Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
Nature and Depth of Overburden
In mining, “overburden” refers to the soil and rocks overlying the target mineral deposit. Miners
have to remove or bore through (constructing shafts through) the overburden to reach the desired
ore body. The overburden in the Roca Honda permit area consists of Upper Jurassic and Upper
Cretaceous rocks that overlie the ore bearing Westwater Canyon Member of the Morrison
Formation. Overburden thicknesses range from 1,600 to 2,800 feet and are primarily controlled
by topography; higher ground surface elevation equals thicker overburden. The thickening of the
overburden reaches 2,800 feet in the southeastern corner of Section 10 and reflects deeper burial
as formations dip eastward along the Fernandez monocline toward the McCartys syncline (RHR,
2009d).
Most of the overburden is composed of shale and sandstone, with lesser amounts of coal and
siltstone. Except for the coal, these rock types are relatively stable and resistant to chemical
alteration.
Soils
General Soil Principles
Soil is a naturally occurring, three-dimensional layer on the surface of the earth that supports
plants. Although the mantle of soil on the earth’s surface varies widely from place to place, all
soils share common traits. They are all comprised of minerals, organic matter, living organisms,
water, and air that occur in varying proportions depending on the type of soil. Soils are the result
of soil-forming processes at work on materials deposited or accumulated by geological processes.
Soil properties at any given site are determined by five factors: (1) physical and mineralogical
composition of the parent material, (2) climate under which the soil material accumulated and has
existed since accumulation, (3) plant and animal life atop and within the soil, (4) topography, or
the “lay of the land,” and (5) length of time that these forces of soil formation have acted on the
parent material (NRCS, 2006).
Soil texture is determined by the proportions of different sized particles—sand, silt, and clay—
found in a particular soil sample (figure 30).
100 DEIS for Roca Honda Mine, Cibola National Forest