Preliminary Lithogeochemical Map Showing Near-Surface Rock Types in the Chesapeake Bay Watershed, Virginia and Maryland

Title:: Preliminary Lithogeochemical Map Showing Near-Surface Rock Types in the Chesapeake Bay Watershed, Virginia and Maryland
Abstract:: This preliminary experimental lithogeochemical map shows the distribution of rock types in the Virginia and Maryland parts of the Chesapeake Bay watershed. The map was produced digitally by classifying geologic-map units according to composition, mineralogy, and texture; rather than by age and stratigraphic relationships as shown on traditional geologic maps. This map differs from most lithologic maps in that the lithogeochemical unit classification distinguishes those rock units having key water-reactive minerals that may induce acid neutralization, or reduction, of hosted water at the weathering interface. The validity of these rock units, however, is independent of water chemistry, because the rock units are derived from geologic maps and rock descriptions. Areas of high soil carbon content, and sulfide metal deposits are also shown.
Water-reactive minerals and their weathering reactions yield five lithogeochemical unit classes: 1) carbonate rock and calcareous rocks and sediments, the most acid-neutralizing; 2)carbonaceous-sulfidic rocks and sediments, oxygen-depleting and reducing; 3) quartzofeldspathic rocks and siliciclastic sediments, relatively weakly reactive with water; 4) mafic silicate rocks/sediments, oxygen consuming and high solute-load delivering; and, 5) the rarer calcareous-sulfidic (carbonaceous) rocks, neutralizing and reducing. Earlier studies in some parts of the map area have related solute loads in ground and stream waters to some aspects of bedrock lithology. More recent preliminary tests of relationships between four of the classes of mapped lithogeochemical units and ground water chemistry, in the Mid-Atlantic area using this map, have focused on and verified the nitrate-reducing and acid-neutralizing properties of some bedrock and unconsolidated aquifer rock types. Sulfide mineral deposits and their mine-tailings effects on waters are beginning to be studied by others. Additional testing of relationships among the lithogeochemical units and aspects of ground and surface water chemistry could help to refine the lithogeochemical classification, and this map. The testing could also improve the usefulness of the map for assessing aquifer reactivity and the transport properties of reactive contaminants such as acid rain, and nitrate from agricultural sources, in the Chesapeake Bay watershed.
Supplemental_Information:: Data were originally collected in UTM coordinates, zone 18, NAD 1927, and projected to geographic coordinates (Lat/Long), NAD 1983. The data base is accompanied by a large format color map, a readme.txt file, and a explanatory PDF pamphlet.

The following is a brief description of the data layers, tables, and their associated attributes that are used in this report. GEOLOGIC DATA: CARBON - polygon data layer of carbon-rich soils: MUID - map unit identifier derived from STATSGO data base OC.M.G.M2 - value for soil organic carbon in square meters GEOLOGY - polygon data layer of the contacts of geologic map units: GEOL_CODE - geologic map unit code LITHOGEO - polygon data layer of the contacts of lithogeochemical map units: LITH_CODE - lithogeochemical map unit code SULFIDE - point data layer of sulfide deposits: STATE_CODE - state code SITE - deposit name LAT - latitude LONG - longitude COMMOD - commodities present MAJOR - major products MINOR - minor products TRACE - trace or occurrence commodity DEP_TYPE - deposit types MODEL - USGS model name DEP_SIZE - size of deposit GEOLLITH - data table that relates GEOL_CODE to LITH_CODE: GEOL_CODE - geologic map unit code LITH_CODE - lithogeochemical map unit code LITHTBL - data table that relates LITH_CODE to ROCK_TYPE_GROUP, SUB_GROUP, MAP_UNIT, ROCK_TYPE_CLASS, MINERALOGY, and WATER_CHEMISTRY: LITH_CODE - lithogeochemical map unit code ROCK_TYPE_GROUP - rock type; e.g., sedimentary rocks and their metamorphic equivalents SUB_GROUP - rock type sub group; e.g., carbonate-rich rock MAP_UNIT - map unit; e.g., limestone ROCK_TYPE_CLASS - rock class; e.g., carbonate-bearing rock MINERALOGY - mineralogical properties; e.g., quartz-, and feldspar-rich rock WATER_CHEMISTRY - suggested water chemistry; e.g., low acid-neutralizing capacity BASE DATA: POLBND - polygon data layer of county boundaries: STATECTY - map unit identifier derived from the Chesapeake Bay Program data ST - state identifier CNTYNAME - county name POLYTYPE - feature identifier derived from the Chesapeake Bay Program data

Data format:	Geologic and lithogeochemical contacts, carbon-rich soils, sulfide deposits, and county boundaries in format Arc/Info export-interchange (version 8.0.2) Size: 7.0 megabytes
Network links:	<http://pubs.usgs.gov/openfile/of01-187/>

Data format:

Geologic and lithogeochemical contacts, carbon-rich soils, sulfide deposits, and county boundaries in format Arc/Info export-interchange (version 8.0.2) Size: 7.0 megabytes

Network links:

<http://pubs.usgs.gov/openfile/of01-187/>

Preliminary Lithogeochemical Map Showing Near-Surface Rock Types in the Chesapeake Bay Watershed, Virginia and Maryland

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