Geologic And Hydrologic Factors Governing ... - Gunnison County

4.3.8 Faults and Joints
Geologic And Hydrologic Factors Governing Impacts Of Development
On The Crystal River Near Marble, Colorado
Gunnison County, Colorado
Faults are fractures in the bedrock along which there has been movement of one side in relation to
the other. They may range from simple breaks, where the sides are only an inch or less apart and the
relative movement was less than a foot, to zones of brecciated rock that are tens of feet wide along
which the movement may have been several thousand feet. Joints are simple fractures in the rock
along which there has been little or no movement. An individual joint does not usually extend for
more than a few feet along its strike. Major faults in some areas can be followed for miles. It can be
presumed, because of the extensive cover of surficial deposits, that there are many more faults in the
bedrock. In the area east of Carbonate Creek several northwest-trending faults were mapped in the
bedrock. These are normal faults with the downslope side of the fault downdropped. The move-
ment along the faults is estimated to be less than 100 feet of vertical displacement.
West of Slate Creek and east of Gallo Hill, and on the southwest slope of Gallo Hill along the sill of
quartz monzonite porphyry, are two major northeast-trending faults. These faults are normal faults
with the southeast side downdropped in relation to the northwest side. It is estimated that the
movement along these faults is less than 100 feet of vertical displacement.
Fault zones are rarely exposed in the study area because they are zones of weakness and erode eas-
ily. The relation of the faults trends and the topography indicate most faults dip steeply. In general,
the fault zones are vertical and 1 to 3 feet wide. The zone between the walls of the faults is filled
with angular fragments of bedrock in a clay gouge. The fault zones in the sedimentary rocks are
tight and usually do not serve as channels for the movement of groundwater. The faults in the igne-
ous rock probably are more open, and may serve as channels for the groundwater movement.
Joints are conspicuous in the bedrock outcrops. The joints in the sedimentary rocks—mostly meas-
ured in the ski area east and west of Carbonate Creek—strike about N. 45° W. and dip steeply
southwest or northeast. The strike of the joints is about parallel to the faults and the axis of the syn-
cline to the southwest, and are probably related to these major structural features in origin. The
joints, in general, are tight. A thin selvage of clay usually occurs along a typical joint.
951-110.000 Wright Water Engineers, Inc. Page 30