Charles E. Meyer

Correlation of upper Cenozoic tephra layers between sediments of the western United States and eastern Pacific Ocean and comparison with biostratigraphic and magnetostratigraphic age data

Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei )], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Implications of the northwestwardly younger age of the volcanic rocks of west-central California

Erosional remnants of volcanic fields in west-central California form a linear northwest-trending belt growing younger in age to the northwest. Major fields within the belt are represented by the Neenach Volcanics, Pinnacles Volcanic Formation, Quien Sabe Volcanics, volcanic rocks in the Berkeley Hills, Tolay Volcanics, Sonoma Volcanics, and Clear Lake Volcanics. Dispersion in the age-distance relation is reduced by restoration of inferred offsets on transecting right-lateral fault systems. The offsets include 115 km on the San Gregorio–Hosgri fault, 314 km on the San Andreas fault, 43 km on the Hayward-Rodgers Creek fault, and 28 km on the Carneros-Franklin-Sunol-Calaveras fault. On the basis of the age and restored position of the volcanic rocks, we judge that the locus of initial active volcanism migrated northwestward ∼3.75 cm/yr from 25 to 12 Ma, and ∼1.35 cm/yr from 12 Ma to the present. The volcanic rocks apparently formed south of the northwardly retreating edge of the subducted part of the Juan de Fuca plate, corroborating one corollary of a published model of an expanding hole in the subducted Farallon-Juan de Fuca-Cocos plate. The present position of the locus of melting at Clear Lake, California, requires substantial overthrusting of the Juan de Fuca plate by the Pacific plate, as was postulated on the basis of foreshortening of magnetic anomalies in the Gorda basin. The change in rate of northwestward migration ∼12 Ma reflects a change in spreading direction of the Juan de Fuca plate vis-a-vis the Pacific plate, previously recognized from changes in orientation of oceanic magnetic anomalies. From the migration rates, it can be inferred that the relative movement between the Pacific plate and the westernmost fringe of the North American plate averaged ∼3.5 cm/yr from 27 m.y. ago to the present.

Correlation of the Rockland ash bed, a 400,000-year-old stratigraphic marker in northern California and western Nevada, and implications for middle Pleistocene paleogeography of central California

Abstract Outcrops of an ash bed at several localities in northern California and western Nevada belong to a single air-fall ash layer, the informally named Rockland ash bed, dated at about 400,000 yr B.P. The informal Rockland pumice tuff breccia, a thick, coarse, compound tephra deposit southwest of Lassen Peak in northeastern California, is the near-source equivalent of the Rockland ash bed. Relations between initial thickness of the Rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the Mazama ash from Crater Lake, Oregon. Identification of the Rockland tephra allows temporal correlation of associated middle Pleistocene strata of diverse facies in separate depositional basins. Specifically, marine, littoral, estuarine, and fluvial strata of the Hookton and type Merced formations correlate with fluvial strata of the Santa Clara Formation and unnamed alluvium of Willits Valley and the Hollister area, in northwestern and west-central California, and with lacustrine beds of Mohawk Valley, fluvial deposits of the Red Bluff Formation of the eastern Sacramento Valley, and fluvial and glaciofluvial deposits of Fales Hot Spring, Carson City, and Washoe Valley areas in northeastern California and western Nevada. Stratigraphic relations of the Rockland ash bed and older tephra layers in the Great Valley and near San Francisco suggest that the southern Great Valley emerged above sea level about 2 my ago, that its southerly outlet to the ocean was closed sometime after about 2 my ago, and that drainage from the Great Valley to the ocean was established near the present, northerly outlet in the vicinity of San Francisco Bay about 0.6 my ago.

Correlation of Pliocene and Pleistocene tephra layers between the Turkana Basin of East Africa and the Gulf of Aden

Abstract Electron-microprobe analyses of glass shards from volcanic ash in Pliocene and Pleistocene deep-sea sediments in the Gulf of Aden and the Somali Basin demonstrate that most of the tephra layers correlate with tephra layers known on land in the Turkana Basin of northern Kenya and southern Ethiopia. Previous correlations are reviewed, and new correlations proposed. Together these data provide correlations between the deep-sea cores, and to the land-based sections at eight levels ranging in age from about 4 to 0.7 Ma. Specifically, we correlate the Moiti Tuff (⩽4.1 Ma) with a tephra layer at 188.6 m depth in DSDP hole 231 and with a tephra layer at 150 m depth in DSDP hole 241, the Wargolo Tuff with a tephra layer at 179.7 m in DSDP Hole 231 and with a tephra layer at 155.3 m depth in DSDP Hole 232, the Lomogol Tuff (defined here) with a tephra layer at 165 m in DSDP Hole 232A, the Lokochot Tuff with a tephra layer at 140.1 m depth in DSDP Hole 232, the Tulu Bor Tuff with a tephra layer at 160.8 m depth in DSDP Hole 231, the Kokiselei Tuff with a tephra layer at 120 m depth in DSDP Hole 231 and with a tephra layer at 90.3 m depth in DSDP Hole 232, the Silbo Tuff (0.74 Ma) with a tephra layer at 35.5 m depth in DSDP Hole 231 and possibly with a tephra layer at 10.9 m depth in DSDP Hole 241. We also present analyses of other tephra from the deep sea cores for which correlative units on land are not yet known. The correlated tephra layers provide eight chronostratigraphic horizons that make it possible to temporally correlate paleoecological and paleoclimatic data between the terrestrial and deep-sea sites. Such correlations may make it possible to interpret faunal evolution in the Lake Turkana basin and other sites in East Africa within a broader regional or global paleoclimatic context.

Surface faulting and paleoseismic history of the 1932 Cedar Mountain earthquake area, west-central Nevada, and implications for modern tectonics of the Walker Lane

The 1932 Cedar Mountain earthquake (M s 7.2) was one of the largest historical events in the Walker Lane region of western Nevada, and it produced a complicated strike-slip rupture pattern on multiple Quaternary faults distributed through three valleys. Primary, right-lateral surface ruptures occurred on north-striking faults in Monte Cristo Valley; small-scale lateral and normal offsets occurred in Stewart Valley; and secondary, normal faulting occurred on north-northeast–striking faults in the Gabbs Valley epicentral region. A reexamination of the surface ruptures provides new displacement and fault-zone data: maximum cumulative offset is estimated to be 2.7 m, and newly recognized faults extend the maximum width and end-to-end length of the rupture zone to 17 and 75 km, respectively. A detailed Quaternary allostratigraphic chronology based on regional alluvial-geomorphic relationships, tephrochronology, and radiocarbon dating provides a framework for interpreting the paleoseismic history of the fault zone. A late Wisconsinan alluvial-fan and piedmont unit containing a 32–36 ka tephra layer is a key stratigraphic datum for paleoseismic measurements. Exploratory trenching and radiocarbon dating of tectonic stratigraphy provide the first estimates for timing of late Quaternary faulting along the Cedar Mountain fault zone. Three trenches display evidence for six faulting events, including that in 1932, during the past 32–36 ka. Radiocarbon dating of organic soils interstratified with tectonically ponded silts establishes best-fit ages of the pre-1932 events at 4, 5, 12, 15, and 18 ka, each with ±2 ka uncertainties. On the basis of an estimated cumulative net slip of 6–12 m for the six faulting events, minimum and maximum late Quaternary slip rates are 0.2 and 0.7 mm/yr, respectively, and the preferred rate is 0.4–0.5 mm/yr. The average recurrence (interseismic) interval is 3600 yr. The relatively uniform thickness of the ponded deposits suggests that similar-size, characteristic rupture events may characterize late Quaternary slip on the zone. A comparison of event timing with the average late Quaternary recurrence interval indicates that slip has been largely regular (periodic) rather than temporally clustered. To account for the spatial separation of the primary surface faulting in Monte Cristo Valley from the epicenter and for a factor-of-two-to-three disparity between the instrumentally and geologically determined seismic moments associated with the earthquake, we hypothesize two alternative tectonic models containing undetected subevents. Either model would adequately account for the observed faulting on the basis of wrench-fault kinematics that may be associated with the Walker Lane. The 1932 Cedar Mountain earthquake is considered an important modern analogue for seismotectonic modeling and estimating seismic hazard in the Walker Lane region. In contrast to most other historical events in the Basin and Range province, the 1932 event did not occur along a major range-bounding fault, and no single, throughgoing basement structure can account for the observed rupture pattern. The 1932 faulting supports the concept that major earthquakes in the Basin and Range province can exhibit complicated distributive rupture patterns and that slip rate may not be a reliable criterion for modeling seismic hazard.

Repeating waveform initiated by a 180–190 ka geomagnetic excursion in western North America: Implications for field behavior during polarity transitions and subsequent secular variation

New paleomagnetic, lithologic, and stratigraphic data are presented from the sediments of Lake Chewaucan in the Summer Lake Basin, Oregon. The new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality. A complex, yet distinct, waveform is observed in all three components of the paleomagnetic vector. The waveform begins as the 180–190 ka Pringle Falls/Long Valley/Summer Lake II geomagnetic excursion and continues for two cycles after the excursion, until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6/5e boundary. The waveforms directional morphology in virtual geomagnetic pole (VGP) space is defined by two clockwise loops followed by a distinctive counterclockwise, clockwise, counterclockwise looping sequence. The VGP paths of the two cycles after the excursion are rotated 180° about Earths spin axis with respect to the VGP paths of the excursion cycle. The waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion. The average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion. This observation indicates that excursion-initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions. Because of the similarities noted previously between this excursion and full polarity transitions (Trie et al., 1991), our new observations constrain models for a wide range of field behavior including polarity transitions, excursions, and secular variation.

Pseudotachylite from the Vredefort Ring, South Africa, and the origins of some lunar breccias

Pseudotachylite veins in the Vredefort impact structure occur in rocks of widely varying composition, and in stratigraphic units as thin as 10 m. The compositions of the pseudotachylites, however, are the same as those of the rocks in which they occur, with some systematic variations, thus indicating in situ formation. The textures of the pseudotachylites indicate that they formed predominantly by mechanical processes, but temperatures were locally high enough to cause thermal metamorphism and possibly fusion. The host rocks of the pseudotachylites are extensively mylonitized and show selective crushing of different mineral species. Greater resistance of quartz and feldspar to crushing partly, but not entirely, explains the systematic differences in composition between host rock and pseudotachylite. Shock features in the host rocks are generally of a low order but indicate pressures far in excess of those reasonably to be expected in crustal rocks deformed by terrestrial processes. Characteristics of the pseudotachylites, breccias formed by one impact, provide simple models for lunar breccias that may have undergone more than one cycle of brecciation.

The Mount Edgecumbe tephra deposits, a marker horizon in southeastern Alaska near the Pleistocene-Holocene boundary

Late Pleistocene tephra deposits found from Sitka to Juneau and Lituya Bay are assigned to a source at the Mount Edgecumbe volcanic field, based on similarity of glass compositions to nearvent deposits and on thinning away from Kruzof Island. The sequence of near-vent layers is basaltic andesite and andesite at the base, rhyolite, and mixed dacite and rhyolite on top. The only breaks in the tephra sequence are two 1-mm-thick silt partings in a lake-sediment core, indicating a depositional interval from basaltic andesite to dacite of no more than about a millennium. Tephra deposits at sites >30 km from the vent are solely dacite and rhyolite and are 10,600 to 11,400 14C yr old based on interpretation of 18 radiocarbon ages, including 5 by accelerator mass spectrometry (AMS). Basaltic andesite and andesite deposits nearer the vent are as much as 12,000 yr old. Discrepancy among radiocarbon ages of upland tephra deposits provisionally correlated as the same grainfall is resolvable within ±2 σ of analytical uncertainty. Comparison of bulk and AMS ages in one sediment core indicates a systematic bias of +600 to +1100 yr for the bulk ages; correlation of tephra deposits among upland and lacustrine sites implies an additional discrepancy of 200–400 yr between upland (relatively too young) and lacustrine ages. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic marker that serves to emphasize the uncertainty in dating biogenic material from southeastern Alaska.

Fission-track age (400,000 yr) of the Rockland tephra, based on inclusion of zirco grains lacking fossil fission tracks

Abstract A zircon fission-track age of about 400,000 yr B.P. has been determined for the Rockland tephra, a widespread pyroclastic layer in northern California and western Nevada. New ages of zircon separates from both proximal and distal exposures of this layer range from 370,000 to 460,000 yr; ages of the best material provide a narrower range, from 370,000 yr for unwelded ash-flow tuff to 420,000 yr for distal air-fall ash that appears to be uncontaminated by clastic detritus or xenocrysts. Detrital or xenocrystic grains in the ash-flow tuff may have been annealed during emplacement and cooling of the tuff. Detrital and xenocrystic zircons are identified on the basis of their physical characteristics and distinctly older ages. Independent stratigraphic and magnetostratigraphic data constrain the age of the Rockland tephra between 300,000 and 600,000 yr, a range that is compatible with the fission-track age. Zircon grains containing no spontaneous (fossil) tracks are regarded as part of the normal population of comagmatic grains because maximum ages calculated for these grains form a population that mimics the distribution of ages of individual zircon grains that contain fossil tracks; modal ages of both groups fall between 250,000 and 500,000 yr. Induced fission tracks from grains that lack fossil tracks are included in the age calculations, resulting in significantly younger and more coherent dates than would result if these tracks had been omitted, especially those of the finer-grained distal samples.

A late Pliocene to middle Pleistocene pluvial lake in Fish Lake Valley, Nevada and California

The question of whether a pluvial lake existed in Fish Lake Valley, Nevada and California, has been debated for more than 100 yr. New stratigraphic evidence indicates that a lake did exist in this valley at intervals during late Pliocene to middle Pleistocene time. This lake may have drained northward, or it may have been periodically contiguous with a pluvial lake to the north in Columbus Salt Marsh. Proof of the existence of this lake, informally named Pluvial Lake Rennie, is derived from three principal outcrops of shallow-water deposits, two outcrops of deep-water deposits, and several drilling logs. The deposits contain beds of silicic tephra, which provide age control. On the basis of thickness, grain size, major-oxide chemistry of glass shards, and paleomagnetism, three of the shallow-water deposits, including deltaic(?), beach, and siliceous hot-spring sediments, consist mainly of Bishop ash derived from the 0.77 Ma eruption of the Long Valley caldera. A fourth shallow-water deposit(?) is associated with ∼1 Ma Glass Mountain tephra beds. The exposed deep-water deposits consist of green claystone, siltstone, and fine-grained sandstone containing tephra derived from the eruptions of the ∼2.1 Ma tuff of Taylor Canyon and the ∼2.0 Ma Huckleberry Ridge Tuff. The drilling logs record numerous thick beds of clay and sandy clay inferred to be deep-water lacustrine deposits. Pluvial Lake Rennie fluctuated in size and depth beginning prior to 2 Ma and continuing until sometime alter 0.77 Ma. At about 0.77 Ma, the lake had a highstand at an elevation of ∼1,460 m, covered an area of 400-500 km2, and had a maximum depth of ∼250 m. The lake level dropped just after the eruption of the Bishop ash, but the lake may have persisted at a lower level until ∼0.5 Ma. No large, long-lived lake existed in Fish Lake Valley in late Pleistocene time, probably due to the increasing rain-shadow effect caused by the relative uplift of the White Mountains and Sierra Nevada in the Pleistocene. These results indicate that the late middle to late Pleistocene history of Pluvial Lake Rennie is similar to that of Lake Tecopa but is quite different from those of Lake Lahontan and Searles Lake.