ABSTRACT This guide describes a three-day field trip to the Paleogene sedimentary and volcanic rocks exposed between the Straight Creek–Fraser River and Entiat faults in the central Washington Cascades. These rocks record a history of deposition, deformation, and magmatism that can be linked to tectonic events along the North American margin using a robust chronology coupled with detailed sedimentological, stratigraphic, and structural studies. These events include deposition in a large sedimentary basin (Swauk basin) that formed in the forearc from <59.9–50 Ma; disruption and deformation of this basin related to the accretion of the Siletzia oceanic plateau between 51 and 49 Ma; the initiation, or acceleration of right-lateral, strike-slip faulting and the development of at least one strike-slip sedimentary basin (Chumstick basin) starting ca. 49 Ma; and the re-establishment of a regional depositional system after ca. 45–44 Ma (Roslyn basin) as strike-slip faulting was localized on the Straight Creek–Fraser River fault. These events are compatible with the presence of the Kula-Farallon ridge near the latitude of Washington ca. 50 Ma and its southward movement, or jump, following the accretion of Siletzia. This trip visits key outcrops that highlight this history and links them to regional studies of sedimentation, faulting, and magmatism to better understand the geologic record of this tectonic setting.

The Lake Mead region contains major Miocene disruptions of structures formed during Mesozoic tectonic shortening. Erosion by the Colorado River and its tributaries has produced exceptional exposures of diverse structures and basin deposits recording the disruptions. Here we provide an overview of the results of studies of these features that started in earnest in 1934 when Chester Longwell began assessing the geology of the reservoir floor prior to impoundment of Lake Mead. The analysis was reinvigorated in the 1970s and early 1980s with geological mapping and structural and stratigraphic studies by Ernie Anderson and Bob Bohannon, as well as geochemical and volcanological studies by Gene Smith and his students, and has culminated in numerous subsequent studies.

Regional stratigraphic units and structural features of the Lake Mead region are presented as a 1:250,000 scale map, and as a Geographic Information System database. The map, which was compiled from existing geologic maps of various scales, depicts geologic units, bedding and foliation attitudes, faults and folds. Units and structural features were generalized to highlight the regional stratigraphic and tectonic aspects of the geology of the Lake Mead region. This map was prepared in support of the papers presented in this volume, Special Paper 463, as well as to facilitate future investigations in the region. Stratigraphic units exposed within the area record 1800 million years of geologic history and include Proterozoic crystalline rocks, Paleozoic and Mesozoic sedimentary rocks, Mesozoic plutonic rocks, Cenozoic volcanic and intrusive rocks, sedimentary rocks and surficial deposits. Following passive margin sedimentation in the Paleozoic and Mesozoic, late Mesozoic (Sevier) thrusting and Late Cretaceous and early Tertiary compression produced major folding, reverse faulting, and thrust faulting in the Basin and Range, and resulted in regional uplift and monoclinal folding in the Colorado Plateau. Cenozoic extensional deformation, accompanied by sedimentation and volcanism, resulted in large-magnitude high- and low-angle normal faulting and strike-slip faulting in the Basin and Range; on the Colorado Plateau, extension produced north-trending high-angle normal faults. The latest history includes integration of the Colorado River system, dissection, development of alluvial fans, extensive pediment surfaces, and young faulting.