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

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.

本数据集为配合美国地质调查局（USGS）西科罗拉多I-70走廊协同地质填图项目所制作的大章克申（Grand Junction）7.5'标准图幅1:24000比例尺地质图，针对科罗拉多河与甘尼森河汇流区域的地层、构造与地质灾害开展了全新的解译工作。区域基岩地层涵盖上白垩统曼科斯页岩（Mancos Shale）至下侏罗统温盖特砂岩（Wingate Sandstone）等多个地层单元。作为格兰德谷基底的曼科斯页岩之下，上、下（？）白垩统达科他组（Dakota Formation）与下白垩统布罗峡谷组（Burro Canyon Formation）共同支撑了昂帕格雷隆起（Uncompahgre uplift）东北侧大部分抗侵蚀性的北东向倾斜单斜构造。其下发育的侏罗系地层序列完整且特征显著，包括上侏罗统莫里逊组（Morrison Formation）的布什盆地段、盐洗段与蒂德维尔段，中侏罗统瓦纳卡组（Wanakah Formation）、非正式命名的"board beds"单元，以及恩特拉达组（Entrada Formation）的滑岩段；同时还包含下侏罗统凯延塔组（Kayenta Formation）与温盖特砂岩。出露于西侧科罗拉多国家纪念地（Colorado National Monument）图幅的上三叠统钦利组（Chinle Formation）、古元古代早期火成变质片麻岩及混合岩化变质沉积岩，在本图幅区域内并无露头。昂帕格雷隆起东北缘的单斜倾斜坡，显然属于拉拉米造山期（Laramide）构造特征。不同于西侧隆起前缘科罗拉多国家纪念地7.5'图幅中所见的元古宙基底南西向高角度逆断层，以及上覆地层中的S形断层传播褶皱，本图幅区域内的单斜构造仅在两处地点出现断裂，整体保持完整。其中一处地点存在小型不对称地堑，地层向西南方向陷落；该构造的断裂特征向西北方向逐渐消失，过渡为一处同样使地层向西南陷落的不对称断层传播褶皱。推测该构造的两个区段均发育北东向高角度逆断层作为基底；另一处地点则存在第二处类似的不对称褶皱。本区域未发现拉拉米造山期后倾斜或抬升的相关证据，但位于图幅区域南南西方向仅30公里处的先成乌纳韦普峡谷（Unaweep Canyon），则提供了晚新生代（若未更新世时期）抬升的明确证据。本区域主要地质灾害包括发育于倾斜坡基底、富含蒙脱石的莫里逊组布什盆地段，以及上覆达科他组与布罗峡谷组相关的大型滑坡。活跃的滑坡现象见于科罗拉多河南岸区域，河流的侧蚀作用与曼科斯页岩中的蒙脱石黏土共同诱发了滑坡灾害。瓦纳卡组、莫里逊组、达科他组以及曼科斯页岩中含有膨胀性蒙脱石黏土，对房屋及其他建构筑物构成显著威胁。除科罗拉多河与甘尼森河季节性春季洪水外，夏季突发雷暴引发的严重山洪灾害，同样威胁着单斜倾斜坡汇水区内的间歇性冲沟。