U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
Characterizing Bedrock Fractures in Outcrop for Studies of Ground-Water
Hydrology: An Example from Mirror Lake, Grafton County, New Hampshire
by
Christopher C. Barton (U.S. Geological Survey, Denver, Colo.)
Abstract
The fractured-rock-hydrology study site at Mirror Lake, New Hampshire
provides an opportunity to use the pavement method to characterize bedrock
fractures at four highway roadcuts and one glacial pavement adjacent to
the bedrock-hydrology test wells at Mirror Lake. The method entails the
mapping of the fracture-trace network down to fracture-trace lengths of
1 meter on a detailed geologic base map. These maps, in turn, form
the basis for quantifying the scaling, interconnectedness, spatial distribution,
and trace-length distribution for the fracture network. The characteristics
of each mapped fracture include orientation, roughness, mineralization,
aperture, and tectonic features, such as slickensides. The combined fracture
data for the five pavement maps made to date indicate the following: Fracture
orientation is highly variable with a preferred strike azimuth of approximately
30°E., preferred fracture dips of 7°NW., 50°SE., and 82°SE.
Fracture roughness coefficients (RC) range from 0 to 18 RC values, with
a mode at 5 to 6. Fracture aperture ranges from a lower cutoff of 0.005
mm to a maximum of 20.6 mm, and the frequency distribution of fractures
follows a power-law function with a scaling exponent of -1.5. Fracture-trace
length ranges from a lower cutoff of 1 m to a maximum of 24.6 m, and
the frequency distribution follows a power-law function with a scaling exponent
of -2.4. Fracture connectivity within the network is low compared to that
of other sites around the country. Fracture mineralization includes iron-oxide
coatings that appear have been biologically precipitated by iron-fixing
bacteria in the ground water and Liesegang bands that extend up to about
1 meter into the rock matrix. A paleohydrologic map of the iron-stained
fracture traces reveals that ground water flowed in only part of the available
network of the open and interconnected fractures.
|
|