Geologic Map of Colorado National Monument and Adjacent Areas, Mesa County, Colorado

To update the interpretation and increase the scale of geologic mapping,
provide a geologic map for the public to use at Colorado National Monument, and
to provide sufficient geologic information for land-use and land-management
decisions.

New 1:24,000-scale geologic mapping in the Colorado National Monument
Quadrangle and adjacent areas, in support of the USGS Western Colorado I-70
Corridor Cooperative Geologic Mapping Project, provides new interpretations of
and data for the stratigraphy, structure, geologic hazards in the area from the
Colorado River in Grand Valley onto the Uncompahgre Plateau. The plateau drops
abruptly along northwest-trending structures toward the northeast 800 m to the
Redlands area and the Colorado River in Grand Valley.

In addition to common alluvial and colluvial deposits, surficial deposits
include Holocene and late Pleistocene charcoal-bearing valley-fill deposits,
late to middle Pleistocene river-gravel terrace deposits, Holocene to middle
Pleistocene younger, intermediate, and old fan-alluvium deposits, late to
middle Pleistocene local gravel deposits, Holocene to late Pleistocene
rock-fall deposits, Holocene to middle Pleistocene young and old landslide
deposits, Holocene to late Pleistocene sheetwash deposits and eolian deposits,
and Holocene Cienga-type deposits.

Only the lowest part of the Upper Cretaceous Mancos Shale is exposed in the map
area near the Colorado River. The Upper and Lower Cretaceous Dakota Formation
and the Lower Cretaceous Burro Canyon Formation form resistant dipslopes in the
Grand Valley and a prominent ridge on the plateau. Less resistant strata of
the Upper Jurassic Morrison Formation consisting of the Brushy Basin, Salt
Wash, and Tidwell Members form slopes on the plateau and low areas below the
mountain front of the plateau. The Middle Jurassic Wanakah Formation
nomenclature replaces the previously used Summerville Formation. Because an
upper part of the Middle Jurassic Entrada Formation is not obviously correlated
with strata found elsewhere, it is therefore not formally named; however, the
lower rounded cliff former Slickrock Member is clearly present. The Lower
Jurassic silica-cemented Kayenta Formation forms the cap rock for the Lower
Jurassic carbonate-cemented Wingate Sandstone, which forms the impressive
cliffs of the monument. The Upper Triassic Chinle Formation was deposited on
the eroded and weathered Middle Proterozoic meta-igneous gneiss, pegmatite
dikes, and migmatitic gneiss.

Structurally the area is deceptively challenging. Nearly flat-lying strata on
the plateau are folded by northwest-trending fault-propagation folds into at
least two S-shaped folds along the mountain front of the plateau. Strata under
Grand Valley dip at about 6 degrees to the northeast. In the absence of local
evidence, the uplifted plateau is attributed to Laramide deformation by dated
analogous structures elsewhere in the Colorado Plateau. The major exposed
fault records high-angle reverse relationships in the basement rocks but
dissipates strain as a triangular zone of distributed microfractures and
cataclastic flow into overlying Mesozoic strata that absorb the fault strain,
leaving only folds. Evidence for younger, probably late Pliocene or early
Pleistocene, uplift does exist at the antecedent Unaweep Canyon south and east
of the map area. To what degree this younger deformation affected the map area
is unknown.

Several geologic hazards affect the area. Middle and late Pleistocene
landslides involving the smectite-bearing Brushy Basin Member of the Morrison
Formation are extensive on the plateau and common in the Redlands below the
plateau. Expansive clay in the Brushy Basin and other strata create foundation
stability problems for roads and homes. Flash floods create a serious hazard to
people on foot in narrow canyons in the monument and to homes close to water
courses downstream from narrow restrictions close to the monument boundary.

Map political location: Mesa County, Colorado
Compilation scale: 1:24,000
Geology mapped in 1998.

Geospatial data files included in this data set:
cnmpoly: geologic units
cnmline: faults, fold axes, dikes, and other line features
cnmpoint: strike and dip measurements and other point features
cnmsym: cartographic decorations-strike/dip symbols, leaders, line
decorations, etc.
cnmtext: text labels for map units
cnmborder: neatline of map
cnmboundary: boundary of Colorado National Monument
cnmhydro: hydrologic features
cnmhypso: elevation contours
cnmrailroads: railroads
cnmroads: roads