The Ibex Hollow Tuff, 12.08 ± 0.03 Ma (40Ar/39Ar), is a widespread tephra layer erupted from the Bruneau-Jarbidge volcanic field of southern Idaho. Tephra from this eruption was deposited across much of western and central North America and adjacent ocean areas. We identified the Ibex Hollow Tuff at Trapper Creek, Idaho, near its eruption site, and at 15 distal sites, from the Pacific Ocean to the Gulf of Mexico, by the chemical composition of its glass shards, using electron-microprobe analysis, instrumental neutron activation analysis, and laser-ablation–inductively coupled plasma–mass spectrometry. By these methods, we distinguished the Ibex Hollow Tuff from overlying and underlying tephra layers near its source and at distal sites. Fluvially reworked Ibex Hollow Tuff ash was transported by the ancestral Mississippi River drainage from the interior of the North American continent to the Gulf of Mexico, where it is present within an ~50-m-thick deposit in marine sediments in the subsurface. The minimum fallout area covered by the ash is ~2.7 million km2, with a minimum volume of ~800 km3, and potential dispersal farther to the north and northeast. The areal distribution for the Ibex Hollow Tuff is similar to that of the Lava Creek B (0.63 Ma) supereruption. The Ibex Hollow Tuff represents a unique chronostratigraphic marker allowing a synoptic view of paleoenvironments at a virtual moment in time across a large terrestrial and marine region. The Ibex Hollow Tuff is also an important marker bed for North American Land Mammal Ages, and it coincides with climatic cooling in the middle to late Miocene documented in marine cores.

The question of whether a pluvial lake existed in Fish Lake Valley, Nevada and California, has been debated for over 100 years. We have obtained stratigraphic evidence that a lake did exist in this valley at intervals during late Pliocene to middle Pleistocene time. This lake may have overflowed 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, rests primarily on four outcrops of shallow-water lacustnne sediments, two outcro~s of deepwater sediments, and drilling logs of sediments. The exposed and buried sedIments contain beds of silicic tephra, which provide age control. Based on thickness, grain size, and majoroxide chemistry of glass shards, three of the shallow-water deposits consist mainly of tephra that was most likely derived from the 0.74-Ma eruption of the Bishop Tuff. These three deposits include deltaic, beach, and siliceous hot-spring sediments. One outcrop of beach sand is underlain by lacustrine (?) sediments believed to be about 1 Ma. The exposed deep-water sediments 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, provisionally, of the -2.0-Ma Huckleberry Ridge Tuff. The drilling logs record numerous thick beds of clay and sandy clay, some containing beds interpreted to be volcanic ash; these clay beds are inferred to be deep-water lacustrine sediments. From the outcrops and drilling logs, the history of Pluvial Lake Rennie is as follows: (1) At around 2 Ma, the lake was dee~ enough in its northeastern part that clay was deposited. The lake level in early PleIstocene time is not known, but a lake probably eXIsted around 1 Ma. (2) At about 0.74 Ma, the lake had a high stand at an elevation of about 1440 m. The lake level must have dropped during or just after the eruption of the Bishop ash. (3) The lake may have persisted sporadically at a lower level until about 0.5 Ma, but no long-lived lake existed in Fish Lake Valley in late Pleistocene time. The late Pliocene and Pleistocene record of Pluvial Lake Rennie is reasonably parallel to that of Lake Tecopa, 200 km to the southeast.

Recent geologic mapping and dating of upper Tertiary and Quaternary deposits in Fish Lake Valley, northern Eureka Valley, and Deep Springs Valley, Nevada and California, provide information on the early history of motion on the northern end of the right-lateral oblique Furnace Creek fault zone (herein called the Fish Lake Valley fault zone) and other associated faults. These deposits are interpreted to suggest the following history: Faulting began on the southern end of the Fish Lake Valley fault zone about 8-12 Ma. During the late Miocene and Pliocene, uplift caused deposition of coarse angular gravel on the southeast side of the fault zone. This gravel graded laterally into a drainage system flowing south from the White Mountains into Eureka Valley; Deep Springs Valley did not exist. Motion on the Fish Lake Valley fault zone and associated faults created another depositional basin on the northeast side of the fault zone into which streams flowed from the northwest end of the Sylvania Mountains. Sometime after 0.74 Ma, streams draining from the White Mountains into Eureka Valley were defeated by motion on the bounding faults of Deep Springs Valley and either began to pond in Deep Springs Valley or were captured by streams flowing into Fish Lake valley. Estimated lateral and vertical slip rates on the Fish Lake Valley fault zone and associated faults vary both temporally and spatially. The post-late Miocene right-lateral slip rate in southern Fish Lake Valley is most likely about 4-6 mm/yr, whereas in northern Fish Lake Valley it is about 1-3 mm/yr. The post-Bishop-ash (0.74 Ma) vertical slip rates are much higher than the long-term slip rate. Post-0.74-Ma vertical slip rates range from 0.3 to 0.7 mm/yr on several parts of the fault zone, whereas poorly constrained post-late Miocene vertical slip rates range from 0.05 to 0.2 mm/yr. These relations suggest that slip rates increased markedly in the Quaternary. The post-0.74-Ma vertical slip rate at the northern end of the Fish Lake Valley fault zone is about half the rate measured at the southern end.

Numerical ages have been determined for a stratigraphic sequence of silicic tephra layers exposed at the Cowan Pumice Mine in Blind Spring Valley, near Benton Hot Springs, east-central California, as well as at Chalk Cliffs, north of Bishop, Calif. The tephra layers at these sites were deposited after eruptions from nearby sources, most of them from near Glass Mountain, and some from unknown sources. The ages were determined primarily by the laser-fusion 40Ar/39Ar method, mostly on sanidine feldspar; two were determined by conventional K-Ar analysis on obsidian clasts. These tephra layers, all underlying the Bishop ash bed and listed in order of concordant age and stratigraphic position, are: A table is presented. The above tephra layers were also petrographically examined and the volcanic glass shards of the layers were chemically analyzed using the electron microprobe and, for some samples, instrumental neutron activation analysis and X-ray fluorescence. The same types of chemical and petrographic analyses were conducted on stratigraphic sequences of tephra layers of suspected upper Pliocene and Pleistocene age in several past and present depositional basins within the region outside of Blind Spring Valley. Chemical characterization, combined with additional dates and with magnetostratigraphy of thick sections at two of the distal sites, allow correlation of the tephra layers at the Cowan Pumice Mine with layers present at the distal sites and provide age constraints for other intercalated tephra layers and sediments for which age data were previously lacking. The identification at several sections of the widespread Huckleberry Ridge ash bed, derived from the Yellowstone eruptive source area in Wyoming, as well as a new 40Ar/39Ar age on this ash bed from a proximal locality, provide additional age constraints to several of the distal sections. The dated or temporally bracketed distal units, in order of concordant age and stratigraphic position, are: A table is presented. At the Cowan Pumice Mine, only a partial section of the eruptive record is preserved, but the best materials for laser-fusion 40Ar/39Ar and other isotopic dating methods were obtained. In the more distal Willow Wash and Confidence Hills sections, both persistent depositional basins for most of late Pliocene time, more complete sections of upper Pliocene tephra layers were preserved. In the region of Glass Mountain, the tephra layers that make up each of the mapped and dated pyroclastic units are multiple and complex, but a progressive simplification of the stratigraphy away from the source area was observed for more distal sites in southern and southwestern California and in Utah. This progressive simplification is attributed to both variable explosiveness and magnitude of individual tephra eruptions, as well as to variable dispersal of the tephra by winds during an eruptive episode. Lake beds present at several localities in the western Great Basin (Fish Lake Valley, Nev.; the Waucoba Road area near Big Pine, Calif.; and Confidence Hills, Death Valley, Calif.) are dated between ∼2.15 and ∼2.04 Ma and indicate that wetter or colder conditions than present existed at these sites. Age and correlation data presented here provide a chronostratigraphic framework for studies of late Neogene stratigraphy, tectonics, and environmental change in the southwestern United States. Two Reunion magnetic events are bracketed between ∼2.18 and ∼2.17 Ma for the older (Reunion 1) and between ∼2.16 and ∼2.14 Ma for the younger (Reunion II). A third, older excursion associated with these two is at 2.22 Ma, but it does not reach reversed magnetization.

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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 waveform's 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 Earth's 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.

Late Cenozoic fault geometry, structure, paleoseismicity, and patterns of recent seismicity at two seismic zones along the Olympic-Wallowa lineament (OWL) of western Idaho, northeast Oregon, and southeast Washington indicate limited right-oblique slip displacement along multiple northwest-striking faults that constitute the lineament. The southern end of the OWL originates in the Long Valley fault system and western Snake River Plain in western Idaho, where it accommodates basin-and-range extension. Several kilometers west of the Long Valley fault system, post-middle Miocene displacement along the OWL has resulted in adjacent areas of uplift and subsidence.The OWL in northeast Oregon consists of a wide zone of northwest-striking faults and is associated with several large, inferred, pull-apart basins. The OWL then emerges from the Blue Mountain uplift as a much narrower zone of faults in the Columbia Plateau known as the Wallula fault zone (WFZ). Structural relationships in the WFZ strongly suggest that it is a right-slip extensional duplex. An ash horizon that correlates with the Mount St. Helens ''J'' composition (10.7 Ka) is displaced vertically 5 m by a normal fault along a segment of the WFZ. The intensity center for a damaging M = 6.1 earthquake in 1936 is located in the WFZ only a few kilometers northeast of this Holocene fault. Other youthful-appearing, strike-slip and thrust faults are located farther northwest along the OWL and may pose a seismic hazard to adjacent population centers and critical facilities.

High‐resolution paleomagnetic records from two sites near Pringle Falls, Oregon, are compared with similar records from Summer Lake, Oregon, ∼170 km to the southeast: Paoha Island, in Mono Lake, ∼660 km to the southeast and Benton Crossing, in Long Valley, approximately 700 km to the southeast, in east‐central California. The sequences at Pringle Falls contain a distinctive coarse pumice‐lapilli tephra layer which we have dated as 218±10 ka by 40Ar/39Ar step‐heating of plagioclase feldspar. Stratigraphically, this tephra is closely associated with a suite of several other tephra layers that bracket the interval studied paleomagnetically. Each tephra layer is distinguished by the unique chemical composition of its volcanic glass shards. The pumice layer dated at Pringle Falls is correlated with layers at three of the other localities. Using all the tephra layers, we can correlate the lake stratigraphic sequences and associated paleomagnetic records among the four distant localities. Additional age control is obtained from a fifth locality at Tulelake in northern California, where the stratigraphic interval of interest is bracketed between ∼171±43 and approximately 140 ka. Characteristics of the paleomagnetic records indicate virtually identical paleofield variation, particularly the geometry of a normal to normal (N‐N) geomagnetic polarity episode. The observed paleofield behavior resembles the Blake geomagnetic polarity episode, but is significantly older than the generally accepted age of the Blake episode. Either the age of the Blake episode is significantly underestimated, or the polarity episode documented here is older, perhaps the Jamaica episode, or is an as yet unreported episode. A corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths, a conclusion implying that a common mechanism is involved.

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 approximately 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 approximately 2.1 Ma tuff of Taylor Canyon and the approximately 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 after 0.77 Ma. At about 0.77 Ma, the lake had a highstand at an elevation of approximately 1,460 m, covered an area of 400-500 km2, and had a maximum depth of approximately 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 approximately 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.

Combined paleomagnetic and tephra chronologies of one of the most complete middle Pliocene through Holocene stratigraphic records yet recovered in western Noth America provide a reference section for much of northwestern North America and adjacent Pacific Ocean. Five long drill cores of lacustrine sediments at Tulelake, northern California, recovered a nearly continuous 331-m-thick record spanning the past 3 m.y. The Brunhes Normal-Polarity, Matuyama Reversed-Polarity, and Gauss Normal-Polarity Chronozones are recognized; within these, the Jaramillo, Olduvai, Reunion(?), and Kaena(?) Subchronozones are present. Six short stratigraphic intervals exhibit anomalous remanent inclinations that may record excursions and brief subchrons within the Brunhes and Matuyama Chronozones. Age estimates suggest correlation of five of the anomalous intervals with (1) one of the Biwa excursions at about 18,000 yr B.P., (2) the Mono Lake excursion at about 27,000 yr B.P., (3) the Blake Reversed-Polarity Subchron at about 114,000 yr B.P., (4) the Kamikatsura Normal-Polarity Subchron at about 850,000 yr B.P., and (5) the Cobb Mountain Normal-Polarity Subchron at about 1.10 Ma. Age of the sixth interval of anomalous inclination is broadly constrained between 117,000 and 180,000 yr B.P.Sixty-three individual tephra layers were characterized by electron-microprobe and X-ray fluorescence analyses of volcanic glass shards. Identified tephra of relatively well known age include (1) the basal airfall pumice at Llao Rock, 7015 yr B.P.; (2) the Trego Hot Springs Bed, 23,400 yr B.P.; (3) the Olema ash bed, between 55,000 and 75,000 yr B.P.; (4) the airfall pumice at Cloudcap Road ("Pumice Castle-like tephra 2"), about 120,000 yr B.P.; (5) the Rockland ash bed, about 410,000 yr B.P.; (6) the Lava Creek-B ash bed, 620,000 yr B.P.; (7) the Rio Dell ash bed, about 1.45 Ma; and (8) the Bear Gulch ash bed, about 1.9 Ma. A sedimentation-rate curve based on independently dated tephra and polarity reversals is used to infer age estimates of undated or previously unidentified ash beds. Some of these ash beds are found over large areas of the western United States and eastern Pacific Ocean basin and provide widespread horizons for correlation. Most of the tephra at Tulelake records eruptions from the nearby southern and central Cascade Range of Oregon and northern California, and the Medicine Lake Highland of northern California. Deposition took place during most of the past 3 m.y. within the Tulelake basin; notable periods of slow or sporadic accumulation, or erosion, occurred between about 620,000 and 200,000 yr B.P. and between about 2.5 and 2.1 Ma. Rapid deposition occurred during marine oxygen-isotope stage 6, between about 170 and 125 ka. Regional volcanism during the past 3 m.y. was markedly episodic, with notable volcanic activity from about 2.1 to 1.9 Ma and from 0.4 Ma to the present.

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.

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.

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.

We have identified ash beds in sediment cores of Clear Lake, California, by the chemistry of their volcanic glasses and petrography. These identifications enable us to correlate between cores, and to correlate three ash beds to several localities outside the Clear Lake basin where they have been isotopically dated or their ages estimated by stratigraphically bracketing dates. The three dated ash beds are ash bed 1 (Olema ash bed), estimated to be between 55 and 75 ka, in two deep cores CL-80-1 and CL-73-4, and two ash beds in core CL-80-1, ash bed 6 (Loleta ash bed), estimated to be...

Research Article| February 01, 1987 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 A. M. SARNA-WOJCICKI; A. M. SARNA-WOJCICKI 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar S. D. MORRISON; S. D. MORRISON 21308 Janes Road, Arcata, California 95521 Search for other works by this author on: GSW Google Scholar C. E. MEYER; C. E. MEYER 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. W. HILLHOUSE J. W. HILLHOUSE 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1987) 98 (2): 207–223. https://doi.org/10.1130/0016-7606(1987)98<207:COUCTL>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. M. SARNA-WOJCICKI, S. D. MORRISON, C. E. MEYER, J. W. HILLHOUSE; 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. GSA Bulletin 1987;; 98 (2): 207–223. doi: https://doi.org/10.1130/0016-7606(1987)98<207:COUCTL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

Research Article| May 01, 1985 Implications of the northwestwardly younger age of the volcanic rocks of west-central California KENNETH F. FOX, JR.; KENNETH F. FOX, JR. 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar ROBERT J. FLECK; ROBERT J. FLECK 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GARNISS H. CURTIS; GARNISS H. CURTIS 2Department of Geology and Geophysics, University of California at Berkeley, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar CHARLES E. MEYER CHARLES E. MEYER 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (5): 647–654. https://doi.org/10.1130/0016-7606(1985)96<647:IOTNYA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation KENNETH F. FOX, ROBERT J. FLECK, GARNISS H. CURTIS, CHARLES E. MEYER; Implications of the northwestwardly younger age of the volcanic rocks of west-central California. GSA Bulletin 1985;; 96 (5): 647–654. doi: https://doi.org/10.1130/0016-7606(1985)96<647:IOTNYA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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-à-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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Studies of sulfide vein mineralization at Point Delgada, California, suggest that the mineralizing process was intimately associated with the San Andreas fault system and/or plate-margin tectonism. The deposits occur along north-south to northeast-southwest-trending tension faults that intersect and cross a major northwest-trending fault previously considered to be a major trace of the San Andreas fault. Galena and sphalerite are concentrated near the intersection of the principal mineralized vein with the fault, suggesting that brecciated rocks at this intersection acted as a major conduit for hydrothermal circulation.Fluid inclusion data show that the sulfide mineralization occurred at a depth of at least 400 m, probably at a temperature of about 250 degrees C. Mineralization fluids were in the salinity range of 10.5 to 5.2 equivalent weight percent NaCl. Sodium and potassium were depleted from the mineralizing fluids in the early stages of mineralization, and traces of argentian tetrahedrite and cassiterite in the fluid inclusions probably account for some of the anomalous values of silver and tin in galena and sphalerite.The age of mineralization, based on K-Ar dating of adularia, is 13.8 + or - 0.4 m.y. This age suggests that the purported trace of the San Andreas fault crossed by the sulfide veins has been largely inactive since middle Miocene time.Lead isotope and geochemical data suggest that base and precious metals in the sulfide veins were derived largely from arc-related sedimentary rocks. The heat source for hydrothermal circulation may have been associated with intrusion of asthenosphere into a triangular slab window opened southeast of the northward-propagating Mendocino triple junction shortly before 13.8 m.y. ago.

The early hominids of East Africa were dated by determining the ages of tuff beds at the sites. Despite much research using palaeomagnetic and K/Ar-dating techniques, some of those ages are still controversial1,2. To obtain independent age estimates for these tephra layers, we have examined cores from DSDP Sites 231 and 232 in the Gulf of Aden (Fig. 1a) which consist mainly of calcareous nannofossil ooze, but also contain rare tephra horizons3 dated by interpolation from the established nannofossil stratigraphy (Fig. 1b). Chemical analysis confirms that the identity and sequence of these horizons is the same as that at the East African sites. We conclude that the age of the Tulu Bor Tuff is <3.4 Myr and hence that the Hadar hominid specimens are also <∼3.4 Myr old.