Volcaniclastic Strata Research Articles

Sevier Fault at Red Canyon

The Sevier fault is spectacularly displayed on the north side of Utah Highway 12 at the entrance to Red Canyon, where it offsets a 500,000-year-old basaltic lava flow. The fault is one of several active, major faults that break apart the western margin of the Colorado Plateau in southwestern Utah. The Sevier fault is a “normal” fault, a type of fault that forms during extension of the earth’s crust, where one side of the fault moves down relative to the other side. In this case, the down-dropped side (the hanging wall) is west of the fault; the upthrown side (the footwall) lies to the east. The contrasting colors of rocks across the fault make the fault stand out in vivid detail. Immediately south of Red Canyon, the 5-million-year-old Rock Canyon lava flow, which erupted on the eastern slope of the Markagunt Plateau, flowed eastward and crossed the fault (which at the time juxtaposed non-resistant fan alluvium against coarse-grained volcaniclastic deposits) (Biek and others, 2015). The flow is now offset 775 to 1130 feet (235-345 m) along the main strand of the fault, yielding an anomalously low vertical slip rate of about 0.05 mm/yr (Lund and others, 2008). However, this eastern branch of the Sevier fault accounts for only part of the total displacement on the fault zone. A concealed, down-to-the-west fault is present west of coarse-grained volcaniclastic strata at the base of the Claron cliffs. Seismic reflection data indicate that the total displacement on the fault zone in this area is about 3000 feet (900 m) (Lundin, 1987, 1989; Davis, 1999).

Chronostratigraphic model of a high-resolution drill core record of the past million years from the Koora Basin, south Kenya Rift: Overcoming the difficulties of variable sedimentation rate and hiatuses

The Olorgesailie Drilling Project and the related Hominin Sites and Paleolakes Drilling Project in East Africa were initiated to test hypotheses and models linking environmental change to hominin evolution by drilling lake basin sediments adjacent to important archeological and paleoanthropological sites. Drill core OLO12-1A recovered 139 m of sedimentary and volcaniclastic strata from the Koora paleolake basin, southern Kenya Rift, providing the opportunity to compare paleoenvironmental influences over the past million years with the parallel record exposed at the nearby Olorgesailie archeological site. To refine our ability to link core-to-outcrop paleoenvironmental records, we institute here a methodological framework for deriving a robust age model for the complex lithostratigraphy of OLO12-1A. Firstly, chronostratigraphic control points for the core were established based on 40Ar/39Ar ages from intercalated tephra deposits and a basal trachyte flow, as well as the stratigraphic position of the Brunhes-Matuyama geomagnetic reversal. This dataset was combined with the position and duration of paleosols, and analyzed using a new Bayesian algorithm for high-resolution age-depth modeling of hiatus-bearing stratigraphic sections. This model addresses three important aspects relevant to highly dynamic, non-linear depositional environments: 1) correcting for variable rates of deposition, 2) accommodating hiatuses, and 3) quantifying realistic age uncertainty with centimetric resolution. Our method is applicable to typical depositional systems in extensional rifts as well as to drill cores from other dynamic terrestrial or aquatic environments. We use the core age model and lithostratigraphy to examine the interconnectivity of the Koora Basin to adjacent areas and sources of volcanism.

Geologic map of the east-central Meadow Valley Mountains, and implications for reconstruction of the Mormon Peak detachment, Nevada

The role of low-angle faults in accommodating extension within the upper crust remains controversial because the existence of these faults markedly defies extant continuum theories of how crustal faults form, and once initiated, how they continue to slip. Accordingly, for many proposed examples, basic kinematic problems like slip direction, dip angle while active, and magnitude of offset are keenly debated. A well-known example is the Miocene Mormon Peak detachment and overlying Mormon Peak allochthon of southern Nevada (USA), whose origin and evolution have been debated for several decades. Here, we use geologic mapping in the Meadow Valley Mountains to help define the geometry and kinematics of emplacement of the Mormon Peak allochthon, the hanging wall of the Mormon Peak detachment. Pre-exten­sion structural markers, inherited from the east-vergent Sevier thrust belt of Meso­zoic age, are well suited to constrain the geometry and kine­matics of the detachment. In this study, we add to these markers a newly mapped Sevier-­age monoclinal flexure preserved in the hanging wall of the detachment. The bounding axial surfaces of the flexure can be readily matched to the base and top of the frontal Sevier thrust ramp, which is exposed in the footwall of the detachment to the east in the Mormon Mountains and Tule Springs Hills. Multiple proxies for the slip direction of the detachment, including the mean tilt direction of hanging wall fault blocks, the trend of striations measured on the fault plane, and other structural features, indicate that it is approximately S77°W (257°). Given the observed structural separation lines between the hanging wall and footwall, this slip direction indicates 12–13 km of horizontal displacement on the detachment (14–15 km net slip), lower than a previous estimate of 20–22 km, which was based on erroneous assumptions in regard to the geometry of the thrust system. Based on a new detailed map compilation of the region and recently published low-temperature thermochronologic data, palinspastic constraints also preclude earlier suggestions that the Mormon Peak allochthon is a composite of diachronously emplaced, surficial landslide deposits. Although earlier suggestions that the initiation angle of the detachment in the central Mormon Mountains is ∼ 20°–25° remain valid, the geometry of the Sevier-age monocline in the Meadow Valley Mountains and other structural data suggest that the initial dip of the detachment steepens toward the north beneath the southernmost Clover Mountains, where the hanging wall includes kilometer-scale accumulations of volcanic and volcaniclastic strata.

KUCUKDA : A NEW, HIGH SULFIDATION EPITHERMAL Au-Ag-Cu DEPOSIT AT THE TV TOWER PROPERTY IN WESTERN TURKEY

The Biga Peninsula in northwestern Turkey hosts a large number of high and low sulfidation epithermal gold-silver-(copper) and associated copper-gold porphyry deposits and prospects, associated with voluminous, Eocene-Miocene calc-alkaline volcanism and plutonism. Approximately 15 million gold-equiv oz have been discovered within the last 10 years in this area, with a number of new discoveries now in the drill-testing stage and two deposits in the development stage. At the TV Tower property in the central Biga Peninsula, Eocene(?) to Oligocene intermediate to felsic volcanic and volcaniclastic strata and associated intrusive rocks overlie metamorphosed basement rocks of the Camlica Group. The volcanic sequence is broadly divisible into a lower sequence of andesitic dikes and flows intruded by monzonite porphyry stocks and dikes, a middle sequence of andesitic breccias and flows, and an upper sequence consisting largely of dacitic tuffs, flows, and epiclastic strata. The middle and upper sequences are strongly altered over an area covering several tens of km2, including argillic, advanced argillic, and massive to vuggy quartz alteration, the latter largely manifested as ledges hosted in dacitic volcaniclastic rocks. Moderate- and high-angle normal and oblique faults are common in the area and may have influenced the distribution of mineralization. The Kucukdag zone was drill-tested in 2010 on the basis of strong surface soil and rock sample data. The second hole returned a near-surface intercept of 136.2 m averaging 4.30 g/t Au, 15.0 g/t Ag, and 0.68% Cu. As of the date of this communication, 169 holes have been drilled to test the zone, which measures approximately 600 × 400 m and consists of a lower gold-rich zone and an upper silver-rich zone. Total mineral endowment is estimated at 966,000 indicated and 351,000 inferred gold-equiv oz. Mineralization is strongly controlled by stratigraphy, with gold hosted primarily within a massive lithic lapilli-bearing tuff unit and silver largely constrained to an overlying fluvial-lacustrine sequence capped by andesite breccias. Copper is distributed throughout the zone in association with both gold and silver. The zone is located primarily in the hanging wall of a moderate-angle fault that separates the dacitic volcaniclastic sequence from andesite breccias in the footwall. Gold is associated with tectonic-hydrothermal breccias, zones of vuggy quartz and veins, and alteration minerals consisting primarily of alunite and dickite with sulfides including pyrite, enargite, and covellite. Spatially, silver is largely separate from gold, and is associated with silicification, very fine grained pyrite, and/or marcasite and enargite. Paragenetic evidence suggests that the silver event may predate the gold event, but that the two may have shared common fluid pathways. A number of other targets are currently being explored on the property, including silicified ledge or rib-style, vuggy quartz-hosted oxide gold, Cu-Au porphyry, and low sulfidation epithermal gold-silver veins.

Eruptive history of an alkali basaltic diatreme from Elie Ness, Fife, Scotland

The Elie Ness diatreme (Fife, Scotland) is an ideal place to study the internal architecture and emplacement processes of diatremes. Elie Ness is one of approximately 100 alkali basaltic diatremes and intrusions in the East Fife area, emplaced during Upper Carboniferous to Early Permian times into an extensive rift system in the northern Variscan foreland. Within the diatreme, seven lithofacies and three lithofacies associations (LFAs 1–3) are recognised. Field, petrographic and geochemical studies demonstrate that the diatreme experienced a protracted history of eruption and infill, initially driven by volatile expansion and later by magma–water interaction. Massive lapilli tuffs of LFA 1 contain abundant highly vesicular juvenile scoria and magma-coated clasts, which are best explained by a magmatic origin for the early explosive eruptions. On a large-scale, the tuffs are well mixed and locally exhibit small-scale degassing structures attributed to fluidisation processes occurring within the diatreme fill. The occurrence of abundant volcaniclastic autoliths and megablocks within LFA 1 can be explained by subsidence of volcaniclastic strata from the maar crater and upper diatreme during emplacement. Pyroclastic density current deposits of LFA 2 form a series of continuous sheets across the diatreme, some of which may have originated from phreatomagmatic explosions in a neighbouring vent. We attribute the overall bedding pattern to a combination of primary volcanic processes and post-depositional folding related to movement along an adjacent fault. Minor steeply inclined breccias and tuffs of LFA 3 cross-cut the LFA 2 succession and are interpreted as late-stage volcaniclastic dykes and conduits, signalling the final phase of eruptive activity at Elie Ness. The study offers new insights into the volcanic evolution of diatremes fed by low viscosity, alkali-rich magmas.

Geologic Setting of Volcanic-Associated Massive Sulfide Deposits in the Kamiskotia Area, Abitibi Subprovince, Canada

The Upper Archean volcanic succession in the Kamiskotia area (Abitibi greenstone belt, Timmins region) hosts a series of past-producing copper-zinc volcanic-associated massive sulfide (VMS) deposits. All of these occur within a restricted, east-facing stratigraphic interval in the upper part of the Kamiskotia Volcanic Complex. New U-Pb ages for this interval, ranging from 2701.1 ± 1.4 to 2698.6 ± 1.3 Ma, and an age of 2703.1 ± 1.2 Ma from the lower part of the Kamiskotia Volcanic Complex, indicate that the complex is likely part of the Blake River assemblage (2701–2697 Ma) rather than the older Tisdale assemblage (2710–2703 Ma). The Kamiskotia Volcanic Complex consists largely of felsic and mafic lava flows, and VMS mineralization appears to have generally developed at or near the sea floor close to inferred synvolcanic faults. New U-Pb ages of 2714.6 ± 1.2 and 2712.3 ± 2.8 Ma from the northeast-facing volcanic succession in the northern part of the study area (Loveland, Macdiarmid, and Thorburn Townships) indicate that it forms part of the Kidd-Munro assemblage (2719–2710 Ma). A west-northwest–trending faulted contact is inferred between this older succession and the Kamiskotia Volcanic Complex rocks to the south. The Kidd-Munro assemblage rocks are coeval with the Kidd Volcanic Complex, which hosts the giant Kidd Creek VMS deposit 30 km to the east of the study area. The lower part of the succession, in south-central Loveland Township, consists of high silica FIIIb rhyolites. These rocks are geochemically similar to ore-associated FIIIb rocks from Kidd Creek and seem likely to represent the most prospective part of this succession. Future exploration in the Kamiskotia Volcanic Complex is probably best focused on the along-strike extension of the VMS-hosting interval and, in particular, on areas close to the intersections of synvolcanic faults. Mafic and felsic volcaniclastic strata which can be replaced by VMS mineralization, and felsic coherent facies flows and/or domes, appear to be important potential targets.

Geological constraints on the eruption of the Jwaneng Centre kimberlite pipe, Botswana

Geological mapping has allowed constraints to be placed on the eruption mechanisms involved in the formation of the Late Permian–Early Triassic Jwaneng Centre kimberlite pipe, Botswana. Twelve lithofacies and three lithofacies associations (LFA 1–3) are recognised. LFA 1 comprises massive to bedded volcaniclastic kimberlite and marginal shale breccias and outcrops over 65% of the surface area of the pipe. It is characterised by a lithic population dominated by Transvaal shale clasts. LFA 1 grades into LFA 2, which comprises massive and bedded volcaniclastic kimberlite and volcaniclastic breccias and outcrops over 19% of the surface area of the pipe. The lithic population of LFA 2 is dominated by contorted and fluidal-outlined Karoo-age mudstones and siltstones. LFA 3 comprises a wedge-shaped unit in the north of the pipe and consists of a series of allochthonous megablocks of sedimentary and volcaniclastic strata. The juvenile clast type and matrix mineral assemblages of the volcaniclastic deposits in the Centre Pipe differ from those in many other southern African kimberlite pipes. Various emplacement models for kimberlite pipes are discussed and evaluated in the light of the new geological data. Both a maar–diatreme model and an explosive volatile-driven eruption model could account for much of the geology of the Centre Pipe and distinguishing between the two models based on deposits alone is difficult. There is strong circumstantial evidence for ambient conditions favourable to phreatomagmatism at the time of the eruption, and the influence of external water may explain the differences between the Jwaneng Centre Pipe and the Class 1 kimberlites common across Southern Africa in terms of both the juvenile clasts and of the inter-clast cement. However, low abundances of some types of lithic inclusions derived from major country rock units pose an unresolved problem for a classic maar–diatreme model of pipe formation.

Contact metamorphism of the White Mountain Peak metavolcanic complex, eastern California

The NNW-trending White-Inyo Range represents the eastern continuation of the Sierran volcanic-plutonic arc along the central California-Nevada state line. South of White Mountain Peak, the NE-trending, steeply SE-dipping Barcroft structural break transects the orogenic belt. This high-angle reverse fault downdropped deformed, weakly recrystallized, mid-Mesozoic, mildly alkaline, White Mountain Peak mafic and felsic metavolcanic flows, interlayered volcanogenic metasedimentary strata, and minor subvolcanic plugs against a folded Neoproterozoic-lower Paleozoic quartzite and carbonate metasedimentary section on the south. The Middle Jurassic Barcroft granodioritic pluton later intruded the Barcroft break; the Middle Jurassic Cottonwood, and Late Cretaceous McAfee Creek + Pellisier Flat granitoids also invaded the superjacent section on the NE. Heating of the White Mountain Peak metavolcanic complex by the Barcroft and nearby plutons caused a southeastward progressive increase in metamorphic grade, producing neoblastic biotite, epidote, and hornblende in mafic volcanic rocks + minor hypabyssal stocks, and biotite + epidote ± andalusite in felsic volcanic rocks + volcaniclastic strata. Gradual increases in biotite Ti and plagioclase An contents reflect this SE metamorphic zonation. Utilizing hornblende-actinolite, muscovite-celadonite, and biotite solid solutions, thermobarometry indicates lithostatic pressures of 2–3 kbar during recrystallization, with temperatures ranging from a background value of ~350 °C far from the Barcroft pluton, to >500 °C along the intrusive contact. Physical conditions were similar to those developed in the Neoproterozoic-lower Paleozoic platform strata directly south of the Barcroft granodiorite. The region constitutes a typical example of the P-T evolution of the late Mesozoic Californian crustal margin, where episodic magma emplacement caused widespread contact metamorphism.

Late Ordovician island‐arc volcanic rocks, northern Capertee Zone, Lachlan Fold Belt, New South Wales

Late Ordovician (ca 450 Ma) units (Sofala Volcanics, Coomber Formation, Burranah Formation, Tucklan Formation) in the northern Capertee Zone of the northeastern Lachlan Fold Belt in central New South Wales are part of an extensive pile of mafic volcanic and volcaniclastic strata and associated shallow intrusions known collectively as the Molong volcanic province. Lavas and large clasts in volcaniclastic rocks comprise basalt (clinopyroxene‐phyric) and basaltic andesite (plagioclasephyric). Alteration mineral assemblages suggest prehnite‐pumpellyite to prehnite‐actinolite facies metamorphism. Geochemical data for samples with minimal alteration indicate shoshonitic affinity for these Ordovician magmatic rocks and are characterised by depletion of Ta, Nb, Zr, Ti and Y relative to Rb, Ba and K. Initial 87Sr/86Sr ratios and ϵNd values are similar to those reported from other Ordovician volcanic suites in the Molong volcanic province and indicate minimal crustal input. The geochemical and isotopic signature of these rocks indicates their genesis in a subduction‐related island‐arc setting.

Volcanic spreading on Mauna Loa volcano, Hawaii: Evidence from accretion, alteration, and exhumation of volcaniclastic sediments

A transect of submersible dives across the submarine west flank of Mauna Loa volcano yields compelling evidence for volcanic spreading and associated hydrothermal circulation during volcano growth. A frontal bench at the toe of the flank, formerly thought to be a downdropped block of Mauna Loa, contains a mix of volcaniclastic lithologies, including distally derived siltstone, mudstone, and hyaloclastite. The bench is overlain by bedded gravels and subaerially erupted pillow flows derived from local shoreline-crossing lava flows. The volcaniclastic strata in the bench were offscraped, uplifted, and accreted to the edge of the flank, as it plowed seaward into the surrounding moat. The accreted strata underwent significant diagenesis, through deep burial and circulation of hydrothermal fluids expelled from porous sediments beneath the volcano. Timing constraints for bench growth and breakup suggest that catastrophic failure of the subaerial edifice ca. 250–200 ka triggered volcanic spreading by reducing stresses resisting basal sliding and rift-zone inflation. Increased eruptive activity, and westward migration of Mauna Loa9s southwest rift zone, gradually rebuilt the massive flank, arresting slip prior to detachment of the Alika 2 debris avalanche ca. 120 ka.

Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the structural evolution of the Silver Peak extensional complex, west-central Nevada

Abstract The Silver Peak extensional complex, located in the Silver Peak Range of west- central Nevada, is a displacement-transfer system linking the Furnace Creek–Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene–Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal- axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual- polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329°, inclination I = 37°, α95 = 4.3°, number N = 30 sites; in situ) (α95 = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation (D = 021°, I = 57°, α95 = 4.3°, N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation (D = 032°, I = 53°, α95 = 8.8°, N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight 40Ar/39Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ∼250 °C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The 40Ar/39Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 °C to 100 °C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system.

A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador

Despite the presence of numerous active volcanoes in the northern half of Ecuador, few, if any, distal tephras have been previously recognized in the southern one third of the country. In this article, we document the presence of thin (0.1–1.0-cm-thick) distal tephras comprising glass and/or phenocrysts of hornblende and feldspar in sediment cores from five glacial lakes and one bog in Las Cajas National Park (2°40′–3°00′S, 79°00′–79°25′W). The lake cores contain from 5 to 7 tephras, and each has a diagnostic major element geochemistry as determined from electron microprobe analysis of ∼710 glass shards and ∼440 phenocrysts of feldspar and hornblende. The loss of sodium with exposure to the electron microbeam causes a 10±7 wt.% (±1σ) reduction in Na content, which we empirically determined and corrected for before correlating tephras among the sediment cores. We use a similarity coefficient to correlate among the sediment cores; pair-wise comparison of all tephras generally yields an unambiguous correlation among the cores. Six tephras can be traced among all or most of the cores, and several tephras are present in only one or two of the cores. Twenty-six accelerator mass spectrometry 14C dates on macrofossils preserved in the sediment cores provide the basis for establishing a regional tephrochronology. The widespread tephras were deposited ∼9900, 8800, 7300, 5300, 2500, and 2200 cal yr B.P. The oldest tephras were deposited ∼15,500 and 15,100 cal yr B.P., but these are not found in all cores. Two of the tephras appear correlative with volcaniclastic strata on the flanks of Volcán Cotopaxi and one tephra may correlate with strata on the flanks of Volcán Ninahuilca; both volcanoes are in central Ecuador. The absence of tephras in sediment cores correlative with the numerous eruptions of active volcanoes of the past two millennia implies that the earlier eruptions, which did deposit tephras in the lakes, must have been either especially voluminous, or southerly winds must have prevailed at the time of the eruption, or both.

Stratigraphy, age relationships and tectonic setting of rift‐phase infill in the Drummond Basin, central Queensland*

The Drummond Basin represents a major, backarc extensional system located at the inboard margin of the northern New England Orogen. Its synrift (cycle 1) infill is distinctively volcanic and volcani‐clastic in character and displays complex facies relationships and considerable variations in thickness controlled by the history and fabric of extensional faulting and the distribution of coeval volcanic centres. Subtle inheritance signatures in the age spectra obtained by SHRIMP (II) Pb‐U dating of zircons from volcanic units have impeded age assignment. New geochronologic data indicate that basinal subsidence was initiated in the north in latest Devonian (Famennian) time but was delayed until the Early Carboniferous (Tournaisian) in the south. Northern successions are dominated by volcaniclastic strata that accumulated distal to the loci of contemporary volcanism, whereas southern successions are dominated by silicic flows and ash‐flow tuffs and associated hypabyssal intrusive suites proximal to, or coincident with, volcanic loci. The Burdekin, Clarke River and Bundock Creek Basins located north of the Drummond Basin are broadly coeval features with comparable Infill. They likewise represent backarc basins developed inboard of the northern New England Orogen which trends offshore at latitude 20°S and appears to be represented in basement cores recovered from the Coral Sea. Calc‐alkaline magmatism of Late Devonian‐Early Carboniferous age extended at least 400 km inboard of the Gondwanan plate margin now represented in Queensland and related to an acute angle of subduction along the active margin at that time.

Interpretation of neovolcanic versus palaeovolcanic sand grains: an example from Miocene deep‐marine sandstone of the Topanga Group (Southern California)

ABSTRACTDespite abundant data on volcaniclastic sand(stone), the compositional, spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by active volcanism (subaqueous and/or subaerial) during sedimentation (neovolcanic, coeval grains).Deep‐marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000‐m‐thick volcanic deposits (both subaqueous and subaerial lava and pyroclastic rocks, ranging from basalt, andesite to dacite). These rocks overlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topanga Group, derived from deep erosion of a Mesozoic magmatic arc.Changes in sandstone composition in the Middle Topanga Group provide an example of the influence of coeval volcanism on deep‐marine sedimentation. Volcaniclastic strata were deposited in deep‐marine portions of a turbidite complex (volcaniclastic apron) built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks.The Middle Topanga Group sandstones are vertically organized into four distinctive petrofacies (2–5). Directly overlying basalt and basaltic‐andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwork volcanic grains, and neovolcanic crystals (plagioclase, pyroxene and olivine). The abundance of quenched glass (palagonite) fragments suggests a subaqueous neovolcanic provenance, whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance. This petrofacies probably was deposited during syneruptive Periods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcanolithic petrofacies 3 includes both neovolcanic (85%) and older detritus derived from plutonic, metamorphic and palaeovolcanic rocks. During post‐eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48–14%) and increase of plutonic‐metamorphic and palaeovolcanic detritus.The Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcanic detritus is similar to percentages of the Lower Topanga Group (petrofacies 1).In general, the volcaniclastic contribution represents a well‐defined marker in the sedimentary record. Detailed compositional study of volcaniclastic strata and volcanic particles (including both compositional and textural attributes) provides important constraints on deciphering spatial (extrabasinal vs. intrabasinal) and temporal relationships between neovolcanic events (pre‐, syn‐, inter‐ and post‐eruptive periods) and older detritus.

VOLCANOGENIC AND VOLCANICLASTIC RESERVOIR ROCKS IN MESOZOIC‐CENOZOIC ISLAND ARCS: EXAMPLES FROM THE CAUCASUS AND THE NW PACIFIC

Subduction of the Tethys oceanic plate beneath the Lesser Caucasus island are in the Late Creataceous — Eocene produced conditions favourable for the generation and accumulation of hydrocarbons. Subduction of crust in the Transcaucasus Massif led to the formation of various types of trap. Also, geothermal gradients here were high, resulting in the generation of hydrocarbons in shallow‐water sediments on the margins of the Massif, and their accumulation in both sedimentary and volcaniclastic reservoirs (e.g. in the Samgori‐Patardzeuli and Muradkhanly fields).The geodynamic setting of the NW margins of the Pacific Ocean was similar in the Neogene to that of the Transcaucasus Massif. Oceanic crust was subducted during the Oligo‐Miocene, and a series of inter‐are rifts were formed. The principal oilfields of Japan, where accumulations are resrvoired in volcaniclastic strata (Neogene‐Pleistocene) are located here. A possible analogue is the rift located in the southern East Kuril Basin, where the occurrence of petroleum has been inferred. Lithological studies of the Komandorsky Islands, eastern Kamchatka, the Kuril Islands and western Sakhalin indicate that the distribution of the reservoirs depends on the stage of evolution of the rifts and adjacent island arcs.

Glass chemistry in volcaniclastic sediments of ODP Leg 107, Site 650, sedimentary sequence: provenance and chronological implications

A detailed chemical investigation of volcanic glass fragments from volcaniclastic strata (6 tephras, 1 volcanic debris flow, 12 volcanic turbidites) of ODP Leg 107, Site 650, sedimentary sequence, leads to a varied pattern in terms of both provenance and age constraints. The six analyzed tephra strata indicate a provenance from at least three different volcanic provinces: Aeolian, Campanian, and Sicilian Channel (Pantelleria Island). The older tephra strata (021, 018, 012) have a large amount of “orogenic” rhyodacite/rhyolite deposits that may be attributed to the Aeolian province, although no subaerial coeval volcanic activity of similar composition has so far been documented in the Aeolian Arc. Tephra 007 is related to the Pantelleria Island activity and, particularly, to an ignimbrite episode dated circa 130 ka. Tephra strata 005 and 003, have a clear Campanian provenance, and are correlated with analogous tephra layers, observed in the Tyrrhenian and Ionian seas, dated circa 107 and 60 ka respectively. In the oldest portion of the sequence (from 1.3 to 0.13 Ma), the volcaniclastic sediments were only derived from the Aeolian domain whereas in the latest 130 ka, the Campanian influx becomes much more predominant. Therefore, a general K-enrichment trend is observed in the temporal sequence of all the analyzed samples (almost 700 point analyses) which may be related both to a variation in the source area and to the specific Pleistocene magmatic evolution of the peri-Tyrrhenian volcanic provinces.

Silicification and metal leaching in semiconformable alteration beneath the Chisel Lake massive sulfide deposit, Snow Lake, Manitoba

Regionally extensive semiconformable zones of silicified, Fe-Mg metasomatized and epidotized, dominantly mafic, volcaniclastic strata and lava flows are exposed 1 to 2 km stratigraphically beneath the Chisel Lake Zn-Cu massive sulfide deposit, Snow Lake district, Manitoba. The alteration zones occur within a Lower Proterozoic medium-grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments, and lie between subvolcanic tonalite sills and the massive sulfide deposit.Semiconformable alteration occurs at two main stratigraphic positions. The lower zone of silicification in pillowed and massive mafic lavas is inferred to have occurred close to the sea floor and probably was not related directly to formation of the Chisel Lake massive sulfide deposit. The stratigraphically higher zone, which may be spatially and temporally associated with massive sulfide deposition, occurs in a approximately 300-m-thick heterolithic mafic volcanic breccia and wacke unit. This volcaniclastic-hosted alteration is zoned laterally from dominantly silicification and epidotization to mainly Fe-Mg metasomatized, garnet-chlorite + or - biotite + or - staurolite rocks nearer the Chisel Lake sulfide deposit. A discordant footwall Fe-Mg alteration zone directly beneath the sulfide deposit extends toward, and may meet, the semiconformable Fe-Mg metasomatized zone.Silicification contributed to partial to complete replacement of volcanic clasts, beds in the volcaniclastic unit, rocks adjacent to some felsic dikes and pillow interiors by quartz and sodic plagioclase. Mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds, dikes, and pillows indicate that SiO 2 increased by up to 50 percent of its initial value, and Na 2 O by up to 30 percent. Up to 80 percent of the FeO, MgO, CaO, and Zn was removed during silicification. Elemental fluxes during Fe-Mg metasomatism are generally opposite those characterizing silicification and are of comparable magnitude. Epidotization resulted in depletion of Na, total Fe (but increased the Fe (super 2+) /Fe (super 3+) ratio), Mg, Mn, K, Zn, and Ba, and enrichment in Ca and Sr relative to least altered rocks. Almost constant interelement ratios of Ti, Zr, and Al in altered and less altered rocks indicate that these elements were essentially immobile during metasomatism and subsequent medium-grade regional metamorphisrm. Limited data suggest that the heavy REE were also immobile during silicification.The subconcordant silicification in the mafic volcaniclastic unit is interpreted to have formed at subsea-floor depths of 1 to 2 km where a felsic dike swarm and subvolcanic tonalite sills heated Si-rich evolved seawater above the temperature of the silica solubility maximum ( approximately 340 degrees -450 degrees C at pressures below 900 bars) causing silica deposition and metal leaching. A portion of the Fe, Mg, and possibly Zn that was leached from the subconcordant silicified zone may have been transported laterally away from this environment, thereby producing the semiconformable Fe-Mg metasomatized zones. Cross stratal structures similar to the hydrothermally altered synvolcanic faults that are known to cut the semiconformable alteration may represent fluid flow paths from a subconcordant metal reservoir in the volcaniclastic unit to the sea floor, where massive sulfides were deposited.Semiconformable pervasively silicified zones, particularly those associated with Fe-Mg metasomatized and/or epidotized rocks, are significantly larger exploration targets than areas of proximal alteration, and indicate large-scale hydrothermal mass transfer. Zonation of silicification to Fe-Mg metasomatism laterally within the alteration may provide a vector toward the discordant Fe-Mg-enriched alteration zones that commonly underlie volcanic-associated massive sulfide deposits.

Stratigraphic relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

A previously unrecognized angular unconformity divides the Jurassic and Cretaceous McCoy Mountains Formation into a lower and an upper unit in the Dome Rock Mountains and Livingston Hills of western Arizona. The lower unit of the McCoy Mountains Formation consists of generally fine-grained quartzose and volcaniclastic strata that were deposited after cessation of Middle Jurassic explosive volcanism. The basal contact of the lower unit is disconformable in most places, but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks. The upper unit is a fining-upward sequence of quartzo-feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane. A tuff in the lower part of the upper unit has a U-Pb crystallization age of 79 ± 2 Ma (Late Cretaceous). Rocks of the lower unit are deformed by pre-80 Ma thrust faults of the generally southward-vergent Maria fold and thrust belt, which bounds the outcrop belt of the McCoy Mountains Formation on the north. The upper unit is exposed only south of the fold and thrust belt. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area.

Depositional Environments and Paleogeographic Significance of Mesozoic Volcanic and Volcaniclastic Strata in the Southern Inyo Mountains, East-Central California

Depositional Environments and Paleogeographic Significance of Mesozoic Volcanic and Volcaniclastic Strata in the Southern Inyo Mountains, East-Central California

Chronostratigraphic Assignment of Volcanopelagic Strata Above the Coast Range Ophiolite

The chronostratigraphic assignments of radiolarian cherts, tuffaceous mudstones and volcaniclastic strata (referred to herein as volcanopelagic succession)' overlying the Coast Range ophiolite in the Coast Ranges of California have been refined on the basis of additional field data and recent revision of the Pessagno and others radiolarian biostratigraphic zonation for the Middle and Upper Jurassic of North America. Revision of this zonation scheme, accomplished through the integration of radiolarian data with ammonite biostratigraphy and geochronometry, has enabled close correlation of radiolarian assemblages in North America, Europe, and Japan. However, major differences still exist in the stage assignments of these assemblages among different zonations. The Pessagno and others scheme is utilized herein to discuss the current chronostratigraphic assignments at five localities in the Coast Ranges: Point Sal, Stanley Mountain, Cuesta Ridge, and Llanada in the southern Coast Ranges, and Black Mountain (Geyser Peak area) in the northern Coast Ranges of California. Stage assignments for volcanopelagic successions in these areas range from uppermost Callovian/lower Oxfordian to upper Tithonian.