms.

The 1987 current measurements coincided with the largest April discharge on record from the Merrimack River, located just north of the study area. Seasurface temperature maps obtained from satellite observations were examined to aid in the interpretation of the current measurements; the images showed a large surface plume extending southward from the Merrimack River and into Massachusetts Bay. A strong northwestward flow was observed that was associated with the western edge of the plume (fig. 6). These observations show the important influence of nonlocal forcing mechanisms, such as freshwater inflow from inland rivers, on circulation and sediment transport in Massachusetts Bay. These observations emphasize the need for expanding these studies to gain a broader regional perspective on what those mechanisms are and what their effects might be. Long-term synoptic

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Figure 5. Western area of Massachusetts Bay showing mean flow at three locations (A, B, and C) measured during the winter and spring of 1987. Although weak, the observations are consistent with a mean shore-parallel flow near the surface and a weak net flow near the bottom. Note the onshore flow observed at station A south of Nahant at all instrument depths.

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A number of radioactive isotopes have been analyzed in this study in order to determine the rates of sediment accumulation, the rates of sediment mixing (bioturbation), and the potential for pollutant accumulation. Lead-210, a sediment reactive isotope that behaves like many contaminants in the marine environment, can be used as a contaminant tracer. Measurements of lead-210 concentrations in a number of cores taken from undisturbed sediments from Boston Harbor and Massachusetts Bay show excess lead-210, between 0.6 and 2.3 times the amount predicted from atmospheric and seawater sources. In areas where increased accumulation of lead210 has occurred, increased accumulation of other contaminants would be

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The critical question to address is whether or not these contaminated sediments will remain as a long-term source

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Figure 6. Sea-surface temperature derived from satellite observations showing plume of warmer water extending southward into Massachusetts Bay from the Merrimack River. Blue arrows show inferred flow; black arrows show nearsurface flow measured by current meters at stations A, B and C. Note the strong northwestward flow associated with the western side of the river plume. Red arrow indicates the mouth of the Merrimack River. (Courtesy of University of Rhode Island.)

The geochemical and geotechnical studies within the cooperative program typically examine sediment characteristics that have developed over a time frame of years to centuries. Heavy metals (chromium, copper, lead, zinc, and so on) are found in significantly higher concentrations in surface sediments of the harbor in comparison with those in sediments deposited before the industrial revolution. The critical question to address is whether or not these contaminated sediments will remain as a long-term source of pollution after the principal sources are reduced or eliminated. Measurements of geotechnical properties are used to characterize the sediments at different locations to determine their suitability for use as fill material and as foundations for structures.

expected. Other isotopes such as carbon14, thorium-234, and plutonium contribute to our understanding of where sediments are actively accumulating. Areas identified to look at sediment accumulations include Stellwagen basin offshore and protected areas of the harbor inshore. These areas of accumulation will be monitored to evaluate the long-term environmental effects of the new ocean outfall.

A detailed sidescan-sonar survey over 31 square miles in Massachusetts Bay was conducted in April 1989 on the RV Anderson, a research vessel provided by the EPA. The sidescan imagery showed a complex pattern of sediment texture and roughness. A preliminary map, drawn at sea as the sidescan data were generated, outlined zones based on different acoustic signatures. Each zone was interpreted as representing different sediment types: areas of coarse gravel and (or) boulders, rippled sand, and smooth, fine-grained sediment. These interpretations were confirmed by photo

Analyzing Nevada's Undiscovered Resources

By Donald A. Singer and Robert C. Jachens

graphing and sampling target locations identified aboard ship.

The preliminary map has been of use to State officials who are charged with deciding the location of the outfall for Boston's treated sewage effluent. An unexpected discovery made during the sidescan-sonar mapping operation was the existence of a previously uncharted shipwreck, approximately 100 feet long, at the edge of the survey area. A digital mosaic of the sidescan sonar data is currently being developed; once completed the digital data can be merged with other data sets, such as texture, bathymetry, or biological habitats, to provide a concise description of the sea floor.

The USGS study in coastal Massachusetts is closely coordinated with other agencies and research organizations through the Massachusetts Bay's Management Committee. USGS participates on this committee along with representatives from each of the Federal and State agencies that has an interest in the coastal environment of Massachusetts. The committee, which is spending $1.6 million for a carefully coordinated research program through 1991, has applied to EPA to include Massachusetts Bay/Cape Cod Bay into the National Bays Program.

The USGS has just concluded a Cooperative Agreement with the Massachusetts Water Resources Authority (MWRA), the independent State agency that provides water and sewer service to the Boston Metropolitan area, to establish a long-term current and sedimenttransport monitoring station in western Massachusetts Bay near the proposed site of the new ocean outfall. This station will provide the first long-term measurements in Massachusetts Bay, and the observations will be used to assess the importance of seasonal variability and infrequent catastrophic events on sediment transport. New conditional sampling instrumentation will be developed to collect samples of suspended material at the height of major storms. This new equipment will improve estimates of the concentration and composition of suspended matter during storms when resuspension and transport of bottom sediments (and any associated contaminants) may be most significant and when sampling from a surface ship is impossible.

Nevada was the Nation's largest silver producer 120 years ago. Today, in spite of the fact that over 50 percent of Nevada's 110,500-square-mile surface is covered with what appears to be barren rocks and gravels, the State is the largest gold producer. Because the majority of metal-bearing mineral deposits exposed at the surface are believed to have already been found, a prime concern of a joint project of the USGS and Nevada Bureau of Mines and Geology has been to disclose the nature of and the depth to possible mineral deposits under this apparently barren cover.

The overall goal of the team of geologists, geophysicists, and geochemists is to provide an analysis of Nevada's mineral resources that can be used to help plan economic development, consider alternate uses of land, plan exploration, and estimate the availability of minerals under different conditions. Because of the extent to which potential mineral resources are covered, an important condition affecting the value of minerals in Nevada is the depth at which the deposits are located. Depth affects the chances of discovery, because deeper deposits are much more difficult and, therefore, more costly to discover; depth affects economic potential, because deeper deposits are significantly more costly to mine. The cooperative analysis currently underway is limited to the deposits and their permissive geologic environments that occur within the upper 1 kilometer (0.6 mile) of the Earth's crust.

A three-part resource assessment process is used because of its ability to respond to each of the diverse problems mentioned above and to use a variety of information and resource assessment methods. In this three-part assessment process (1) areas are delineated according to the types of deposits their geology will permit; (2) grade-tonnage models

are used to estimate the amount of metal and some characteristics of ore; and, (3) estimates are made of the number of deposits of each type in the delineated areas.

Areas or domains are delineated that may contain particular deposit types as inferred by analogy with deposits in similar geologic settings elsewhere. In order to construct the boundaries of these areas, it is necessary to have a geologic map, and it is desirable to have accompanying mineral occurrence, geophysical, exploration, and geochemical information. This information must be integrated with information about the geologic environment of different types of mineral deposits to delineate the area. The key

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stone to combining all of this diverse information is the use of a mineral deposit model. The USGS has published such deposit models (USGS Bulletin 1693), which allow linkage of deposit types to geologic environments.

In order to make the connection of deposit type to geologic environment, it is necessary to recognize and map the relevant geologic settings in Nevada. This task is the primary purpose of producing maps that address geology, gravity field, magnetic field, pre-Tertiary geology (older than 63 million years), Tertiary geology (63 million to 2 million years old), ages of young volcanic deposits, intrusive rocks, neotectonics, known mineral deposits, and mineral resources. Each of the maps is being prepared in digital form so that hypotheses can be quickly tested, derivative maps generated (fig. 7), and the information disseminated to other researchers.

The geologic map of the State being used in this analysis is modified from that published in 1978 in that rock units are grouped to represent geologic environments that would permit the formation of different types of mineral deposits. It also includes new information on the ages of igneous rocks. Because the geologic map represents the geology that is exposed and therefore best known, it is the foundation for most of the other studies.

Analysis of regional gravity data is being used to estimate the thickness of younger rocks that have most recently been deposited. From these gravity data, the scientists were able to produce another gravity map from which the gravity components of thick deposits of young rock and unconsolidated sediments have been removed. This map is used to help identify the rock types of the concealed basement layer, to delineate major crustal structures and boundaries, and to identify concealed plutons (intrusions of igneous rocks) and calderas (large basin-shaped volcanic depressions), all of which can reflect geologic environments permissive for the formation of certain types of mineral deposits.

The analysis of magnetic data focuses on the distribution of near-surface magnetic sources in order to delineate bodies of shallowly buried magnetic rock. Typically these are Tertiary and

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Quaternary (2 million to 10,000 years old) volcanic rocks. The threedimensional information provided by the analysis affects the mineral resource assessment in that certain types of mineral deposits, such as porphyry and skarn copper deposits and platinum, are associated with magnetic rocks.

Many kinds of mineral deposits owe their origin to intrusive igneous rocks. Knowledge of where these plutonic rocks occur is critical in identifying where these types of deposits could exist. A new geophysical tool, which relies primarily on magnetic data, is used to locate unexposed plutonic rocks.

In order to explicitly consider depth in this study, we must deal with volumes of rock and must combine the rock units so that they represent consistent geologic environments. A new type of geologic map is required to portray these rock groups because a number of different geologic environments may overlap in the 1 kilometer (0.6 mile) beneath any given locality on the surface. The complexity of display requires two different maps. The first map of the pre-Tertiary geology shows older rocks that may host mineral deposits related to later igneous, activity or may contain mineral deposits that formed at the same time as the rocks. The second map, Tertiary geology of Nevada, concentrates on the young igneous rocks and related calderas that are closely related to many of the mineral deposits of Nevada.

Ages of young volcanic rocks when compared with the ages of different kinds of mineral deposits provide key information about the development and nature of the mineral deposits and also provide new light on the geologic development of Nevada.

Geologic, geomorphic, geophysical, and well-log data are being analyzed to infer the approximate subsurface geometry of fault-bounded basins in Nevada. By analyzing this more recent tectonic activity, scientists can better determine the depth of environments permissive for older deposits and about the spatial distributions of younger rocks that may be associated with the mineral deposits formed at shallow depths, possibly near faults related to the basins.

Types of mineral deposits and occurrences that have already been found in

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specific geologic environments in Nevada not only confirm that the environments are permissive for the same deposit types, but they also suggest the possibility of genetically related deposit types. For the first time, approximately 1,600 mineral deposits and occurrences have been classified by deposit type. Maps of the locations of Cenozoic (63 million to 10,000 years old) deposits show a remarkable pattern in which volcanichosted epithermal deposits of gold, silver, mercury, and manganese in Nevada are distributed in a C-shaped pattern (fig. 8) with the interior of the C generally devoid of these deposit types, despite the widespread distribution of volcanic rocks. Sediment-hosted gold deposits, such as the Carlin deposit, are restricted (with minor overlap) to the interior of the C. Because the sediment-hosted gold deposits originally formed at a deeper level in the crust, are related to different types of faults, and were formed earlier than the epithermal deposits, important new information about the geologic evolution of Nevada and about where different types of deposits might exist has been revealed.

As noted above, specific geologic environments suggest the possibility of certain types of mineral deposits. The converse is also true; the distribution of different types of known mineral deposits suggest the presence of geologic

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