Tuff Beds Research Articles

Sedimentation rates, basin analysis and regional correlations of three Neoarchaean and Palaeoproterozoic sub-basins of the Kaapvaal craton as inferred from precise U–Pb zircon ages from volcaniclastic sediments

Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of 2588±6 Ma and 2549±7 Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of 2516±4 Ma for the upper Gamohaan Formation and 2555±19 for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.

Mesozoic stratigraphy of the Marokopa area, southwest Auckland, New Zealand

The Late Triassic and Jurassic rocks of the Marokopa area, southwest Auckland, form part of the western limb of the Kawhia Syncline. Within the area mapped, the Late Triassic ‐ Early Jurassic Newcastle Group consists of 3100 m of generally fine to very fine sandstone and siltstone with common tuff beds, and occasional coarser sandstones and shellbeds. Faunas are marine, generally sparse to moderate, and dominated by bivalves and brachiopods. The Arawi Shellbed, Ngutunui Formation, Tewharau Formation, Ururoa Shellbed, and Ururoa Formation are recognised. The Middle Jurassic Rengarenga Group marks a change to shallow water, marginal marine, and nonmarine conditions. Lithologies are generally coarse to medium sandstone with abundant fine plant fragments, with occasional pebble‐granule conglomerates, tuffs, and shellbeds. Thin coal and plant beds mark the southern limit of nonmarine conditions in the Rengarenga Group. Fully marine conditions return with the Late Jurassic Kirikiri Group. Oraka Sandstone and Ohineruru Formation consist mainly of slightly calcareous, concretionary fine sandstone and sandy siltstone, with a rich molluscan fauna dominated by bivalves. These formations are separated by the 1 m thick Captain King's Shellbed, with a glauconitic sandstone matrix and a distinctive molluscan and brachiopod fauna. The uppermost part of the sequence in the mapped area is the interfingering Kiwi Sandstone and Waikutakuta Formation. Strata generally dip regularly to the east‐northeast at between 20 and 45°. Minor folding may be present in the Kairimu valley. The north‐south trending Whareorino Fault is part of the Taharoa Fault Zone. A NE‐ENE trend is shown by two faults in the Marokopa valley and two in the eastern part of the area. A fault in the Paraohanga valley shows a northwest trend.

The sandstone-hosted Pb occurrence of Rio Pischinappiu (Sardinia, Italy): a Pb-carbonate end-member

The discovery of a new type of Pb–Ba deposit in a clastic complex near Monti Ferru, Sardinia, Italy is discussed with respect to source of metals and conditions of deposition, in a near-shore oxidizing and high-pH environment, as well as conditions of diagenesis and metal concentration. Unlike most red-bed-type Pb deposits, this occurrence is characterized by oxidized minerals, without sulphides, with both clastic material and metals coming from a Tertiary volcanic edifice. The main anions, comprising carbonate, sulphate and chloride species, are of both continental and marine origin. The reconstruction of the mineralizing phenomena in this coastal environment is based on evidence of: transport of metals, composition of the pore waters, pH–Eh conditions, and differences in solubility of the possible species accounting for the predominance of barite and cerussite. The main mineralized stratum was originally an ash tuff bed transformed into an aggregate of authigenic K-feldspar and cerussite during diagenesis. In this less-permeable `trap-level', intercalated in the normal sedimentary series, the mobility of Pb was restricted, resulting in deposition of ore-grade quantities of cerussite.

Lower Pliocene polarity transitions from the Suva Marl, Fiji

We present a paleomagnetic record of the lower Cochiti polarity transition and new results from the lower and upper Nunivak transitions obtained from the Lower Pliocene Suva Marl ( Fiji, (18°S, 178°E)). This formation has yielded a well‐defined magnetostratigraphy that, combined with 40Ar/39Ar ages from interbedded tuff beds, provides excellent chronostratigraphic control [Clement et al, 1997]. We previously presented the upper Cochiti and the upper and lower Nunivak polarity transition records from this unit [Clement et al., 1995, 1996]. New results were obtained from oriented block samples spanning these reversals. Each of these reversals was sampled at duplicate sections separated by 4 to 8 km. The agreement between the records obtained from different sections suggests that the paleomagnetic records are not artifacts of local sedimentary disturbances or random degrees of incompletely removed overprints. Instead the similarities in these and other records of these same reversals from different types of paleomagnetic recorders suggest that they provide information about the behavior of the geomagnetic field as it reverses.

Geochronology of basin phases in the western Mt Isa Inlier, and correlation with the McArthur Basin*

Zircon geochronology—using U‐Pb SHRIMP methods—has been applied to investigate the Palaeoproterozoic history of basin development and sedimentation in the western Mt Isa Inlier and eastern McArthur Basin. Ages of the sediment packages have been measured using the abundant felsic volcanics in lower parts of the sequences, and overlying sediments have been dated through the use of zircons in sparse, concordant tuff beds that punctuate these successions. The earliest volcanic phases north and south of the Murphy Inlier are close to 1725 Ma, and may be part of a much larger extensional tectonic regime now identified in the central and eastern Mt Isa Inlier. A somewhat younger episode of volcanism took place in the central Lawn Hill Platform (Fiery Creek Volcanics and Bigie Formation) at about 1710 Ma, approximately coeval with volcanism throughout much of the McArthur Basin. The most southerly volcanic suite below the Mt Isa Group is 30–40 million years younger, closer to the ca 1650 Ma age of overlying strata and suggesting a rather complex pattern of basin evolution. There is evidence that the present concept of discrete cover sequences over the entire Mt Isa Inlier may no longer be sustainable. SHRIMP U‐Pb zircon results for the McNamara/Mt Isa Groups which host syngenetic or late diagen‐etic Pb‐Zn‐Ag mineralisation constrain the time(s) of deposition of associated stratabound ores. For the Mt Isa and Hilton orebodies, the depositional ages are 1652 ± 7 Ma and 1654 ± 5 Ma, confirming correlation to the lower McNamara Group which contains tuff interbeds dated at 1653 ± 7 Ma. The turbiditic facies of the upper McNamara Group is at least 20 million years younger than this. The deeper water basin phase represented by the Lawn Hill Formation in the uppermost McNamara Group is younger (1595 ± 6 Ma) and has no analogue in the exposed Mt Isa Group. The new results provide a good time connection between parts of the Fickling and McArthur Groups that host the Walford Creek and McArthur Pb‐Zn deposits. Tuffs in these host sequences have U‐Pb zircon ages of 1640 ± 7 Ma and 1639 ± 3 Ma, respectively. Thus, deposition of the predominantly shallow‐water sediments was approximately synchronous across both the Mt Isa and McArthur regions in the interval 1655–1640 Ma.

Tectonic and geochronological constraints on late Archaean and Palaeoproterozoic stratigraphic correlation within and between the Kaapvaal and Pilbara Cratons

Recent radiometric dating of carbonates and banded iron-formations on the Kaapvaal Craton, southern Africa, has suggested that late Archaean and Palaeoproterozoic carbonate sedimentation was diachronous across the craton. We present new SHRIMP U–Pb zircon ages of 2583±5 Ma and 2588±7 Ma for two samples of a tuff bed, 320 km apart in the upper Oak Tree Formation, Transvaal Supergroup. These ages, in conjunction with published data, support previously established sequence stratigraphic correlations that show the temporal equivalence of late Archaean carbonates in the region. A review of the sequence stratigraphy and geochronology of the Mount Bruce Megasequence Set on the Pilbara Craton, Western Australia, and the newly named Highveld Megasequence Set on the Kaapvaal Craton, indicates a remarkably similar geohistory. Both cratons are characterized by a divergent megacycle (Hamersley Range and Vaal Megasequences) followed by a convergent megacycle (Chichester Range and Drakensberg Megasequences). Our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time ( c. 2590 Ma), which permits more meaningful geotectonic correlations between them.

An Eifelian U-Pb zircon date for the Enagh Tuff Bed from the Old Red Sandstone of the Munster Basin in NW Iveragh, SW Ireland

The first geochronological data from the Minister Basin (SW Ireland) contradict the accepted biostratigraphical maximum age (late Givetian-early Frasnian) of the oldest alluvium in the basin depocentre. Magmatic zircons from the Enagh Tuff Bed, a sub-plinian air-fall tuff within fluviatile Old Red Sandstone of NW Iveragh, give a Pb/Pb isotope age of 384.9 ± 0.7 Ma. This confirms Middle Devonian (probably Eifelian) sedimentation and allows possible Early Devonian basin initiation. Associated second-cycle gravels suggest a pre-385 Ma minimum age for the Caherbla Group of the Dingle Basin. Correlation with alluvium containing tetrapod trackways indicates a pre-385 Ma age for the trace maker.

Regional depth-controlled hydrothermal metamorphism in the Zymoetz River area, British Columbia

An estimated 6 km of basic to silicic volcanic flows and clastic rocks of the Early Jurassic Telkwa Formation is exposed in moderately east-dipping fault blocks along the Zymoetz River, British Columbia. Extensive wholesale zeolitic replacement of porous tuff beds suggests widespread hydrothermal activity. Metamorphic grade increases regionally from laumontite–albite facies to prehnite–pumpellyite facies with increasing stratigraphic depth. Telkwa Formation strata and the imparted metamorphic zonation are cut and tilted by rotational block faulting, and are repeated in each of the upturned blocks. Late Mesozoic to Tertiary plutonism locally thermally overprinted the regional facies, particularly in the western part of the area. Fluid-inclusion isochores, combined with calculated mineral equilibria, suggest that metamorphism took place at fluid pressures of 2 kbar (1 kbar = 100 MPa) or less, consistent with estimates of stratigraphic burial. Metamorphic fluids were H2O rich and low in dissolved salts. Maximum temperatures during regional depth-controlled hydrothermal metamorphism, based upon the widespread presence of laumontite and the lack of wairakite in the middle to upper parts of the Telkwa Formation, probably did not exceed about 250 °C at H2O pressures of 2 kbar. Mineral zones, estimated paleotemperatures, and geothermal gradients are comparable to regional hydrothermal metamorphism in active volcanic settings such as Iceland.

Sedimentology, geochemistry and palaeogeographic implications of volcanic rocks in the Upper Archaean Campbell Group, western Kaapvaal craton, South Africa

Tuffs and lava interbedded in Campbell Group carbonates and shales are investigated for their sedimentological and geochemical properties. The most proximal tuffs occur at the southwestern margin of the Kaapvaal craton, in Griqualand West. They were deposited in a shallow-marine to tidal carbonate environment from hydroclastic eruptions. The tuffs along this margin are subdivided into two main facies types: (1) graded fallout-lapilli to ash tuffs, containing accretionary lapilli; and (2) current-reworked, laminated and rippled lapilli to ash tuffs. Calculation of water depth from settling velocities of tuff particles indicates deposition in depth of a maximum of some 30 m. The tuff beds generally thin out towards the north and northwest, but distal tuff layers are widespread in the Campbell Group and allow for some lateral correlation within regionally narrow limits. Single zircon age dates suggest that correlation over the entire basin is unreliable. Two geochemical groups of tuffs are tentatively distinguished. One group includes andesites and basalts and tends to concentrate in higher stratigraphic levels than the other, which consists exclusively of basalts. Volcanism was of a tholeiitic, within-plate character, situated in a rifted cratonic setting that failed to develop an oceanic basin. Geochemical similarity to some Karoo volcanic rocks is striking. A cratonic rift basin model for the Griqualand West depository in which carbonate sedimentation in shallow environments prevailed is discussed. The sedimentation of carbonates was periodically interrupted by hydroclastic eruptions along a rifting graben or half graben system.

New Late Miocene mammal sitescorrelated with the radiometric scale from the Bicorp Basin (Eastern Spain)

The basin-fill sequence from the Tertiary Bicorp Basin (Eastern Spain) comprises a lower alluvial unit andan upper lacustrine unit. The lower part of the lacustrine unit includes a tuff bed, interbedded between two mammal-bearing horizons which have yielded fossil rodents characteristic of the MN 10 standard Mammal Unit (Late Vallesian, Late Miocene). K-Ar age determinations on the tuff have provided a radiometric age averaging 9.6 ± 0.2 Ma (weighted average of two samples) for this ash fall deposit. The stratigraphic correlation between the fossil mammal sites and the radiometrically-dated tuff allows a more precise calibration between the Late Miocene biochronologic and geochronometric scales. The Bicorp tie point constrains the calibration of the Late Vallesian to around 9.7 Ma, the ensemble of data suggesting ages of 10–11 Ma for the Early/Late Vallesian boundary and 9.5–8.5 Ma for the Vallesian/Turolian boundary.

Cycles of Doming and Eruption of the Miocene Kisingiri Volcano, Southwest Kenya

Volcanic cycles of doming and eruption of the Miocene Kisingiri volcano produced three sedimentary cycles recorded in the volcaniclastic strata of Rusinga and Mfangano Islands, Lake Victoria, Kenya. Each of the three cycles began with the deposition of cobble and boulder conglomerates shed from the volcanically domed Precambrian basement, followed by deposition of pyroclastic and volcaniclastic strata, representing nephelinite-carbonatite eruption of the Kisingiri volcano. Volcanogenic strata produced by the first two cycles (lower two-thirds of the Rusinga Group) are predominantly fine-grained tuffs and medium-grained volcaniclastic deposits, indicating alluvial deposition from a low-relief volcanic edifice. The third cycle is dominated by boulder debris flows, lava flows, and minor tuffaceous beds (upper part of the Rusinga Group and overlying Kisingiri Group). This last cycle records the formation of the high-relief Kisingiri stratocone, much of which is preserved in the dissected flanks of the volcano. The first two cycles are recorded in distal apron deposits but are not well preserved in the core of the volcano. Second-order sedimentary cycles, consisting of fining-upward sequences (5-10 m thick) of granular tuffaceous sandstones and conglomerates that fine into siltstones, stacked floodplain paleosols, and airfall tuff beds, dominate the strata of the first two cycles. These fining-upward sequences represent alluvial aggradation that accompanied and followed eruptive episodes that were much shorter in duration than the main cycles of doming and eruption.

Stratigraphy, mineralogy, and genesis of the Bigadic borate deposits, western Turkey

The Bigadic borates are the largest colemanite and ulexite deposits in the world and the high-grade colemanite and ulexite ores (30 and 29% B 2 O 3 , respectively) should supply a substantial proportion of the world9s needs for many years. The Bigadic deposits formed within Neogene perennial saline lakes sediments located in a northeast-southwest-trending basin. The volcano-sedimentary sequence in the deposits consists of (from bottom to top) basement volcanics, lower limestone, lower tuff, lower borate zone, upper tuff, upper borate zone, and olivine basalt. The borate deposits formed under arid conditions in perennial saline lakes fed by hydrothermal springs associated with local volcanic activity. The deposits are interbedded with tuffs, clay, and limestones. Borate minerals formed in two zones separated by thick tuff beds that have been transformed to montmorillonite, chlorite, and zeolites (mainly heulandite) during diagenesis. Colemanite and ulexite predominate in both borate zones, but other borates, including howlite, probertite, and hydroboracite are present in the lower borate zone; whereas inyoite, meyerhofferite, pandermite, tertschite (?), hydroboracite, howlite, tunellite, and rivadavite are found in the upper borate zone. Calcite, anydrite, gypsum, celestite, K feldspar, analcime, heulandite, clinoptilolite, quartz, opal-CT, montmorillonite, chlorite, and illite are also found in the deposit. Colemanite nodules in both borate zones probably formed directly from solution, within unconsolidated sediments just below the sediment-water interface, and continued to grow as the sediments were compacted. It is unlikely that the colemanite formed by dehydration of inyoite and/or by replacement of ulexite after burial. Later generations of colemanite and ulexite are found in vugs and veins and as fibrous margins of early formed nodules. Other diagenetic changes include the partial replacement of colemanite by howlite and hydroboracite and ulexite by tunellite. Nodular-shaped colemanite and ulexite minerals predominate in both borate zones. Colemanite and ulexite show alternating horizons, and the transformation of one mineral to another has not been observed and the...

Connection between igneous activity and extension in the central Mojave metamorphic core complex, California

The development of metamorphic core complexes and associated low‐angle detachment faults commonly is intimately associated with synextensional igneous activity. In most areas studied to date, the relation of magmatism to extension is obscured by imprecise dating and by the overprint of later tectonic events. We present data from the early Miocene central Mojave metamorphic core complex (CMMCC) which indicate that extension was accompanied by igneous activity, as reflected by prekinematic, synkinematic, and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins. The principal intrusion is an early Miocene granite pluton exposed in outcrops across an area greater than 400 km2. Dikes adjacent to the pluton are common in the Mitchel Range, at The Buttes, and at Fremont Peak. The overall orientation of the pluton and associated dikes is west‐northwest, roughly perpendicular to the extension direction. Results of U‐Pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 Ma. Two other dike samples yield inconclusive results. Synextensional basins formed by detachment faulting during the core complex development. Rocks in these basins compose the Jackhammer and Pickhandle formations and filled an elongate, NW trending trough more than 50 km long. The 40Ar/39Ar ages for tuff beds are as old as 23.8±0.3 Ma near the base of the lower Pickhandle Formation and as young as 21.3±0.5 Ma in the uppermost lower Pickhandle. Hence volcanism and plutonism are coeval. The diversity of intrusive relations relative to the timing and development of the mylonitic fabric in the CMMCC precludes any simple cause‐and‐effect relationship between magmatism and extensional deformation. Rather, magmatism and extension may have been localized at a releasing bend in a transfer‐fault system which links extension in the CMMCC with extension in the Colorado River area to the east.

Some evidence for the base-metal potential of the Pretoria Group: stratigraphic targets, tectonic setting and REE patterns

The Timeball Hill and Silverton Formations of the 2.1–2.3 Ga Pretoria Group have regional lithological associations which are thought to have been favourable for the genesis of stratiform sulphide deposits. The observed association of carboniferous and pyritic black shales, tuffaceous material, stromatolitic carbonates and inferred turbidity current deposits is common in stratiform sulphide deposits of the sedimentary exhalative group. Massive sulphides in the Silverton Formation are compatible with a syngenetic brine discharge, probably related to deep fracture systems. The basal shales of the Timeball Hill Formation are significantly enriched in base-metals and Ba. Interlayered tuff beds at this stratigraphic level have PGE-contents of up to 1 g/t. The REE-geochemistry of Pretoria Group sedimentary rocks supports hydrothermal activity as an important factor in both stratigraphic units.

Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.

Subaqueous Pyroclastic Deposits in an Ordovician Fore-Arc Basin: An Example from the Saint-Victor Formation, Quebec Appalachians, Canada

ABSTRACT The Saint-Victor Formation is mainly composed of graptolite-bearing, nonvolcanic turbidite sequences, 7 km thick, deposited in a fore-arc basin during the Ordovician Taconian orogeny. The bedded tuff and lapilli tuff (BTL) facies, 10-30 m thick, is the principal volcaniclastic facies in the Saint-Victor Formation. The BTL, which is repeated at several stratigraphic levels within the formation, contains two divisions. The lower division is a well sorted, fines-poor, mostly massive, lapilli tuff representing 50% of the BTL. It was emplaced as a single, subaqueous volcanic debris flow. The upper division is made up of upward-thinning and-fining tuff beds emplaced by frequently recurring subaqueous ash turbidites. The abundance of pyroclast (shards and pumice) in the BTL, good sorting of the framework constituents, and the absence of nonvolcanic background sediment interbeds favor direct sedimentation from one subaqueous explosion. A bedded lapilli tuff (BL) or bedded tuff (BT) facies (1-5 m) is locally present below the BTL facies. The BL contains the same constituents and sedimentary structures as the lower division of the BTL and is interpreted to have formed from similar processes. Similarly, the BT is similar to the upper division of the BTL and is interpreted to have formed as ash turbidites. Either they resulted from flow decoupling during emplacement of the overlying BTL or they represent dislodged debris from an eruptive event that occurred before the main eruptive event that led to deposition of the BTL.

The Udo tuff cone, Cheju Island, South Korea: transformation of pyroclastic fall into debris fall and grain flow on a steep volcanic cone slope

ABSTRACTThe Udo tuff cone of Cheju Island, South Korea, is a middle Pleistocene basalt tuff cone that has formed by early Surtseyan‐type eruptions and later drier hydroclastic eruptions. The tuff cone comprises steep (20–30°) and planar beds of lapillistone, lapilli tuff and tuff that can be grouped into seven sedimentary facies (A‐G). Facies A and B comprise continuous to lenticular layers of grain‐supported and openwork lapillistone that are inversely graded and coarsen downslope. They suggest emplacement by grain flows that are maintained by gravity‐induced stress and grain collisions. Facies C includes poorly sorted, crudely bedded and locally inversely graded lapilli tuff, also suggestive of rapid deposition from highly concentrated grain flows. Facies D includes thinly stratified and mantle‐bedded tuff that was probably deposited by fallout of wind‐borne ash. Other facies include massive lapilli tuff (Facies E), chaotic lapilli tuff (Facies F) and cross‐bedded tuffaceous sandstone (Facies G) that were deposited by resedimentation processes such as debris flow, slide/slump and stream flow, respectively.The grain flows that produced Facies A, B and C are interpreted to have originated from falling pyroclasts, which initially generated highly dispersed, saltating avalanches, in which momentum was transferred by the particles themselves. This transport mechanism is similar to that of debris fall. As the slope gradient was too low to maintain a highly dispersed flow, the debris fall decelerated and contracted due to a decrease in dispersive pressure. The mode of momentum transfer changed to one of collision because contraction of the debris fall resulted in an increase in particle concentration. This transport mechanism is similar to that of common grain flows. Grain segregation occurred in several ways. Initial segregation of ash from lapilli occurred due to their differing terminal fall velocities, and their contrasting degrees of sliding friction with the bed. Percolation of ash into interstices of lapilli during flow (kinematic sieving) augmented further segregation of ash from lapilli. The latter process, along with a dispersive pressure effect, gave rise to vertical inverse size grading. Downdip inverse grading was produced by particle overpassing.

Two examples of subaqueously welded ash-flow tuffs: the Visean of southern Vosges (France) and the Upper Cretaceous of northern Anatolia (Turkey)

Pyroclastic deposits interpreted as subaqueous ash-flow tuff have been recognized within Archean to Recent marine and lacustrine sequences. Several authors proposed a high-temperature emplacement for some of these tuffs. However, the subaqueous welding of pyroclastic deposits remains controversial. The Visean marine volcaniclastic formations of southern Vosges (France) contain several layers of rhyolitic and rhyodacitic ash-flow tuff. These deposits include, from proximal to distal settings, breccia, lapilli and fine-ash tuff. The breccia and lapilli tuff are partly welded, as indicated by the presence of fiamme, fluidal and axiolitic structures. The lapilli tuff form idealized sections with a lower, coarse and welded unit and an upper, bedded and unwelded fine-ash tuff. Sedimentary structures suggest that the fine-ash tuff units were deposited by turbidity currents. Welded breccias, interbedded in a thick submarine volcanic complex, indicate the close proximity of the volcanic source. The lapilli and fine-ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales. Presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm-wave base. In northern Anatolia, thick massive sequences of rhyodacitic crystal tuff are interbedded with the Upper Cretaceous marine turbidites of the Mudurnu basin. Some of these tuffs are welded. As in southern Vosges, partial welding is attested by the presence of fiamme and fluidal structures. The latter are frequent in the fresh vitric matrix. These tuff units contain a high proportion of vitroclasis, and were emplaced by ash flows. Welded tuff units are associated with non-welded crystal tuff, and contain abundant bioclasts which indicate mixing with water during flowage. At the base, basaltic breccia beds are associated with micritic beds containing a marine fauna. The welded and non-welded tuff sequences are interbedded in an alternation of limestones and marls. These limestones are rich in pelagic microfossils. The evidence above strongly suggest that in both examples, tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment. The sources of the pyroclastic material are unknown in both cases. We propose that the ash flows were produced during submarine fissure eruptions. Such eruptions could produce non-turbulent flows which were insulated by a steam carapace before deposition and welding. The welded ash-flow tuff deposits of southern Vosges and northern Anatolia give strong evidence for existence of subaqueous welding.

Uplift, subsidence, and volcanism in the southern Neiva Basin, Colombia, Part 1: Influence on fluvial deposition in the Miocene Honda Formation

In the southern Neiva Basin, four large-scale sedimentary sequences are recognized in the deposits of the Miocene Honda Formation on the basis of maximum clast diameter, fluvial channel/overbank deposit ratio, thickness of the overbank and channel sequences, and lithology of the sandstones. These sequences can be related to tectonic activity of the Central Cordillera and the Garzón Massif, to volcanism, and to the rate of basin subsidence. They may be interpreted in terms of different alluvial fan systems and volcanic aprons that prograded onto a broad plain during periods of active uplift or volcanism and braided and meandering river systems that developed during the intervening periods of greater volcanic and tectonic quiescence. The lower two sequences were deposited under relatively quiet tectonic conditions, following the major and final phase of thrust faulting of the Central Cordillera. During deposition of these sequences, sediment loading let to subsidence of the basin. The depositional processes of these sequences, furthermore, were influenced by activity of the Central Cordillera volcanic arc. Radiometric age determinations on two tuff beds located some 300 meters above the base of the formation in the lowermost sequence gave ages of 15.8 ± 0.6 and 14.3 ± 0.5 Ma, respectively. Conglomerates of the third sequence record pronounced uplift of the source area, i.e., the Central Cordillera. Deposition of this sequence very probably coincided with uplift of the Garzón Massif, the southermost extension of the Eastern Cordillera, which took place at about 12 Ma. Uplift of both cordilleras led to rapid subsidence of the basin. The upper sequence was deposited under conditions of waning tectonic activity.

Synsedimentary volcanic-ash-derived illite tonsteins in Late Permian coal-bearing formations of southwestern China

Illite claystones (tonsteins) of Late permian coal-bearing formations are well developed and extensively distributed in southwestern China. Over the past decades it has been recognized that they are synsedimentary volcanic ash-falls in origin (altered tuff beds), based on the data derived from the comprehensive investigations on their stratigraphic and geographic distribution, petrographic types, chemical composition, accessory-mineral assemblage, as well as on their morphological characteristics. Recently, mineralogical and petrological identifications and X-ray diffraction analyses on these peculiar tonsteins further suggest that they can be classified mainly into three categories based on their clay-mineral constitution: kaolinitic, kaolinitic-illitic (transitional type), and illitic. As coal partings in most cases, these different types of tonsteins show a distinct zonal distribution on a regional scale. Their mineral constitution is profoundly modified by the combined effects of many factors during deuterogenic diagenesis, as indicated by the variation in volatile-component percentage (VM) of the adjacent coal seams (VM is calculated on a combustible-component basis). Those tonsteins intercalated in coal with a VM > 10% are dominated by kaolinite. But they contain increasing amount of illite at the expense of kaolinite when VM of coal decreases from 10% to 8%. When VM declines to less than 8% the clay minerals in these tonsteins are almost entirely illite with some sort of accompanying chloritization. It is clear that, from a regional point of view, the variation in clay-mineral constitution of these tonsteins is in response to the continual and progressive change in geological processes. In addition to the temperature and pressure conditions, the existence of a certain amount of alkali-metal ions and ferrous ions in solution is one of the controlling factors.