ABSTRACT Tuff beds (volcanic ash beds and tuffs) have been known in the Miocene Monterey and Modelo Formations since they were initially described nearly 100 yr ago. Yet, these tephra layers have remained largely ignored. The ages and correlation of the Monterey and Modelo Formations are predominantly based on associated biostratigraphy. Here, we combined tephrochronology and biostratigraphy to provide more precise numerical age control for eight sedimentary sequences of the Monterey and Modelo Formations from Monterey County to Orange County in California. We correlated 38 tephra beds in the Monterey and Modelo Formations to 26 different dated tephra layers found mainly in nonmarine sequences in Nevada, Idaho, and New Mexico. We also present geochemical data for an additional 19 tephra layers in the Monterey and Modelo Formations, for which there are no known correlative tephra layers, and geochemical data for another 11 previously uncharacterized tephra layers in other areas of western North America. Correlated tephra layers range in age from 16 to 7 Ma; 31 tephra layers erupted from volcanic centers of the Snake River Plain, northern Nevada to eastern Idaho; 13 other tephra layers erupted from the Southern Nevada volcanic field; and the eruptive source is unknown for 12 other tephra layers. These tephra layers provide new time-stratigraphic markers for the Monterey and Modelo Formations and for other marine and nonmarine sequences in western North America. We identified tephra deposits of four supereruptions as much as 1200 km from the eruptive sources: Rainier Mesa (Southern Nevada volcanic field) and Cougar Point Tuff XI, Cougar Point Tuff XIII, and McMullen Creek (all Snake River Plain).

ABSTRACT The structure and stratigraphy of the Miocene formations east of San Francisco Bay have been described in multiple studies for over a century. We integrated the results of past investigations and provide new data that improve understanding of formation age, the timing of deformation, and the amount of dextral displacement on selected faults. New geologic mapping and better age control show that formations previously inferred to be separate units of different ages are correlative, and new names are proposed for these units. Miocene structures associated with the development of the San Andreas transform system exerted significant control on Miocene deposition in the East Bay area. The developing structure created five distinct stratigraphic sections that are differentiated on the basis of differences in the stratigraphic sequence, lithology, and age. The stratigraphic changes are attributed to significant dextral displacement, syndepositional faulting, and distal interfingering of sediment from tectonically elevated source areas. New stratigraphic evaluations and age control show that prior to ca. 6 Ma, the developing fault system created local tectonically induced uplift as well as spatially restricted subbasins. Regional folding did not occur until after 6 Ma. Past evaluations have inferred significant dextral displacement on some of the faults in the East Bay. The spatial relationships between unique conglomerate clasts and known source areas, as well as the distribution of well-dated and unique tuffs, suggest that dextral displacement on some faults in the East Bay is less than previously reported.

ABSTRACT We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

ABSTRACT We used geologic mapping, tephrochronology, and 40 Ar/ 39 Ar dating to describe evidence of a ca. 3.5 Ma pluvial lake in Eureka Valley, eastern California, that we informally name herein Lake Andrei. We identified six different tuffs in the Eureka Valley drainage basin, including two previously undescribed tuffs: the 3.509 ± 0.009 Ma tuff of Hanging Rock Canyon and the 3.506 ± 0.010 Ma tuff of Last Chance (informal names). We focused on four Pliocene stratigraphic sequences. Three sequences are composed of fluvial sandstone and conglomerate, with basalt flows in two of these sequences. The fourth sequence, located ~1.5 km south of the Death Valley/Big Pine Road along the western piedmont of the Last Chance Range, included green, fine-grained, gypsiferous lacustrine deposits interbedded with the 3.506 Ma tuff of Last Chance that we interpret as evidence of a pluvial lake. Pluvial Lake Andrei is similar in age to pluvial lakes in Searles Valley, Amargosa Valley, Fish Lake Valley, and Death Valley of the western Great Basin. We interpret these simultaneous lakes in the region as indirect evidence of a significant glacial climate in western North America during marine isotope stages Mammoth/Gilbert 5 to Mammoth 2 (MIS MG5/M2) and a persistent Pacific jet stream south of 37°N.

We describe the structure of the eastern Española Basin and use stratigraphic and stratal attitude data to interpret its tectonic development. This area consists of a west-dipping half graben in the northern Rio Grande rift that includes several intrabasinal grabens, faults, and folds. The Embudo–Santa Clara–Pajarito fault system, a collection of northeast- and north-striking faults in the center of the Española Basin, defines the western boundary of the half graben and was active throughout rifting. Throw rates near the middle of the fault system (i.e., the Santa Clara and north Pajarito faults) and associated hanging-wall tilt rates progressively increased during the middle Miocene. East of Española, hanging-wall tilt rates decreased after 10–12 Ma, coinciding with increased throw rates on the Cañada del Almagre fault. This fault may have temporarily shunted slip from the north Pajarito fault during ca. 8–11 Ma, resulting in lower strain rates on the Santa Clara fault. East of the Embudo–Santa Clara–Pajarito fault system, deformation of the southern Barrancos monocline and the Cañada Ancha graben peaked during the early–middle Miocene and effectively ceased by the late Pliocene. The north-striking Gabeldon faulted monocline lies at the base of the Sangre de Cristo Mountains, where stratal dip relations indicate late Oligocene and Miocene tilting. Shifting of strain toward the Embudo–Santa Clara–Pajarito fault system culminated during the late Pliocene–Quaternary. Collectively, our data suggest that extensional tectonism in the eastern Española Basin increased in the early Miocene and probably peaked between 14–15 Ma and 9–10 Ma, preceding and partly accompanying major volcanism, and decreased in the Plio-Pleistocene.

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.

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Full article available in PDF version.