Lithogeochemical Character of Near-Surface Bedrock in the New England Coastal Basins

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. 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 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.

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 Basins (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, Androscogginn,
Saco, Merrimack, Charles, and Blackstone River Basins, as well as all of Cape
Cod. Bedrock units in the NECB study area are classified into 38
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.

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 classification scheme provides flexibility
because the user can reclassify the 38 lithogeochemical units into other groups
for other types of data analysis.

The bedrock units in this study area have been mapped defined by time-
stratigraphic and other geologic criteria which may not be directly relevant to
water quality. Bedrock units depicted on the State geologic maps are
inconsistent across state boundaries in some areas (See
Data_Quality_Information section of this document for explanation on how these
discrepancies were addressed with the classification scheme). Thus, a
study-area-wide coding scheme was developed to classify the geologic map units
according to mineralogical and chemical characteristics that are relevant for
water-quality investigations.

Bedrock units were classified for water-quality purposes according to the
chemical composition and relative susceptibility to weathering of their
constituent minerals. Although weathering rates may vary, the relative
stability of different minerals during weathering in moist climates is
generally consistent (Robinson, 1997). However, the degree to which a rock
weathers reflects the proportions of its constituent mineral as well as many
other factors such as degree of induration and relative amount of mineral
surfaces exposed to water through primary and secondary porosity. Thus,
although largely based on the relative stability of rock constituent minerals,
the classification scheme to group bedrock units according to effects on water
quality is more complex than mineral- stability sequences. Most common
rock-forming minerals are only sparingly soluble, so that small amounts of
highly reactive minerals can have large effects of water quality (Robinson,
1997). For example, carbonate minerals are more rapidly weathered and tend to
produce higher solute concentrations in natural waters than other rock types.
In contrast, granites, schists and quartzites, which are rich in
alkali-feldspar, muscovites, and quartz, produce low solute concentrations
because they react to a lesser degree and at slower rates than other rock types
in humid temperate climates (Robinson, 1997). The lithogeochemical
classification scheme used in this data set incorporates the relative stability
of minerals classifications criteria such as used in previous studies, and the
characteristics of bedrock geology specific to the study area (such as the
presence of a discrete fault bounded sedimentary basins of Mississipian or
younger age). Further description of the lithogeochemical classification
scheme and the expected water- quality and ecosystem characteristics associated
with each lithogeochemical unit is explained in Robinson (1997).

Thirty-eight lithogeochemical units have been defined for the NECB study area
based on the mineral and textural properties of the bedrock unit's constituent
minerals, presence of carbonate and sulfide minerals and for some of the
granitic units, relative age. The classification scheme used descriptions from
State geologic maps (Osberg and others, 1985; Lyons and others, 1997; Zen and
others, 1985;Hermes and others, 1994; and Rogers, 1985) of the lithology,
mineralogy, and weathering characteristics of the bedrock units. For example,
"rusty-weathering" serves as an indicator of sulfidic-bearing bedrock units
(Robinson, 1997). Carbonate and sulfide minerals predominate in the
classification scheme because these highly reactive minerals have a
disproportionately large effect on water chemistry compared to other minerals
commonly found in the rocks of this region. In the Maine data set, information
about metamorphic grade was also used to classify bedrock units. A digital data
layer of generalized regional metamorphic zones (Guidotti, 1985, shown in
Osberg and others,1985), was obtained from the Maine Geological Survey. This
layer was intersected with the digital bedrock geology to determine the
regional metamorphic grade of each polygon in the bedrock geology data layer.
Polygons lying within two metamorphic zones were split at the metamorphic-zone
boundary. Metamorphic grade and geochemical composition of the protolith
(pre-metamorphism source rock) were used to classify polygons into
lithogeochemical units. For example, bedrock units with protoliths of
"limestone and(or) dolostone" were classified as "limestone, dolomite, and
carbonate-rich clastic sediments" (lithogeochemical unit "11u") in areas of
none or weak regional metamorphism and as "marble, may include some
calc-silicate rock" (lithogeochemical unit "12u") in areas of greenschist
facies or high grade metamorphism.

The 38 lithogeochemical units defined for the NECB study area result from the
combination of a lithology code (numeric) with a modifier code (alphabetic).
There are 17 lithology codes that represent the influences on water chemistry
of lithology, metamorphic grade, and geologic setting. Each bedrock unit is
assigned one of 17 lithology codes based on the description of the bedrock unit
from the State bedrock geologic maps. There are 13 modifier codes used to
identify minor amounts of carbonate and(or) sulfide minerals, and subdivide
granitic units into subgroups based on their chemical and mineral
characteristics and relative age. A description of the 38 lithogoechemical
units in the NECB study area and their potental effects on water quality can be
found in the Supplemental_Information section of this document.

The 38 lithogeochemical units are generalized into 7 major groups that share
similarities in overall geochemistry and lithology: (1) carbonate-bearing
metasedimentary rocks; (2) primarily noncalcareous, clastic sedimentary rocks
deposited in 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. Major group 7 encompasses areas in the south-coastal part of the
NECB study area where the bedrock is overlain by thick glacial sediments at the
surface. These surficial glacial deposits are the primary aquifer for these
areas. An example of how this data set has been used in study design strategies
and in analyzing water-quality characteristic by lithogeochemical units and
major groups is provided in Ayotte and others (1999).

The bedrock units shown on the individual State maps for the NECB were
classified according to a lithogeochemical scheme modified from Robinson and
others (1999). Specifically, the modification included the subdivision of
granitic bedrock units into additional lithogeochemical units with modifying
attributes to indicate relative age. However, this modification to the
classification system is evident in the lithogeochemical units. Thus, the CONN
and the NECB data set can be readily merged together to create a larger
regional product with these difference being more frequent when the data set is
viewed with the lithogeochemical units showing and less frequent when the data
set is viewed with the major groups showing. Overall, the bedrock units in the
two study units are classified in a consistent manner to a create regional
product that can be used to evaluate the influences of bedrock geology on
water-quality characteristics.

Quality Assurance procedures: The scientific content of this digital data set
underwent technical review by two USGS scientists who have knowledge of the
regional geology,and GIS and spatial-data production. The data set was
evaluated on positional accuracy, contextual accuracy, attribute accuracy, and
topological consistency.