Tuffaceous Deposits Research Articles

Isotopes of carbon and oxygen of siderite and their genetic indications for the Late Permian critical-metal tuffaceous deposits (Nb-Zr-REY-Ga) from Yunnan, southwestern China

This paper describes numerous siderite occurrences from Nb-Zr-REY-Ga-enriched beds of tuffaceous mudstone from the Yujingshan borehole, eastern Yunnan, southwestern China. The mineral phases in the samples were identified and characterized through macroscopic observation, XRD (X-ray diffraction) analysis, optical microscopy, and FE-SEM (field emission-scanning electron microscopy). The results show that the siderite occurs in the drill core from depths of 664.07 to 729.21 m, where its content varies from 15.1 to 65.7%, 31.3% on average. Modes of occurrence of authigenic siderite include irregular banded or massive aggregates, discrete fine particles, oolitic aggregates, and double or multi-ring ellipsoid-shaped or rhombohedral aggregates. The mineralogical association with quartz, anatase, ilmenite, chromite, and zircon with sharp-edged outlines, high-temperature cracks and/or corroded embayments, and REE-bearing minerals and geochemical characteristics such as Al2O3/TiO2 between 3 and 8, less commonly 8–23.77, indicate that abundant siderite was produced by hydrothermal alteration of airborne volcaniclastic materials. The δ13CPDB, δ18OPDB, and δ18OSMOW values of siderite in the argillized tuff range from −12.56 to −1.32‰, −8.98 to −5.88‰, and 21.60 to 24.80‰, respectively. In the δ13CPDB-δ18OSMOW diagram, plots of the studied siderite are distributed between the fields of marine carbonate rocks, sedimentary organic matter, and carbonates of hydrothermal ores. Some siderite from the upper part of the studied borehole is isotopically close to REY carbonates from the Abramovka lignite deposit, Far Eastern Russia, which precipitated from evolved basinal waters mixed with CO2 from a mantle source. In summary, this indicates a hybrid origin for the studied siderite including carbonate dissolution, dehydroxylation of sedimentary organic matter, and hydrothermal activity. Furthermore, the paleosalinity (Z) and paleotemperature (T) of the ore-forming fluids were calculated based on the age-corrected stable carbon and oxygen isotopic data in the samples. The Z values range from 96.65 to 121.5‰, suggesting the formation of siderite under brackish-water conditions. The T values vary from 19.47 to 47.12 °C, with some sharp fluctuations that may be due to ash falls and/or hydrothermal injections. Overall, our data support the model for critical-metal Nb(Ta)-Zr(Hf)-REY-Ga enrichment in tuffaceous deposits of southwest China including a combination of volcanic activity and terrestrial organic-rich sedimentation under warm and humid conditions, followed by low-temperature alteration evolved from acidic leaching, which is responsible for the Nb-Zr-REY-Ga enrichment, and, subsequently, formation of authigenic siderite with associated quartz, sulfide minerals, and phosphate minerals under reducing neutral/weakly alkaline conditions.

Submarine volcanic activity and giant amygdale formation along the Panama island arc as a precursor to 6000-year-old agate exploitation on Pedro González Island

AbstractAn extensive deposit of agate occurs in Pedro González Island in the Gulf of Panama. Previous archaeological research showed that the agate was exploited between 6200 and 5600 cal BP to make stone tools found at the oldest known Preceramic human settlement in the Pearl Island archipelago. We constrain here the origin and geological context of the agate through a geological and geochemical study of the island. We show that it includes primary volcanic breccias, lavas, and tuffaceous marine deposits with sedimentary conglomerates and debris flow deposits, which we define as the Pedro González Formation. This formation records submarine to subaerial volcanic activity along an island arc during the Oligo-Miocene, confirming previous regional models that favour progressive emergence of the isthmus in the early Miocene. The igneous rocks have an extreme tholeiitic character that is interpreted to reflect magmatic cessation in eastern Panama during the early Miocene. The agate is hosted in andesitic lavas in unusually large amygdales up to 20–40 cm in diameter, as well as small amygdales (0.1–1.0 cm) in a bimodal distribution, and in veins. The large size of the agates made them suitable for tool manufacture. Field evidence suggests that the formation of large amygdales resulted from subaqueous lava–sediment interaction, in which water released from unconsolidated tuffaceous deposits at the base of lava flows rose through the lavas, coalesced, and accumulated below the chilled lava top, with subsequent hydrothermal mineralization. These amygdales could therefore be regarded as an unusual result of combined peperitic and hydrothermal processes.

Diverse evidence for grasslands since the Eocene in Patagonia

Timing and ecological steps of the rise and expansion of grasslands differ not only geographically but also according to the type of available information. Patagonia (Argentina) was largely considered the region where grasslands and grazers early evolved in the Eocene, mostly based on functional morphology of mammals. However, recent paleobotanical studies question this traditional view and maintain that this occurred after early Miocene. To tackle this discrepancy, we conduct a multidisciplinary and integrative approach using diverse lines of evidence obtained from middle Eocene - middle Miocene tuffaceous deposits of central and northern Patagonia. Gathered data correspond to sedimentary facies, paleosols, insect trace fossils, opal phytoliths and fossil mammals (tooth morphology, body size). Long-lasting and frequent volcanic ashfalls caused permanent disturbances in physical scenarios, biotic systems and grassy habitats; just like herbivory of mammals through coevolutionary grass-grazer mechanisms. The results indicate that soils, vegetation, insects and mammal herbivores begun synchronously to record diverse traits related to grasslands showing mosaic vegetation since middle Eocene (~44 Ma). These traits include mollic paleosols, abundant and persistent dung beetle trace fossils (Coprinisphaera ichnofacies), phytolith assemblages attributable to savannas and medium to large-size hypsodont mammals. We also track the evolutionary history after the reduction of closed forests, recognizing alternating subhumid savannas with riparian forests, where the relative frequency between grasses, palms, trees and shrubs fluctuated in time and space; and desert vegetation (shrublands) dominated by palms and shrubs during dryer periods. The advent and expansion of grasslands can be linked to the cooling and drying period subsequent to the Early Eocene Climate Optimum.

A magnetostratigraphic age constraint for the proximal synorogenic conglomerates of the Late Cretaceous Cordilleran foreland basin, northeast Utah, USA

AbstractReliable ages of proximal conglomerates in the Cordilleran foreland basin that are associated with emplacement and erosion of major thrust sheets are essential for reconstructing the kinematic history of the Sevier fold-thrust belt. Although these conglomerates have been dated by palynology, their absolute ages have been difficult to determine because of their coarse-grained texture and a lack of marine interbeds and tuffaceous deposits. We collected sets of oriented samples from outcrops in northeastern Utah, USA, to construct an overall magnetic polarity stratigraphy that can be correlated to the geomagnetic polarity time scale (GPTS). We sampled fine-grained, hematitic interbeds in the Upper Cretaceous Echo Canyon Conglomerate and Weber Canyon Conglomerate. Common paleomagnetic and rock magnetic analyses were conducted, and several rock magnetic results indicated that the dominant magnetic carriers in these weakly magnetized rocks are hematite and very subordinate magnetite/titanomagnetite/maghemite and goethite. Demagnetization results show that hematitic, fine-grained sandstone to siltstone intervals carry a geologically stable magnetization with directions and polarity consistent with the Late Cretaceous geomagnetic field. A small percentage of samples carry a low laboratory unblocking temperature secondary overprint residing primarily in goethite. Magnetic polarity results indicate that the Echo Canyon Conglomerate is exclusively of normal polarity and that the younger Weber Canyon Conglomerate is of normal polarity in its lowermost part, reverse polarity in the middle, and normal polarity in the upper part of the sequence. The new data indicate that these coarse-grained strata were most likely deposited over the time span of the magnetic polarity Chron (C) 34n to C33r interval and the younger C33r to C33n interval; the former interval includes the Santonian-Campanian stage boundary (ca. 83.4 Ma/83.1 Ma). Palynological data suggest that these rocks span Coniacian-Santonian time (ca. 89–84 Ma); thus, the most parsimonious correlation of the normal polarity magnetozone of the Echo Canyon Conglomerate is with the youngest part of C34n Superchron, which is of ca. 30 Ma duration (ca. 115 Ma to 83.4 Ma/83.1 Ma). The normal polarity magnetozone of the lower part of the younger Weber Canyon Conglomerate likely correlates to the youngest part of C34n, whereas the reverse and normal magnetozone from the middle and upper parts of the Weber Canyon Conglomerate likely correlate to C33r and C33n, respectively. We infer that the Santonian-Campanian boundary resides in the lower Weber Canyon Conglomerate, which implies that deposition of the unit started prior to the C34n/C33r boundary age (ca. 83.4 Ma/83.1 Ma) and continued through the C33r and C33n chrons. Sediment provenance data and growth structures tie the Echo Canyon and Weber Canyon Conglomerates to emplacement of the Crawford thrust sheet. Based on the magnetic polarity data, as constrained by the biostratigraphic age estimates from these synorogenic deposits, we hypothesize that the principal displacement along the Crawford thrust started during the Coniacian (>C34n/C33r boundary age) and continued into the middle Campanian (<C33r/C33n boundary age), from ca. 90–75 Ma, which is nearly 10 Ma longer than previously thought. The new age constraints demonstrate complete temporal overlap between proximal and distal coarse-grained deposits in this part of the Cordilleran foreland basin, coeval with active thrust displacement and rapid hinterland exhumation.

Mineralogical characterization and diagenetic history of Permian marine tuffaceous deposits in Guangyuan area, northern Sichuan basin, China

Based on thin section identification, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis of samples from cuttings and cores in drillings, tuffaceous deposits were identified to intercalate in marine carbonate rocks and mudstones in the upper part of Maokou Formation and the middle to lower part of the Wujiaping Formation in Guangyuan area, northern Sichuan basin, China. These tuffaceous deposits are of significance to understand the influence range of magmatic activity of the Emeishan Large Igneous Provinces (ELIP), and explore and exploit oil and gas reservoirs in Permian strata in the Sichuan basin. The research results indicated that tuffs formed 0.1–10 mm-thick laminae, and they formed lamellar interbedding with organic mudstones and carbonaceous mudstones, which were sandwiched among bioclast limestones and marls. Tuffaceous deposits in the study area include tuff, sedimentary tuff and tuffaceous sedimentary rock. Under the influence of sulfate reduction, acidic hydrotherm and alkaline fluid, tuffaceous deposits were altered by diagenesis such as dolomitization, argillization, pyritization and dissolution, and formed a variety of secondary rocks and pores which characterized by the micron-scale solution pores in tuff components and the nanometer-scale intercrystal pores among authigenic clay minerals. In addition, the primary directional sedimentary structure of tuffs and sedimentary tuffs and developed laminae in the rocks are conducive to the formation of fissures parallel to laminae, and effectively improve the reservoir property of rocks. The tuffaceous deposits in the study area are mainly the products of subaqueous pyroclastic flow that were dominated by acidic magmatism and indicated that they may not be directly related to the ELIP, which is located in the southwest Sichuan-Yunnan provinces and dominated by continental eruption of basalt.

Late Toarcian continental palaeoenvironmental conditions: An example from the Cañadón Asfalto Formation in southern Argentina

The Cañadón Asfalto continental Basin preserves terrestrial deposits interbedded with volcanic rocks, providing a unique opportunity for palaeoenvironmental reconstructions of the Jurassic continental realm. This study presents a sedimentological, mineralogical, and geochemical analysis of three lacustrine successions in the Cerro Cóndor area of the Cañadón Asfalto Basin, Argentina. New CA ID-TIMS U–Pb data from tuffaceous deposits indicate a late Toarcian age (179.481 ± 0.059, 179.41 ± 0.13, and 177.27 ± 0.40 Ma), suggesting that the palaeolakes developed contemporaneously to the Chon Aike volcanic activity. The sedimentary successions are composed of carbonate, organic matter-rich mudstone — with up to 8 wt% total organic carbon (TOC) content — sandstone and conglomerate, all with an important contribution of volcanic and volcanogenic material. The clay mineral assemblage dominated by corrensite (chlorite-vermiculite mixed layers), smectite and vermiculite is likely related to hydrothermal activity and alteration of volcanic material, rather than to change in weathering processes and climate. Organic matter preserved in the sedimentary successions has a lacustrine and terrestrial/reworked origin. Changes in the source of carbon dioxide (CO2) and in the type and/or degree of preservation of the organic matter had a major impact on the δ13Corg values. This further exemplifies the necessity to evaluate the influence of the depositional environment before interpretation of the organic carbon isotope records. The combined high phosphorus and TOC contents suggest that episodes of increased nutrient availability into the basin enhanced lacustrine primary productivity, which favoured the development of oxygen-depleted conditions bottom waters and ultimately organic matter preservation and burial. High Corg/Ptot ratios indicate that phosphorus was likely released back into the water column in a positive feedback loop, further sustaining primary productivity. This study provides an important clue to understand the palaeoenvironmental conditions prevailing on land during the late Toarcian and on how volcanism exerted a control on the depositional conditions.

Selenium and molybdenum enrichment in uranium roll-front deposits of Wyoming and Colorado, USA

Sandstone uranium (U) roll-front deposits of Wyoming and Colorado (USA) are important U resources, and may provide a terrestrial source for critical accessory elements, such as selenium (Se), molybdenum (Mo), and tellurium (Te). Due to their associated toxicity, MoSeTe occurrences in roll-fronts should also be carefully monitored during U leaching and ore processing. While elevated MoSe concentrations in roll-fronts are well established, very little is known about Te occurrence in such deposits. This study aims to establish MoSeTe concentrations in Wyoming and Colorado roll-fronts, and assess the significance of these deposits in an environmental and mineral exploration context.Sampled roll-front deposits, produced by oxidized groundwater transportation through a sandstone, show high MoSe content in specific redox zones, and low Te, relative to crustal means. High Se concentrations (up to 168ppm) are restricted to a narrow band of alteration at the redox front. High Mo content (up to 115ppm) is typically associated with the reduced mineralized nose and seepage zones of the roll-front, ahead of the U orebody. Elevated trace element concentrations are likely sourced from proximal granitic intrusions, tuffaceous deposits, and local pyritic mudstones. Elevated MoSe content in the sampled roll fronts may be regarded as a contaminant in U in-situ recovery and leaching processing, and may pose an environmental threat in groundwaters and soils, so extraction should be carefully monitored. The identification of peak concentrations of MoSe can also act as a pathfinder for the redox front of a roll-front, and help to isolate the U orebody, particularly in the absence of gamma signatures.

Jurassic dinosaurs of Transbaikalia and prospects of searching for them in Mongolia

Three Transbaikalian localities of dinosaur remains confined to concretions of the Tarbagatai Coal Field, coal deposits of the Arbagarskaya deposits, and tuffaceous sedimentary deposits of the Ukureiskaya Formation of the Olov Depression (Kulinda locality) are considered. In concretions, dinosaur fossils co-occur with fragmentary and complete skeletons of Salmo and Palaeoniscidae and tuffaceous sedimentary beds of volcanic lakes have yielded fossils of temporary inhabitants, such as notostracans, conchostracans, izophlebiid dragonflies, ostracods, and plants with index species of the Late Jurassic Unda–Daya Assemblage. These characteristics can be used in searching for Jurassic dinosaur burials in Mongolia. The horizons with concretions are known in the Tormkhon, Tevsh, Unduruhin, Anda-Khuduk, and Ulanereg formations. Remains of notostracans and izophlebiid dragonflies were recorded in the Ulugei Formation and terrigenous deposits of the Khalgyn-Ula locality and mass burials of large izophlebiid dragonflies and Palaeoniscidae have occur in the Dorogot Formation.

EARLY CRETACEOUS APTIAN CHARCOAL FROM XINCHANG PETRIFIED WOOD NATIONAL GEOPARK OF ZHEJIANG PROVINCE, EASTERN SOUTH CHINA

ABSTRACT We report on the distribution and character of fossil charcoal found in the Lower Cretaceous Guantou Formation in the Xinchang Petrified Wood National Geopark, eastern South China. All charcoal fragments studied so far exhibit well-preserved anatomical features of Araucarioxylon, consistent with the taxonomic classification assigned to this geopark's widely known silicified tree trunks. Homogenized cell walls and high vitrinite reflectance values of charcoal particles, and the synchronous presence of tuffaceous deposits within the associated sequence, suggest that charcoal in the Xinchang Geopark may have mainly formed under conditions of complete exclusion of air by entombment in hot pyroclastic flows. Scattered charcoal fragments in several sandstone beds overlying the lahar sediments and locally high concentrations of fragments on channel-scoured surfaces are consistent with postformation reworking and transport of charcoal material. Zircon U–Pb SHRIMP dating of the volcanic interbeds of the G...

Geological summary of the Busidima Formation (Plio-Pleistocene) at the Hadar paleoanthropological site, Afar Depression, Ethiopia

The Hadar paleoanthropological site in Ethiopia preserves a record of hominin evolution spanning from approximately 3.45Ma to 0.8Ma. An angular unconformity just above the ca. 2.95Ma BKT-2 complex divides the sediments into the Hadar Formation (ca. 3.8–2.9Ma) and the Busidima Formation (ca. 2.7–0.15Ma). The unconformity is likely a response to a major tectonic reorganization in the Afar Depression, and activation of the As Duma fault near the Ethiopian Escarpment (west of Hadar) created a half-graben in which the Busidima Formation was deposited. The pattern and character of sedimentation in the region changed dramatically above the unconformity, as cut-and-fill channel conglomerates and silt-dominated paleosols that comprise the Busidima Formation stand in sharp contrast to the underlying deposits of the Hadar Formation. Conglomerate deposition has been related to both the perennial, axial paleo-Awash and ephemeral, escarpment-draining tributaries. Overbank silts have yielded fossils attributed to early Homo and Oldowan stone tools. Numerous tuffaceous deposits exist within the Busidima Formation, but they are often spatially limited, fine-grained, and reworked. Recent work on the tephrostratigraphic framework of the Busidima Formation at Hadar has identified at least 12 distinct vitric tephras and established the first geochemical-based correlations between Hadar and the neighboring project areas of Gona and Dikika. Compared to Gona and Dikika, where Busidima Formation sediments are exposed over large areas, the highly discontinuous sediments at Hadar comprise less than 40m in composite section and are exposed over an area of <20km2, providing only snapshots into the 2.7–0.15Ma window. The stratigraphic record at Hadar confirms the complex depositional history of the Busidima Formation, and also provides important details on regional stratigraphic correlations and the pattern of deposition and erosion in the lower Awash Valley reflective of its tectonic history.

Revision of rock fossils of Dryinidae and Embolemidae (Hymenoptera: Chrysidoidea)

New fossil taxa of Embolemidae and Dryinidae (Hymenoptera: Chrysidoidea) are described. For Embolemidae, Embolemopsis baissensis n. gen. n. sp. and Baissobius minutus n. sp., are described from Central Siberia, West Transbaikalia, left bank of Vitim River (facing downstream), Baissa, lake deposits from earlier Early Cretaceous (Neocomian stage) (130–140 mybp). For Dryinidae, Anteonopsis antiquus n. gen. n. sp., Bocchus ? cenomanianus n. sp. and Gonatopus ? cretacicus n. sp., are described from Siberia, Magadan Region, Obeschchayushchiyi, tuffaceous deposits from Upper Cretaceous (Cenomanian) (90–95 mybp); Deinodryinus ? aptianus n. sp., is described from Central Mongolia, Bayanhongor Aimag, 5–8 Km N Bon Tsagan Nuur Lake, from impressed in lake deposits of the Khurilt rock unit probably of Aptian (Early Cretaceous) (100–115 mybp). In addition, an unidentified fossil belonging to Deinodryinus ? or Dryinus ? is recorded from Russian Far East, Maritime Province, Velikaya Kema (later Earlier or early Middle Miocene) (about 30 mybp). The known species of Baissobius Rasnitsyn are reviewed and a key is presented.

Massive volcanic SO2 oxidation and sulphate aerosol deposition in Cenozoic North America

Volcanic eruptions release a large amount of sulphur dioxide (SO(2)) into the atmosphere. SO(2) is oxidized to sulphate and can subsequently form sulphate aerosol, which can affect the Earth's radiation balance, biologic productivity and high-altitude ozone concentrations, as is evident from recent volcanic eruptions. SO(2) oxidation can occur via several different pathways that depend on its flux and the atmospheric conditions. An investigation into how SO(2) is oxidized to sulphate-the oxidation product preserved in the rock record-can therefore shed light on past volcanic eruptions and atmospheric conditions. Here we use sulphur and triple oxygen isotope measurements of atmospheric sulphate extracted from tuffaceous deposits to investigate the specific oxidation pathways from which the sulphate was formed. We find that seven eruption-related sulphate aerosol deposition events have occurred during the mid-Cenozoic era (34 to 7 million years ago) in the northern High Plains, North America. Two extensively sampled ash beds display a similar sulphate mixing pattern that has two distinct atmospheric secondary sulphates. A three-dimensional atmospheric sulphur chemistry and transport model study reveals that the observed, isotopically discrete sulphates in sediments can be produced only in initially alkaline cloudwater that favours an ozone-dominated SO(2) oxidation pathway in the troposphere. Our finding suggests that, in contrast to the weakly acidic conditions today, cloudwater in the northern High Plains may frequently have been alkaline during the mid-Cenozoic era. We propose that atmospheric secondary sulphate preserved in continental deposits represents an unexploited geological archive for atmospheric SO(2) oxidation chemistry linked to volcanism and atmospheric conditions in the past.

The basaltic to trachydacitic upper Diliman Tuff in Manila: Petrogenesis and comparison with deposits from Taal and Laguna Calderas

The basaltic to trachydacitic (50–65 wt.% SiO 2) upper Diliman Tuff is the youngest deposit of a sequence of tuffaceous deposits in Metro Manila. The deposit is located north of Taal Caldera and northwest of Laguna Caldera, which are both within the Southwest Luzon Volcanic Field. Chemical variations in the pumice fragments within the upper Diliman Tuff include medium-K basalt to basaltic andesite, high-K basaltic andesite to andesite and trachyandesite to trachydacite. Magma mixing/mingling is ubiquitous and is shown by banding textures in some pumice fragments, considerable range in groundmass glass composition (54 to 65 wt.% SiO 2) in a single pumice fragment, and zoning in plagioclase phenocrysts. Simple binary mixing modeling and polytopic vector analysis were used to further evaluate magma mixing. Trace-element variations are inconsistent with the medium-K and high-K magmas being related by crystal fractionation. The medium-K basalts represent hotter intrusions, which induced small degrees of partial melting in older crystallized medium-K basaltic material within the crust to produce the high-K magmas. All melts likely differentiated in the crust but the emplaced and new basaltic intrusions originated from the mantle wedge and were generated by subduction zone processes. The volcanic source vent for the upper Diliman Tuff has not been identified. In comparisons with the deposits from adjacent Taal and Laguna Calderas it is chemically distinct with respect to both major- and trace-element concentrations.

Alunite alteration of tuffaceous layers and zircon dating, Upper Permian Kennedy Group, Carnarvon Basin, Western Australia

Alunite layers were found in two locations in the Binthalya Formation, Upper Kennedy Group, of Late Permian age, in the Merlinleigh Sub‐basin, onshore Carnarvon Basin. Alunite has not previously been reported from the Kennedy Group, or from a similar setting in Western Australia. The alunite occurs in discrete layers that have different chemical signatures to surrounding, unaltered sedimentary rocks. The lack of alteration in surrounding sedimentary rocks suggests that alteration was localised by initial differences in lithology, rather than by fortuitous localisation of alteration in a single layer. Alunite is thought to be the product of alteration of K‐feldspars, clay minerals and pyrite under acidic, specifically sulfuric, conditions. It is found in hydrothermal systems and weathering profiles. Because the precursor minerals of alunite are believed to be feldspars, clays and sulfur‐rich minerals, tuffaceous layers are the probable precursors of the alunite layers. Sulfur for the reaction was derived from within the tuffaceous deposits or from the oxidation of pyrite, which was probably present as cementation of trace fossils in other beds. The alteration is thought to have occurred during extensive oxidation, weathering and laterite formation during the Cenozoic, because the low density and powdery nature of the alunite indicates that it developed at a shallow depth. U–Pb dates obtained from zircons extracted from the alunite layers returned ages ranging from ca 2690 Ma to ca 270 Ma. The majority of zircons are inherited from the same source as the Kennedy Group sediments, indicating mixing of the initial tuffaceous layers with background sedimentation, either during the sedimentation process or afterwards by reworking or biological disturbance.

Tectonic history of a Jurassic backarc-basin sequence (the Gran Cañon Formation, Cedros Island, Mexico), based on compositional modes of tuffaceous deposits

The Jurassic Gran Canon Formation (Cedros Island, Baja California, Mexico) constitutes an unusually well preserved and exposed example of ancient backarc-basin fill. Petrofacies analysis conducted on tuff- aceous sandstone and tuff samples from this formation complement and reinforce prior lithofacies interpretations, but with some modification. When temporal and spatial trends in petrographic data (detrital modes) are analyzed and compared to mod- els based on data collected from Deep Sea Drilling Project and Ocean Drilling Pro- gram cores, the trends indicate a second, heretofore unrecognized, phase of backarc rifting. Basalt lavas interstratified with da- citic pyroclastic rocks of the primary vol- canic lithofacies, previously interpreted to record the eruption of differentiated mag- mas at the climax of growth of the Gran Canon island arc, are now as a result of this study considered to be the product of arc extension and rifting. Our method of modal analysis uniquely combines the quantification of textural at- tributes of pyroclastic and epiclastic debris that reflect eruption style and magma com- position, as well as the effects of reworking and mixing in marine settings. This study demonstrates that detailed petrographic analysis is useful in the interpretation of ancient volcaniclastic deposits suspected of having formed in backarc-basin settings.

Fossil bee nests, coleopteran pupal chambers and tuffaceous paleosols from the Late Cretaceous Laguna Palacios Formation, Central Patagonia (Argentina)

The Late Cretaceous Laguna Palacios Formation in Central Patagonia (San Jorge Basin), southern South America, is composed of tuffaceous deposits supplied by periodical volcanic ash falls partly reworked by rivers, on broad plains. Variations in ash-fall rates allowed the formation of stacked, mature paleosols, which are one of the most characteristic features of this formation. The mature paleosols show well-developed horizons, ped structure and bear an intricate network of trace fossils mostly produced by small roots and invertebrates. Two different insect trace fossils could be recognized in this formation: sweat bee nests and coleopteran pupal chambers. Fossil bee nests are composed of inclined tunnels with cells attached to them by means of short necks, a typical construction of bees of the subfamily Halictinae. Similar halictine constructions were reported from the Cretaceous of the USA. Coleopteran pupal chambers are discrete, ovoid structures, having an internal cavity with a smooth surface, and an outer wall of lumpy appearance composed of different layers of soil material. They are commonly constructed by the larvae of different families of Coleoptera. Similar trace fossils were previously reported from the Asencio Formation (Late Cretaceous–Early Tertiary) of Uruguay and from the Djadokhta Formation of Mongolia (Late Cretaceous). These trace fossils constitute some of the only paleontological data from the Laguna Palacios Formation, allowing inferences about its paleoecology, paleoclimatology and paleogeography. Ecological preferences of Halictinae, as well as some features of the nests, suggest a temperate, seasonal climate and an environment dominated by low vegetation for the Laguna Palacios Formation, which is also compatible with sedimentologic and pedogenic evidence. The morphology of the nests, typical of North American halictinae, adds more evidence to the hypothesis of the existence of faunal interchange between North and South America by the Late Cretaceous. The fossil nests constitute some of the oldest evidence of bees in the fossil record, the third known record of bees of Cretaceous age and the first for the Southern Hemisphere. The two traces described are, together with those of Dakota and the Gobi Desert, the only trace fossils from paleosols of Cretaceous age that can be certainly attributed to insects.

Tuffaceous beds in glaciogenic argillites of the Late Palæozoic Dwyka Group of KwaZulu-Natal, South Africa

Two centimetre thick tuffaceous beds of the Late Palæozoic Dwyka Group were identified in an outcrop 2 km east of Nondweni in KwaZulu-Natal, South Africa. Their occurrence is significant as they represent the only known tuffaceous deposits in the Dwyka Group in the eastern part of the Karoo Basin. The tuffaceous beds probably originated from ash-fall into a proglacial marine environment. They were preserved in argillites, which were accumulating in a low energy environment. Small glass shard relics and quartz splinters testify to the volcanic origin of the inferred tuffaceous deposits. As the base of the tuffaceous beds is mostly undulatory and cuts into the underlying shales, it is possible that they represent volcanic ash-fall material that was redeposited by traction current activity together with local glacially-derived clastic sedimentary material containing angular quartz fragments, plagioclase and muscovite. The source area of the ash is not exactly known, but the tuffaceous beds provide scant testimony to contemporaneous volcanic activity that probably persisted at intervals throughout the Palæozoic and occurred along a zone of crustal weakness which was a precursor to the Mesozoic fragmentation of Gondwana along the southeastern margin of Africa.

Archaeological implications of the geology and chronology of the Soa basin, Flores, Indonesia

The timing of arrival of early hominids in Southeast Asia has major implications for models of hominid evolution. The majority of evidence for the earliest appearance of hominids in the region has previously come from Java in western Indonesia. Much of this evidence remains controversial owing to a poor understanding of the stratigraphic and chronologic relationships of the depositional units from which the material was derived. Before artifacts may be placed into their proper archaeological context, the geologic history of archaeological sites must be thoroughly understood, and deposits containing artifacts must be properly dated. An extensive investigation has been undertaken on the island of Flores, in eastern Indonesia, to determine the depositional and chronological history of stratigraphic units within the Soa basin; many of the units are associated with stone artifacts attributed to Homo erectus. Zircon fission-track dates of tuffaceous deposits within this lacustrine basin now provide the most reliable data concerning the true time of arrival of Homo erectus into Southeast Asia and indicate that these early hominids must have successfully begun colonizing eastern Indonesia by ca. 840 ka.

Geology, lithogeochemistry, and alteration of the Battle volcanogenic massive sulfide zone, Buttle Lake mining camp, Vancouver Island, British Columbia

Volcanogenic, polymetallic massive sulfide deposits within the Buttle Lake mining camp occur within the felsic volcanic and volcaniclastic rocks of the Myra formation that immediately overlies Price formation andesite. These units comprise the lower exposed part of the Paleozoic Sicker Group in the Buttle Lake uplift in central Vancouver Island. The Sicker Group represents a volcanic island arc that forms the base of the allochthonous Wrangell terrane.The Battle Main zone contains 2.3 million tons (Mt) of proven and probable reserves grading 1.3 g/t Au, 25.3 g/t Ag, 2.5 percent Cu, 0.5 percent Pb, and 13.2 percent Zn. Overall, the Buttle Lake camp hosts geologic reserves of about 12.5 Mt. The Battle Main massive sulfide zone occurs within the H-W horizon, the lowest member of the Myra formation, immediately above the Price formation andesite.Price formation andesite is over 300 m thick in the vicinity of the camp and consists of feldspar + or - pyroxene porphyritic basaltic andesite flows. Synvolcanic block faulting of this unit formed the regional-scale Buttle Lake camp basin with a strike length of at least 10 km. Local sub-basins in this major structure contain the sulfides and felsic volcanic rocks of the H-W horizon.The H-W horizon is a 15- to 200-m-thick felsic package that extends throughout the basin. It consists of eight members, three of which represent periods of massive sulfide deposition. From the stratigraphic base to the top of these members are (1) the Battle Main massive sulfide lens, (2) fine rhyolitic tuffaceous deposits, (3) H-W mafic sills, (4) Gap massive sulfide lens, (5) coarse rhyolite pyroclastic deposits, (6) rhyolite tuffaceous and cherry sediments, (7) Battle Upper massive sulfide lenses, and (8) a tripartite rhyolite flow-dome complex consisting of quartz porphyritic rhyolite, quartz-feldspar porphyritic rhyolite, and green quartz-feldspar porphyritic rhyolite. Volcaniclastic units in the felsic package evolved with interfingering of subaerial and submarine pyroclastic processes. Flow-dome units evolved by fractionation of quartz and feldspar.Alteration, evaluated petrographically and with Pearce element ratios, is most intense close to synvolcanic and synmineral feeder faults that channeled solutions through the Price formation andesite proximal to the Battle Main massive sulfide lens. Envelopes to the feeder faults have distinctive Mg addition anomalies and contain the mineral assemblage pyrite > sericite > quartz > chlorite. Discharge feeder stockworks immediately below the Battle Main massive sulfide lens contain varying proportions of sericite, quartz, and pyrite but lack significant chlorite. Deposition of the H-W horizon rhyolitic units on top of the Battle Main massive sulfide lens did not halt mineralization. Rather, fluids continued to percolate upward through the newly deposited rhyolitic units and deposited the Battle Upper lenses close to the new sediment-water interface. Alteration below the Battle Upper lenses consists of quartz, sericite, and pyrite with minor galena, sphalerite, and tennantite. Overall, hydrolysis of feldspar in both the Price formation and the H-W horizon resulted in a halo surrounding the ore lenses marked by addition of K and concomitant Na and Ca depletion.

Depositional environments and paleolimnology of an ancient caldera lake: Oligocene Creede Formation, Colorado

The Oligocene Creede Formation represents an exceptionally well-preserved intracaldera sedimentary sequence within a large, resurgent caldera. The tuffaceous, epiclastic, and limestone deposits observed in surface exposures and Continental Scientific Drilling Program (CSDP) core provide a record of depositional processes and basin evolution following caldera collapse. The basal part of the Creede Formation is characterized by interbedded caldera wall–derived debris-flow breccias and ephemeral lake deposits. This unit is succeeded by deep-water lacustrine beds that constitute the bulk of the Creede Formation. Interbedded fallout tuffs from intracaldera volcanic eruptions significantly affected lacustrine sedimentation patterns and provide a means of basinwide correlation. Prominent sublacustrine travertine accumulations circumscribe the margins of the moat basin and probably outline the structural margin of the caldera. Deposition in a cool, montane climate is indicated by the presence of pseudomorphs after ikaite in the travertine and lake beds, and corroborative paleofloral analyses. Distinctive lowstand (shallow lake) and highstand (deep lake) facies-association distributions were deposited. Changes in the proportion and character of lowstand versus highstand sequences imply an evolution from a narrow, deep, steep-walled basin to a broad, shallow, incised basin through time. Carbonate minerals were deposited in the lake as travertine at spring-orifices, and as suspension-fallout laminae across the bottom. Little evidence for shelly or fish fauna is preserved in the lake beds. The character of the laminated strata and abundance of depositional carbonate suggest that the lake evolved into a permanently stratified, bicarbonate lake. The volume and mineralogy of depositional carbonate changed through time, but evidence for development of hypersaline, alkaline lake-water compositions is not observed. The level of basin-fill and geomorphic considerations preclude control of lake level by a hydrologic sill; thus, the basin hydrology appears to have been controlled through a combination of climate and groundwater seepage. The deposits of the Creede Formation closely resemble those in modern and ancient caldera and crater lake basins. The effects ofvolcanism on detrital lacustrine sedimentation are profound; however, the influences on lake chemistry and biology are less clear. The extensive travertine in the Creede Formation appears to be related to the volcanic setting, although the origin is as of yet unclear.