Geologic map of the Grand Junction quadrangle, Mesa County, Colorado

To update and reinterpret earlier geologic mapping, and provide sufficient
geologic information for land-use decisions for private land and for areas
managed by the Bureau of Land Management and the National Park Service. Use of
these data at scales greater than 1:24,000 would be inappropriate because
mapping was performed at that scale.

This 1:24,000-scale geologic map of the Grand Junction 7.5' quadrangle, in
support of the USGS Western Colorado I-70 Corridor Cooperative Geologic Mapping
Project, provides new interpretations of the stratigraphy, structure, and
geologic hazards in the area of the junction of the Colorado River and the
Gunnison River. Bedrock strata include the Upper Cretaceous Mancos Shale
through the Lower Jurassic Wingate Sandstone units. Below the Mancos Shale,
which floors the Grand Valley, the Upper and Lower(?)Cretaceous Dakota
Formation and the Lower Cretaceous Burro Canyon Formation hold up much of the
resistant northeast- dipping monocline along the northeast side of the
Uncompahgre uplift. The impressive sequence of Jurassic strata below include
the Brushy Basin, Salt Wash, and Tidwell Members of the Upper Jurassic Morrison
Formation, the Middle Jurassic Wanakah Formation and informal "board beds" unit
and Slick Rock Member of the Entrada Formation, and the Lower Jurassic Kayenta
Formation and Wingate Sandstone. The Upper Triassic Chinle Formation and Early
Proterozoic meta-igneous gneiss and migmatitic meta- sedimentary rocks, which
are exposed in the Colorado National Monument quadrangle to the west, do not
crop out here. The monoclinal dip slope of the northeastern margin of the
Uncompahgre uplift is apparently a Laramide structural feature. Unlike the
southwest-dipping, high-angle reverse faults in the Proterozoic basement and
s-shaped fault- propagation folds in the overlying strata found in the Colorado
National Monument 7.5' quadrangle along the front of the uplift to the west,
the monocline in the map area is unbroken except at two localities. One
locality displays a small asymmetrical graben that drops strata to the
southwest. This faulted character of the structure dies out to the northwest
into an asymmetric fault-propagation fold that also drops strata to the
southwest. Probably both parts of this structure are underlain by a
northeast-dipping high-angle reverse fault. The other locality displays a
second similar asymmetric fold. No evidence of post-Laramide tilting or uplift
exists here, but the antecedent Unaweep Canyon, only 30 km to the
south-southwest of the map area, provides clear evidence of Late Cenozoic, if
not Pleistocene, uplift. The major geologic hazards in the area include large
landslides associated with the dip-slope-underlain, smectite-rich Brushy Basin
Member of the Morrison Formation and overlying Dakota and Burro Canyon
Formations. Active landslides affect the southern bank of the Colorado River
where undercutting by the river and smectitic clays in the Mancos trigger
landslides. The Wanakah, Morrison, and Dakota Formations and the Mancos Shale
create a significant hazard to houses and other structures by containing
expansive smectitic clay. In addition to seasonal spring floods associated
with the Colorado and Gunnison Rivers, a serious flash flood hazard associated
with sudden summer thunderstorms threatens the intermittent washes that drain
the dip slope of the monocline.

Although this Federal Geographic Data Committee-compliant metadata file is
intended to document the data set in nonproprietary form, as well as in
ARC/INFO format, this metadata file may include some ARC/INFO-specific
terminology.

Geospatial data files of this data set:
> gj24k: geology polygons, contacts, faults
> gjpnt: point data representing bedding attitudes
> gjline: line representing location of the cross section,
> and fold axes

Symbolsets used for plotting in ArcInfo:
> wpgcmykg.shd: shadeset
> geol_sfo.lin: lineset
> geoscamp1.mrk: markerset