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Abstract Cambrian and Ordovician shelf, platform, and basin rocks are present in Sonora, México, and southern Arizona and were deposited on the southwestern continental margin of North America (Laurentia). Cambrian and Ordovician rocks in Sonora, México, are mostly exposed in scattered outcrops in the northern half of the state. Their discontinuous nature results from extensive Quaternary and Tertiary surficial cover, from Tertiary and Mesozoic granitic batholiths in western Sonora, and from widespread Tertiary volcanic deposits in the Sierra Madre Occidental in eastern Sonora. Cambrian and Ordovician shelf rocks were deposited as part of the southern Laurentian miogeocline on the southwestern continental margin of North America. Lower Cambrian shelf units in Sonora consist mainly of quartzite, siltstone, and silty limestone; limestone increases upward in the sequence. Middle Cambrian shelf rocks consist mostly of limestone, dolostone, and siltstone. Upper Cambrian shelf rocks are sparse in Sonora; where present, they consist chiefly of siltstone and minor limestone. Cambrian shelf rocks display subtle facies changes from west to east across Sonora. In northwestern Sonora, these rocks attain their maximum thickness and may represent the Early Cambrian shelf margin. At the Sierra Agua Verde section, 110 km (68 mi) east of Hermosillo, these rocks thin, have greater proportions of clastic material, and were probably deposited in an inner-shelf setting. A major unconformity is present near the base of the Cambrian in Sonora and is similar to the Sauk I unconformity in the Wood Canyon Formation in Nevada and California. The top of the Cambrian is transitional with overlying Ordovician strata. Cambrian cratonic platform rocks are exposed in northern Sonora and southern Arizona and include the Middle Cambrian Bolsa Quartzite and Middle and Upper Cambrian Abrigo Limestone. The most complete sections of Ordovician shelf rocks in Sonora are 50 km (31 mi) northwest of Hermosillo. In these sections, the Lower Ordovician is characterized by intraclastic limestone, siltstone, shale, and chert. The Middle Ordovician is mostly silty limestone and quartzite, and the Upper Ordovician is cherty limestone and some argillaceous limestone. A major disconformity separates the Middle Ordovician quartzite from the overlying Upper Ordovician carbonate rocks and is similar to the disconformity between the Middle and Upper Ordovician Eureka Quartzite and Upper Ordovician Ely Springs Dolomite in Nevada and California. In parts of northwestern Sonora, Ordovician rocks are disconformably overlain by Upper Silurian rocks. Northeastward in Sonora and Arizona, toward the craton, Ordovician rocks are progressively truncated by a major onlap unconformity and are overlain by Devonian rocks. Except in local areas, Ordovician rocks are generally absent in cratonic platform sequences in northern Sonora and southern Arizona.

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.

The Lost Basin Range in the eastern Lake Mead domain consists of Proterozoic rocks that bound the west side of the Grand Wash Trough. Exhumation of the Proterozoic rocks of the Lost Basin Range occurred from ca. 18 to 15 Ma based on seven apatite fission-track ages that range from 20 to 15 Ma. The Lost Basin Range fault lies along the west side of the Lost Basin Range and steps to the east to the southern end of the Wheeler fault, which then runs north for 60 km, where it joins the Grand Wash fault. The geometry of the southern Wheeler–Lost Basin Range fault system is that of a relay ramp between two, west-dipping, high-angle normal faults. The intervening area of the fault step over, Gregg Basin, is interpreted as a relay ramp basin. New interpreted ages from stratigraphic units on the north and east sides of the Lost Basin Range integrated with existing structural data from the eastern Lake Mead domain reveal that faulting, sedimentation, and tilting of hanging-wall and footwall blocks along the southern Wheeler–Lost Basin Range fault system began by 15.3 Ma. Sedimentation continued until after 13 Ma along the southeastern Lost Basin Range, while the age of continuing sedimentation in Gregg Basin is poorly constrained. A paleocanyon in the footwall of the southern Wheeler fault filled with conglomerate and minor breccia between ca. 15.3 and ca. 14 Ma and then overtopped to the south to cover the Paleozoic rocks of south Wheeler Ridge. The Paleozoic strata of the south Wheeler Ridge area tilted east 20°–30° more than the Miocene strata that overlie them, and therefore this tilting occurred before ca. 14 Ma. Upward-decreasing (fanning) bedding attitudes in the overlapping Miocene conglomerate indicate that Paleozoic strata were being tilted along with the Miocene strata by ca. 14 Ma. Gentle (5° and less) east dips in the lower beds of the Hualapai Limestone above and east of the paleocanyon suggest that most tilting in the western Grand Wash Trough ceased by ca. 11 Ma. The lower conglomerate of Gregg Basin lies below, and interfingers with, the limestone of Gregg Basin, which is undated but correlates with the 11–7 Ma Hualapai Limestone in the adjacent Grand Wash Trough. The syncline in upper Gregg Basin strata is linked spatially to the Wheeler and Lost Basin Range faults and indicates that these faults were likely active at 11–7 Ma. The two faults appear to cut the Gregg Basin limestone, and therefore post–7 Ma fault activity at lower rates is likely.

The southeastern margin of the Colorado Plateau (CP) lies in southwestern New Mexico and southeastern Arizona. It is defined as the boundary between the CP, the Rio Grande rift, and the Basin and Range (BRP) provinces. Along its western and southern margins, the CP is physically distinguishable from the BRP by major escarpments. Because of a thick cover of volcanic material associated with the Mogollon-Datil volcanic field (MDVF), no such demarcation exists in the study area. To accurately determine lithospheric structure in this area, two seismic refraction lines that intersect within the MDVF were studied. In addition, gravity data along these lines were simultaneously analyzed. Based on these data, velocity and density profiles were created via an iterative forward modeling processes. Major crustal boundaries for each paired gravity/refraction profile were constrained to agree at the point of intersection and with other geological and geophysical control. Structurally, the CP margin appears as an abrupt northward deepening of the Moho just south of Datil, New Mexico. This lies beneath a major low density-low velocity upper crustal body, interpreted to be a plutonic complex approximately 230 by 150 by 6 km. It is very likely that this complex provided the source material for the MDVF. Stoping appears to be the vehicle for emplacement of this body, implying a major crustal reorganization in middle Tertiary time.

Geochemical and isotopic data obtained from three bimodal basalt-rhyolite fields in west-central Arizona point to significant variation in the sources of the siliceous products despite petrological similarities between them. The studied fields, which straddle the boundary between the Basin and Range and Colorado Plateau physiographic provinces, include Castaneda Hills (CH) in the Basin and Range, Kaiser Spring (KS) in the Transition Zone, and Mount Floyd (MF) in the Colorado Plateau. Two types of rhyolite (high-silica [HSR] and low-silica [LSR]) occur as lavas in the KS and MF fields, whereas all analysed CH samples are HSR. These lavas generally post-date low-angle extension of the Basin and Range crust and become generally younger towards the Colorado Plateau. Our isotopic data illustrate that the CH rhyolites require a source with comparatively radiogenic Sr, that most rhyolites require a source with comparatively unradiogenic 206 Pb/ 204 Pb, and that the MF–HSR require a source with comparatively radiogenic Nd and 206 Pb/ 204 Pb. The isotopic data clearly indicate that the western Arizona rhyolites contain a large crustal component. Importantly, the Pb isotopic compositions of the rhyolites illustrate the transition between two crustal provinces identified by other workers on the basis of Nd and Pb isotopic studies of Proterozoic granites. The Sr, Nd, and Pb isotopic compositions of our rhyolites can be modelled by mixing a basaltic component with three hypothetical crustal end-members which themselves may be formed by mixing two crustal components. One crustal component, which is characterised by unradiogenic Pb and Sr, may be similar to the mafic gneiss xenoliths of Tule Tank on the Colorado Plateau. The second crustal component, which has more radiogenic Sr and Pb, may be similar to the Proterozoic Fenner Gneiss of southeastern California. This conceptual model indicates an apparent increase in the contribution of the depleted Tule Tank source from the Basin and Range onto the Colorado Plateau and accommodates the notion that there is an underlying consistency to the petrogeneses of the western Arizona rhyolites as suggested by their chemistry and mineralogy. Although the CH and KS lavas require only one crustal end-member, the MF–HSR and LSR require two isotopically distinct sources. This difference may be attributed to the homogenisation of diverse crustal lithologies by ductile stretching during crustal extension.