Search

A plethora of names has been applied to upper Cambrian and Lower Ordovician lithostratigraphic units distributed from western Utah to southeastern and central Nevada. We report comprehensive stratigraphic analysis utilizing lithologic, conodont, brachiopod, sequence stratigraphic, and carbon isotope data from several ranges in southeastern Nevada. Those data demonstrate that upper Cambrian to Lower Ordovician strata in that area belong to a variety of depositional systems that can be integrated into a comprehensive and functional depositional model that span carbonate tidal flat to shallow marine carbonate ramp to shelf to deep marine settings. Most of these strata should be placed within a single lithostratigraphic nomenclature based on the well-known Notch Peak Formation and House Limestone of western Utah. Strata of the Whipple Cave Formation in the South Egan Range, Nevada are reassigned to the three members of the Notch Peak Formation. Strata assigned to the Nopah Formation and Pogonip Group by geologic mappers who worked in the Delamar Mountains, Meadow Valley Mountains, Sheep Range, and Arrow Canyon Range in Nevada are reassigned to the Notch Peak Formation and overlying House Limestone. Coeval strata farther west record a deeper marine depositional setting and are appropriately assigned to the Windfall Formation and Goodwin Limestone. Conodonts and sequence stratigraphic packages identified from slope deposits of the Hales Limestone at Tybo Canyon in the Hot Creek Range allow correlation to the Notch Peak Formation and House Limestone in western Utah. [ABSTRACT FROM AUTHOR]

• Carbonate δ44/40Ca and Sr/Ca from Meiklejohn Peak, Nevada during positive δ13C shift. • Significant covariation between δ44/40Ca – Sr/Ca, but not δ44/40Ca – δ13C. • Numerical models used to characterize contributions from mineralogy and diagenesis. • Variations in mineralogy and diagenesis may influence δ44/40Ca and Sr/Ca. • A change in DIC (locally or globbaly) is needed to reproduce the δ13C trend. The Middle Ordovician Darriwilian Stage (∼469 – 458 Ma) records a ∼2‰ positive carbon isotope shift known as the MDICE (Mid-Darriwilian Carbon Isotope Excursion). Although studies have shown that the MDICE is a globally synchronous event, the link between the MDICE and changes in the global carbon cycle remains unclear. This is largely due to possible local processes including diagenesis and variations in carbonate polymorphs that can obscure δ13C signals recorded in shallow marine carbonate rock from the global dissolved inorganic carbon reservoir. Here, we use paired measurements of δ13C, δ44/40Ca, and Sr/Ca from a stratigraphic section at Meiklejohn Peak in southwest Nevada (USA) to constrain the potential for local processes in decoupling the recorded δ13C signals from the global carbon cycle. We find that variations in δ44/40Ca and Sr/Ca in this section at Meiklejohn Peak can in part be explained by changes in primary mineralogy and diagenesis. However, these processes are unable to reproduce the entire shift observed in δ13C, with the remaining portion of the δ13C curve driven by either local changes in platform water mass dissolved inorganic carbon (DIC) or global carbon cycling. While we cannot completely deconvolve local signals from the recorded δ13C, we argue that the MDICE may, at least in part, reflect a primary signal of increase organic carbon burial related to changes in primary productivity and nutrient delivery during increased basaltic weathering associated with the Taconic uplift. The proposed link between tectonic uplift and increase in primary productivity strengthens the notion that tectonic processes played a significant role in modulating changes in the global carbon cycle during the Ordovician Period. [ABSTRACT FROM AUTHOR]

It remains unclear whether waning of the volcanic degassing CO2 source or enhancement of the mafic (Ca, Mg-silicate) weathering CO2 sink, or both, caused global cooling leading to the Ordovician greenhouse--icehouse transition. We present a uniquely age-constrained and integrated Middle--Late Ordovician (470-450 Ma) continental weathering isotopic proxy data set (87Sr/86Sr and εNd(t)) from carbonate rocks of the Antelope Range of central Nevada, USA, paired with published paleotemperature proxy measurements (δ18O) of conodont apatite from the same locality. This suite of proxy records signals an increase in mafic weathering of the Taconic mountains (eastern United States) at ca. 463 Ma, which forced a period of global cooling. We adapt a 87Sr/86Sr and pCO2 mass balance approach to model CO2 drawdown during the Ordovician, and show that a combined decrease in volcanic degassing and increase in mafic weathering approximately halves pCO2 in agreement with δ18O trends and paleotemperature reconstructions. [ABSTRACT FROM AUTHOR]

Heckethornia n. gen. is a morphologically striking clade of dimeropygid trilobites known from the Pogonip Group (Ordovician; Ibexian Series, Tulean and Blackhillsian stages) of western Utah and eastern Nevada. It includes seven species based on silicified material. All occur in the Fillmore Formation of western Utah, and four are also known from the Yellow Hill Limestone of eastern Nevada. Diagnostic features of Heckethornia include a highly vaulted exoskeleton with a tall pygidial “wall” made of fused outer pleurae, three pairs of large glabellar spines, two to three pairs of prominent pygidial spines, a single (or paired) large occipital spine(s), and an arc of tubercles on the librigenal field. Cladistic parsimony analysis suggests that the genus comprises two subclades, one including species with a single median occipital spine, and species with paired occipital spines or tubercles in the other. New species are H. smithi, H. hyndeae, H. numani, H. bowiei, H. morrisseyi, and H. ballionae. [ABSTRACT FROM AUTHOR]

Focuses and characterizes the earliest Whiterockian faunas during the Middle Ordovician period in Nevada. Prominence of trilobites as component of the Ordovician faunas; Meiklejohn Peak; Type species; Etymology; Holotype; Species variation.

Analyzes the three stratigraphic sections in central Nevada in relation to the Late Ordovician mass extinction. Conduct of integrated sequence stratigraphic, biostratigraphic, and chemostratigraphic analyses; Formation of diachronous, stepwise faunal turnover in graptolites, conodonts, chitinozoans and radiolarians.

Field-based revision and phylogenetic analysis demonstrate that the pliomerid trilobite taxon Ibexaspis Přibyl and Vaněk (in Přibyl et al., 1985), previously known from a single formally named species (I. brevis [Young, 1973]), belongs to a complex of 14 mostly newly discovered, related species from the Early Ordovician (Floian; Tulean and Blackhillsian) of the northern Laurentian margin. The species are known from silicified samples recovered from sections in eastern Nevada, western Utah, and southeastern Idaho. The stratigraphically early Tuleaspis n. gen. (type species: T. jeneki n. sp.; Tulean; low Protopliomerella contracta Zone) includes its type and two species described in open nomenclature. Tuleaspis is sister to the remainder of the clade. Ibexaspis now includes three additional species: I. coadyi n. sp. (Blackhillsian; Carolinites nevadensis Zone), I. leuppi n. sp. (Blackhillsian; Presbynileus ibexensis Zone), and I. rupauli n. sp. (Blackhillsian; "Pseudocybele nasuta Zone"). Ibexapsis is sister to a clade of Millardaspis n. gen. + Deltapliomera n. gen. Millardaspis (type species M. milsteadi n. sp.; Tulean; Heckethornia hyndeae Zone) also includes M. knoxi n. sp. (Tulean; Panisaspis sevierensis Zone). Deltapliomera (type species D. humphriesi n. sp.; Blackhillsian, Carolinites nevadensis Zone) also includes D. inglei n. sp. (Tulean; Heckethornia bowiei Zone), D. heimbergi (Tulean; Panisaspis sevierensis Zone), D. eppersoni n. sp. (Blackhillsian; Bathyurina plicolabeona Zone), and a species described in open nomenclature.

Identifies conodonts representing several species and genera from the Upper Ordovician rocks in central Nevada. Geographic distribution; Amorphognathus ordovicicus; Gamachignathus ensifer; Oulodus ulrichi; Morphological features.

The outcrop at Whiterock Canyon Narrows, Nevada, is the stratotype for the Middle Ordovician Whiterockian Series in Laurentia. Contrasts in the distribution of trilobites and conodonts between two parallel sections at the stratotype demonstrate the presence of an unconformity separating Ibexian faunas (the historic trilobite Zone J, the more recent restricted “Pseudocybele nasuta” trilobite Zone, the Reutterodus andinus conodont Zone) from overlying Whiterockian faunas (Zone L, the Psephosthenaspis pseudobathyurus trilobite Zone, the Orthidiella brachiopod Zone, the Tripodus combsi conodont Zone). The unconformity represents the erosional loss of a minimum of 10 m of shale and bedded limestone within the upper Ninemile Formation. In comparison to more continuous section in the Ibex region, Utah, the equivalent to the thin Psephosthenaspis microspinosa trilobite Zone and, possibly, the uppermost interval of the underlying “P. nasuta” Zone are missing. Illaenus welchi new species is described from the Whiterockian faunas of the upper Ninemile and Antelope Valley formations. [ABSTRACT FROM AUTHOR]

The new edrioasterine Pseudedriophus guensburgi n. gen. n. sp., is described from the Lower Ordovician Ninemile Shale of central Nevada based on three complete to partial small specimens, a well-preserved large ambulacrum, and an isolated ambulacral floor plate. The weathered-out holotype of this edrioasterine exposes the bottom surface of the theca that bears an aboral collar, peduncular stalk, and attachment disk, features that are poorly known in this clade. These specimens were found with a single specimen of a new edrioblastoid, Porosublastus inexpectus n. gen. n. sp., only the second edrioblastoid ever found in the Early Ordovician. Some of the ambulacral cover plates are stripped off one of the ambulacal grooves, revealing new information about how the ambulacra are built in this rare group of bud-shaped edrioasteroids. [ABSTRACT FROM PUBLISHER]

A large drop in seawater 87Sr/86Sr during the Middle Ordovician was among the most rapid in the entire Phanerozoic. New 87Sr/86Sr measurements from Nevada indicate that the rapid shift began in the Pygodus serra conodont zone of the upper Darriwilian Stage. We use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic Sr to the oceans. A close balance between volcanic outgassing and CO2 consumption from weathering produced steady pCO2 levels and climate through the middle Katian, consistent with recent Ordovician paleotemperature estimates. In the late Katian, outgassing was reduced while volcanic weathering continued, and resulted in a cooling episode leading into the well-known end-Ordovician glaciation. [ABSTRACT FROM AUTHOR]

Sulfur isotopes measured from sedimentary rocks are used to reconstruct the global sulfur cycle and chemostratigraphic correlation. Relative and absolute changes in the sulfur isotopic composition of the ocean (δ34S seawater) are essential for estimating changes in redox conditions in ancient environments, particularly for geochemical models that utilize isotope mass balance. Thus, accurate estimates of δ34S seawater must be measured using a laboratory protocol optimized for the extraction of primary sulfate (preserved as carbonate-associated sulfate; CAS) by removing any contaminant secondary sulfate (e.g., from diagenetic pyrite oxidation). Here we use two CAS-extraction protocols on Lower–Middle Ordovician carbonate rocks to test the degree to which similar absolute values and stratigraphic trends in δ34S CAS are produced by different CAS extraction methods. One method processes carbonate powders once with a single rinse of a 10% NaCl solution to remove sulfate minerals or weakly bonded sulfate ions, followed by a rinse in 5–6% bleach to remove organically bound sulfur before dissolution in hydrochloric acid (HCl). The second method treats carbonate powders with three rinses of NaCl, without a bleach rinse, to more aggressively remove secondary sulfate before dissolution in HCl. Isotopic results show that samples treated in three NaCl rinses produce δ34S CAS values that are on average 7‰ more positive than samples treated with a single NaCl rinse, but a similar stratigraphic trend is preserved in samples processed with either method. We interpret the difference in δ34S CAS between methods to reflect the incomplete removal of secondary sulfate derived from the oxidation of 32S-enriched pyrite when employing only a single NaCl rinse. Surprisingly, rocks with low CAS concentrations (≤10 mg/kg) using both methods show no significant difference in δ34S CAS values, as well as no significant difference in pyrite concentrations and pyrite δ34S (δ34S PY) between methods. Samples with low CAS concentrations are likely altered and secondary sulfate sourced from pyrite oxidation has likely become incorporated into recrystallized carbonate minerals or incorporated into pore-filling calcite cement, all of which is captured as CAS. Results suggest that both CAS-extraction protocols can produce broadly similar δ34S CAS trends, but a single NaCl rinse may not completely remove contaminant sulfate sorbed onto carbonate grains and thus will not yield meaningful data for reconstructing δ34S seawater. This has clear implications for modeling studies aimed at reconstructing paleo-redox conditions using δ34S data assumed to record global δ34S seawater trends if these δ34S data are generated using a non-optimized CAS protocol. [ABSTRACT FROM AUTHOR]

A continuous trench exposure within the uppermost type Vinini Formation at Vinini Creek, Roberts Mountains, Nevada, provides an unparalleled opportunity to examine the fate of graptolites, prominent Paleozoic zooplankton, during most of the Hirnantian mass extinction event. On the basis of a detailed biostratigraphic and sedimentological dataset, the relatively complete extinction record is examined in the context of ecological constraints, and it is found to reflect an ecological collapse driven by glacio-eustatic sea-level fall and associated changes in oceanic circulation. Diverse graptolite populations of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which flourished in denitrifying waters within the oceanic oxygen-minimum zone (OMZ) during sea-level highstand, largely vanished with the loss of these conditions during glacio-eustatic sea-level fall. However, populations of one clade, the normalograptids, which inhabited the oxygenated waters of the photic zone, not only survived but diversified. These survivors gave rise to rapid recolonization and diversification with re-establishment of the oxygen-minimum and denitrifying conditions during post-Hirnantian sea-level rise. This ecological model also applies globally to other well-documented coeval stratigraphic intervals, representing both oceanic and platform sea settings. [ABSTRACT FROM AUTHOR]

Microbialite morphostratigraphy is a new tool for intrabasinal correlation using diverse microbialite structures (morphotypes). The recognition of the succession of morphotypes over constrained temporal intervals and broad areas is a function of the complex interactions that operate to create the structure. Because so many nonlinked variables (e.g., biotic, sedimentological, physicochemical) are involved, similar morphotypes do not reoccur over long temporal intervals. To demonstrate the technique, the upper Cambrian-lowermost Ordovician shelf strata of the Great Basin, United States, were correlated using both morphostratigraphy and standard lithostratigraphy. Six morphozones and one morphosubzone were recognized, as were four main lithologic successions. Because the boundaries between the morphozones and lithologic successions did not coincide, it is inferred that the characteristics of the various microbialite structures are not solely controlled by physical factors. The principles for establishing a morphostratigraphy outlined in this article allow for the potential to correlate along other ancient marine margins in both the same Cambrian and Ordovician interval, as well as any interval in the Phanerozoic in which diverse microbialite structures occur.

The Eureka Quartzite is a sheet-like quartzarenite up to 200-m thick that was deposited on the eastern shelf of the Cordilleran miogeocline from Canada to California. It is the only sandstone lithosome from the Middle Cambrian through Devonian succession in the Great Basin and is remarkable in its purity of detrital and authigenic quartz, scarcity of bedding, and heterogeneity of both grain packing and quartz cement abundance. Sand sources ranged from the Peace River Arch in Canada to the Transcontinental Arch in mid-continental North America. The Eureka represents a third-order regressive--transgressive stratigraphic sequence, although whether the regression formed in response to eustasy or epirogenic uplift of western North America is unresolved. The near absence of detrital clay, body fossils, and subaerial features in addition to the presence of herringbone cross-beds indicate that the Eureka was deposited in intertidal and shallow subtidal environments except for minor eolian deposits. Bioturbation destroyed most primary stratification, although discrete burrows are rare. Textural features of quartz (superb roundness, bean grain shape, crescentic impact scars) indicate a prolonged episode of eolian abrasion prior to marine deposition. Regionally the detrital composition is 99.5 percent detrital monocrystalline quartz, 0.5 percent K-feldspar and carbonate allochems, and a trace of heavy minerals. From 2 percent to 4 percent feldspar and carbonate allochems that were initially present have been replaced by quartz during burial. The chief authigenic phases are quartz overgrowths with minor calcite (now dolomite) and illite. Spheroidal and amoeboid calcite-cemented concretions up to 3 cm in diameter formed at shallow burial depths, but all carbonate in the concretions has been leached in outcrop. The heterogeneity of grain compaction and amount of quartz cement resulted in beds that range from semifriable to sedimentary quartzites in the same outcrop. Compaction by the combination of grain rearrangement, pressure dissolution, and grain fracturing generated anomalously low intergranular volumes that average 14 percent in Nevada and 21 percent in Utah. The normal evolution of microquartz overgrowths (<10 μm) to meso and macroquartz overgrowths (>10 μm) during cementation was retarded; consequently, microquartz and mesoquartz cement (80 percent) dominate over macroquartz (20 percent), and they are the only cements in the least-cemented beds and laminations. Illite co-precipitated with microquartz and impeded quartz cementation by coating quartz crystal faces. Despite reaching temperatures >135° C for ~100 million years, much of the Eureka, especially in Utah, remains incompletely cemented and retains porosity of ~2 percent. The chief cause of cement heterogeneity appears to be authigenic illite abundance. Pressure dissolution of quartz at shale beds and clay drapes that formed stylolites was the most likely major source of silica for quartz cement. Invasion by hydrocarbons and hydrogen sulfide (H2S) resulted in the reduction of iron in hematite grain coats on quartz grains, a remnant of their eolian dune formation, the bleaching of beds, and the generation of pyrite (now hematite). The abundance of iron that now resides in pyrite/hematite suggests that red beds were once widespread. Outcrop and near-outcrop processes generated Liesegang bands of iron oxide, desert varnish, "pockmarks" where carbonate cement in centimeter-scale concretions dissolved, hematite pseudomorphs of pyrite, and minor opal cement. [ABSTRACT FROM AUTHOR]

Abstract: The impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the Late Ordovician Hirnantian glaciation. Three sections were studied: two from Nevada, one proximal and one distal; and one from the Yukon. The opportunity to compare δ 13 C carb, δ 13 C org and δ 13 C grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater δ 13 C values at each stratigraphic section. Using the best preserved sedimentary components from the two Nevada sections, a lowstand sedimentary gradient of ∼4‰ was found between platform and ocean settings during the Hirnantian glaciation. Gradients of similar magnitude have been reported from Hirnantian platforms in the Canadian Arctic and Baltic basins. Taking the outboard sections as the most representative of the ocean carbon pool, a mean, positive δ 13 C shift of just 2.7±0.4‰ is calculated, which is significantly smaller than the 5–7‰ positive shifts recorded in epeiric platform settings. The discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas. It is hypothesized that increased carbonate weathering caused δ 13 C values of local carbon weathering fluxes to increase, which in turn caused seawater δ 13 C values to increase in basin proximal settings. In addition, nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings, driving sedimentary δ 13 C values even higher—much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive. A δ 15 N profile reconstructed from deposits of the distal Vinini Creek section records a 1.5‰ positive shift through the Hirnantian glacial interval. Interpreted as a major Late Ordovician upwelling zone, high organic carbon (>1 wt.%) and high hydrogen indices (400–600) make the Vinini Creek section a suitable candidate for the preservation of original sedimentary δ 15 N records. To explain the results, a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers. It is hypothesized that in greenhouse climates, the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean''s expansive oxygen minimum zones. This caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone, yielding sedimentary δ 15 N values between 0 and −2‰. By contrast, the cooler icehouse climate of the Hirnantian led to increased ocean ventilation, greater partitioning of atmospheric oxygen into downwelling surface waters, and oxygen minimum zone shrinkage. As a result, fixed nitrogen levels climbed in the ocean''s abyssal depths due to the lower denitrification rates. This in turn allowed nitrogen with high δ 15 N values to recycle back into the photic zone, driving sedimentary δ 15 N values higher during the Hirnantian glaciation. [Copyright &y& Elsevier]

Abstract Cambrian–Ordovician sponge-microbial mounds in the Great Basin of the western USA reveal reef structure and composition immediately prior to the Great Ordovician Biodiversification Event (GOBE). Here we describe lithistid sponge-microbial reefs from the upper Cambrian (Furongian, Stage 10) strata of the Arrow Canyon Range, Nevada. The reefs are mound-like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin-walled, bowl-shaped anthaspidellid sponge, columnar microstromatolite fabric, and the calcified microbe Angusticellularia. The reefs formed in low-energy, subtidal environments in which lime mud filled spongocoels and inter-reef spaces around undisturbed, in place, thin-walled sponges. The reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals. The mutually attached lithistid sponges form the main framework of the reefs. These thin-walled and bowl-shaped lithistids most likely were adapted to low-energy environments. Spaces beneath the overhanging sponge walls were filled by microbial carbonates. These include pendent micro-dendritic Angusticellularia attached to dermal sponge surfaces and upward-growing masses of microstromatolites. After death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft-bodied burrowing organisms and keratose-like sponges. These lithistid sponge-microbial reefs, together with an earlier example of late Cambrian (Paibian) dendrolite-lithistid reefs in the same area, characterize skeletal-microbial reefs immediately prior to the GOBE. Highlights • Lithistid sponges formed reef frameworks in the upper Cambrian of Nevada, USA. • Reefs dominated by thin-walled sponges developed under low-energy conditions. • Spaces beneath the overhanging sponge walls were filled by microbial carbonates. • In the Great Basin, gradual reef transition occurred across Cambro-Ordovician boundary. • Regional disparities in reef evolution pattern during early stage of the GOBE are recognized. [ABSTRACT FROM AUTHOR]

Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny. [ABSTRACT FROM AUTHOR]

Carlin-type gold deposits are among the most important gold-bearing hydrothermal ore systems and are mainly located in Nevada, USA, and southwestern China. However, the source and evolution of the ore-forming fluids for these deposits remain controversial, especially those found within China. In this study, lithium and oxygen isotopic analyses of quartz-hosted fluid inclusions are used to elucidate the source and evolution of the giant Shuiyindong Carlin-type gold deposit. Fluid inclusions trapped in quartz of three distinct genetic stages have low salinity (0.8–6.3wt% NaCl equiv.) and moderate temperature (154–343 °C), but display variable Li and O isotope signatures. The Li and O isotopes of stage I fluids (δ7Li values from + 5.1 to + 9.1‰; δ18O values from + 6.3 to + 10.0‰) indicate predominantly a magmatic source for the initial ore-forming fluids. The large variations of Li and O isotopes of stage II fluids (δ7Li values from + 9.3 to + 16.0‰; δ18O values from + 0.1 to + 7.7‰) suggest that the fluids are controlled by mixing of magmatic fluids and meteoric water, which in turn triggered the precipitation of gold-bearing sulfides. The isotopic compositions of stage III fluids (δ7Li values from + 15.5 to + 22.8‰; δ18O values from − 5.4 to − 2.8‰) confirm that the final fluids are dominated by meteoric water. Furthermore, this work demonstrates that the combined Li–O isotopic analysis of fluid inclusions is a powerful tracer to decode the source and evolution of ore-forming fluids in hydrothermal mineralizing systems. [ABSTRACT FROM AUTHOR]

Abundant cryptospore-like microfossils have been collected from a sample of the 'Middle' Cambrian Log Cabin Member of the Pioche Shale in eastern Nevada, USA. Compared with organic-walled microfossils obtained from the Cambrian Kaili Formation in eastern Guizhou Province, China, some specimens of leiosphers may be inferred to be similar cryptospore-like microfossils. The new fossil record would be the oldest known specimens of cryptospore-like microfossils during the Cambrian Period. [ABSTRACT FROM AUTHOR]

Two non-trilobite arthropods are described from the Emigrant Formation (Lower Cambrian-Lower Ordovician) in the Silver Peak Range, Esmeralda County, Nevada. A Middle or Upper Cambrian "arachnomorph" arthropod with a phosphatic exoskeleton has been noted in previous faunal lists, but has not been previously described. This fossil is here named Quasimodaspis brentsae gen. et sp. nov. Q. brentsae belongs in the Aglaspidida, a close outgroup to the true chelicerates; this is the second report of an aglaspidid from the Great Basin. Esmeraldacaris richardsonae gen. et sp. nov. is a newly discovered arthropod from the lower Ordovician, from beds transitional between the Emigrant Formation and the overlying Palmetto Formation. It is a survivor of an early arthropod lineage that does not belong in any extant taxon, but which may also include the Ordovician Corcorania and the Cambrian Mollisonia. [ABSTRACT FROM AUTHOR]

This study describes an assemblage of Dzieduszyckia brachiopods preserved in sedimentary barite deposits from the Famennian, Upper Devonian of Nevada, USA. The brachiopods lived in a methane-seep environment like those described from Sonora, Mexico, and the Western Meseta, Morocco. Although the original carbonate fabrics are not preserved, pockets of limestone within the brachiopod barite are depleted in with δ13C values ranging from -31.1 to -27.3‰ VPDB. The fossils are preserved as internal and external molds only but record enough detail to demonstrate key internal features of Dzieduszyckia such as a pronounced dorsal septum, septalium, and vertical dental plates. The lack of common bifurcation or trifurcation of the costae as well as larger size of the shells recommends against inclusion within D. sonora as previously published. Based on the number of costae per width of shell, it is possible that there are two distinct species in Nevada, like other localities in Morocco, the Urals, and South China. These new data expand the body of knowledge of these earliest dimerelloids and it is hypothesized that this seep-dwelling macrofauna arose from depauperate-successful lineages between the Famennian biotic crises. As nearly all global Dzieduszyckia deposits are associated with active tectonic margins, it is also suggested that tectonically-driven methane expulsion in low nutrient waters spurred the evolution of dimerelloids. [ABSTRACT FROM AUTHOR]

The Emigrant Formation at Clayton Ridge, Nevada, consists of 420 m of variably interbedded shale, siltstone, and limestone representing an outer shelf environment. It ranges in age from the youngest portion of the Dyeran Stage (Laurentian Waucoban Series; Cambrian Stage 4, Series 2) to Skullrockian Stage (Ibexian Series; Tremadocian Stage, Lower Ordovician). Consequently, it is condensed compared to other localities. For example, the age-equivalent succession in the House Range and the Wendover Range of Utah and Nevada, respectively, is seven times thicker. The 100m thick lower limestone and siltstone member of the Emigrant Formation at Clayton Ridge ranges in age from the Bolbolenellus euryparia/Nephrolenellus multinodus Zone (Laurentian Dyeran Stage) to the Cedaria brevifrons Zone (Laurentian Marjuman Stage; Drumian Stage; this study). The 40 trilobites described herein from two closely spaced sections of the lower member belong to the Glossopleura walcotti/Ptychagnostus praecurrens Zone (Laurentian Delamaran Stage; Stage 5), Ehmaniella Zone (Laurentian Topazan Stage; Stage 5), and Cedaria selwyni to C. brevifrons zones (Laurentian Marjuman Stage; Drumian Stage) zones. The lower Emigrant Formation is <10% as thick as ageequivalent age strata elsewhere in Nevada, California, and Utah. This condensed nature of the lower Emigrant Formation began with the flooding event at the beginning of the Oryctocephalus indicus biochron and the subsequent build-ups of the thick carbonate platforms to the south and northeast. [ABSTRACT FROM AUTHOR]

Several similar indicators in Nevada (USA) and South Yakutia (Russia) gold-bearing provinces have been identified based on modern tectonic, geophysical and seismic tomography observations, followed by the analysis of the main geodynamic factors of the formation and distribution of large gold-bearing provinces in the North Pacific. One of the significant metallogenic peculiarities is a wide variety of formational and mineral deposits concentrated in the areas. Both provinces are situated at active margins surrounded by fold-thrust belts. In South Yakutia, a combination of sublatitudinal Baikal-Elkon-Ulkan and submeridional Seligdar-Verkhnetimpon gravity field gradient zones is recorded. In contrast, significant positive gravity anomalies of the Northern Nevada Rift and higher-order gradient zones are presented in Nevada. Large pluton and transform fault zones in both provinces support a conclusion about the fundamental role of geodynamic factors in developing ore-magmatic systems in the regions. Significant differences in the scale of the gold mineralization in the considered provinces are explained by the existence under the North American continent not only of the Mendocino transform fault zone but also of the Juan de Fuca paleo-spreading center. In contrast, the Inagli-Konder-Feklistov magmatic-metallogenic belt alone controls mineralization under the Asian continent. [ABSTRACT FROM AUTHOR]

The geologically rapid appearance of fossils of modern animal phyla within Cambrian strata is a defining characteristic of the history of life on Earth. However, temporal calibration of the base of the Cambrian Period remains uncertain within millions of years, which has resulted in mounting challenges to the concept of a discrete Cambrian explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation, Nevada. These data demonstrate the base of the Cambrian Period, as defined by both ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than 533 Mya, at least 6 My later than currently recognized. This new geochronology condenses previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated with global records, demonstrates an explosive tempo to the early radiation of modern animal phyla. [ABSTRACT FROM AUTHOR]

In surface mining operations, geological pit wall mapping is important since it provides significant information on the surficial geological features throughout the pit wall faces, thereby improving geological certainty and operational planning. Conventional pit wall geological mapping techniques generally rely on close visual observations and laboratory testing results, which can be both time- and labour-intensive and can expose the technical staff to different safety hazards on the ground. In this work, a case study was conducted by investigating the use of drone-acquired RGB images for pit wall mapping. High spatial resolution RGB image data were collected using a commercially available unmanned aerial vehicle (UAV) at two gold mines in Nevada, USA. Cluster maps were produced using unsupervised learning algorithms, including the implementation of convolutional autoencoders, to explore the use of unlabelled image data for pit wall geological mapping purposes. While the results are promising for simple geological settings, they deviate from human-labelled ground truth maps in more complex geological conditions. This indicates the need to further optimize and explore the algorithms to increase robustness for more complex geological cases. [ABSTRACT FROM AUTHOR]

The North American Newark Canyon Formation (NCF; ∼113–98 Ma) presents an opportunity to examine how terrestrial carbonate facies reflect different aspects of paleoclimate during one of the hottest periods of Earth's history. The lower NCF type section preserves heterogeneous palustrine facies and the upper NCF preserves lacustrine deposits. We combined carbonate facies analysis with δ13C, δ18O, and Δ47 data sets to assess which carbonate facies preserve stable isotope signals that are most representative of climatic conditions. Palustrine facies record the heterogeneity of the original wetland environment in which they formed. Using the pelmicrite facies that formed in deeper wetlands, we interpret a lower temperature zone (35–40°C) to reflect warm season water temperatures. In contrast, a mottled micrite facies which formed in shallower wetlands records hotter temperatures (36–68°C). These hotter temperatures reflect radiatively heated "bare‐skin" temperatures that occurred in a shallow depositional setting. The lower lacustrine unit has been secondarily altered by hydrothermal fluids while the upper lacustrine unit likely preserves primary temperatures and δ18Owater of catchment‐integrated precipitation. Resultantly, the palustrine pelmicrite and lacustrine micrite are the facies most likely to reflect ambient climate conditions, and therefore, are the best facies to use for paleoclimate interpretations. Average warm season water temperatures of 41.1 ± 3.6°C and 37.8 ± 2.5°C are preserved by the palustrine pelmicrite (∼113–112 Ma) and lacustrine micrite (∼112–103 Ma), respectively. These data support previous interpretations of the mid‐Cretaceous as a hothouse climate and demonstrate the importance of characterizing facies for identifying the data most representative of past climates. Plain Language Summary: Considered a "supergreenhouse" world, the Cretaceous (145–65 Mya) was one of the hottest periods in geologic history. Understanding how environments respond to extreme global warmth provides insights that will help to mitigate the harmful effects of modern climate change. Geochemical signals, like stable isotopes, preserved in rocks allow us to reconstruct past climate conditions. We use stable isotope geochemistry on limestones (rocks made of carbonate minerals) to estimate average mid‐Cretaceous (∼110 Mya) summer temperatures in Nevada, USA. We examine limestones that formed in wetlands and lakes to assess which types record signals representative of the original landscape and which have values that were altered after the rock formed. We find that carbonates deposited in the shallowest parts of wetland environments record temperatures much hotter than air or deeper water temperatures due to extreme land surface heating, while deeper wetland limestones preserve temperatures more representative of ambient conditions. Average warm season water temperatures for wetland and lake carbonates are approximately 41 and 38°C, respectively, revealing hot conditions that support our understanding of the Cretaceous as a greenhouse world. This research provides an example of how to interpret useful climate information from complex carbonate data sets. Key Points: Deeper palustrine and unaltered lacustrine carbonates record warm season water temperatures of approximately 41 and 38°C, respectivelyHigh temperatures from well‐preserved shallower palustrine carbonates likely reflect radiative heatingCareful facies analysis is imperative for robust paleoclimate interpretation of heterogeneous terrestrial carbonate archives [ABSTRACT FROM AUTHOR]

The Devonian to Carboniferous (DC) transition coincided with a green-to-ice house climatic shift, anoxia, disappearance of lower latitude carbonate banks, and turnover from warm-to-cool water carbonate factories. In western Laurentia, the switch to carbonate factories dominated by cool-water biota was contemporaneous with a tectonically driven palaeogeographic change. To investigate this depositional shift and infer the relative impact of climate vs tectonics, a continental-scale sedimentological and geochemical study was conducted on twelve stratigraphic sections of DC strata from western Canada to southern Nevada (USA). The spatial–temporal distribution of microfacies records the turnover from [i] a Famennian lime mud-rich, shallow warm-water carbonate ramp with low sedimentation rates, mesotrophic conditions and tabular geometry to [ii] Tournaisian to Viséan lime mud-depleted and grainstone dominated cool-water carbonate ramp with anomalous high sedimentation rates, oligotrophic conditions and a pronounced slope. Positive excursions of δ18Ocarb (+ 2‰ V-PDB) and δ13Ccarb (+ 4‰ V-PDB) of Lower Mississippian carbonates likely correspond to the first cooling peak of the Carboniferous-Permian icehouse climate, following carbon withdrawal during black shale deposition during the late Famennian and early Tournaisian. However, late Tournaisian return of photozoan elements and their persistence throughout the Viséan suggests that warmer surface water existed, revealing a decoupling of the lower latitude ocean and the atmosphere. Shoaling of the thermocline was likely a result of cold-water upwelling along an open coast, as the Antler orogen no longer provided an oceanic obstruction to the west. This study shows that carbonate platforms are more susceptible to regional changes than global shifts. [ABSTRACT FROM AUTHOR]

The Hilltop, Lewis, and Bullion mining districts, located within the northern Shoshone Range, Lander County, Nevada, are part of the greater Battle Mountain-Eureka trend. Historic production of mostly Ag with An, Cu, Pb, and Zn from all three districts exploited ≤3-m-wide quartz veins hosted mainly within upper plate Ordovician-Devonian siliceous and siliciclastic rocks of the Roberts Mountains allochthon. Eocene felsic intrusive rocks, located both proximal and distal to the vein deposits, were emplaced throughout the northern Shoshone Range along a west-northwest trend; molybdenite mineralization is associated with some intrusive rocks, and some intrusive rocks host vein-type mineralization. New and previously published radiometric ages for primary igneous minerals from unaltered rocks within and/or proximal to mineralized areas are nearly coincident with published Re-Os ages for several northern Shoshone Range molybdenite occurrences (40.1 ± 0.6 Ma, avg), supporting a relationship between pluton emplacement and molybdenite mineralization. Age differences between quartz vein-hosted gold ± base metal mineralization (e.g., 38.3 ± 0.1 Ma: Gray Eagle mine, Bullion district) and nearby intrusive rocks (e.g., 39.0 ± 0.1 Ma: Granite Mountain) imply that vein mineralization probably formed during prolonged hydrothermal activity, synchronous with or subsequent to igneous rock emplacement, or from heat associated with a second, slightly younger phase of intrusive igneous activity. Primary fluid inclusion salinity (0-6.4 wt % NaCl equiv) and Th (115°-425°C) data from ore-bearing vein quartz, in conjunction with measured δ18OVSMOW data (-0.97 to +17.3‰), indicate that vein-forming fluids contained variable mixtures of magmatic and meteoric components (calculated δ18OVSMOW values of-16.0 to + 13.3‰) and spanned the epithermal-porphyry temperature continuum. New and previously published stable isotope (C, O, and S) data from vein-hosted ore and gangue minerals, unaltered and altered siliceous (upper plate) host rocks, and unaltered carbonate (lower plate and upper plate) rocks indicate mixed and variable sources of vein-hosted ore and gangue components and variable interaction between some vein-forming fluids and nonsiliceous rock types (carbonates, bedded barite) that underlie the northern Shoshone Range. Large vein widths, high vein densities, and lack of evidence supporting fluid boiling in primary fluid inclusions suggest that vein deposits of the northern Shoshone Range probably formed at shallow depths (≤1 kin) under lithostatic conditions. Geochronology and stable isotope data support a temporal and genetic link between Eocene magmatism, quartz vein-hosted precious and base metal mineralization, and molybdenite (porphyry) mineralization within the Hilltop, Lewis, and Bullion mining districts. [ABSTRACT FROM AUTHOR]

Lead isotope compositions of bulk mineral samples (fluorite, orpiment, and realgar) determined using conventional techniques and of ore-stage arsenian pyrite using the Sensitive High Resolution Ion-Microprobe (SHRIMP) in the Getchell and Turouoise Ridge Carlin-tyne gold denosits (Osgood Mountains) require contribution from two different Pb sources. One Pb source dominates the ore stage. It has a limited Pb isotope range characterized by [sup208]Pb/[sup206]Pb values of 2.000 to 2.005 and [sup207]Pb/[sup206]Pb values of 0.8031 to 0.8075, as recorded by 10-/μm-diameter spot SHRIMP analyses of ore-stage arsenian pyrite. These values approximately correspond to [sup206]Pb/[sup204]Pb of 19.3 to 19.6, [sup207]Pb/[sup204]Pb of 15.65 to 15.75, and [sup208]Pb/[sup206]Pb of 39.2 to 39.5. This Pb source is isotopically similar to that in average Neoproterozoic and Cambrian clastic rocks but not to any potential magmatic sources. Whether those clastic rocks provided Pb to the ore fluid cannot be unequivocally proven because their Pb isotope compositions over the same range as in ore-stage arsenian pyrite are similar to those of Ordovician to Devonian siliciclastic and calcareous rocks. The Pb source in the calcareous rocks most likely is largely detrital minerals, since that detritus was derived from the same sources as the detritus in the Neoproterozoic and Cambrian clastic rocks. The second Pb source is characterized by a large range of [sup206]Pb/[sup204]Pb values (18-34) with a limited range of [sup208]Pb/[sup204]Pb values (38.1-39.5), indicating low but variable Th/U and high and variable U/Pb... [ABSTRACT FROM AUTHOR]

The Ordovician Meiklejohn Peak mud-mound in western Nevada contains sedimentary structures (e.g., stromatactis and “zebra-rock”) that resemble features found in modern methane hydrate deposits. However, the δ13C of the mud-mound carbonates (−1 to 1‰ PDB) does not suggest interaction with δ13C-depleted methane. Consequently, Krause (Sedimentary Geology 145 (2001) 189) proposed that the Meiklejohn structures might have been produced by the formation and dissociation of CO2 gas hydrate rather than methane gas hydrate. Structures in Neoproterozoic cap carbonates, sheet cracks and tepee-like structures have also been suggested to represent gas hydrate interaction with carbonate sediments (Geology 29 (2001) 443), but, similarly, δ13C values do not suggest direct interaction with methane. The objective of this contribution is to see if the presence of CO2 gas hydrate is plausible for either setting.To test the feasibility of CO2 gas hydrate involvement in the formation of these structures, necessary concentrations of TCO2 were calculated based on required phase equilibria using the quadruple points Q1 (−1.73 °C, 10.2 atm) and Q2 (10.2 °C, 44.5 atm) from the phase diagram for CO2 gas hydrate (Miller, S.L., 1974. The Nature and Occurrence of Clathrate Hydrates. In: Kaplan, I.R. (Ed.), Natural Gases in Marine Sediments. Plenum, New York, NY, pp. 151–178). A limit for Ca+2 was established based on the lack of evidence for gypsum formation in this environment and the assumption that sulfate was up to 100% of its present oceanic value; the calculated TCO2 concentration, however, is relatively insensitive to the concentration of SO4−2 used. The lowest permissible value of pCO2 and TCO2 were found at Q1, where TCO2 must be at least 300 times the present value. These conditions are implausible for the formation of the Meiklejohn Peak mud-mound. Although some models for Neoproterozoic “Snowball Earth” require highly elevated atmospheric CO2, these models predict pCO2 that is still too low to permit the formation of CO2 gas hydrates in carbonate sediments. In addition, if the required conditions could be achieved during “Snowball Earth”, one would expect to find the associated structures only at the base of the section, while they are commonly observed meters to tens of meters above the base. Therefore, we conclude that it is unlikely these structures were produced by the formation and dissociation of CO2 gas hydrate. [Copyright &y& Elsevier]

Discusses the use of a radiolarian biostratigraphic tool for dating Paleozoic cherts from Roberts Mountains allochthon, Nevada. Anomalous Devonian ages obtained from the section; Structural interleaving of Ordovician and Devonian strata within the allochthon.

Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene "Lake Adobe" of limited aerial extent and a laterally extensive middle Eocene "Lake Elko," are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ18O, δ13C and 87Sr/86Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of 12C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of 87Sr/86Sr in the upper Elko Formation reflects an increasing aerial extent of low‐87Sr/86Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate. Plain Language Summary: Large lakes formed in the region that is now northeast Nevada between 50 and 40 million years ago. This region was at high elevation and undergoing a transition from compressional to extensional plate tectonic forces at that time. The geologic record shows that lakes formation started locally at 49.5 million years ago with a short‐lived freshwater Lake Adobe. Later, approximately 43.5 million years ago, a broad Lake Elko formed and then became increasingly evaporative over the next 3 million years, likely due to removal of the shallowly subducting Farallon slab from beneath the region. Key Points: Lakes formed in the hinterland of the North American Cordillera during the Eocene during a transition from contraction to extensionLake Elko expanded and became increasingly evaporative across 3.5 Ma from 43.5–40.5 Ma, coincident with an episode of explosive siliceous magmatismLake formation and drainage isolation likely records the progressive removal of the flattened Farallon slab from beneath the Cordillera [ABSTRACT FROM AUTHOR]

Copyright of Mine Water & the Environment is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Mine Water & the Environment is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Multi-year research on Carlin-type noble-metal mineralization has allowed the determination of deposit distribution trends, as well as the identification of several stratigraphic, geological-structural, and physicochemical conditions for the formation of the deposits. However, empirical conclusions do not fully encompass the results of modern geophysical and seismotomographic investigations. The involvement of the latter in the determination of the formation and distribution characteristics of known trends enables us to acknowledge that the Carlin world-class gold-ore province (Nevada, USA) is situated in the rear zone of the marginal-continental volcanic-plutonic belt with late Mesozoic–Cenozoic magmatism manifestations and linear belts of endogenous deposits of different formational and mineral types, which are diffusely scattered across the area of accretionary orogens. The formation of late Mesozoic–Cenozoic ore-magmatic systems in the province is predetermined by retinues of underslab and overslab asthenospheres that have been positioned as two layers beneath the continental lithosphere. The lower asthenosphere located in the mantle transition zone under the subducted Farallon plate contacted the higher asthenosphere by the upwelling of toroidal and poloidal fluid-energetic flows. Known trends of Carlin-type Au deposits were controlled by two permeable structures of deep-level geodynamics: the sublatitudinal structure of the Mendocino transform-type fault zone (Getchell and Jerritt Canyon trends), which bounds the Farallon plate in the north, and the paleo-spreading center of the East Pacific Rise of a NNW strike (Battle Mountain Eureka, Carlin, and Alligator Ridge trends). [ABSTRACT FROM AUTHOR]

Reports on the paleomagnetic investigations of Tertiary dikes and Ordovician carbonate rocks at Bare Mountain and in the surrounding region of southwestern Nevada. Exposed Proterozoic and Paleozoic stratigraphy of the Cordilleran miogeocline; Bare Mountain as footwall of at least two Neogene extensional-fault systems; Thermal history.

The lower member of the Poleta Formation is well known for its abundant archaeocyaths, including an exceptionally preserved reef at Stewart's Mill in Nevada. Large archaeocyath-bearing reefs are relatively uncommon, with most occurring as small nearshore patch reefs. Here, we describe a new 70 m-thick archaeocyath reef from the lower Poleta Formation, herein referred to as the Gold Point Reef, located near the townsite of Gold Point, Nevada. In spite of broad similarities with the Stewart's Mill exposure, the Gold Point Reef shows an interplay between shoal and reef development over the ~ 70 m exposure that differs from what is observed at Stewart's Mill. This reef also preserves a small shelly fauna in its uppermost 10 m, dominated by Hyolithellus. Three successive ecological stages of reef development are observed here: the pioneer stage characterized by a single rare frame-building species; the diversification stage involving about nine species of both frame-building and mud-sticking archaeocyaths; and the final foundering stage represented by one to two species of archaeocyaths and facies indicating reef demise, including abundant grainstone/packstone shoals and a silicified upper bed with a settlement of hyolitheminthes. During the apex of reef development here, there are large modular archaeocyaths, which were able to produce thick secondary skeletal tissue for a firm attachment; these archaeocyaths were capable of building large reefs in a variety of settings. Additionally, the carbon isotope profile recorded in the Gold Point Reef shows similarities to nearby exposures of the equivalent lower Poleta Formation, with the exception of a small negative downturn at the top of the reef that is not found in other locations. Combined with the documentation of facies changes over short lateral distances, the isotope data suggest that archaeocyathan reefs may have been time-transgressive across the shelf during deposition of the lower Poleta. Additionally, our work suggests that both here and globally, the specific biotic assemblage reflected the local influences of the environment on reef development, creating highly variable coeval paleocommunities even within a restricted area. [ABSTRACT FROM AUTHOR]

Chancelloriids are a poorly understood group of phylogenetically problematic Cambrian metazoans; complete specimens show they were sessile, radially symmetrical, club‐shaped organisms covered with sclerites in the form of rosettes of spines. While isolated sclerites are common components of Cambrian shelly assemblages, they have been relatively little studied. We describe chancelloriid sclerites from a series of nine sections spanning the upper Dyeran to lower Delamaran stages (latest Stage 4 to perhaps basal Wuliuan) from the Pioche–Caliente region of east‐central Nevada, USA. Acid maceration of samples from the Combined Metals, Comet Shale and Susan Duster Limestone members of the Pioche Formation yielded more than 2000 sclerites. Based on careful examination of these sclerites and statistical analyses of co‐occurring sclerite types, we distinguish six species, each with a restricted stratigraphic range. Chancelloria impar Moore sp. nov. is the dominant species in most upper Dyeran samples. Archiasterella cometensis Moore sp. nov. and A. auriculata Moore sp. nov. are rare in the upper Dyeran but abundant in the lowest Delamaran; A. uncinata Moore sp. nov. and C. lilioides Moore sp. nov. replace these in younger samples. A. auriculata is noteworthy for sharing features with species of both Archiasterella and Chancelloria. These results provide further support for the taxonomic tractability and biostratigraphical utility of chancelloriid sclerites; large collections from single horizons allow intraspecific variability to be assessed and species to be distinguished. Our results document a taxonomic turnover in chancelloriids at the Dyeran–Delamaran boundary, showing that not only trilobites were affected at this time. [ABSTRACT FROM AUTHOR]

Copyright of Mine Water & the Environment is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

An important interval for reef evolution occurs in the Triassic Period. However, Late Triassic reefs along the edge of the Panthalassa Ocean receive much less attention than their Tethys region counterpart. To improve our understanding of reef evolution along the Panthalassa, we present a quantitative microfacies analysis to characterize reef facies types from a central Nevada locality. We point counted thin sections to quantitatively assess the relative abundance of reef and interstitial components from six Carnian reef mounds. Reef mound formation occurs at three different stratigraphic intervals. Each interval records a different reef mound size and shape. The oldest stratigraphic interval consists of four separate reef mounds; each mound is approximately 3–12 m wide and 2–4 m high. The second youngest stratigraphic interval consists of one large ~ 10-m wide and 15.5-m high domal reef mound. The third, and youngest, reef mound is smaller in comparison (i.e., 1-m wide and 1.5-m high). We recognize two reef facies types within these reef mounds. Different calcareous sponges, bound together by microproblematica and clotted micrite, form the overall reef framework. We suspect that changes in relative sea level affect the initial colonization, different growth capacities, and overall preservation of the South Canyon reef mounds. When South Canyon reefs are compared to other Panthalassa reefs, it seems that "Wetterstein-type" reef–sponge bindstone facies containing biogenic crusts dominate and corals are less abundant and persist generally in both shallow and deeper water environments. However, significant differences in primary and secondary framebuilder abundances exist due to differences in where the reefs grew in relation to fair-weather wave base (FWWB). Thus, changes in relative sea level may affect the initial colonization, different growth capacities, and overall preservation of reef but water depth may also control the primary and secondary reef builders. Consequently, quantitative assessment of reef ecosystems within a sequence stratigraphic framework undoubtedly facilitates fewer problematic comparisons between reefs but it also enhances our understanding of reef development. [ABSTRACT FROM AUTHOR]

Owing to their temporal position during the decline of the classic Ediacara biota and the appearance of the more recognizable metazoans of the Cambrian Period, the terminal Ediacaran (∼551–539 Ma) assemblages of tubular fossil forms hold potential to improve understanding of biotic turnover near the end of the Ediacaran Period. Cloudinid taxa, including the terminal Ediacaran index fossil Cloudina, are the most well studied of these Ediacaran tubular forms due to their global palaeogeographical distribution. Recent reports revealed ecosystems of tubicolous organisms from the Great Basin region, Nevada, USA, and assigned taxa to such genera as Conotubus, Gaojiashania and Wutubus, well known from contemporaneous Chinese deposits. Appreciating the role that these organisms may have played in the evolutionary history of metazoans and recognizing their potential global distribution, however, requires careful taxonomic study. Here, we detail pyritized fossils from the Deep Spring and Wood Canyon formations of Esmeralda and Nye counties, Nevada, USA, respectively. Our investigation focused on the most abundant tubular taxon from the Nevada sites, which was previously generically assigned to Conotubus. While outwardly similar in morphology to this Gaojiashan taxon, our investigation determines that those fossils previously figured as Conotubus are instead two distinct taxa. The first represents a new species of Saarina – Saarina hagadorni sp. nov. – a genus previously known only from the East European platform, and the second is established as a novel genus and species, Costatubus bibendi gen. et sp. nov. [ABSTRACT FROM AUTHOR]

The seismic reflection method provides high‐resolution data that are especially useful for discovering mineral deposits under deep cover. A hindrance to the wider adoption of the seismic reflection method in mineral exploration is that the data are often interpreted differently and independently of other geophysical data unless common earth models are used to link the methods during geological interpretation. Model‐based inversion of post‐stack seismic data allows rock units with common petrophysical properties to be identified and permits increased bandwidth to enhance the spatial resolution of the acoustic‐impedance model. However, as seismic reflection data are naturally bandlimited, any inversion scheme depends upon an initial model, and must deal with non‐unique solutions for the inversion. Both issues can be largely overcome by using constraints and integrating prior information. We exploit the abilities of fuzzy c‐means clustering to constrain and to include prior information in the inversion. The use of a clustering constraint for petrophysical values pushes the inversion process to select models that are primarily composed of several discrete rock units and the fuzzy c‐means algorithm allows some properties to overlap by varying degrees. Imposing the fuzzy clustering techniques in the inversion process allows solutions that are similar to the natural geologic patterns that often have a few rock units represented by distinct combinations of petrophysical characteristics. Our tests on synthetic models, with clear and distinct boundaries, show that our methodology effectively recovers the true model. Accurate model recovery can be obtained even when the data are highly contaminated by random noise, where the initial model is homogeneous, or there is minimal prior petrophysical information available. We demonstrate the abilities of fuzzy c‐means clustering to constrain and to include prior information in the acoustic‐impedance inversion of a challenging magnetotelluric/seismic data set from the Carlin Gold District, USA. Using fuzzy c‐means guided inversion of magnetotelluric data to create a starting model for acoustic‐impedance proved important in obtaining the best result. Our inversion results correlate with borehole data and provided a better basis for geological interpretation than the seismic reflection images alone. Low values of the acoustic impedance in the basement rocks were shown to be prospective by geochemical analysis of rock cores, as would be predicted for later gold mineralization in weak, decalcified rocks. [ABSTRACT FROM AUTHOR]

Archaeocyathan sponges were the dominant metazoan framework builders during Series 2 of the Cambrian. After their near extinction during the Toyonian stage (middle Stage 4), this important ecological role was eventually filled by robustly skeletonized lithistid sponges. However, the exact timing of ecological restructuring is not well understood and was likely not contemporaneous across different paleocontinents. For example, reefs from the Wuliuan of China appear to show rapid replacement of archaeocyaths with lithistid sponges, yet the earliest occurrence of lithistids in Laurentia is not until the early Furongian. In this study, we explore the Mule Spring Limestone of Nevada, which contains shallow water carbonate environments from the immediate aftermath of the regional archaeocyathan extinction, for signs of reef-building activity. Within this formation, we find evidence of sparse microbial-built leolites and some potentially poorly preserved metazoan organisms. However, the totality of our field observations and thin-section point counts suggest that there was no substantial reef-building activity by either microbial or metazoan organisms within our study locality. Our data suggest the occurrence of a local reef eclipse during this interval for the locality investigated. We also incorporate geochemical proxies to determine paleoredox conditions, which suggest well-oxygenated marine conditions through the period of interest. Lack of hardground substrate is proposed as the cause for this gap in the reef record. These results show that a temporary loss of framework-building activity occurred after the regional extinction of archaeocyaths and demonstrates the ecological impact of losing framework builders on a reef environment. [ABSTRACT FROM AUTHOR]

In 2001, researchers from the National Institute for Occupational Safety and Health (NIOSH) installed instruments at the Turquoise Ridge Mine in cooperation with Placer Dome, Inc. to monitor the geomechanical behavior and stability of a cemented rockfill (CRF) sill and the surrounding host rock during test mining of a large undercut span beneath backfill. Six parallel, adjacent drifts were mined and backfilled to construct a CRF sill, approximately 22.9 m (75 ft) wide by 30.5 m (100 ft) long. The sill was then partially undercut, successfully creating a 13.7-m (45-ft) wide by 30.5-m (100-ft) long span beneath the CRF. Only small vertical displacements were measured in the overlying host rock during mining, with most of the movement occurring at shallow depths in the mine roof. Because the back above the CRF sill remained stable, the majority of the mining-induced stress was transferred to the host rock abutments rather than to the backfilled drifts. During retreat mining of the undercut span, the CRF sill and the mine roof remained stable. Most of the measured vertical displacement was caused by separation of the backfill from the overlying host rock, or deflection of the CRF sill, which was comparable to the deflection of a monolithic, elastic plate having similar dimensions, material properties, and undercut spans. The CRF sill moved in mass as a single unit rather than as individual drift segments, and the vertical cold joints between adjacent backfill drifts did not adversely affect their stability. Additional measurements collected from the instruments have shown that the backfill span is still intact and in stable condition more than 16 years after the completion of undercut mining. Displacements in the mine roof and abutments have stabilized, and vertical stress and deformation within the CRF have generally leveled off or decreased. Although only slight mining-induced loads were transferred to the backfilled drifts, the CRF has confined the abutment ribs and mine roof, thereby improving their long-term stability. Results of compressive and tensile strength tests conducted with CRF samples from the test site indicate that the long-term compressive strength gain for CRF is similar to that of concrete, and that the tensile-to-compressive strength ratio for CRF is about 1/6 rather than 1/10. Assuming the in-place CRF gained strength at the same rate as the lab samples, an analytical analysis of the flexural stability of the CRF undercut span shows that the Factor of Safety for the span should have logically increased over time. By providing a better understanding of the long-term strength properties and geomechanical behavior of CRF, these research findings help improve the methods that are used for designing stable, long-term undercut entries beneath cemented backfill. [ABSTRACT FROM AUTHOR]

A measured section of the Upper Pennsylvanian-Lower Permian Riepe Spring Limestone at Spruce Mountain Ridge, Elko County, Nevada, presents a stratigraphic record of the coastal evolution and the shoreline-to-open marine depositional history of a portion of the post-Antler foreland. The 420 m thick “North Spruce Mountain Ridge” (NSMR) section consists of highly variable lower Missourian (Upper Pennsylvanian)-uppermost Wolfcampian (Lower Permian) strata. The NSMR section, which follows Section 1 of Hope (1972), was remeasured and sampled by the authors for stratigraphic revision of the Hope’s Section 1 with an accompanying biostratigraphic study of the fusulinid and conodont assemblages. As a result, a new lithostratigraphic interpretation of the Riepe Spring Limestone at Spruce Mountain Ridge, consisting of three lithologically distinguishable informal members, is presented herein. The so-called lower, middle, and upper informal members of the Riepe Spring Limestone are primarily composed of thick, alternating units of limestone, calcareous sandstone, and quartzose conglomerate. The mixed carbonate-siliciclastic strata of the NSMR section are biostratigraphically constrained by a fusulinid succession comprising eight assemblage zones (lowest to highest stratigraphically): Eowaeringella assemblage zone; Triticites preoryziformis n. sp. assemblage zone; Triticites cf. jacksboroensis assemblage zone; Triticites cullomensis-Dunbarinella kauffmani assemblage zone; Triticites creekensis-Triticites pinguis assemblage zone; Schwagerina wellsensis assemblage zone; Eoparafusulina linearis assemblage zone; Schwagerina franklinensis-Schwagerina nowitzkii n. sp. assemblage zone. Fusulinid collections include specimens from new field samples and loaned material from the Smithsonian National Museum of Natural History’s Douglass-Henbest collection. Descriptions of 39 species are provided, including two species of Eowaeringella, 14 species of Triticites, three species of Pseudofusulinella, one species of Dunbarinella, one species of Douglassites, one species of Schubertella, one species of Pseudofusulina, two species of Cuniculinella, seven species of Schwagerina, three species of Alaskanella, two species of Pseudoschwagerina, one species of Eoparafusulina, and one species of Leptotriticites. New early Missourian-late Wolfcampian taxa described herein include Eowaeringella nevadaensis n. sp., Eowaeringella riepensis n. sp., Triticites contrariatheca n. sp., Triticites postspringvillensis n. sp., Triticites preoryziformis n. sp., Pseudofusulinella wahlmani n. sp., Cuniculinella hiatus n. sp., Schwagerina celeris n. sp., Schwagerina margerumi n. sp., Schwagerina nowitzkii n. sp., Alaskanella hopei n. sp., Alaskanella lata n. sp., and Leptotriticites nevadaensis n. sp. [ABSTRACT FROM AUTHOR]

Copyright of Mine Water & the Environment is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Global Stratotype Section and Point of the mid-Carboniferous boundary was selected in the Arrow Canyon section within the lower Bird Spring Formation at the first evolutionary appearance of the conodont Declinognathodus noduliferus (Ellison and Graves) sensu lato. The studied boundary beds, spanning a 38.8-m interval from the UpperMississippian into the Lower Pennsylvanian, occur in bryozoan-brachiopod-crinoidal grainstones/packstones with conodonts and foraminifers. The conodonts of the studied interval belong to the shallow-water Adetognathus-Rhachistognathus biofacies. The Adetognathus and Rhachistognathus species/subspecies A. lautus (=A. gigantus), A. spathus, R. minutus minutus, R. muricatus, R. primus, R. prolixus, R. websteri and transitional forms dominate the succession. Gnathodus girtyi simplex, G. lanei n. sp., G. defectus and transitional forms between them are abundant. The elements of the boundary-marker D. noduliferus s. l. and transitional forms between G. girtyi simplex and D. inaequalis are less common. The occurrences of Idiognathoides spp. are very rare. Neognathodus spp. only occur in the upper part of the studied interval. The mid-Carboniferous boundary stratotype has been studied by many workers but its conodonts have been rarely described systematically. The detailed updated descriptions of themost stratigraphically important taxa are given in this paper. The evolution of Gnathodus, Adetognathus and Rhachistognathus species within the mid-Carboniferous interval is discussed. Four lineages are summarized: 1) C. unicornis, A. unicornis, A. lautus and A. spathus; 2) G. girtyi girtyi, G. g. simplex and D. inaequalis; 3) G. lanei and G. defectus; and 4) R. prolixus, R. muricatus and R. websteri/R. primus/R. minutus. [ABSTRACT FROM AUTHOR]

Eighty measured stratigraphic sections across the Late Devonian carbonate platform of Nevada (USA) document the uppermost terminal Alamo Breccia and overlying sediments, which record the waning energy at the end of an impact and the recolonization of the post-impact seafloor. Four sedimentary styles of terminal breccia, recognized by the continuity of normal grading versus reworking, and dolomitization, define patterns of sedimentary accommodation across the platform. Examined in combination with the first post-impact facies, the field area can be divided into deep subtidal, shallow subtidal, and peritidal zones with increased distance from the inferred crater center. Farthest away, peritidal outcrops have very low accommodation, and the terminal breccia is physically reworked and dolomitized. Biotic signals are rare in this zone. However, we find rare but exceptional deposits of impact fallout lapilli at or above the top of the breccia. The shallow subtidal region records reworked and pristine grading of the terminal breccia, which at several locations document burrowing directly into the top of the Alamo Breccia, confirming infaunal recolonization prior to post-impact sediment accumulation. Rare occurrences of erosional terminal breccia produced rockgrounds at the seafloor, some showing evidence of bioerosion. Deeper subtidal localities, closest to the crater center, preserve the thickest, continuously graded terminal breccias owing to high accommodation at the end of the event. Body and trace fossils are common in the first overlying lithofacies, although the abundance of oxidized firmground surfaces in deep water settings suggest these deposits were sediment-starved and fossil assemblages are strongly time-averaged. [ABSTRACT FROM AUTHOR]