We used tephrochronology for upper Neogene deposits in the Española Basin and the adjoining Jemez Mountains volcanic field in the Rio Grande rift, northern New Mexico, to correlate key tephra strata in the study area, identify the sources for many of these tephra, and refine the maximum age of an important stratigraphic unit. Electron-microprobe analyses on volcanic glass separated from 146 pumice-fall, ash-fall, and ash-flow tephra units and layers show that they are mainly rhyolites and dacites. Jemez Mountains tephra units range in age from Miocene to Quaternary. From oldest to youngest these are: (1) the Canovas Canyon Rhyolite and the Paliza Canyon Formation of the lower Keres Group (ca. <12.4–7.4 Ma); (2) the Peralta Tuff Member of the Bearhead Rhyolite of the upper Keres Group (ca. 6.96–6.76 Ma); (3) Puye Formation tephra layers (ca. 5.3–1.75 Ma); (4) the informal San Diego Canyon ignimbrites (ca. 1.87–1.84 Ma); (5) the Otowi Member of the Bandelier Tuff, including the basal Guaje Pumice Bed (both ca. 1.68–1.61 Ma); (6) the Cerro Toledo Rhyolite (ca. 1.59–1.22 Ma); (7) the Tshirege Member of the Bandelier Tuff, including the basal Tsankawi Pumice Bed (both ca. 1.25–1.21 Ma); and (8) the El Cajete Member of the Valles Rhyolite (ca. 60–50 ka). The Paliza Canyon volcaniclastic rocks are chemically variable; they range in composition from dacite to dacitic andesite and differ in chemical composition from the younger units. The Bearhead Rhyolite is highly evolved and can be readily distinguished from the younger units. Tuffs in the Puye Formation are dacitic rather than rhyolitic in composition, and their glasses contain significantly higher Fe, Ca, Mg, and Ti, and lower contents of Si, Na, and K. We conclude that the Puye is entirely younger than the Bearhead Rhyolite and that its minimum age is ca. 1.75 Ma. The San Diego Canyon ignimbrites can be distinguished from all members of the overlying Bandelier Tuff on the basis of Fe and Ca. The Cerro Toledo tephra layers are readily distinguishable from the overlying and underlying units of the Bandelier Tuff primarily by lower Fe and Ca contents. The Tshirege and Otowi Members of the Bandelier Tuff are difficult to distinguish from each other on the basis of electron-microprobe analysis of the volcanic glass; the Tshirege Member contains on average more Fe than the Otowi Member. Tephra layers in the Española Basin that correlate to the Lava Creek B ash bed (ca. 640 ka) and the Nomlaki Tuff (Member of the Tuscan and Tehama Formations, ca. 3.3 Ma) indicate how far tephra from these eruptions traveled (the Yellowstone caldera of northwestern Wyoming and the southern Cascade Range of northern California, respectively). Tephra layers of Miocene age (16–10 Ma) sampled from the Tesuque Formation of the Santa Fe Group in the Española Basin correlate to sources associated with the southern Nevada volcanic field (Timber Mountain, Black Mountain, and Oasis Valley calderas) and the Snake River Plain–Yellowstone hot spot track in Idaho and northwestern Wyoming. Correlations of these tephra layers across the Santa Clara fault provide timelines through various stratigraphic sections despite differences in stratigraphy and lithology. We use tephra correlations to constrain the age of the base of the Ojo Caliente Sandstone Member of the Tesuque Formation to 13.5–13.3 Ma.

Late Miocene and early Pliocene sediments exposed along the lower Colorado River near Laughlin, Nevada, contain evidence that establishment of this reach of the river after 5.6 Ma involved flooding from lake spillover through a bedrock divide between Cottonwood Valley to the north and Mohave Valley to the south. Lacustrine marls interfingered with and conformably overlying a sequence of post–5.6 Ma fine-grained valley-fill deposits record an early phase of intermittent lacustrine inundation restricted to Cottonwood Valley. Limestone, mud, sand, and minor gravel of the Bouse Formation were subsequently deposited above an unconformity. At the north end of Mohave Valley, a coarse-grained, lithologically distinct fluvial conglomerate separates subaerial, locally derived fan deposits from subaqueous deposits of the Bouse Formation. We interpret this key unit as evidence for overtopping and catastrophic breaching of the paleodivide immediately before deep lacustrine inundation of both valleys. Exposures in both valleys reveal a substantial erosional unconformity that records drainage of the lake and predates the arrival of sediment of the through-going Colorado River. Subsequent river aggradation culminated in the Pliocene between 4.1 and 3.3 Ma. The stratigraphic associations and timing of this drainage transition are consistent with geochemical evidence linking lacustrine conditions to the early Colorado River, the timings of drainage integration and canyon incision on the Colorado Plateau, the arrival of Colorado River sand at its terminus in the Salton Trough, and a downstream-directed mode of river integration common in areas of crustal extension.

Abstract A detailed record of the late Cenozoic history of the lower Colorado River can be inferred from alluvial and (likely) lacustrine stratigraphy exposed in dissected alluvial basins below the mouth of the Grand Canyon. Numerous sites in Mohave, Cottonwood, and Detrital valleys contain stratigraphic records that directly bear on the mode, timing, and consequences of the river’s inception and integration in the latest Miocene–early Pliocene and its subsequent evolution through the Pleistocene. This field trip guide describes and illustrates many of these key stratigraphic relationships and, in particular, highlights evidence that supports the hypothesis of cascading lake-overflow as the principal formative mechanism of the river’s course downstream from the Grand Canyon.