Toiyabe Range Research Articles

Testing climate tracking of montane rodent distributions over the past century within the Great Basin ecoregion

Geographic ranges of many taxa have shifted poleward or up-slope under warming. Little is known, however, about the consistency of these dynamics among multiple mountains within a species’ geographic range. We assess climate tracking over the past century for four montane rodents, across the Ruby Mountains and Toiyabe Range within the Great Basin ecoregion of western North America. To do this, we generated species distribution models (SDMs, Maxent) in each mountain range during historical (1923–1940) and modern (2005–2015) time periods. Nineteen bioclimatic variables were derived from 800 m resolution monthly PRISM climate group data. SDMs trained on historic occurrences and climate were projected onto modern climates to estimate expected species distributions along each elevational gradient. Occurrence-based Observed Modern SDMs were compared to these Expected Modern SDMs, with congruence between the two representing the degree of climate tracking. Climate tracking predicted up-slope range expansions and contractions in the Ruby Mountains, while stasis or down-slope contractions were predicted in the Toiyabe Range. Observed Modern SDMs rarely matched climate tracking expectations, and the same species showed variable responses among mountains. Deviations are partially explained by changes in the contribution of temperature vs. precipitation over time. Additionally, optimal niche space in the Ruby Mountains shifted in elevation, but shifted in both elevation and latitude in the Toiyabe Range. Our results indicate that data from one elevational gradient may not be predictive of range dynamics elsewhere, even across similar environmental conditions and habitats, even for the same species. Conservation efforts will be better served by strategies that consider both inter- and intraspecific variation in responses to climate change over space and time.

Habitat Use of the Piñon Mouse (Peromyscus truei) in the Toiyabe Range, Central Nevada

Pinon-juniper woodland is expanding across much of western North America. In the Great Basin, woodland expansion has encroached on native shrublands, threatening species that have close associations with the sagebrush ecosystem. Pinon-juniper woodlands also harbor great biodiversity, and the response of woodland specialists to expansion has been less well studied than that of shrubland specialists. Here, we use occupancy and abundance modeling, accounting for imperfect detection, to assess habitat use of a woodland specialist, the pinon mouse (Peromyscus truei). Our study occurred in the Toiyabe Range of central Nevada, an area P. truei is thought to have recently colonized. Understanding habitat use at an expanding range margin can have important implications for dispersal-mediated woodland expansion. Peromyscus truei was documented in a wide range of conditions, including nonwoodland and woodland habitats, across the latitudinal and elevational extent of the mountain range. Occupancy models suggest that P. truei occurrences are most associated with the presence of pinon pine. For abundance, our global model was the best supported, indicating that no one environmental factor or set of factors considered were found to structure abundance. Chi-square tests indicate that use of woodland versus nonwoodland habitats by P. truei is not structured by age or sex. Although P. truei was found in a range of habitat types, we cannot conclude whether it is more appropriate to characterize this species as a habitat generalist, or whether postcolonization abiotic and biotic filtering is not yet complete. Research on woodland specialists at expanding local and landscape-scale range margins provides a unique opportunity to study how habitat selection and ecological filtering impact community assembly under environmental change.

ORIGINAL ARTICLE: Comparison of predictor sets for species richness and the number of rare species of butterflies and birds

AbstractAim Accurate inventories of biota are typically restricted to few locations within an extensive region. Accordingly, effective planning must involve some form of surrogate measures coupled with spatial modelling. We conducted a simultaneous comparison of models of both species richness and the number of rare species using three types of surrogates (indicator species, vegetation composition and structure, and topoclimate) as predictors. We evaluated each type of surrogate alone and in combination with others.Location Data for our analyses were collected from 1996–2004 in three adjacent mountain ranges in the central Great Basin (Lander and Nye counties, Nevada, USA), the Shoshone Mountains, Toiyabe Range and Toquima Range.Methods Data on species richness and species composition of butterflies and birds and measures of vegetation composition and structure were obtained in the field. Topoclimatic variables were derived by GIS from digital sources and satellite images. We used Poisson regression with Bayesian model averaging to predict species richness and the number of rare species. We compared the expected prediction success of all models on the basis of internal and external validation trials.Results Same‐taxon indicator species were the most accurate predictors of species richness and of the number of rare species of butterflies and birds. Cross‐taxon indicator species and topoclimate variables were reasonably accurate predictors of species richness of butterflies and birds and of the number of rare butterfly species. Although vegetation variables were more effective for predicting species richness and number of rare species of birds than of butterflies, they were the least accurate predictors overall.Main conclusions Although indicator species may provide the most accurate predictions of species richness, their practical value, like any surrogate measure, depends greatly on ecological considerations and land‐use context. In general, the ability to predict numbers of rare species based on any set of candidate predictors was weaker than the ability to predict species richness, which may result from the high degree of stochasticity that often characterizes distributions of rare species. Our statistical approach for objective examination of different candidate predictors can help ensure that selection of species‐richness surrogates in any system is scientifically reliable and cost‐effective.

Comparative influence of spatial scale on beta diversity within regional assemblages of birds and butterflies

AbstractAim We examined whether variation in species composition of breeding birds and resident butterflies in the Great Basin of North America depended on sampling grain (the smallest resolvable unit of study) and on the relative proximity of sampling units across the landscape. We also compared patterns between the two taxonomic groups with reference to their life‐history characteristics.Location Data for our analyses were collected from 1996 to 2003 in three adjacent mountain ranges in the central Great Basin (Lander and Nye counties, Nevada, USA): the Shoshone Mountains, Toiyabe Range and Toquima Range.Methods Data on species composition for both taxonomic groups were collecting using standard inventory methods for birds and butterflies in temperate regions. Data were compiled at three sampling grains, sites (average 12 ha), canyons (average 74 ha) and mountain ranges. For each sampling grain in turn, we calculated similarity of species composition using the Jaccard index. First, we investigated whether mean similarity of species composition among the three ranges differed as a function of the grain size at which data were compiled. Secondly, we explored whether mean similarity of species composition was greater for canyons within the same mountain range than for canyons within different mountain ranges. Thirdly, we examined whether mean similarity of species composition at the site level was different for sites within the same canyon, sites within different canyons in the same mountain range, and sites within canyons in different mountain ranges. We used a Bayesian model to analyse these comparisons.Results For both taxonomic groups, mean similarity of species composition increased as the sampling grain increased. The effect of spatial grain was somewhat greater for birds than for butterflies, especially when the intermediate sampling grain was compared with the smallest sampling grain. Similarity of species composition of butterflies at each sampling grain was greater than similarity of species composition of birds at the same grain. Mean similarity of species composition of both birds and butterflies at the canyon level and site level was affected by relative proximity of sampling locations; beta diversity increased as the relative isolation of sampling locations increased.Main conclusions The sensitivity of beta diversity to sampling grain likely reflects the effect of local environmental heterogeneity: as sampling grain increases, biotic assemblages appear more homogeneous. Although breeding birds in our study system have larger home ranges than resident butterflies, birds may have more specialized resource requirements related to vegetation structure and composition, especially at small sampling scales. The degree of variation in species composition of both taxonomic groups suggests that spatially extensive sampling will be more effective for drawing inferences about regional patterns of species diversity than intensive sampling at relatively few, smaller sites.

The “veiled gradients” problem in ecology

Apparent regular variation in the distributions of organisms along geographical gradients has always fascinated ecologists. Variation in species richness with latitude has received the most attention, of course, but variation in species richness with elevation also has been of considerable interest and study. Precisely how and why species richness varies with elevation remains controversial (Wolda 1987, McCoy 1990, Colwell and Hurtt 1994, Rahbek 1995, 1997, Fleishman et al. 1998). Part of the reason for the continuing controversy is the fact that species richness is only an index, a manageable substitute for the set of distributions of individual species along an elevational gradient. Any explanation proposed for apparent regular variation in species richness with elevation can be, at best, only a loose reflection of the complicated autecological relationships between the individual species and elevation. The more species the proposed explanation is able to accommodate, the closer it will come to explaining variation in species richness, but any single explanation may not be suitable for a wide range of species (Lawton 1996). Another part of the reason for the continuing controversy about how and why species richness varies with elevation involves sampling problems (McCoy 1990, Rahbek 1995). Simply comparing species richness from one location to another can involve many sampling problems (Gotelli and Colwell 2001), but when such comparisons are made along gradients, the problems multiply. Understanding the causes of variation in species richness with elevation may have been hampered, for example, because different studies typically do not encompass the same elevational range and because the elevation of true maximum species richness sometimes occurs outside the sampled range (McCoy 1990). Ultimately, sampling problems even could lead to erroneous views of how species richness varies along elevational gradients (Rahbek 1995). Sampling problems such as those just described for studies of variation in species richness along elevational gradients are instances of a general problem in ecology that I term gradients. Veiled gradients arise from the fact that the outcome of any examination of a local gradient elevation, in this case is contingent upon the temporal and spatial frameworks employed. Over short time periods or across small geographical scales, only a portion of the gradient is likely to be revealed, the remainder of the gradient being veiled in time or space. Failure to realize that only a portion of the gradient is likely to be revealed in studies that are temporally or spatially restricted could obscure any true relationship that might exist (Allen and Starr 1982, McCoy 1990, McCoy and Bell 1991, Korner 2000). A ready example of how veiled gradients can influence interpretation of the variation in species richness with elevation can be found in the recent literature. Fleishman et al. (2000) determined that butterfly species richness increased with elevation in one of the canyons they studied but decreased in the other (Fig. 1). They attributed the apparent difference in the distribution of species richness with elevation to differences in climatic severity between the two canyons. The elevational ranges that were sampled were not the same in the two canyons, however, raising the possibility of a sampling problem. Samples (n = 102) from the first canyon (Toiyabe Range) spanned an elevational range of 1917-3272 m, whereas samples (n = 49) from the second canyon (Toquima Range) spanned an elevational range of 1872-2750 m. Ideally, we would like to unveil the distribution of species richness with elevation by extending the gradient in the Toquima Range to 3272 m, but we cannot do so with the available data. We can find out what the distribution of species richness with elevation would look like if the gradient in the Toiyabe Range were veiled to 2750 m, however. Examining changes in richness with elevation when the gradients are of the same length dramatically alters the interpretation of the relationship (Fig. 1). The apparent decline in species richness with elevation in the Toiyabe Range essentially disappears. Furthermore, an alternative interpretation of the data now suggests itself: that

Modeling and Predicting Species Occurrence Using Broad‐Scale Environmental Variables: an Example with Butterflies of the Great Basin

Abstract:If occurrence of individual species can be modeled as a function of easily quantified environmental variables (e.g., derived from a geographic information system [GIS]) and the predictions of these models are demonstrably successful, then the scientific foundation for management planning will be strengthened. We used Bayesian logistic regression to develop predictive models for resident butterflies in the central Great Basin of western North America. Species inventory data and values for 14 environmental variables from 49 locations (segments of canyons) in the Toquima Range ( Nevada, U.S.A.) were used to build the models. Squares of the environmental variables were also used to accommodate possibly nonmonotonic responses. We obtained statistically significant models for 36 of 56 (64%) resident species of butterflies. The models explained 8–72% of the deviance in occurrence of those species. Each of the independent variables was significant in at least one model, and squared versions of five variables contributed to models. Elevation was included in more than half of the models. Models included one to four variables; only one variable was significant in about half the models. We conducted preliminary tests of two of our models by using an existing set of data on the occurrence of butterflies in the neighboring Toiyabe Range. We compared conventional logistic classification with posterior probability distributions derived from Bayesian modeling. For the latter, we restricted our predictions to locations with a high ( 70%) probability of predicted presence or absence. We will perform further tests after conducting inventories at new locations in the Toquima Range and nearby Shoshone Mountains, for which we have computed environmental variables by using remotely acquired topographic data, digital‐terrain and microclimatic models, and GIS computation.

Upsides and downsides: contrasting topographic gradients in species richness and associated scenarios for climate change

AbstractAimWe tested whether montane butterflies exhibit similar responses to elevation in two adjacent mountain ranges in the Great Basin; whether surface water availability, canyon depth and canyon width vary predictably with elevation; and whether those factors act in conjunction with elevation to generate the observed gradients in species richness. These variables might reasonably be expected to affect species richness of butterflies and can be derived readily for most landscapes. We also explored how climate change may affect species distributions in both mountain ranges.LocationField research was conducted in the Toiyabe and Toquima ranges, Lander and Nye counties, Nevada, USA.MethodsWe obtained data for 102 locations in the Toiyabe Range and 49 locations in the Toquima Range. These locations covered an elevational gradient of c.1350 m in the Toiyabe Range and 900 m in the Toquima Range. Species richness data were based upon comprehensive field inventories. Elevation was measured by differentially corrected global positioning systems. Values for the remaining predictive variables were derived using geographic information systems. Availability of surface water was quantified as the distance from the centre of the inventory route to the nearest permanent source of running or standing water.ResultsIn the Toiyabe Range, species richness decreases as elevation increases. Water availability and canyon depth and width vary predictably with elevation, but do not interact with elevation to affect species richness patterns. In the Toquima range, in contrast, species richness increases as elevation increases, and no other predictive variable covaries with elevation.Main conclusionsWe suggest that range‐specific gradients in climatic severity contribute to the distinct elevational gradients in species richness. In the face of climate change, species richness patterns in the Toquima range will likely remain similar. The elevational gradient in species richness in the Toiyabe range may flatten, or the patterns in the two ranges may converge.

Demographic analysis of the Columbia spotted frog (Rana luteiventris): case study in spatiotemporal variation

This study examined size, mass, sex ratios, and age structure of the Columbia spotted frog (Rana luteiventris) at seven sites in the Toiyabe Range in central Nevada from 1994 through 1996. Age structure was estimated using skeletochronology. Demographic composition was found to be significantly different among sites, suggesting that localized factors influence recruitment and mortality rates. Demographics among years at the sites were also significantly different, indicating that the population dynamics of this system are complex and are also driven by one or more temporal factors. Knowledge of local land-use patterns and anecdotal observations were incorporated in an attempt to identify potential stress agents in need of further research and possible intensive management. Differences in recruitment, survivorship, and mortality rates among sites may be due to microclimate, food availability, and predation rates. Introduction of exotic trout and cattle are likely the most important anthropogenic factors limiting the distribution and persistence of R. luteiventris in the study area. Extreme variations in annual weather patterns may account for many differences observed at some sites. This study demonstrates that adequate assessment of amphibian population status requires knowledge of subpopulation demographics across a broad landscape.

Bryophytes of riparian areas in the Toiyabe Range of Central Nevada

Bryophytes of riparian areas in the Toiyabe Range of Central Nevada

Patterns and processes of nestedness in a Great Basin butterfly community.

We examined nestedness and potential mechanisms causing that distributional pattern in resident butterfly communities of the Toiyabe Range, a mountain range in the central Great Basin of western North America. We tested whether life history characteristics, including habitat use and vagility, affected the relative degree of nestedness or mean species incidence. We also tested whether nestedness at the level of individual species was independent of life history. Relationships between distributional patterns and habitat use, particularly in ecologically sensitive riparian areas, are relevant to ongoing conservation planning in the Great Basin. The distributional pattern of the 68 resident butterfly species in 19 Toiyabe Range canyons was significantly nested, as was the distribution of all functional subgroups that we tested. Life history affected neither relative nestedness of species groups nor mean species incidence. More than 80% of the individual butterfly species that inhabit the Toiyabe Range had distributions that were more nested than expected. Colonization does not appear to have played an important role in determining the composition of butterfly communities in Toiyabe Range canyons. Likewise, selective dispersal has probably played a minor role in producing nested distributions of Toiyabe Range butterflies. Our results suggest either that impacts to riparian areas are not jeopardizing species viability, or that highly sensitive butterfly species have already been extirpated from the Toiyabe Range.

AN EMPIRICAL TEST OF RAPOPORT’S RULE: ELEVATIONAL GRADIENTS IN MONTANE BUTTERFLY COMMUNITIES

We examined the response of montane butterflies to a 1300-m elevational gradient in the Toiyabe Range, a mountain range in the central Great Basin. We tested whether elevational ranges of montane butterflies conform to Rapoport’s elevational rule (species at higher elevations have greater elevational ranges). We also tested whether the Rapoport-rescue hypothesis (emigrants from higher elevation populations inflate the species richness of lower-elevation sites) could explain the species richness patterns we observed. In addition, we predicted whether global climate change is likely to perturb current elevational gradients in butterfly species richness. Species presence data were collected from 105 100-m vertical elevational bands in 17 canyons. Elevation and species richness were significantly negatively correlated, although species richness peaked at intermediate elevations, and butterflies in species-rich areas had significantly narrower elevational ranges than species in more depauperate areas. Toiyabe ...

History and kinematics of Cenozoic extension in the northern Toiyabe Range, Lander County, Nevada

Since earliest Oligocene time, ∼15 km of extension has occurred along a major gently dipping normal fault system and a younger more steeply dipping system of normal faults in the northern Toiyabe Range in central Nevada. The Bernd Canyon fault is the structurally lowest of the gently dipping faults in this area. It dips 12°-28° west and is similar to well-known detachment faults. Paleozoic rocks in the hanging wall are cut by subsidary gently dipping normal faults and are tilted 40° to more than 90° to the east. The early Oligocene intracaldera tuff of Hall Creek is the oldest Cenozoic unit present in the hanging wall and is tilted 28°-58° eastward. Successively younger units have systematically decreasing dips and record a history of progressive, possibly episodic, extension. Bedding in greenschist-grade lower Paleozoic rocks in the footwall of the Bernd Canyon fault intersects the fault at angles of 5° to 25° and is arched across the range. Unlike many other detachment faults, the Bernd Canyon fault contains early Oligocene and younger volcanic and sedimentary units in the footwall that may be correlated to units in the hanging wall, and they record a similar tilting history. Sequential untilting of successively older Tertiary units suggests that the Bernd Canyon fault was initiated at a dip of 50°-60° and was rotated to a dip of ∼25°-30° during progressive extension before being cut and further rotated by younger faults. Restoration of Tertiary units also suggests that the doming of Paleozoic units across the is a Tertiary feature and that associated low bedding-to-fault angles in the footwall of the Bernd Canyon fault are the result of penetrative inhomogeneous simple shear deformation of the footwall. Including faults outside the area of detailed study, ∼20 km of east-west extension occurred across a domain that is now ∼45 km long (north-south) by ∼35 km wide (east-west). Extension apparently began in earliest Oligocene time, before or perhaps synchronously with, the onset of volcanism, and it continued into the Pliocene epoch. Tilt directions of Oligocene volcanic rocks, calcite veining near the detachment, and elongate fossils in footwall units indicate that the extension direction on the detachment and related faults may have been nearly east-west to possibly east-northeast-west-southwest. Changing tilt directions in younger rocks suggest that the extension direction switched to west-northwest-east-southeast at some time. Palinspastic restoration of normal faults also suggests that Oligocene basins were initially bounded by high-angle faults and had topographically elevated footwalls. The restoration implies that the styles of Miocene to Recent basin and range faulting and pre-Miocene pre-basin and range faulting are closely similar.

Depositional Setting and Paleogeography of Ordovician Vinini Formation, Central Nevada

The eugeoclinal strata of the Ordovician Vinini Formation composes most of the Roberts Mountains allochthon (RMA). Its stratigraphy, reconstructed in the Roberts Mountains, can be precisely correlated throughout much of the RMA and into coeval strata of the autochthonous miogeocline by means of graptolite and conodont biostratigraphy. The Vinini is a mixture of clastic lithologies with some limestone and greenstone. Coarse clastics, characterized by well-rounded and well-sorted sands derived from cratonic sources, occur in two separate intervals and provide critical data on depositional and paleogeographic setting. The lower interval, hundreds of meters thick, was deposited in the latest Ibexian-earliest Whiterockian and can be closely correlated throughout central Nevada. It is a mixture of quartz sandstone, siltstone, limestone, and calcareous sandstone, deposited by turbidity flows during a lowstand of sea level. Both eastern and western sources have been interpreted for these sands. The upper interval is a prominent, pure quartzite up to 20 m thick deposited as a prograding blanket of sand in the latest Whiterockian-earliest Mohawkian. It occurs in the Vinini in the northern Toquima Range and in a parautochthonous sequence of transitional strata in the southern Toiyabe Range. This interval represents the most basinward edge of the Eureka Quartzite,more » which was also deposited across the miogeocline. The presence of sand of the Eureka Quartzite in the RMA indicates that the basin in which the RMA strata were deposited was immediately adjacent to the margin of North America in the Ordovician.« less

Paleogeography and Evolution of the Ordovician/Silurian (Whiterockian-Llandoverian) Continental Margin in Central Nevada

In central Nevada, stratigraphic successions of Whiterockian-Llandoverian lithofacies, transitional with autochthonous platform/shelf carbonates to the east, occur in isolated windows in outer slope to basinal lithotopes of the Roberts Mountains allochthon. Petrologic, chronostratigraphic and lithostratigraphic, and paleontologic comparison of those successions with platform/shelf facies to the east is integral for reconstruction of Ordovician-Silurian platform margin paleogeography and pre-Antler genesis of the western North American continental margin. Numerous facies changes and/or stratigraphic omissions in central Nevada can be related to sea level fluctuation and aggradation/progradation of the carbonate platform to the east, and not to a postulated, offshore geanticline (i.e., the Toiyabe Ridge). Stratigraphic omission of the Eureka Quartzite above Pogonip equivalents in transitional successions of the Toquima Range and the presence of correlative quartzite in outer slope/basinal parautochthonous facies of the Toiyabe Range suggest development of a possible bypass-margin during the Middle Ordovician. Deposition of Late Ordovician platform margin dolostones (Ely Springs Dolostone) and upper ramp limestones (Hanson Creek Formation and Martin Ridge strata) followed Late Ordovician transgression that drowned the margin and reestablished the carbonate factory. Glacioeustatic drawdown of Late Ordovician-earliest Silurian seas due to the Gondwanan glacial fluctuation can be recognized in strata along the platform margin andmore » upper ramp. Rapid, Early Silurian transgression produced dark-gray carbonates and may have induced marginal flexure and regional, massive slope failure in central Nevada, generating stratigraphic hiatuses west of the platform margin.« less

Origin of kink bands in the Golconda allochthon, Toiyabe Range, Nevada

The orientation of monoclinal and conjugate kink bands in three thrust sheets of the Golconda allochthon in the southern Toiyabe Range, Nevada, provides information about shortening directions during the late-stage emplacement of the allochthon. The kink bands are the youngest of the structures within the allochthon and postdate the stacking of internal thrust sheets. The reoriented attitudes of the kink bands along the Golconda thrust, the sole of the allochthon, indicate that the kinks formed before the latest movement along the thrust. Calculated shortening directions from kink bands are interpreted to represent synchronous orthogonal contractions that led to the formation of two discrete groups of monoclinal and local conjugate kink bands. One group apparently was caused by an east-west shortening subparallel to the direction of tectonic transport of the allochthon, and the other, by an approximately north-south shortening subparallel to the strike of the allochthon. The biaxial contraction may have been generated owing to the spoon-shaped configuration of the sole thrust of the allochthon. Kink bands probably were developed during ramping of the allochthon up the continental margin when cleavage rotated to gentler dips and erosional unroofing reduced the normal stress across the cleavage for slip to occur parallel to cleavage.

Late Paleozoic extension in the Great Basin, western United States

Geologic mapping in the Toiyabe Range in central Nevada has revealed the existence of normal faults of probable mid-Mississippian to Early Permian age that strike roughly east-west and dip northward. Additional evidence of uplift and erosion followed by mafic volcanism and subsidence suggests that much of the central and southern Toiyabe Range was affected by late Paleozoic extension. Similar patterns of late Paleozoic uplift and subsidence, together with local basaltic volcanism, are widespread in the western United States, suggesting that the continental margin was dominated by extension or transtension in Mississippian to Permian time. This extension was coeval with convergence between North America and South America across the Ouachita and Marathon belts, and the dynamic interaction of these two margins may, by analogy with the Cenozoic tectonics of Asia, have given rise to complex late Paleozoic deformation in the Ancestral Rocky Mountains and adjacent areas of the interior western United States.

Glacial Deposits and Tephra in the Toiyabe Range, Nevada, U.S.A.

The Toiyabe Range is a prominent north–south-trending range in the Great Basin, a region whose glacial history is poorly known. The most complete sequences of glacial deposits occur in north- and e...

Glacial Deposits and Tephra in the Toiyabe Range, Nevada, U.S.A.

The Toiyabe Range is a prominent north–south-trending range in the Great Basin, a region whose glacial history is poorly known. The most complete sequences of glacial deposits occur in north- and e...

Paleogeographic and tectonic implications of the Golconda allochthon, southern Toiyabe Range, Nevada

The Golconda allochthon in the southern Toiyabe Range, Nevada, is composed of several thrust slices of rocks that belong to three age categories: early Paleozoic, Early Triassic, and late Paleozoic. These slices were stacked mainly during the Sonoma orogeny that occurred between Permian and late Early Triassic time. The late Paleozoic rocks, composing the bulk of the allochthon, range in age between Mississippian and Late Permian and contain four lithotypes that occur in the following structural order above the Golconda thrust: terrigenous, hemipelagic, volcanogenic, and pelagic. The upper Paleozoic rocks were deformed several times during their eastward emplacement onto the North American continental margin. The deformation involved isoclinal folding and thrusting of these rocks into slices. The slices were openly and gently folded during emplacement, and sporadic mesoscopic folds were formed along their-boundaries. Post-thrust movements along slice boundaries formed kink bands and related folds in slate and chert interbeds. The kinks were later deformed when the Golconda thrust was reactivated. Further contraction, during the Sonoma orogeny or possibly later, was accommodated by imbrication of slices of Early Triassic and their unconformably underlying lower Paleozoic rocks.

Paleozoic Stratigraphy of Toiyabe Range, Southern Lander County, Nevada

A sequence of Paleozoic sedimentary rocks, ranging in age from Cambrian to Carboniferous, was deposited in a transitional zone between the mio- and eugeosynclinal belts and now is exposed in a large area of the central Toiyabe Range south of Austin, Nevada. A maximum of 3,250 ft of quartzite, graywacke, and some limestone constitutes the Gold Hill Formation. Archaeocyathids (Archaeocyathus atlanticus Billings and Ethmophyllum whitneyi Meek), locally abundant at the top of the lower Gold Hill Formation, indicate an Early Cambrian age. The Cambro-Ordovician Broad Canyon Formation (new name), consisting of intensely deformed limestone, argillaceous siltstone, and phyllite, is about 2,000 ft thick. Antelope Valley Limestone, up to 800 ft thick, contains the Palliseria assembl ge (Middle Ordovician) near the top in two areas. Remnants of the Silurian Masket Shale (Roberts Mountains Formation) overlie the Antelope Valley with regional disconformity. Devonian corals are present in a massively bedded limestone overlying the Masket in one locality. A Tertiary conglomerate preserves boulders of coralline limestone and chert pebbles of the Carboniferous Wildcat Peak Formation. An allochthonous sequence lies tectonically above and next to the transitional succession which is considered to be autochthonous. It consists mainly of Paleozoic chert and argillite about 10,000 ft thick, and, in one small area, of volcanic rocks, chert, and siltstone, perhaps of the Charcoal Canyon Formation. Only poorly preserved radiolarians have been found in these rocks.