Resedimented Volcaniclastic Deposits Research Articles

Physical and chemical depositional processes when volcanoes meet lacustrine environments: the Cretaceous Imjado Volcanics, Jeungdo, southwestern Korea

Abstract A Cretaceous volcano-sedimentary succession (Imjado Volcanics, Jeungdo, SW Korea) was analysed to understand volcanic influences on physical and chemical depositional processes of a shallow alkaline lake during and after explosive eruptions. The succession is composed of primary and resedimented volcaniclastic deposits interbedded with fine-grained sediments and a bedded chert. The primary volcaniclastic deposits are characterized by two end-members: thick (20 m), welded lapilli tuff and thin (0.9 m), planar stratified tuff deposits. The first member deposits were accumulated by steady pyroclastic density currents that displaced the lake water from the shoreline. The second end-member deposits were accumulated by unsteady pyroclastic density currents that rapidly disintegrated at the shoreline and transformed into turbidity currents. The reverse to normally graded volcaniclastic sandstone is one of the resedimented volcaniclastic deposits accumulated by hyperpycnal flows originating from subaerial discharge. On top of this deposit, a bedded chert is exposed and composed of microcrystalline texture without biogenic remains. The microscopic analysis and stratigraphic relationship suggest that the chert bed was formed by chemical precipitation as a result of changes in the hydrochemistry of lake water by interaction between fresh water (hyperpycnal flows) and the alkaline bottom water of the lake.

Recalibrating the breakup history of SW Gondwana: U–Pb radioisotopic age constraints from the southern Cape of South Africa

Syn-rift deposits often provide the only means to determine the chronology of rift initiation and evolution. However, the earliest syn-rift packages deposited in Jurassic – Cretaceous rift basins that formed during the breakup of SW Gondwana are poorly understood because they are deeply buried beneath overlying passive margin sequences. The exhumed remnants of several such rift basins are exposed in the southern Cape of South Africa and contain the Suurberg and Uitenhage groups, which are predominantly continental, taphrogenic, fossiliferous strata interbedded with volcaniclastics. Here we present the first robust U–Pb chronostratigraphic framework for these groups by dating zircon in nine pyroclastic and five resedimented volcaniclastic deposits using Laser Ablation – Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). To further improve the precision and accuracy of the results, we utilize Chemical Abrasion – Thermal Ionisation Mass Spectrometry (CA-TIMS) on four selected samples minimizing the effects of Pb-loss and further constraining depositional uncertainties. We thereby show that the Suurberg Group was deposited rapidly during the emplacement of the Karoo Large Igneous Province in the Early Jurassic and likely predates the main phase of rifting, whereas the Uitenhage Group was deposited over a prolonged (>40 Ma) period beginning in the Early Jurassic and continuing into the Early Cretaceous. The Uitenhage Group records two phases of rifting: an initial Jurassic episode that roughly coincides with the separation of East and West Gondwana and is contemporaneous with widespread volcanism in SW Gondwana, and a subsequent period of renewed rifting during the Early Cretaceous opening of the South Atlantic and initiation of the Agulhas Falkland Transform. This framework illustrates the complexity of long-lived rift-basin sedimentation and highlights the importance of high-resolution chronostratigraphy when investigating and integrating the tectonic, palaeogeographic and palaeontological records from the final stages of a unified SW Gondwana.

The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member, Wido Island, Korea

AbstractThe Daeri Member, a Cretaceous volcanic–sedimentary succession, can be divided into lower, middle, and upper parts based on vertical changes in its lithologic characters. The lower Daeri Member is composed of siliciclastic deposits formed in a semi‐arid floodplain environment, which is overlain by the middle Daeri Member consisting mainly of andesite lava flow. After the emplacement of the andesite, activities of intrabasinal normal faults created accommodation on hanging wall blocks together with the development of intrabasinal topographic relief. The upper Daeri Member occurs only in hanging wall blocks and is composed of rhyolitic volcaniclastic sediments formed during an explosive volcanic eruption. Following the eruption, owing to semi‐arid climatic conditions and the destruction of vegetation, the eruptive materials were easily remobilized and deposited by episodic sediment gravity flows, resulting in deposition of the resedimented volcaniclastic deposits with sheet‐like geometry. Away from the intrabasinal normal faults, the resedimented volcaniclastic deposits show a decrease in grain size together with changes in inferred depositional processes from debris flows to hyperconcentrated flows and supercritical sheetfloods. This suggests that the resedimented volcaniclastic deposits were stacked on alluvial fan environments induced by intrabasinal topographic relief associated with normal fault activities. In addition, episodic movement of the faults gave rise to periodic fluctuation of the accommodation and an increase in gradient of the alluvial fan surface, resulting in the development of coarsening‐upward trends in the resedimented volcaniclastic deposits. The development of the alluvial fan and the coarsening‐upward trends indicate that dynamic tectonic subsidence and concomitant changes in the intrabasinal physiographic relief influenced the depositional processes and sizes of the transported volcaniclastic sediments of the upper Daeri Member. Thus, it is necessary to carefully observe tectonic signatures in volcaniclastic successions, particularly the syneruptive lithofacies, in order to reconstruct the tectonic and volcanic histories of receiving basins.

Hydrothermal alteration of chlorite to randomly interstratified corrensite-chlorite: Geological evidence from the Oligocene Smrekovec Volcanic Complex, Slovenia

Chlorite, ordered mixed-layer chlorite-smectites, laumontite, quartz and albite are the most widespread alteration assemblage in cone-building and near-vent successions of lavas, autoclastic, pyroclastic and resedimented volcaniclastic deposits of the Oligocene Smrekovec Volcanic Complex, Slovenia. Randomly interstratified corrensite-chlorite with ~70–80% of corrensite layers (R0 Cr-Ch) is less common in occurrence and associated either with clinoptilolite and heulandite in fine-grained vitric tuffs, or with prehnite, laumontite, actinolite, analcime and albite in extensively altered and/or fractured host-rocks. The assemblage with clinoptilolite and heulandite indicates the temperatures of formation of <~120°C and can be regarded as intermediate in the prograde alteration process of volcanic glass that leads in progressively increasing temperature regime to the formation of chlorite, laumontite, prehnite and albite. The assemblage of R0 Cr-Ch, prehnite, laumontite and actinolite developed in different superimposed temperature regimes and/or stages of hydrothermal activity related to the emplacement and subsequent gradual cooling of the Kramarica Sill. The formation of prehnite and actinolite signifies the highest-temperature conditions attaining up to ~300°C. Laumontite and R0 Cr-Ch extensively developed during the cooling period and as a result of the incursion of alkaline Ca-Mg hydrothermal fluids that possibly evolved from deep ground-waters convecting through Triassic dolomite in the basement of the submarine Smrekovec stratovolcano-hosted hydrothermal system.

The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings.

Early Miocene volcanic activity and paleoenvironment conditions recorded in tephra layers of the AND-2A core (southern McMurdo Sound, Antarctica)

The ANtarctic geological DRILLing Program (ANDRILL) successfully recovered 1138.54 m of core from drill hole AND-2A in the Ross Sea sediments (Antarctica). The core is composed of terrigenous claystones, siltstones, sandstones, conglomerates, breccias, and diamictites with abundant volcanic material. We present sedimentological, morphoscopic, petrographic, and geochemical data on pyroclasts recovered from core AND-2A that provide insights on eruption styles, volcanic sources, and environments of deposition. One pyroclastic fall deposit, 12 resedimented volcaniclastic deposits and 14 volcanogenic sedimentary deposits record a history of intense explosive volcanic activity in southern Victoria Land during the Early Miocene. Tephra were ejected during Subplinian and Plinian eruptions fed by trachytic to rhyolitic magmas and during Strombolian to Hawaiian eruptions fed by basaltic to mugearitic magmas in submarine and/or subglacial to subaerial environments. The long-lived Mount Morning eruptive center, located ∼80 km south of the drill site, was recognized as the probable volcanic source for these products on the basis of volcanological, geochemical, and age constraints. The study of tephra in the AND-2A core provides important paleoenvironment information by revealing that the deposition of primary and moderately reworked tephra occurred in a proglacial setting under generally open-water marine conditions.

Geology of the Orion South kimberlite, Fort à la Corne, Canada

A wide variety of primary pyroclastic, volcaniclastic and re-sedimented volcaniclastic deposits derived from multiple kimberlite eruptions are exceptionally well preserved at the Orion South kimberlite body in the Fort à la Corne field in central Saskatchewan. Construction of this kimberlite complex involved episodic kimberlite volcanic events punctuated by periods of volcanic quiescence, erosion and sedimentary deposition within the Western Canadian Sedimentary Basin. The Orion South kimberlite complex erupted into deltaic coastal plain/transitional estuarine (Mannville Group: Cantuar and Pense formations) and shallow marine (Lower Colorado Group: Joli Fou, Viking and Westgate formations) environments. At Orion South, eight distinct kimberlite phases erupted over 6 to 7 million years from ca. 106 Ma (Cantuar Formation equivalent) to ca. 99 Ma (Viking Formation equivalent). Detailed core logging, geochemistry, chronostratigraphy, petrography and geophysics were undertaken to define distinct eruptive phases and reconstruct the depositional environment, volcanic styles and 3-D architecture of the complex. The eight main kimberlite deposits record eruptive styles ranging from magmatic, to ‘wet’ and ‘dry’ phreatomagmatic, to submarine eruptions, a consequence of the varying interaction between kimberlite magma and seawater and/or groundwater.

Impacts of explosive volcanism on distal alluvial sedimentation: Examples from the Pliocene–Holocene volcaniclastic successions of Japan

Volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation (lahar) processes after the cessation of eruptive activity. Although there are many studies dealing with the assessment and mitigation of volcanic hazards, these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes. However, the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence, and can ultimately result in severe hazards in downstream areas. Examination of the volcaniclastic successions of non-marine Pliocene–Holocene sedimentary basins in Japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano. Impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material. Typical volcaniclastic beds comprise centimetre- to decimetre-thick primary pyroclastic fall deposits overlain by metre- to 10s of metres-thick resedimented volcaniclastic deposits, intercalated in sedimentary successions of non-volcanic provenance. The relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that: 1) potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone; and 2) the majority of resedimented material was likely derived from erosion of non-welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes. The nature, distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of: 1) proximity to ignimbrite, but not directly with the distance to the eruptive centre; 2) ignimbrite nature (non-welded or welded) and volume; 3) temporal changes in sediment flux from the source area; 4) the physiography and drainage patterns of the source area and the receiving basin, and any intervening areas; and 5) the formation of ephemeral dam-lakes and intra-caldera lakes whose potential catastrophic failure can impact distal areas. Models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation.

Volcaniclastic resedimentation in distal fluvial basins induced by large‐volume explosive volcanism: the Ebisutoge–Fukuda tephra, Plio‐Pleistocene boundary, central Japan

The Ebisutoge–Fukuda tephra (Plio‐Pleistocene boundary, central Japan) has a well‐recorded eruptive style, history, magnitude and resedimentation styles, despite the absence of a correlative volcanic edifice. This tephra was ejected by an extremely large‐magnitude and complex volcanic eruption producing more than 400 km3 total volume of volcanic materials (volcanic explosivity index=7), which extended more than 300 km away from the probable eruption centre. Remobilization of these ejecta occurred progressively after the completion of a series of eruptions, resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins, more than 100 km from the source. Facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins. The distal tephra (≈100–300 km from the source) comprises two different lithofacies, primary pyroclastic‐fall deposits and reworked volcaniclastic deposits. The resedimented volcaniclastic succession shows five distinct sedimentary facies, interpreted as debris‐flow deposits (facies A), hyperconcentrated flow deposits (facies B), channel‐fill deposits (facies C), floodplain deposits with abundant flood‐flow deposits (facies D) and floodplain deposits with rare flood deposits (facies E). Resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic, large ignimbrite‐forming eruption. Factors controlling inter‐ and intrabasinal facies changes are (1) temporal change of introduced volcaniclastic materials into the basin; (2) proximal–distal relationship; and (3) distribution pattern of pyroclastic‐flow deposits relative to drainage basins. Thus, studies of the Ebisutoge–Fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large‐magnitude eruptions, an aspect of volcaniclastic deposits that has often been ignored or poorly understood.

Tuffaceous Conglomerates: Syn-Eruptive Resedimented Debris Flow and Hyperconcentrated Flow Deposits from the "Rhyodacitic Formation", Gutai Mountains, Eastern Carpathians

The first volcanic products from Gutâi Mts, Eastern Carpathians belong to the Rhyodacitic Formation. The physical constituents of this formation are as follows: pyroclastic deposits (ignimbrites and fall tuffs), syn-eruptive resedimented volcaniclastic deposits, volcanogenic and non-volcanogenic sedimentary deposits. The tuffaceous conglomerates belong to the syn-eruptive resedimented volcaniclastics and represent the coarsest deposits. They outcrop in Baiţa area, west from Baia-Mare, being underlain by Lower Badenian (15.4±0.6 Ma) ignimbrites and overlain by Lower Sarmatian claystones. The main components of the conglomerates are boulders and cobbles of sedimentary and metamorphic rocks in a tuffaceous matrix. Scarse blocks eroded from the lower ignimbrite unit are present. In the frame of the study of the Rhyodacitic Formation, the conglomerates have been investigated using facies analysis. The facies model concludes that the studied deposits have been emplaced from debris flows and hyperconcentrated flows. They derived from subaerial channelized ignimbrite flows, which underwent strong flow separation while jumping in submarine basin and settling in deep water, below wave base. Such conditions, above the lower subaerial ignimbrite unit, were determined by the subsidence of Sasar-Dealu Crucii compartment, contemporaneously developed with the conglomerates emplacement.

U–Pb dating of silicic lavas, sills and syneruptive resedimented volcaniclastic deposits of the Lower Devonian Crudine Group, Hill End Trough, New South Wales

The Hill End Trough of central‐western New South Wales was an elongate deep marine basin that existed in the Lachlan Fold Belt from the early Late Silurian to late Early Devonian. It is represented by a regionally extensive, unfossiliferous sequence of interbedded turbidites and hemipelagites of substantially silicic volcanic derivation, which passes laterally into contemporaneous shallow‐water sedimentary rocks. The Turondale and Merrions Formations of the Lower Devonian Crudine Group are two prominent volcanogenic formations in the predominantly sedimentary trough sequence. They contain a range of primary and resedimented volcanic facies suitable for U–Pb dating. These include widespread subaqueous silicic lavas and/or lava cryptodomes, and thick sequences of crystal‐rich volcaniclastic sandstone emplaced by a succession of mass‐flows that were generated by interaction between contemporaneous subaerial pyroclastic flows and the sea. Ion microprobe dating of the two volcanogenic formations by means of the commonly used SL 13 zircon standard yields ages ranging between 411.3 ± 5.1 and 404.8 ± 4.8 Ma. Normalising the data against a different zircon standard (QGNG) yields preferred slightly older mean ages that range between 413.4 ± 6.6 and 407.1 ± 6.9 Ma. These ages broadly approximate the Early Devonian age that has been historically associated with the Crudine Group. However, the biostratigraphically inferred late Lochkovian ‐ early Emsian (mid‐Early Devonian) age for the Merrions Formation is inconsistent with the current Australian Phanerozoic Timescale, which assigns an age of 410 Ma to the Silurian‐Devonian boundary, and ages of 404.5 Ma and 395.5 Ma to the base and top of the Pragian, respectively. There is, however, good agreement if the new ages are compared with the most recently published revision of the Devonian time‐scale. This suggests that the Early Devonian stage boundaries of the Australian Phanerozoic Timescale need to be revised downward. The new ages for the Merrions Formation could also provide a time point on this time‐scale for the Pragian to early Emsian, for which no data are presently available.