Michael E. Perkins

Environmental Change in the Great Plains:An Isotopic Record from Fossil Horses

Carbon and oxygen isotope ratios of fossil horse tooth enamel from Nebraska and Texas show evidence for late Neogene environmental changes in the Great Plains. The earliest unambiguous C4 dietary signal among Texas equids coincides with the age of the classic late Hemphillian–age Coffee Ranch fauna, which we suggest is ∼6.6 Ma based on volcanic ash correlations. C4 vegetation was present in the diets of a small fraction of late Hemphillian equids in Nebraska and was thereafter ordinary in the diets of both Nebraska and Texas equids. There is no unequivocal evidence for abundant C4 vegetation in the diets of pre–late Hemphillian equids, and we suggest that the ensuing dietary change reflects C4‐biomass expansion in the latest Miocene. Carbon isotope ratios of post‐Hemphillian horses in Nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co‐occur with horse remains, indicating that the two proxies (carbon isotopes and presence/absence of tortoises) record complementary environmental phenomena. The average δ18O values of late Hemphillian and younger fossil localities in Nebraska trend toward bimodal distribution, but more data are needed to confirm this pattern. Oxygen isotope ratios of Barstovian and Clarendonian horses are significantly enriched in 18O relative to Hemphillian horses, which in turn are significantly enriched relative to Blancan and Irvingtonian horses. A large portion of this oxygen isotope decrease appears to have taken place during late Hemphillian time. Secular variation in the Nebraska δ18O record correlates with changes in ungulate diversity, the disappearance of crocodilians in Nebraska, and global change in the latest Miocene.

Explosive silicic volcanism of the Yellowstone hotspot: The ash fall tuff record

Unaltered silicic ash fall tuffs are abundant in Neogene sedimentary basins of the western U.S. and constitute an important record of explosive silicic volcanism in this region. In particular, ash fall tuffs from silicic volcanic centers along the Yellowstone hotspot track are common in these basins and provide a detailed record of explosive volcanism along the hotspot track. The available hotspot ash fall tuff record commences at ca. 16 Ma, shortly after the initiation of hotspot silicic volcanism at ca. 16.5 Ma, and continues through the most recent explosive eruptions in the late Pleistocene. Post–16 Ma hotspot silicic volcanism has been dominated by eruption of metaluminous ash flow tuffs and rhyolites, and ash fall tuffs produced by these eruptions dominate the Yellowstone hotspot ash fall tuff record. Evaluation of a well-dated composite sequence of 142 of these tuffs reveals systematic variation in magma composition, magma temperature, eruption frequency, and, possibly, volumetric discharge as the hotspot migrated eastward from the western edge of the North America craton to its current location in the Yellowstone Plateau. On the basis of these variations, three primary stages of metaluminous rhyolite magmatism (M stages) are recognized: M1 (16.0–15.2 Ma), M2 (15.2–7.5 Ma), and M3 (7.5–0 Ma). Each of these stages is marked by distinctive magma compositions, eruption frequencies, and magma temperature ranges and trends, with an overall decline in average eruption frequency and magma temperature from stage to stage. The partitioning of explosive hotspot volcanism into stages likely reflects variation in the style and intensity of the interaction between the mantle anomaly powering the hotspot magmatic systems and a spatially and temporally heterogeneous lithosphere along the hotspot track. Although the ash fall tuff record does not provide definitive constraints on the nature of these variable interactions, correlations between changes in eruption frequency, ash fall tuff discharge, and magma temperature point to variable input of mantle basalt into hotspot crustal magmatic systems as a first order control on intensity of explosive volcanism. As future studies reveal more about the processes controlling the variations in hotspot silicic volcanism, a fuller understanding of both the nature of silicic magmatism and the nature of the Yellowstone hotspot should emerge.

Fallout tuffs of Trapper Creek, Idaho—A record of Miocene explosive volcanism in the Snake River Plain volcanic province

A 900-m-thick section of tuffaceous sedimentary rock, vitric fallout tuff, and ash-flow tuff is well exposed along Trapper Creek in south-central Idaho. This section provides nearly continuous exposure through the fill of the Goose Creek basin, a major north-trending Miocene extensional basin located along the southern margin of the Snake River Plain volcanic province (SRPVP). Some 51 separate units of vitric fallout tuff are recognized in the Trapper Creek section. Petrographic and chemical characteristics of these vitric tuffs indicate that most are from SRPVP sources. New 40Ar/39Ar laser-fusion dating, along with prior isotopic age determinations, show that the Trapper Creek tuffs span the period ca. 13.9 – 8.6 Ma. Chemical correlation indicates that fallout tuffs in the central part of the Trapper Creek section (12.5 – 10.0 Ma) are from sources in the Bruneau-Jarbidge volcanic field of the SRPVP centered ≈100 km west of Trapper Creek. Underlying fallout tuffs may have had sources in the Owyhee-Humboldt field of the SRPVP centered ≈200 km west of Trapper Creek, while overlying fallout tuffs, interlayered with several ash-flow tuffs, had a relatively proximal source, possibly in the proposed Twin Falls volcanic field centered ≈60 km north of Trapper Creek. The Trapper Creek tuffs provide insight into the characteristics of explosive silicic volcanism within the SRPVP during middle – late Miocene time. From ca. 13.9 to ca. 9.5 Ma, major eruptions (those depositing ≥1.5 m of fallout tuff) were frequent (about one event per 200 k.y.); their products display a trend toward the eruption of progressively less evolved, higher temperature silicic magma after 12.5 Ma. This trend to higher temperature eruptions, termed the Cougar Point “flare-up,” culminated in the eruption of high-temperature (≈1000°C), plagioclase-rich magma during the period 10.5 – 9.5 Ma. In contrast to these eruptions, later (<7.0 Ma) major silicic eruptions within the SRPVP were characterized by the lower temperature (≈850°C) of the erupted magma and by the longer intervals (about one event per ≈500 – 600 k.y.) between eruptions. Variations in the character of SRPVP explosive silicic eruptions may reflect changes in the structure, composition, or state of stress in the crust beneath the eastward propagating SRPVP, or, perhaps, changes in the Yellowstone hot-spot plume that may drive the SRPVP volcanism.

Sequence, age, and source of silicic fallout tuffs in middle to late Miocene basins of the northern Basin and Range province

The latest Cenozoic (<6 Ma) ash beds in the western United States have been intensively studied for several decades. The more widespread of these ash beds are well-documented event horizons that are of great value in studies of the timing and pace of geological, climatological, and biological events throughout the region. Because explosive volcanism was not restricted to latest Neogene time in this region, many older ash beds are likely to prove as useful as younger beds as event horizons, once they are located, characterized, and dated. As a first step in developing a useful chronology of older Cenozoic ash beds in the western United States, we have sampled and analyzed silicic fallout tuffs in middle to late Miocene sedimentary basins across the northern Basin and Range province. The northern Basin and Range basins, ideally situated in the vicinity of major coeval silicic volcanic centers, contain numerous relatively unaltered, silicic fallout tuffs. We have correlated tuffs between all sampled sections on the basis of glass shard composition. The composite stratigraphic sequence established by the correlations contains more than 200 individual tuffs, including 59 widely distributed tuffs termed correlative tuffs. The tuffs vary widely in composition, but most are in one of two compositional groups: gray metaluminous vitric tuffs (Gm tuffs) or white metaluminous vitric tuffs (Wm tuffs). Distribution patterns, compositional characteristics, and correlation with ash-flow tuffs show that the source for most Gm tuffs was the Snake River Plain volcanic province along the northern edge of the northern Basin and Range, and the source for most Wm tuffs was the southwestern Nevada volcanic field in the southern part of the northern Basin and Range. The northern Basin and Range tuffs range in age from ca. 16–6 Ma. The ages of individual tuffs are determined variously by direct isotopic dating, by correlation to previously dated fallout and ash-flow tuffs, or by interpolation age estimation. Ages for most tuffs are known to within 0.25 m.y. (1σ) or less and for many tuffs to within 0.1 m.y. or less. The sequence and ages of tuffs established in this study provide insights into the evolution of the northern Basin and Range basins and patterns of explosive volcanism in coeval volcanic centers, and contribute to the development of a high-resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western United States.

Sierra Nevada–Basin and Range transition near Reno, Nevada: Two-stage development at 12 and 3 Ma

Relative and absolute elevations of the Sierra Nevada and adjacent Basin and Range province, timing of their differentiation, and location, amount, and timing of strike-slip movement between them are controversial. The provincial boundary near Reno developed in two stages. (1) At ca. 12 Ma, the ≥700 km 2 Verdi-Boca sedimentary basin formed across what was to become the boundary, probably as a result of a small-magnitude but regional extensional episode that affected much of the western Basin and Range. (2) At 3 Ma, the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern Sierran structural and topographic boundary in this area. The boundary is really a transition zone with a western edge along the Donner Pass, California, fault zone, which is farther west than previously placed. Both episodes appear to have resulted from east-west extension only, which suggests that northwest motion of the Sierra Nevada relative to the Basin and Range shown by geodetic data began after 3 Ma or was taken up farther east.

CHRONOLOGY OF POLYPHASE EXTENSION IN THE WINDERMERE HILLS, NORTHEAST NEVADA

Fission-track and 40 Ar/ 39 Ar dating and chemical correlation of volcanic strata exposed in the Windermere Hills and northern Pequop Mountains, northeast Nevada, indicate a protracted, polyphase history of Tertiary (late Eocene‐late Miocene) extension along the northern margin of a major Cordilleran metamorphic core complex. Early extension is recorded by a west-tilted half graben filled with early Oligocene (34.79 ± 0.18‐39.18 ± 0.12 Ma) sedimentary rocks in the eastern Windermere Hills above the lowangle Black Mountain detachment fault. The early Oligocene half graben conformably overlies a widespread suite of late Eocene (39.18 ± 0.12‐ 40.38 ± 0.06 Ma) calc-alkaline volcanic rocks, reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare-up. These strata are cut by east-west‐striking normal faults, which are exposed along, and parallel to, the northern margin of the metamorphic complex. Available age data (e.g., between 14.93 ± 0.08 and 34.79 ± 0.18 Ma) permit the interpretation that the east-west‐ striking faults formed at the same time as, or after, large-magnitude unroofing of highgrade rocks. We interpret the east-west‐ striking faults to accommodate differential uplift of greenschist-grade metamorphic rocks in the upper crust, above a lateral ramp in a west-northwest‐directed mylonitic shear zone. Subsequent extension in the Windermere Hills is defined by deep, rapidly filled half grabens of middle Miocene (<7.42 ± 2.0 to 14.93 ± 0.08 Ma) age that unconformably overlie older faults and synextensional deposits. These are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume (e.g., the Yellowstone hotspot) located in southeastern Oregon at this time.

Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40 Ar/ 39 Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies. Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma. We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

Late Cenozoic paleogeographic evolution of northeastern Nevada: Evidence from the sedimentary basins

Field and geochronologic studies of Neogene sedimentary basins in northeastern Nevada document the paleogeographic and geologic evolution of this region and the effects on major mineral deposits. The broad area that includes the four middle Miocene basins studied—Chimney, Ivanhoe, Carlin, and Elko, from west to east—was an upland that underwent prolonged middle Tertiary exposure and moderate erosion. All four basins began to retain sediments at ca. 16 Ma. Eruption of volcanic flows in the Chimney and Ivanhoe basins produced short-lived (ca. 2 Ma), lacustrine-dominated basins before the dams failed and the streams drained to the southwest. In contrast, early, high-angle, normal faulting induced fluvial to lacustrine sedimentation in the Carlin and Elko basins, and volcanic flows further blocked drainage in the Carlin basin until the basin drained at ca. 14.5 Ma. The Elko basin, with continued synsedimentary faulting, retained sediments until ca. 9.8 Ma and then drained west into the Carlin basin. Sediment buildup in all basins progressively buried existing highlands and created a subdued landscape. Relatively minor post-sedimentation extension produced early north-northwest–striking normal faults with variable amounts of offset, and later east-northeast–striking normal faults with up to several kilometers of vertical and left-lateral offset. The earlier faults are more pronounced east of the Tuscarora Mountains, possibly reflecting a hanging-wall influence related to uplift of the Ruby Mountains-East Humboldt core complex on the east side of the Elko basin. The later faults are concentrated along the north-northwest–trending northern Nevada rift west of the Tuscarora Mountains. The area west of the rift contains major tilted horsts and alluvium-filled grabens, and differential extension between this more highly extended region and the less extended area to the east produced the intervening east-northeast–striking faults. The Humboldt River drainage system formed as the four basins became integrated after ca. 9.8 Ma. Flow was into northwestern Nevada, the site of active normal faulting and graben formation. This faulting lowered the base level of the river and induced substantial erosion in upstream regions. Erosion preferentially removed the poorly consolidated Miocene sediments, progressively reexposed the pre-middle Miocene highlands, and transported the sediments to downstream basins. Thus, some ranges in the upstream region are exhumed older highlands rather than newly formed horsts. In addition, the drainage system evolution indicates that northern Nevada has become progressively lower than central Nevada since the middle Miocene. Mineral belts with large Eocene gold deposits are exposed in uplands and concealed beneath Neogene basin units in the study area. Also, numerous epithermal hot-spring deposits formed at and near the paleosurface in the Chimney, Ivanhoe, and Carlin basins as those basins were forming. The Neogene geologic and landscape evolution had variable effects on all of these deposits, including uplift, weathering, supergene enrichment, erosion, and burial, depending on the events at any particular deposit. As such, this study documents the importance of evaluating post-mineralization processes at both regional and local scales when exploring for or evaluating the diverse mineral deposits in this area and other parts of the Basin and Range region.

Neogene Fallout Tuffs from the Yellowstone Hotspot in the Columbia Plateau Region, Oregon, Washington and Idaho, USA

Sedimentary sequences in the Columbia Plateau region of the Pacific Northwest ranging in age from 16–4 Ma contain fallout tuffs whose origins lie in volcanic centers of the Yellowstone hotspot in northwestern Nevada, eastern Oregon and the Snake River Plain in Idaho. Silicic volcanism began in the region contemporaneously with early eruptions of the Columbia River Basalt Group (CRBG), and the abundance of widespread fallout tuffs provides the opportunity to establish a tephrostratigrahic framework for the region. Sedimentary basins with volcaniclastic deposits also contain diverse assemblages of fauna and flora that were preserved during the Mid-Miocene Climatic Optimum, including Sucker Creek, Mascall, Latah, Virgin Valley and Trout Creek. Correlation of ashfall units establish that the lower Bully Creek Formation in eastern Oregon is contemporaneous with the Virgin Valley Formation, the Sucker Creek Formation, Oregon and Idaho, Trout Creek Formation, Oregon, and the Latah Formation in the Clearwater Embayment in Washington and Idaho. In addition, it can be established that the Trout Creek flora are younger than the Mascall and Latah flora. A tentative correlation of a fallout tuff from the Clarkia fossil beds, Idaho, with a pumice bed in the Bully Creek Formation places the remarkably well preserved Clarkia flora assemblage between the Mascall and Trout Creek flora. Large-volume supereruptions that originated between 11.8 and 10.1 Ma from the Bruneau-Jarbidge and Twin Falls volcanic centers of the Yellowstone hotspot in the central Snake River Plain deposited voluminous fallout tuffs in the Ellensberg Formation which forms sedimentary interbeds in the CRBG. These occurrences extend the known distribution of these fallout tuffs 500 km to the northwest of their source in the Snake River Plain. Heretofore, the distal products of these large eruptions had only been recognized to the east of their sources in the High Plains of Nebraska and Kansas.

Middle Miocene to Holocene tectonics, basin evolution, and paleogeography along the southern margin of the Snake River Plain in the Knoll Mountain–Ruby–East Humboldt Range region, northeastern Nevada and south-central Idaho

New geologic mapping and tephrochronologic assessment of strata in extensional basins surrounding Knoll Mountain (Nevada, USA) reveal a geologic history linked to tectonic development of the Yellowstone hotspot and Snake River Plain to the north, and to the Ruby–East Humboldt–Wood Hills metamorphic core complex to the south. Data from these areas are utilized to present a paleogeographic reconstruction of northeastern Nevada–southcentral Idaho depicting the architecture of extensional faulting and basin development during collapse of the Nevadaplano over the past 17 m.y. Knoll Mountain is a northeast-trending horst along the southern margin of the Snake River Plain and track of the Yellowstone hotspot. The horst is bounded on the east by the Thousand Springs fault system and basin, and on the west by the Knoll Mountain fault and basin, where streams currently drain north into the Snake River Plain. The Knoll and Thousand Springs basins form half-grabens that are filled with the ca. 16 Ma to ca. 8–5 Ma Humboldt Formation, which was deposited in alluvial, eolian, and lacustrine environments during slip along range-bounding faults and a series of late-stage synthetic intrabasin faults. Structural, chronologic, and sedimentologic assessment of the Humboldt Formation in the Knoll basin indicates that it records overall southward fluvial drainage with slip along the Knoll Mountain fault beginning ca. 16 Ma and continuing to at least 8 Ma, and that between 8 and ca. 5 Ma, a west-dipping intrabasin fault system had developed. Between ca. 8–5 Ma to ca. 3 Ma, several fundamental changes took place, beginning with the cessation of faulting followed by widespread erosion that in turn was followed by deposition of older alluvium. The reversal of drainage direction from south to north flowing in the Knoll basin also took place during this time period, but its age relative to the widespread erosion or older alluvium is unknown. An integration of our work with previous studies north of Knoll Mountain reveal that the Knoll Mountain and intrabasin faults terminate to the north in the vicinity of the Jurassic Contact pluton, and that this area forms an accommodation zone separating broadly coeval and colinear faults bounding the ca. 10–8 Ma north-trending Rogerson graben, the northern end of which merges with the Snake River Plain. Furthermore, an integration of our work with previous work south of Knoll Mountain reveals that the Knoll Mountain fault formed part of a >190-km long, west-dipping fault zone that included the Ruby–East Humboldt detachment. This fault zone, which we refer to as the Knoll-Ruby fault system, had an extensive hanging-wall basin, the KnollRuby basin. The Knoll-Ruby fault system was a prominent structure facilitating collapse of the Nevadaplano in northeastern Nevada between ca. 16 and ca. 8–5 Ma, and its central part produced partial exhumation of high-grade, mid-crustal metamorphic rocks in the Ruby–East Humboldt–Wood Hills metamorphic core complex. By 8–5 Ma, during the waning stages of extension along the Knoll-Ruby fault system, a series of intrabasin faults developed at about the same time as the integration of streams to form the incipient eastern reaches of the Humboldt River system. Profound changes in tectonics and paleogeography took place between ca. 8–5 Ma and ca. 3 Ma, that included the extinction of the Knoll-Ruby and intrabasin basin fault systems followed by southward migration of significant tectonism away from the Snake River Plain, resulting in development of a set of modern normal faults responsible for uplift of the southern Snake Mountains, Ruby Mountains, East Humboldt Range, and Pequop Mountains. These new faults cut and dismembered the central and southern part of the Knoll-Ruby fault system and basin, effectively ending any fluvial connection between the northern and southern parts of the Knoll-Ruby basin. Since ca. 8–5 Ma to the present, the Knoll Mountain region has remained relatively tectonically quiescent, and continued subsidence in the Snake River Plain to the north induced capture of the drainage system in the Knoll basin and reversed the drainage direction from south to north flowing. Our new findings indicate that (1) the Knoll-Ruby fault system and associated intrabasin faults were active until ca. 8–5 Ma, which is younger than the 12–10 Ma age generally recognized for cessation of major extension elsewhere in the northern Nevada region; (2) although this fault system was responsible for partial exhumation of core-complex metamorphic rocks, it extended well GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01318.1 16 figures; 3 tables; 1 supplemental file CORRESPONDENCE: camillerip@apsu .edu CITATION: Camilleri, P., Deibert, J., and Perkins, M., 2017, Middle Miocene to Holocene tectonics, basin evolution, and paleogeography along the southern margin of the Snake River Plain in the Knoll Mountain– Ruby–East Humboldt Range region, northeastern Nevada and south-central Idaho: Geosphere, v. 13, no. 6, p. 1901–1948, doi:10.1130/GES01318.1. Received 24 January 2016 Revision received 3 April 2017 Accepted 2 August 2017 Published online 25 September 2017 For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.