Air-fall Tuffs Research Articles

Holocene carbonatite-nephelinite tephra deposits of Oldoinyo Lengai, Tanzania

Natrocarbonatite and nephelinite tephra have been erupted together from Oldoinyo Lengai over the past few thousand years. The oldest deposits with recognizable natrocarbonatite are thin tuff beds probably between 2000 and 5000 years in age. Tuffs and agglomerates with an age of about 1250–2000 years contain evidence strongly suggestive of natrocarbonatite ash. The volcano is mantled by nephelinite-carbonatite ash deposits, termed the Footprint Tuff, that were erupted about 600 years ago. Evidence is contradictory as to whether natrocarbonatite was discharged during the cone-building phase of eruptions, which ended about 15,000 years ago. Nephelinite-carbonatite ash erupted in 1966 contains an unidentified mineral, designated NCS, with a chemical composition of Na 4.09Ca 2.76Si 5O 14.81. It may be genetically related to natrocarbonatite magma as it has thus far been identified only in the younger tephra deposits of Oldoinyo Lengai, most of which contain evidence of natrocarbonatite. A tephra deposit termed the Footprint Tuff contains footprints thought to be preserved by the rapid recrystallization of primary natrocarbonatite. Calcite of natrocarbonatite origin forms an estimated 15–20% of the airfall tuffs, and natrocarbonatite probably equalled or exceeded the volume of nephelinite tephra at the time of eruption. Nyerereite ([Na 0.82K 0.18] 2 Ca[CO 3] 2) and gregoryite ([Na 0.78K 0.05] [Ca 0.17CO 3]) were primary minerals in the natrocarbonatite, as in modern lavas of Oldoinyo Lengai. Unlike modern lavas, the groundmass contained a substantial amount of silicate material. Noncarbonate minerals in the Footprint Tuff include nepheline, melilite, augite, wollastonite, melanite, fluorite, and NCS. The Footprint Tuff was cemented soon after deposition, very likely by trona. Gaylussite, pirssonite, or both, were probably later alteration products in the transformation of natrocarbonatite ash to form calcite. Oxygen and carbon isotopic re-equilibration were essentially decoupled in alteration of natrocarbonatite ash. δ 18O values of this calcite suggest extensive to complete oxygen exchange, but δ 13C values are in and near the range for unaltered natrocarbonatite. Carbon exchange may have been retarded in ash alteration by the high pH of pore fluid.

Epithermal sinters of Paleozoic age in north Queensland, Australia

Research Article| August 01, 1989 Epithermal sinters of Paleozoic age in north Queensland, Australia N. C. White; N. C. White 1BHP-UTAH Minerals International, P.O. Box 619, Hawthorn, Victoria 3124, Australia Search for other works by this author on: GSW Google Scholar D. G. Wood; D. G. Wood 2P.O. Box 425, Spring Hill, Queensland 4000, Australia Search for other works by this author on: GSW Google Scholar M. C. Lee M. C. Lee 3BHP Gold Mines Limited, P.O. Box 101, Dunedin, New Zealand Search for other works by this author on: GSW Google Scholar Author and Article Information N. C. White 1BHP-UTAH Minerals International, P.O. Box 619, Hawthorn, Victoria 3124, Australia D. G. Wood 2P.O. Box 425, Spring Hill, Queensland 4000, Australia M. C. Lee 3BHP Gold Mines Limited, P.O. Box 101, Dunedin, New Zealand Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (8): 718–722. https://doi.org/10.1130/0091-7613(1989)017<0718:ESOPAI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation N. C. White, D. G. Wood, M. C. Lee; Epithermal sinters of Paleozoic age in north Queensland, Australia. Geology 1989;; 17 (8): 718–722. doi: https://doi.org/10.1130/0091-7613(1989)017<0718:ESOPAI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Silica sinters deposited from hot springs have been found associated with volcanic rocks of Late Devonian to late Carboniferous age in north Queensland, Australia. These deposits have many features in common with modern hot-spring sinters, such as those of the Taupo Volcanic Zone in New Zealand. They occur associated with subaerial volcanic rocks and are locally found with air-fall tuffs and fluvial and lake deposits in which hydrothermal eruption breccias are common. The associated rocks are extensively hydrothermally altered and are cut by silica veins showing characteristic epithermal vein textures; the silica veins contain low-salinity fluid inclusions trapped at epithermal temperatures, and there is evidence of boiling.Textures preserved in the sinters are identical to those found in modern sinters; they include columnar structures similar to bacterial stromatolites described from Yellowstone National Park, as well as striated surfaces apparently resulting from silica deposition on filamentous algae. Plant fossils, including Oxroadia gracilis, are abundant. Criteria applied to identify these ancient sinters may be applied to other possible sinter deposits and may provide evidence indicating the level of exposure of the former geothermal system, which may be an important guide in exploration for epithermal precious-metal deposits. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

63B. Distinction between welded air-fall tuffs and lava flows.(Abstracts of Papers Presented at the Fall Meeting of the Society)

63B. Distinction between welded air-fall tuffs and lava flows.(Abstracts of Papers Presented at the Fall Meeting of the Society)

The age of the Kambalda greenstones resolved by ion-microprobe: implications for Archaean dating methods

A “chert” horizon within the Archaean basalt pile at Kambalda, Western Australia, contains zircons from air-fall tuffs erupted as the sediment was laid down. Ion-microprobe analyses of these magmatic zircons constrain the long-sought age of the Kambalda lavas to 2692 ± 4Ma(2 σ), and show that regional metamorphism and deformation of the greenstone pile occurred within 30 Ma of deposition. Rare zircon xenocrysts are evidence of pre-greenstone crust as old as 3450 Ma, exposed remnants of which are yet to be identified. Subsets of both the magmatic and xenocryst zircons show evidence of hitherto-unsuspected low-temperature U and/or Pb mobility in the Proterozoic, long after stabilisation of the craton. The ion-microprobe data unravel complexities of isotopic inheritance and alteration, illustrating the hazards of using bulk or whole-rock samples in Archaean isotopic investigations. Identification of interflow “cherts” as a precisely dateable rock type opens the way to direct ion-probe dating of mafic-ultramafic piles, and close numerical age control of the complex, but ill-exposed, stratigraphic and structural relations in this richly-mineralised greenstone belt.

Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo volcanic zone, New Zealand

Erosion and excavations at Ohakuri in the Taupo Volcanic zone have exposed the upper portion (100–150 m) of a hydrothermal system that was active sometime between 700,000 and 160,000 years ago. Extensive hydrothermal alteration occurred within a host sequence of young, relatively undeformed, chemically and lithologically similar unwelded rhyolitic ignimbrite and air-fall tuffs. Mapping and petrologic work have identified six distinct alteration types. An early event formed a concentrically zoned suite of alteration through the pervasive movement of alkaline chloride type water. In the innermost zone, primary rock components were almost entirely converted to quartz + adularia ± illite ± hematite ± leucoxene. Mineralized veins and breccias of quartz ± pyrite ± adularia ± chlorite formed here in response to episodic hydraulic fracturing. This zone grades outward and upward into a zone of less intense, lower rank alteration with a mordenite + clinoptilolite + smectite + opal ± hematite assemblage, then a zone of weak clay alteration and into fresh rock. Calcite is conspicuously absent from the entire suite. Acid-sulphate type water, formed from steam-condensate, dominated the shallow activity in a second stage of alteration that followed local erosion. Widespread but discontinuous alteration converted the ignimbrite to kaolinite + opal ± hematite, with alunite occurring in the more intense zones. This alteration locally overprints the early alkali-chloride produced suite, but the focus of the second-stage activity was north of the focus of the older event. Scattered opaline sinters and silicified surficial deposits are products of either still later activity or the waning part of the second stage. Chemical analysis shows that the various alteration types have characteristic patterns of major element addition and removal; these reflect the key hydrothermal mineral reactions that formed the new assemblages. Quartz-adularia alteration involved mainly silicification, dehydration and cation exchange (K + for Na 2+, H +, Ca 2+, Mg 2+), whereas alteration in the mordenite zone was mostly a moderate hydration process. Kaolinite alteration involved strong hydration, hydrolysis and redistribution of silica. Trace elements show varying degrees of mobility and correlation with major elements. Alteration features identify the important upflow zones, zones of mixing between hydrothermal and shallow groundwater, and changes in alkali chloride water level. They also reflect a transition from diffuse to channel flow as sealing eliminated original rock porosity, and led to hydraulic fracturing which maintained fracture permeability in the system. Mineralogy and fluid inclusion studies indicate that the primary fluid at now-exposed levels was a high-pH (7–8), low-CO 2 and low-H 2S water cooler than 200°C, probably modified by boiling at depth.

The West Waterberg tonstein, South Africa

What is thought to be the first recorded occurrence of a tonstein in the coalfields of South Africa is described from the Volkrust Shale Formation of the Permain Karoo Sequence in the West Waterberg Coalfield. Now, virtually pure kaolin, petrological, mineralogical and geochemical evidence points to its origin being an airfall tuff of acid composition. The concentrations of Ba, Sr, and Pb in the tonstein are unusually high. It is thought the elements were released from the ash and precipitated as phosphates of mixed composition. The phosphate distribution is not uniform and is apparently related to the fragmental texture of the rock.

Miocene Tectono-Stratigraphic History of La Mision Basin, Northwestern Baja California: Implications for Early Tectonic Development of Southern California Continental Borderland: ABSTRACT

The middle Miocene La Mision basin in northwestern Baja California, Mexico, provides a rare opportunity to study an onshore portion of the southern California continental borderland. Stratigraphy, geometry of dispersal, and a variety of lithotypes within the volcanic and volcaniclastic sediments of the Rosarito Beach Formation provide clues to the nature of early tectonic evolution of this area during the Miocene. The elongated, trough-shaped La Mision basin formed in response to peninsular basement uplifts and the formation of volcanic highlands west of the present coastline. Lithologies and depositional environments represented within the basin sediments include: subaerial basalt flows and airfall tuffs, submarine muddy- and sandy-matrix mudflow breccias, lapilli tuffs, crystal tuffs, tuffaceous sandstones,d diatomites, and conglomerates. The environments of deposition range from fluvatile to intertidal to shallow marine. Early basin infilling is characterized by sediments and basalts, with a western source terrane, that were deposited against the faulted seacliffs. progressive infilling against the seacliff resulted in the formation of an extensive eastward-sloping basaltic platform extending eastward to the foothill coastal belt of the Peninsular Ranges. Marine transgression and subsequent regression are recorded by diverse marine volcaniclastic lithologies. Abundant fossils, K-Ar dates, and paleomagnetic data obtained from the La Mision basinmore » allow precise correlation with other areas in the continental borderland and provide conclusive evidence that this block of the borderland was formed and in its present position by 16-14 Ma.« less

Environmental and diagenetic analyses of Lower Permian epiclastic and pyroclastic fan deposits—their role for coal formation and uranium metallogeny in the Stockheim Trough (F.R.G.)

Conglomerate-silt/mudstone successions are of widespread occurrence within Permian riftogenic troughs from mid Europe. Commonly these beds are abundant in volcanic detritus which was partly derived by weathering of volcanic cones, and partly from directly related certain eruptive pulses. The volcanic influence on sedimentation processes in the course of basin evolution decreases. The lowermost units contain abundant ignimbrites and lahars, whereas the overlying strata exhibit a gradual change from debris flow into more stream-dominated fan deposits. The outer-fan depositions built up by silty sediments, locally mixed with air-fall tuffs, contain coal seams. The coal-bearing host rocks may be categorized into four classes: (1) grey siltstones made up of prevalently non-volcanic detritus; (2) grey to black layers consisting of epiclastic and pyroclastic components; (3) silicified coal seams; and (4) brick-red petrified wood. Ore concentrations in these carbonaceous rocks result from the mobilization of elements from parent material containing large quantities of labile constituents. Four modes of strata-bound U concentration may be encountered: (1) U-bearing petrified wood remains; (2) U-bearing coal lenses; (3) U-bearing carbonaceous volcaniclastic conglomerates; and (4) U-bearing carbonaceous fine-grained tuffites (“fish eyes”). These U accumulations are produced during the course of early diagenesis of volcaniclastic material and coal-bearing beds. The reason for the very high prospectivity of these Permian mid-European sediments is the close intertongueing of carbonaceous matter-bearing rocks and volcaniclastic rocks necessary for the U supply (“uraniferous dirty coal”), in basins of rapid subsidence, which causes the bimodality of those clastic deposits.

Lamproites and other potassium-rich igneous rocks: a review of their occurrence, mineralogy and geochemistry

Summary In this paper the geological occurrence, geochemistry and mineralogy of ultrapotassic (K 2 O/Na 2 O &gt; 3 (molar ratio)) and perpotassic (K 2 O/Al 2 O 3 &gt; 1 (molar ratio)) igneous rocks, especially lamproites, are reviewed and discussed in the context of compositionally-similar mantle-derived melts. Lamproites are K- and Mg-rich igneous rocks (typically K 2 O &gt; 5 wt.%, MgO &gt; 5 wt.%) which possess an exotic and diagnostic mineralogy and geochemistry. Known lamproites occur in 21 major suites or localities in continental regions with a variety of geological and tectonic environments; they range in age from the early Proterozoic dykes at Holsteinsborg, W Greenland, and Chelima, India, and Precambrian pipe at Argyle, W Australia, to the Middle Pleistocene flows and the Recent volcanics of the Leucite Hills, Wyoming, and Gaussberg, Antarctica, respectively. Intrusive and extrusive forms of lamproites include flows, a variety of pyroclastics (welded tuffs, piperno, air-fall tuffs, volcanic breccias etc.), cinder cones, dykes, sills and diatremes. Whereas kimberlite diatremes tend to be carrot shaped, the shape of olivine lamproite diatremes approximates a sherbet-glass. The recent discovery of diamondiferous lamproties of large volumetric proportion in the E and W Kimberleys, NW Australia, and the reclassification of the diamondiferous micaceous peridotite at Prairie Creek, Arkansas, as a lamproite substantiate their economic importance. The 21 lamproite suites considered here tend to be localized marginal to continental craton cores in areas that overlie fossil Benioff zones, in contrast with the general occurrence of kimberlites more interior to continental cratons. The petrographic diversity of lamproites has historically hindered the development of a concise and universal classification and nomenclature. Lamproites are distinguished from kimberlites and alkali basalts (and lamprophyres) in terms of mineralogy, mineral chemistry, geochemistry and volcanic extrusive character. Relative to kimberlites, lamproites are enriched in K, Si, Ti, Al, Rb, Sr, Zr and Ba and depleted in CO 2 , Ca, Mg, Fe, Ni, Co and Cr. Lamproites are characterized by the general presence of phlogopite, diopside, leucite and K-richterite, occasional glass, olivine, sanidine, priderite, perovskite, wadeite, apatite and chrome spinel, and very rare ilmenite. Lamproite amphiboles, diopsides and phlogopites are distinctly depleted in Al 2 O 3 relative to those of nearly all other igneous rocks. Lamproite magmas are produced by the partial melting of old refractory mantle peridotite (approaching a dunite or harzburgite in mineralogy) that was enriched in K-bearing and other incompatible-element-enriched phases, such as phlogopite and apatite, most probably as a result of some metasomatic event which occurred prior to melting. In contrast with alkali basalt and kimberlite melts which are apparently produced from the partial melting of a CO 2 -enriched mantle periodtite (i.e. a source with a relatively high CO 2 /H 2 O ratio), water is the key volatile species involved with lamproite petrogenesis (source with a low CO 2 /H 2 O ratio).

Sedimentology, stratigraphy and palaeoenvironments of the Holocene Galana Boi Formation, NE Lake Turkana, Kenya

Summary Lake Turkana lies in a closed drainage basin at the northern end of the Kenya Rift Valley. During the early and middle Holocene the lake was fresh, much expanded and subject to many fluctuations in surface level. Periodically it formed one linkage in a greater Nile drainage network. Late Holocene, climatically-induced regression left a ring of sediments stranded up to 80 m above the modern lake. To the NE, these deposits are known as the Galana Boi Beds and here we propose raising them to formation status. They consist of diatomaceous silts, silty diatomites, quartz-arenites, arkoses, litharenites, coquinas and at least one air-fall tuff. Depositional environments ranged from low-energy offshore, through littoral, lagoonal and deltaic to alluvial and aeolian. A rich, diverse microfossil assemblage of diatoms, pollen and ostracods has been recorded and used in palaeoenvironmental analysis. Macrofossils include molluscs, fish debris and other vertebrates. With their excellent outcrop and diverse environments represented, the Galana Boi Formation can serve as a model for sedimentation of the margins of a large rift lake, and aid in the interpretation of similar, but poorly exposed graben sequences in the geological record.

Depositional context of Plio-Pleistocene hominid-bearing formations in the Middle Awash valley, southern Afar Rift, Ethiopia

Summary The Awash flows from the Ethiopian Highlands via the Ethiopian Rift into Lake Abhe in the Afar Rift. Over 8000 km 2 of Neogene fluvio-lacustrine deposits crop out in the Middle Awash valley, of which we have surveyed 300 km 2 E of the river between 10°25′N and 10°45′N. Pleistocene sediments adjacent to the Awash floodplain consist mostly of unfossiliferous brown clays with sand and gravel interbeds which contain vertebrate (including hominid) fossils and prehistoric stone artefacts (Oldowan and Acheulian). Palaeocurrent directions in these fossiliferous sands invariably parallel the ephemeral sandbed channels that flow W from the basalt escarpment to the Awash. We interpret the brown clays as palaeo-Awash floodplain deposits, in contrast to the fossiliferous gravel-sands, which are mainly of local provenance. Away from the river, alternating Pliocene clays and diatomites, with minor fossiliferous gravel-sands, are capped by a 3.8–4.0 Ma primary air-fall tuff. Facies changes are more frequent above this ‘Cindery Tuff’, indicating tectonic disruption of the early Pliocene lakes, reflected also in a progressive increase in fluvio-volcanic gravels from Pliocene to early Pleistocene.

Submarine‐fan deposited sandstone and rudite in a mid‐Cenozoic interarc basin in Maewo, Vanuatu (New Hebrides)

ABSTRACTThe volcaniclastic, Lower Sarava Formation of north‐eastern Maewo was deposited in a Late Oligocene‐Early Miocene deep sea fan environment. Eight lithofacies can be recognized from a westward‐trending, deep‐sea fan which prograded at a depth of &gt;4.25 km within an extinct interarc basin. These lithofacies include massive spilite‐rich rudite, graded coarse rudite‐arenite, graded fine rudite‐arenite, intraformational rudite, brown and grey siltstone, minor calcarenite and a lithofacies composed of turbidites with Bouma sequences. The sediment was essentially derived from the Vitiaz palaeoarc to the east. Non‐calcareous red and green siltstone and minor airfall tuff was deposited distal to the arenite and rudite, in areas, or during periods, of slower sedimentation.Slightly more than half of the lower part of the Sarava Formation is represented by massive rudite, graded coarse rudite‐arenite, and graded fine rudite‐arenite. These deposits, and some of the calcarenites and massive arenites. were deposited from decelerating high density turbidity currents. Some of the thicker rudite‐arenites may have been induced by tsunamis. Classical turbidites deposited, perhaps, from low density turbidity currents are rare.The petrography of igneous clasts indicates that Late Oligocene‐Early Miocene volcanism on the Vitiaz palaeoarc was largely basaltic‐andesitic but there were some dacitic eruptions.

Airfall Tuff in the Browns Park Formation, Northwestern Colorado and Northeastern Utah

Bedded airfall tuffs, mainly rhyolitic in composition and locally very thick, occur throughout the Browns Park Formation (upper Oligocene to upper Miocene) in northwestern Colorado and northeastern most Utah. First mentioned in the geologic literature by Bradley (1935), they have received only cursory attention other than for the purpose of radiometric dating. The present writer began study of the tuffs in 1980, hoping to use them as time-stratigraphic marker beds within the formation. Several tuff-rich stratigraphy sections were measured and numerous samples were collected. The results of petrographic and petrochemical studies of these samples are presented. Refractive indices of vitric shards from 52 samples increase slightly and irregularly with apparently decreasing age of the samples. Seemingly, the trend points toward less felsic compositions with time. Quartz, feldspar, and especially heavy-mineral suites from these from these 52 samples were studied petrographically. Because airfall components (phenocrystic minerals) are not generally readily distinguishable from contaminating detrital grains, time-composition trends in airfall assemblages are problematical and difficult to establish. Clinopyroxene of likely airfall origin may increase slightly in abundance with decreasing age. The ratio of apatite (mainly of airfall origin) to zircon (mixed origins) appears to decrease with time. Recalculated analyses of major oxides for the 24 whole-rock samples that appear to be least contaminated by detritus show the following variations with decreasing age: increased silica, potash, total alkalis, and titania, and decreased alumina, total iron oxides, magnesia, lime, and soda, and soda: potash ratio. In general, petrochemical results indicate that Browns Park tuffs became increasingly more felsic with time, albeit irregularly. This contradicts the conclusions drawn from refractive-index determinations on the same samples. Preliminary correlations of airfall-tuff beds by petrochemistry, refractive indices of glass, and mineralogy and population of heavy minerals, have been partly successful Minor-element distribution provides useful information, but data are incomplete. Prevailing westerly winds in mid- to late-Tertiary time brought ash into the Browns Park Formation and correlative units elsewhere in the Rocky Mountain region from distant sources m numerous volcanically active areas in the western United States. Bulk mineralogy and major-oxide compositions and variations from selected eruptive centers in the Basin and Range province commonly resemble those of Brown Park tuff. Source areas of silicic eruptive material were in, but not necessarily limited to, Utah, Nevada, eastern California, southwestern Idaho, and southeastern Oregon.

Volcanic stratigraphy of the Los Azufres geothermal area, Mexico

The Los Azufres volcanic center, located 200 km northwest of Mexico City, is one of a number of Pleistocene silicic volcanic centers with active geothermal systems that lie north of the axis of the Mexican Neovolcanic Belt. Silicic volcanism began approximately 1 m.y. ago with eruption of the Agua Fria rhyodacite to high-silica rhyolite lava domes and flows. They are covered by rhyodacite and dacite lava domes and flows of the San Andres volcano, dated at 0.3 m.y. A north- to northeast-trending set of high-angle, normal faults is offset by a younger and more prominent east-trending set that parallels the principal regional structural trend. These faults cut all but the youngest volcanic rocks of the center, the Yerbabuena rhyodacites to high-silica rhyolites. These morphologically well-preserved lava domes are approximately 0.15 m.y. old and are probably associated with air-fall tuffs that blanket the western portion of the volcanic center. Two episodes of high heat flow suggested by studies of drill-core samples and fluids from the Los Azufres geothermal field may correspond to two different episodes of magma injection to high levels, the most recent one resulting in eruption of the Yerbabuena lavas.

Laacher See Tephra: A widespread isochronous late Quaternary tephra layer in central and northern Europe

A late Quaternary tephra layer, widespread in central and northern Europe, resulted from explosive Plinian and phreatomagmatic eruptions of the Laacher See Volcano 11,000 yr B.P. The tephra is distinguished from other late Quaternary andesitic-rhyolitic airfall tuff layers in northern Europe and from basaltic or trachytic tuff deposits in southern Europe by its phonolitic composition and abundance of sanidine, plagioclase, clinopyroxene, amphibole, and sphene. The proximal tephia sequence at Laacher See is divided into three main deposits: the predominantly Plinian deposits of Lower and Middle Laacher See Tephra (LLST and MLST) and phreatomagmatic deposits of the Upper Laacher See Tephra (ULST). The MLST member is further subdivided into beds A, B, and C1, C2, and C3. The chemical composition of the magma is highly differentiated phonolite in the LLST to MLST B sections but mafic phonolite in the MLST C1 to ULST sections. All deposits are considered to be isochronous, the frequency maximum of 16 radiocarbon datings indicating an eruption about 11,000 ±50 yr B.P. Distal ash was deposited in three main fans directed to the northeast (LST traced up to 1,100 km distance), south (LST traced up to 600 km), and southwest (LST traced up to 100 km). Tephrostratigraphic correlation of the distal ash deposits is based on (a) the major-element composition of glass shards, (b) lithology, and (c) heavy-mineral analyses. The northeastern fan consists of deposits from LLST, MLST B, and MLST C1 eruptive phases, the southern fan comprises MLST A, MLST C2, and ULST deposits, and the southwestern fan consists exclusively of ash from the ULST eruptive phase. Northeastern transport of ash during eruptive phases, with high Plinian eruption columns, but southern and southwestern transport of ash along phases of relatively low eruption columns, are interpreted in terms of prevailing southwesterly paleowinds at high altitudes (tropopause level?) but northerly winds dominating in the lower atmosphere. The Laacher See eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions, which also explains the near-vent shifting of LLST, MLST B, and MLST C1 iso-pach axes from east-southeast to northeast within the first 20 km of transport.

Late Quaternary caldera volcanoes of the Kenya Rift Valley

There are seven major, late Quaternary (≤1 m.y.), trachytic caldera volcanoes in the Kenya Rift Valley; the Barrier, Emuruangogolak, Silali, Paka, Menengai, Longonot, and Suswa. These are situated in the inner troughs of the rift. Their positions are not apparently related to transverse lineaments or basement structures. The structural development was complex, involving combinations of the formation of large calderas and small calderas/pit craters, the growth of lava and tuff cones and the eruption of fissure basalts. Two groups of volcanoes are broadly distinguished. A southern group, comprising Suswa, Longonot, and Menengai, are overwhelmingly of trachytic composition. Krakatau‐style formation of calderas approaching 100 km2 in area was accompanied by the eruption of voluminous ash flow and airfall tuffs (20–50 km3). In the remaining, more northerly volcanoes, basalts and mugearites form bimodal associations with trachytes. Caldera formation was accompanied by only sparse pyroclastic activity and the collapse may have been initiated by withdrawal of magma from depth. Knowledge of the geochemical evolution of the complexes is limited by the scarcity of published analytical data. The only detailed study is that of Menengai, where chemical variations were caused, at different structural stages, by magma mixing, crystal fractionation and various liquid‐state differentiation processes

The regional geology and evolution of the Toba volcano-tectonic depression, Indonesia

The late Quaternary Toba volcano-tectonic depression, in the Sunda Arc, is the largest resurgent cauldron in the world and lies in one of the largest ignimbrite fields, the Toba Tuffs. Although composed of at least 3000 km 3 of acid tuffs, spread over 20000 km 2 of mainland Sumatra, the greater part of the Toba Tuffs is believed to be a single ignimbrite cooling unit, formed about 100,000 years ago. The Toba Tuffs also include a 30000-year-old airfall tuff, which was erupted from a centre just N of the Toba depression. The Toba depression developed through the same stages as other resurgent cauldrons. It formed after lithification of the Toba Tuffs, by collapse along regional faults crossing an eastwards bulge in the Sumatran geanticline. Resurgent uplift raised lake sediments within the depression by about 500 m. The uplift, which was not confined to the depression, was probably caused by continuing influx of magma and it is possible that the Toba depression is still in the stage of post-resurgence volcanism. The eruption of the Toba Tuffs and of all post-ignimbrite volcanism took place on the line of the western marginal fault of the depression. This marginal fault once extended northwards offshore into a zone of Miocene back-arc rifting.

Contemporaneous faulting, and the eruption and preservation of the Lough Guitane Volcanic Complex, Co. Kerry

Interbedded with the clastic fluvial sediments of the Munster Basin, the Lough Guitane Volcanic Complex represents a rare volcanic episode in the Upper Devonian of SW Ireland. From three distinctly separate volcanic centres, an exclusively rhyolitic suite of thick lava flows and tuffs were erupted. The largest of the centres possessed a system of contemporaneous faults producing a subcircular depressed area, itself enclosed within a graben 1.5 km across. These faults lead to rapid thickness variations in many volcanic units, and a pooled lava flow reaches a thickness of 300 m. The site of a major elongate vent lies along one such fault, and rapid erosion of its poorly lithified walls generated coarse sandstone boulder agglomerates and is related to the absence throughout the complex of normal air-fall tuffs unmixed with terrigenous sand. The containment of the bulk of the volcanics by contemporaneous faults, and subsequent rapid burial by terrigenous sediments has led to preservation of the volcanic succession virtually intact.

Proximal volcaniclastic sedimentation in a Cretaceous back-arc basin, northern Antarctic Peninsula

Summary During Cretaceous times, the northern Antarctic Peninsula was the site of an active ensialic magmatic arc. Volcanism was dominated by pyroclastic eruptions with rare lava flows. Marine conglomerates and sandstones formed a volcaniclastic apron along the eastern margin of the arc and represent the proximal deposits of an extensive back-arc basin. Volcanogenic material, redeposited by turbidity currents and other sediment gravity flows, forms an important part of the proximal basin fill. Air-fall tuffs and eruption-induced sediment flows form a small but significant part of the succession and large exotic slideblocks of Jurassic sediment are a distinctive feature of the Lower Cretaceous strata on James Ross Island. Aeromagnetic data and regional geology indicate that the arc-back-arc basin boundarv was fault-controlled. Sedimentation within the basin was strongly influenced by both the steep, unstable nature of the faulted arc flanks and the coeval volcanism.

Volcaniclastic rocks of the Reydarfjördur drill hole, eastern Iceland: 1. Primary features

A 1918‐m‐deep bore hole in a late Miocene shield basalt/central volcano mixed sequence contains 58 stratigraphically distinct Volcaniclastic interbeds, constituting 8.8% of the extrusive part of the section. Textural, mineral chemical and bulk chemical criteria are applied to identify the original nature, composition, and origin of these rocks which have been metamorphosed by hydrothermal alteration between 100°C (top) to ca. 300°C (base of the section). Units range from a few centimeters to 18 m, excluding a 30‐m‐thick ignimbrite, with 75% of the units being less than 1 m thick. Formerly vitric to tachylitic shards and lapilli, lithic fragments, and crystals are the main components. Most thin Volcaniclastic units are basaltic vitric to tachylitic tuffs. Thin felsic air fall tuffs, a few millimeters to a few centimeters thick, occur as separate layers and interbedded with basaltic tuffs. Nearly all thicker units are made up dominantly of felsic vitric to lithic clasts and crystals. Phenocrysts, common only in felsic air fall tuffs, are clinopyroxene, ranging from augite to ferrohedenbergite, and plagioclase, ranging from An80–20, both of which span the entire range of compositions previously known from Iceland. Bulk chemical analyses of major and 11 trace elements of 65 samples allow texturally indistinct rocks to be compositionally identified, even though chemical changes due to mixing and alteration are pronounced in many rocks. The rhyolitic ignimbrite cooling unit is composed of four flow units, the upper most being slightly more mafic in composition because of admixture of a basaltic component. The two central flow units show extensive, high‐temperature devitrification with lensoid, tridymite‐rimmed cavities (now laumontite‐filled), representing collapsed pumice lapilli. The composition of the clastic units corresponds in a general way to that of the interbedded lava flows. These data and evidence for weathering and reworking in some units are used to reconstruct the volcanic history of the area: Phase A (1918–1306 m), dominantly mafic shield basalts, contains only few and mostly basaltic tuffs except for sparse rhyolitic air fall tuff from distant central volcanoes; phase B (1306‐653 m), composed dominantly of evolved basalts and icelandites, culminating with the rhyolitic ignimbrite (920–950 m), probably represents the eastern flank of a nearby central volcanic complex, whose center may be situated less than 10 km westward downdip. The upper extrusive 500 m of the section (phase C) are basalt lavas of average tholeiitic composition, containing abundant chiefly felsic and reworked clastic units in the interval between 286 and 410m. These are tentatively interpreted to reflect a phase of structural basin‐forming not compensated by extrusive volcanic activity. Allanite phenocrysts were found for the first time in Icelandic rocks, and the host pumice tuffs may constitute an important stratigraphic marker horizon that may help to tie the drilled section to the subaerial volcanic series in this part of eastern Iceland. The alteration of the clastic layers is discussed in a companion paper.