This file is a digital polygon representation of the depth to (overburden above) abandoned underground coal mines in the Boulder-Weld coal field, Denver Basin, Colorado. This file was created as part of the Front Range Infrastructure Resources Project (FRIRP) of the USGS, and provides information pertaining to energy resource issues within the Colorado Front Range urban corridor in and near the Denver metro area. These data contributed to the publication titled as follows: Maps showing the extent of mining, locations of mine shafts, adits, air shafts, and bedrock faults, and thickness of overburden above abandoned coal mines in the Boulder-Weld coal field, Boulder, Weld, and Adams counties, Colorado: U.S. Geological Survey Geologic Investigations Map I-2735, 1:48000-scale.
In 1996, the U.S. Geological Survey (USGS) initiated a Front Range Infrastructure Resources Project (FRIRP) to provide relevant information to the public, land-use planners, and decision makers regarding the location and characteristics of water, mineral, and energy resources along the Front Range corridor in Colorado. Energy resource issues within the FRIRP project area include the impacts of historic coal mining, particularly in regard to the potential for subsidence over abandoned mines in recently developed areas or in areas currently being evaluated as residential, commercial, and (or) industrial building sites. In order to address this issue, the USGS and the Colorado Geological Survey (CGS) jointly undertook the development of digital data (coverages) based on previously published maps showing the location and depth to abandoned underground coal mines in the Boulder-Weld coal field north and northwest of Denver, Colorado. Mining in the coal field began in the early 1860s and continued into the 1970s with the last mine closing in 1979 because of fire. Although much of the surface expression of mine development has been removed or masked by increasing urban and residential development, subsurface features of underground mining, such as shafts and open mine rooms, are still present. Because of this, there continues to be a potential for subsidence in abandoned mine areas. The primary purpose for compiling the Boulder-Weld coal field coverages is to provide the public and land-use planners with some basic digital data necessary for an initial GIS assessment of areas that have been impacted by coal mining. Such areas may require special considerations with regard to development and surface subsidence potential. It should be noted, however, that these data do not in any way provide the information necessary for a complete and accurate estimation of the subsidence potential in any specific area of the coal field. For this reason, users requiring more detailed information regarding subsidence potential should consult with the Colorado Geological Survey, 1313 Sherman Street, Room 715, Denver, Colo., 80203; 303-866-2611; http://geosurvey.state.co.us.
The digital coverage of the estimated depth to (overburden above) abandoned underground coal mines is based on a previously published map compiled by the consulting firm of Amuedo and Ivey for the Colorado Geological Survey in the early 1970s (see Myers and others, 1975). The primary intent of the coverage is to show the depth of cover (overburden) above the abandoned mine workings within the Boulder-Weld coal field. Overburden polygons are categorized in intervals of 50 ft to a depth greater than 400 ft. We have made no modifications to the original depth of cover interpretations as reported in Myers and others (1975).
publication date
These data are considered more accurate at scales of 1:48,000 or smaller. It should be noted that these data do not in any way provide the information necessary for a complete and accurate estimation of the subsidence potential in any specific area of the coal field. For this reason, users requiring more detailed information regarding subsidence potential should consult with the Colorado Geological Survey, 1313 Sherman Street, Room 715, Denver, Colo., 80203; 303-866-2611; http://geosurvey.state.co.us.
U.S. Geological Survey, Box 25046, MS 939, Denver Federal Center
Users analyzing the depth to abandoned coal mines coverage in conjunction with the USGS-generated digital coverage showing the extent of abandoned coal mines in the Boulder-Weld coal field should note that polygons depicting depth to mines might not correspond completely with the mine extent polygons in some areas. This is because mine extent polygons were modified locally based on more current (post 1970s) unpublished information from the Colorado Geological Survey, whereas overburden polygons were not modified from original interpretations of Myers and others (1975).
The following discussion addresses some of the processes and caveats to the development of the depth of cover map, as reported in Myers and other, 1975. Elevation data for the old mine workings was the most critical element for determining overburden, and the lack of these data in many older mine maps (pre-1920's) hampered interpretations to some degree. In these cases, drill hole data and shaft-depth records were used to approximate the depth to mine workings if those data were available. If such data were lacking altogether, estimations of overburden thickness were made by (1) extrapolating structure contour data for the top of the mined horizon from nearby mines where the same bed was being recovered, (2) obtaining records of water wells penetrating mined horizons, (3) interviewing miners, or (4) basing overburden estimates on similarities between subsidence characteristics in areas of known overburden to subsidence characteristics in areas of unknown overburden. Where sufficient elevation data in the old mine workings were available, the overburden was determined by generating a structure contour map on top of the extracted coal bed horizon. A topographic map was then superimposed on this structure contour data, and elevations on top of the extracted coal horizon were subtracted from surface elevations to create a contourable data set of overburden thickness values. These thickness values were then plotted on the mine extent maps, and contoured in 50-ft intervals. Where multiple coal beds were mined, the overburden represents the depth of cover to the highest mining level. Four factors that contribute to the variation in depth of cover include (1) variations in surface topography, (2) variations in the mined stratigraphic level and the stratigraphic interval separating extracted coal beds, (3) structural offset of coal horizons across faults, and (4) the overall south to southeasterly dip of the strata, which results in increasing overburden in these directions.
The digital polygon data representing depth to abandoned mines were compiled from analog data plotted on a 1:24,000-scale composite base map (clear film) representing all or parts of nine USGS 7.5' topographic quadrangles within the coal field boundary. To facilitate the scanning and digitization procedures required to create the coverage, photographic reproductions were made on scale-stable mylar for each 7.5' quadrangle area within the coal field area rather than creating a single photographic reproduction of the entire composite map. Each 7.5' segment was then electronically scanned, and converted to a Tagged Image File Format (TIFF) image that could be imported into Environmental Systems Research Institute (ESRI) Arc/Info software for coverage development. The depth to abandoned underground mines polygon coverage was developed from the "on-screen" digitization of depth categories as delineated in the 7.5' quadrangle TIFF images. Users analyzing the depth to abandoned coal mines coverage in conjunction with the USGS-generated digital coverage showing the extent of abandoned coal mines in the Boulder-Weld coal field should note that polygons depicting depth to mines might not correspond completely with the mine extent polygons in some areas. This is because mine extent polygons were modified locally based on more current (post 1970s) unpublished information from the Colorado Geological Survey, whereas overburden polygons were not modified from original interpretations of Myers and others (1975).
Feature geometry.
ESRI
Area of feature in internal units squared.
ESRI
Perimeter of feature in internal units.
ESRI
the minimum depth (in feet)
USGS
the range of thickness (in feet)
USGS
the maximum depth (in feet)
USGS
Data from Myers and others (1975) define the upper limit of depth to abandoned mine workings as > 400 ft. Those polygons in the coverage that are within this category are attributed as follows: MINIMUM 400, RANGE >400, and MAXIMUM 450; all values are in feet. The maximum depth in these polygons was arbitrarily set at 450 to maintain consistency with the 50 ft-thick contour interval used for all other polygons. The maximum thickness for these polygons, however, can only be assumed to be between 400 and 450 ft.
Box 25286 Denver Federal Center
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Depth to coal mining geographic features and attribute data for the Colorado Front Range
ArcIMS Image Map Service
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U.S. Geological Survey, Box 25046, MS 939, Denver Federal Center
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