This geographic information system (GIS) data layer shows the generalized lithologic and geochemical, termed lithogeochemical, character of near-surface bedrock in the New England Coastal Basin (NECB) study area of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) Program. The area encompasses 23,000 square miles in western and central Maine, eastern Massachusetts, most of Rhode Island, eastern New Hampshire and a small part of eastern Connecticut. The NECB study area includes the Kennebec, Androscoggin, Saco, Merrimack, Charles, and Blackstone River Basins, as well as all of Cape Cod.
Bedrock units in the NECB study area are classified into lithogeochemical units based on the relative reactivity of their constituent minerals to dissolution and the presence of carbonate or sulfide minerals. The 38 lithogeochemical units are generalized into 7 major groups: (1) carbonate-bearing metasedimentary rocks; (2) primarily noncalcareous, clastic sedimentary rocks with restricted deposition in discrete fault-bounded sedimentary basins of Mississipian or younger age; (3) primarily noncalcareous, clastic sedimentary rocks at or above biotite-grade of regional metamorphism; (4) mafic igneous rocks and their metamorphic equivalents; (5) ultramafic rocks; (6) felsic igneous rocks and their metamorphic equivalents; and (7) unconsolidated and poorly consolidated sediments.
Goals of the NAWQA program are to describe the status and trends of a large representative part of the Nation's surface- and ground-water resources and to identify the natural and human factors that affect the quality of these resources (Leahy and others, 1990). The lithogeochemical data layer was compiled to provide the NECB NAWQA study area with digital geologic information that could be used in the analysis of surface- and ground-water quality. The data layer presented here was intended to characterize the bedrock units in the study area in terms of mineralogic and chemical parameters relevant to water quality, such that the geologic data could be used in GIS to plan NAWQA study-unit activities, and to analyze and interpret water-quality and ecosystem conditions.
The classification scheme used was first developed as part of the USGS's study of the Connecticut, Housatonic, and Thames River Basins (CONN), an adjacent NAWQA study area (Robinson and others, 1999). The classification scheme is based on geochemical principles, previous studies of the relations among water-quality and ecosystem characteristics and rock type, and the regional geology of New England. The classification scheme and data set are intended to provide a general, flexible framework for classifying and mapping bedrock units in the study area for all types of water-quality analysis. The data set is a lithologic map that has been coded to reflect the potential influence of geology on water quality.
The lithogeochemical data were derived and compiled from the five individual State geologic maps in the NECB study area (Osberg and others, 1985; Lyons and others, 1997; Zen and others, 1983; Hermes and others, 1994; Rogers, 1985). Digital versions of the State geologic maps were stored in a geographic information system (GIS) so the bedrock units could be spatially grouped into lithogeochemical units and major groups. A lithogeochemical unit code, major group code, NAWQA-unit code, and State abbreviation were assigned to each bedrock unit in the digital State geologic maps. The digital State geologic maps were joined in the NECB study area so the lithogeochemical digital data set could be created. A table listing which bedrock types from the individual State geologic maps were classified into each lithogeochemical unit and each major group is provided in Robinson and others (2002).
The spatial accuracy of the geologic source information varies. The source geologic map scales range from 1:100,000 to 1:500,000. In addition, analysis near State boundaries may be limited by the geologic map interpretation differences across State boundaries. Because each State bedrock geologic map was interpreted and assembled by different groups of geologists, the bedrock maps are not consistent from state to state.
In particular, there are variations in the scales of the State bedrock maps, the descriptions of mineral assemblages in the formations, and the regional trends in lithology and metamorphic grade. Interpretations combining the NECB NAWQA data set with a similar bordering data set done for the CONN NAWQA (Robinson and others, 1999) are limited by variances related to differences in source materials used and interpretation of geologic information between the CONN and NECB lithogeochemical mapping.
These data can be retrieved as an ArcVIEW shape file or an ARC/INFO export file. The metadata (documentation) of the NECB lithogeochemical data set also is available with the digital data set. The metadata provides detailed information on data quality, spatial-data organization, spatial reference, spatial entities and attributes, and other aspects of the data set. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Ayotte, J.P., Nielsen, M.G., Robinson, G.R., Jr., Moore, R,B., 1999, Relation of arsenic, iron, and manganese in ground water to aquifer type, bedrock lithogeochemistry, and land use in the New England Coastal Basins, U. S. Geological Survey Water-Resources Investigative Report 99-4162, 61 p.
Guidotti, C.V., 1985, Generalized map of regional metamorphic zones: in Osberg, P.H., and others: Augusta, Maine, Maine Geological Survey, 1 map sheet, 1:1,600,000.
Hermes, O.D., Gromet, L.P., Murray, D.P., 1994, Bedrock geologic map of Rhode Island: Kingston, R.I., Office of the Rhode Island State Geologist, Rhode Island Map Series No 1, 1 map sheet, 1:100,000.
Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 1990, Implementation plan for the National Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 90-174, 10 p.
Lyons, J.B., Bothner, W.A., Moench, R.H., and Thompson, J.B., Jr., 1997, Bedrock geologic map of New Hampshire: U.S. Geological Survey Special Map, 2 map sheets, 1:250,000.
Osberg, P.H., Hussey, A.M. II, and Boone, G.M., 1985, Bedrock geologic map of Maine: Augusta, Maine, Maine Geological Survey, 1 map sheet, 1:500,000.
Robinson, G.R., Jr., 1997, Portraying chemical properties of bedrock for water quality and ecosystem analysis: an approach for the New England Region: U.S. Geological Survey Open-File Report 97-154, 17 p.
Robinson, G.R., Jr., Peper, J.D., Steeves, P.A., and DeSimone, L.A., 1999, Lithogeochemical character of near-surface bedrock in the Connecticut, Housatonic, and Thames River Basins: U.S. Geological Survey, Water-Resources Investigations Report 99-4000 digital.
Rogers, John (compiler), 1985, Bedrock geological map of Connecticut: Connecticut Geologic and Natural History Survey, Natural Resource Atlas Map Series, 2 map sheets, 1:125,000.
Zen, E-an, Goldsmith, G.R., Ratcliffe, N.L., Robinson, P., and Stanley, R.S., 1983, Bedrock geologic map of Massachusetts: U.S. Geological Survey, Monograph Series, 3 map sheets, 1:250,000.
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