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1.
The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings.The lithofacies associations at Highway–Reward collectively define a submarine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0.5 km3. Single porphyritic units vary from <10 to 350 m in thickness and some are less than 200 m in diameter. Ten of the porphyritic units studied in the immediate host sequence to the Highway–Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lavas and intrusions. Direct evidence for eruption at the seafloor is limited to a single partly extrusive cryptodome. However, distinctive units of resedimented autoclastic breccia indicate the presence nearby of additional lavas and domes.The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaqueous emplacement environment. Due to rapid quenching and mixing with unconsolidated clastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units. Magma preferentially invaded the sediment, avoiding the older units or conforming to their margins. Large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma. 相似文献
2.
Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several
monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The
diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration
of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism.
Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted
explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment
gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating
in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush
eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically
more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is
common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur
at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or
during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the
development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas.
Received: 5 February 1996 / Accepted: 17 January 1997 相似文献
3.
Luigina Vezzoli Massimo Matteini Natalia Hauser Ricardo Omarini Roberto Mazzuoli Valerio Acocella 《Bulletin of Volcanology》2009,71(5):509-532
The Middle-Upper Miocene Las Burras–Almagro-El Toro (BAT) igneous complex within the Eastern Cordillera of the central Andes
(∼24°S; NW Argentina) has revealed evidence of non-explosive interaction of andesitic magma with water or wet clastic sediments
in a continental setting, including peperite generation. We describe and interpret lithofacies and emplacement mechanisms
in three case studies. The Las Cuevas member (11.8 Ma) comprises facies related to: (i) andesite extruded in a subaqueous
setting and generating lobe-hyaloclastite lava; and (ii) marginal parts of subaerial andesite lava dome(s) in contact with
surface water, comprising fluidal lava lobes, hyaloclastite, and juvenile clasts with glassy rims. The Lampazar member (7.8 Ma)
is represented by a syn-volcanic andesite intrusion and related peperite that formed within unconsolidated, water-saturated,
coarse-grained volcaniclastic conglomerate and breccia. The andesite intrusion is finger-shaped and grades into intrusive
pillows. Pillows are up to 2 m wide, tightly packed near the intrusion fingers, and gradually become dispersed in the host
sediment ≥50 m from the parent intrusion. The Almagro A member (7.2 Ma) shows evidence of mingling between water-saturated,
coarse-grained, volcaniclastic alluvial breccia and intruding andesite magma. The resulting intrusive pillows are characterized
by ellipsoidal and tubular shape and concentric structure. The high-level penetration of magma in this coarse sediment was
unconfined and irregular. Magma was detached in apophyses and lobes with sharp contacts and fluidal shapes, and without quench
fragmentation and formation of a hyaloclastite envelope. The presence of peperite and magma–water contact facies in the BAT
volcanic sequence indicates the possible availability of water in the system between 11–7 Ma and suggests a depositional setting
in this part of the foreland basin of the central Andes characterized by an overall topographically low coastal floodplain
that included extensive wetlands. 相似文献
4.
香港中侏罗世屯门组火山岩主要由安山质熔岩、凝灰岩和凝灰角砾岩夹少量凝灰质砂岩组成。其中凝灰角砾岩初期曾误认为是沉积砾岩。1990年香港地质调查组重新研究后,确认是火山成因的凝灰角砾岩。按岩相分析,屯门组火山岩可分为火山通道相安山-英安质熔岩和爆发角砾岩,还有爆发空落相凝灰角砾岩。火山通道相又可分为火山颈相和岩墙相,分布在屯门组东西两侧。在青山东麓,出露一列NNW向呈断续、线状分布的火山颈群。近年来新出版的《香港地质考察指引》和《香港工程地质实践》等地质著作仍将该区爆发角砾岩误认为是沉积成因的砾岩,在地质勘探和工程设计上造成混乱,导致不应有的经济和时间上的损失。本文综合最新研究成果并与世界各地同类火山岩的特征进行对比,确证火山通道相爆发角砾岩的存在,并发现呈线状分布的古火山颈群。 相似文献
5.
Isolated, Late Miocene volcanogenic sequences in northern Alexander Island, Antarctic Peninsula, form an unusual, cogenetic association of volcaniclastic, sandy-gravelly lithofacies (including tillites) and volcanic (lava/hyalocalstite) lithofacies. Using simple lithofacies analysis and theoretical considerations of hydrodynamic effects of subglacial eruptions, valley-confined volcanic activity beneth thin, wet-based ice is suggested. The Alexander Island successions are complete enough to be regarded as model sequences for this uncommonly recorded type of eruptive/depositional activity. The sedimentary lithofacies represent resedimented tuffs and meltout or flow tills, which were probably deposited in subglacial ice tunnels eroded or enlarged by volcanically heated meltwater. The volcanic lithofacies formed by the interaction of hot magma with the ice tunnel walls (generating abundant meltwater) and water-saturatedsediments, resulting in the formation of heterogeneous masses of lava and hyaloclastite. There is no obvious sequence organisation in the sedimentary sections. This is probably due to a complex interplay of eruption-related and environmental hydrodynamic factors affecting the relative proportions of water and entrained sediment. 相似文献
6.
Detailed facies analysis of hyaloclastites and associated lavas from eight table mountains and similar "hyaloclastite volcanoes"
in the Icelandic rift zone contradict a rapid and continuous, "monogenetic", entirely subglacial evolution of most volcanoes
studied. The majority of the exposed hyaloclastite deposits formed in large, stable lakes as indicated by widespread, up to
300-m-thick, continuous sections of deep water, shallow water and emergent facies. Salient features include extensively layered
or bedded successions comprising mainly debris flow deposits, turbidites, base surge and fallout deposits consisting of texturally
and compositionally variable, slightly altered hyaloclastites, as well as sheet and pillow lavas. In contrast, chaotic assemblages
of coarser-grained, more poorly sorted and more strongly palagonitized hyaloclastite tuffs and breccias, as well as scoria
and lava are interpreted to have formed under sub- or englacial conditions in small, chimney-like ice cavities or ice-bound
lakes. Irregularly shaped and erratically arranged hyaloclastite bodies produced at variable water levels appear to have resulted
mainly from rapid changes of the eruptive environment due to repeated build-up and drainage of ice-bound lakes as well as
the restricted space between the ice walls. We distinguish a "deep water" facies formed during high water levels of the lake,
a hydroclastic shallow water and emergent facies (leakage of the lake or growth of the volcano above the water surface). Our
model implies the temporary existence of large, stable lakes in Iceland probably formed by climatically induced ice melting.
The highly complex edifices of many table mountains and similar volcanoes were constructed during several eruptive periods
in changing environments characterized by contrasting volcanic and sedimentary processes.
Received: 10 June 1997 / Accepted: 28 July 1998 相似文献
7.
Macquarie Island is composed of a complete section of oceanic crust that formed in a slow-spreading mid-ocean ridge 2.0 to
3.5 km below sea level. Vitriclastic facies preserved on the island have both pyroclastic and hyaloclastic characteristics.
Monomict hyaloclastic breccia facies are widespread across the island and are predominantly composed of near-primitive (~7.9 wt%
MgO) subalkaline/transitional (~0.7 wt% K2O) sideromelane shards and crystalline basalt clasts with low vesicularity (LV, < 15% vesicles). Breccias are thick bedded
and structureless with matrix-supported angular pillow fragments, bomb-sized fluidal mini-pillows, and globular glass lapilli.
Clasts are lithologically similar to interbedded pillow basalts and laterally grade into fine-grained sandstone facies. These
sandstones are normal-graded, well-laminated, thin bedded, and interstratified with red pelagic mudstone. Lithofacies associations
indicate that the hyaloclastic breccias were formed proximal to a source vent via quench-fragmentation, and subsequently reworked
by ocean-bottom currents into distal epiclastic sandstone facies. During eruption, co-genetic pillow lava and hypabyssal intrusions
mingled with the breccia, forming fluidal peperite. Rare polymict pyroclastic facies only occur in the highest stratigraphic
levels and are mostly composed of highly vesicular (HV, 15–50% vesicles) sideromelane shards and crystalline basalt clasts
with alkaline (~1.0 wt% K2O) fractionated (~6.8% MgO) compositions. Minor lithic grains are composed of subalkaline (~0.7 wt% K2O) to very highly alkaline (~1.7 wt% K2O) LV sideromelane shards, and amphibole-bearing diabase. The pyroclastic facies contains medium to thick beds of lapilli-tuff
that exhibit both reverse and normal grading, diffuse lamination, and planar-grain fabric. These beds are locally overlain
by thin fine-grained tuff beds entirely composed of cuspate to very thin elongate bubble-wall shards. These characteristics
indicate that explosive deep-marine eruptions produced high-density coarse-grained gravity flows that were covered by slower
suspension settle-out of delicate bubble-wall shards. Stratigraphic relationships suggest that explosive eruptions started
during the waning stages of more alkaline volcanism along the proto-Macquarie spreading center. 相似文献
8.
In this paper we present a model for the growth of a maar-diatreme complex in a shallow marine environment. The Miocene-age
Costa Giardini diatreme near Sortino, in the region of the Iblei Mountains of southern Sicily, has an outer tuff ring formed
by the accumulation of debris flows and surge deposits during hydromagmatic eruptions. Vesicular lava clasts, accretionary
lapilli and bombs in the older ejecta indicate that initial eruptions were of gas-rich magma. Abundant xenoliths in the upper,
late-deposited beds of the ring suggest rapid magma ascent, and deepening of the eruptive vent is shown by the change in slope
of the country rock. The interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts
of variable size and amount. The occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle-lower part
of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity. Intrusions of nonvesicular
magma, forming plugs and dikes, occur on the western side of the diatreme, and at the margins, close to the contact between
breccia deposits and country rock; they indicate involvement of volatile-poor magma, possibly during late stages of activity.
We propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a
rampart that isolated for a short time the inner crater from the surrounding marine environment. This allowed explosive activity
to draw down the water table in the vicinity of the vent and caused deepening of the explosive center. A subsequent decrease
in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water, at which time the hyaloclastite
was deposited. Emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and
unconsolidated, locally forming peperites. The sheltered, low-energy lagoon filled with marine limestones mixed with volcaniclastic
material eroded from the surrounding ramparts. Ultimately, lagoonal sediments accumulated in the crater until subsidence or
erosion of the tuff ring caused a return to normal shallow marine conditions. 相似文献
9.
Lava flux and a low palaeoslope were the critical factors in determining the development of different facies in the Late Permian Blow Hole flow, which comprises a series of shoshonitic basalt lavas and associated volcaniclastic detritus in the southern Sydney Basin of eastern Australia. The unit consists of a lower lobe and sheet facies, a middle tube and breccia facies, and an upper columnar-jointed facies. Close similarities in petrography and geochemistry between the basalt lavas from the three facies suggest similar viscosities at similar temperatures. Sedimentological and palaeontological evidence from the sedimentary units immediately below the Blow Hole flow suggests that the lower part of the volcanic unit was emplaced in a cold water, shallow submarine environment, but at least the top of the uppermost lava was subaerial with some palaeosol development. The lower lobe and sheet facies was emplaced on a low slope (<2°) in a lower to middle shoreface environment with water depths of 20–25 m. Lava may have transgressed from subaerial to subaqueous and was emplaced relatively passively with lava flux sufficiently high and uniform to form lobes and sheets rather than pillows. The middle unit probably originated from a subaerial vent and flowed into a shallow (10–15 m) submarine environment, and wave action probably interacted with the advancing lava front to form a lava delta. Lava flux was sufficiently high to produce well-developed, subcircular lava tubes, which lack evidence for thermal erosion. In some areas, lava ‘burrowed’ into the unconsolidated, water-saturated lava delta and sand pile to produce intrusive contacts. The upper columnar-jointed unit represents a ponded facies probably emplaced initially in water depths <5 m but whose top was subaerial. 相似文献
10.
11.
An exceptionally well-exposed, ancient, intra-arc basin in the Permian Takitimu Group of New Zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition. These are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation. The Takitimu oceanic arc/basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres. Volcanism probably included (1) moderate- to deep-water extrusion of lava and deposition of hyaloclastite, (2) extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin, and (3) Plinian and phreato-Plinian eruptions from more distant subaerial vents along the arc. Much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows, slumping and sliding. Hemipelagic sedimentation predominated on the outer margin of the basin, infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites. Turbidite sedimentation prevailed in the remainder of the basin, producing a thick prograding volcaniclastic apron adjacent to the arc. The volcaniclastic strata closely resemble classic turbidite deposits, and show similar lateral facies variations to submarine fan deposits. Study of such sequences provides insight into poorly understood processes in modern arc-related basins. 相似文献
12.
The Milos volcanic field includes a well-exposed volcaniclastic succession which records a long history of submarine explosive
volcanism. The Bombarda volcano, a rhyolitic monogenetic center, erupted ∼1.7 Ma at a depth <200 m below sea level. The aphyric
products are represented by a volcaniclastic apron (up to 50 m thick) and a lava dome. The apron is composed of pale gray
juvenile fragments and accessory lithic clasts ranging from ash to blocks. The juvenile clasts are highly vesicular to non-vesicular;
the vesicles are dominantly tube vesicles. The volcaniclastic apron is made up of three fades: massive to normally graded
pumice-lithic breccia, stratified pumice-lithic breccia, and laminated ash with pumice blocks. We interpret the apron beds
to be the result of water-supported, volcaniclastic mass-How emplacement, derived directly from the collapse of a small-volume,
subaqueous eruption column and from syn-eruptive, down-slope resedimentation of volcaniclastic debris. During this eruptive
phase, the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion.
The lava dome, emplaced later in the source area, is made up of flow-banded lava and separated from the apron by an obsidian
carapace a few meters thick. The near-vertical orientation of the carapace suggests that the dome was intruded within the
apron. Remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement.
Published online: 30 January 2003
Editorial responsibility: J. McPhie 相似文献
13.
The volcanic island of Milos, Greece, comprises an Upper Pliocene –Pleistocene, thick (up to 700 m), compositionally and texturally diverse succession of calc-alkaline, volcanic, and sedimentary rocks that record a transition from a relatively shallow but dominantly below-wave-base submarine setting to a subaerial one. The volcanic activity began at 2.66±0.07 Ma and has been more or less continuous since then. Subaerial emergence probably occurred at 1.44±0.08 Ma, in response to a combination of volcanic constructional processes and fault-controlled volcano-tectonic uplift. The architecture of the dominantly felsic-intermediate volcanic succession reflects contrasts in eruption style, proximity to source, depositional environment and emplacement processes. The juxtaposition of submarine and subaerial facies indicates that for part of the volcanic history, below-wave base to above-wave base, and shoaling to subaerial depositional environments coexisted in most areas. The volcanic facies architecture comprises interfingering proximal (near vent), medial and distal facies associations related to five main volcano types: (1) submarine felsic cryptodome-pumice cone volcanoes; (2) submarine dacitic and andesitic lava domes; (3) submarine-to-subaerial scoria cones; (4) submarine-to-subaerial dacitic and andesitic lava domes and (5) subaerial lava-pumice cone volcanoes. The volcanic facies are interbedded with a sedimentary facies association comprising sandstone and/or fossiliferous mudstone mainly derived from erosion of pre-existing volcanic deposits. The main facies associations are interpreted to have conformable, disconformable, and interfingering contacts, and there are no mappable angular unconformities or disconformities within the volcanic succession.Electronic Supplementary Material Supplementary material is available for this article at 相似文献
14.
A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km3 of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved en masse downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite. 相似文献
15.
The Llangorse volcanic field is located in northwest British Columbia, Canada, and comprises erosional remnants of Miocene to Holocene volcanic edifices, lava flows or dykes. The focus of this study is a single overthickened, 100-m-thick-valley-filling lava flow that is Middle-Pleistocene in age and located immediately south of Llangorse Mountain. The lava flow is basanitic in composition and contains mantle-derived peridotite xenoliths. The lava directly overlies a sequence of poorly sorted, crudely bedded volcaniclastic debris-flow sediments. The debris flow deposits contain a diverse suite of clast types, including angular clasts of basanite lava, blocks of peridotite coated by basanite, and rounded boulders of granodiorite. Many of the basanite clasts have been palagonitized. The presence and abundance of clasts of vesicular to scoriaceous, palagonitized basanite and peridotite suggest that the debris flows are syngenetic to the overlying lava flow and sampled the same volcanic vent during the early stages of eruption. They may represent lahars or outburst floods related to melting of a snow pack or ice cap during the eruption. The debris flows were water-saturated when deposited. The rapid subsequent emplacement of a thick basanite flow over the sediments heated pore fluids to at least 80–100°C causing in-situ palagonitization of glassy basanite clasts within the sediments. The over-thickened nature of the Llangorse Mountain lavas suggests ponding of the lava against a down-stream barrier. The distribution of similar-aged glaciovolcanic features in the cordillera suggests the possibility that the barrier was a lower-elevation, valley-wide ice-sheet. 相似文献
16.
Evolution of an englacial volcanic ridge: Pillow Ridge tindar, Mount Edziza volcanic complex, NCVP, British Columbia, Canada 总被引：2，自引：1，他引：1
Benjamin R. Edwards Ian P. Skilling Barry Cameron Courtney Haynes Alex Lloyd Jefferson H.D. Hungerford 《Journal of Volcanology and Geothermal Research》2009,185(4):251
Glaciovolcanic deposits are critical for documenting the presence and thickness of terrestrial ice-sheets, and for testing hypotheses about inferred terrestrial ice volumes based on the marine record. Deposits formed by the coincidence of volcanism and ice at the Mount Edziza volcanic complex (MEVC) in northern British Columbia, Canada, preserve an important record for documenting local and possibly regional ice dynamics. Pillow Ridge, located at the northwestern end of the MEVC, formed by ice-confined, fissure-fed eruptions. It comprises predominantly pillow lavas and volcanic breccias of alkaline basalt composition, with subordinate finer-grained volcaniclastic deposits and dykes. The ridge is presently 4 km long, 1000 m in maximum width, and 600 m high. Fifteen syn- and post-eruptive lithofacies are recognized in excellent exposures along the glacially dissected western side of the ridge. We recognize five lithofacies associations: (1) poorly sorted tuff breccia and dykes, (2) proximal pillow lava, dykes and tuff breccia, (3) distal pillow lava, poorly sorted conglomerate and well-sorted volcanic sandstone, (4) interbedded tuff, lapilli tuff, and tuff breccia units, and (5) heterolithic volcanogenic conglomerate and sandstone. Given the abundance of pillow lavas and the lack of surrounding topographic barriers capable of impounding water, we agree with Souther [Souther, J.G., 1992. The late Cenozoic Mount Edziza volcanic complex. Geol. Soc. Can. Mem., vol. 420. 320 pp] that the bulk of the edifice formed while confined by ice, but have found evidence for a more complex and variable eruption history than that which he proposed. Preliminary estimates of water-ice depths derived from FTIR analyses of H2O give ranges of 300 to 680 m assuming 0 ppm CO2, and 857 to 1297 m assuming 25 ppm CO2. Variations in depth estimates among samples may indicate that water/ice depths changed during the evolution of the ridge, which is consistent with our interpretations for the origins of different lithofacies associations. Given that the age of the units are likely to be ca. 0.9 Ma [Souther, J.G., 1992. The late Cenozoic Mount Edziza volcanic complex. Geol. Soc. Can. Mem., vol. 420. 320 pp], Pillow Ridge may be the best documentation of a regional high stand of the Cordilleran Ice Sheet (CIS) in the middle Pleistocene, and an excellent example of the lithofacies and stratigraphic complexities produced by variations in water levels during a prolonged glaciovolcanic eruption. 相似文献
17.
THE CHARACTERISTICS OF VOLCANIC ROCKS STRUCTURE AND LITHOFACIES OF DALIUCHONG VOLCANO IN THE TENGCHONG VOLCANIC FIELD,YUNNAN PROVINCE 
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下载免费PDF全文 A set of grey-purple layered volcanic rocks are found widely distributed from the mountain flank to the main peak of Daliuchong volcano, but it's difficult to identify whether they are volcaniclastic rock or lava rock just by field investigation and the crystal structure observation under microscope. The study of matrix microstructure of the volcanic rocks can help to identify the volcanic facies. We recognize the eruptive facies rocks through observation of the matrix microstructure and pore shape with comparison to those of the volcanic vent facies, extrusive facies and effusive facies rocks under microscope, thus the mentioned layered volcanic rocks could be named as dacitic crystal fragment tuff. Combining the joint work of field investigation, systematic sampling, chemical analyzing and microscopic observation, we summary the Daliuchong volcanic facies as follows:1. The effusive facies lava constitutes the base of Daliuchong volcano and was produced by early eruption.2. The explosive facies is composed of dacite crystal fragment welded tuff and volcanic breccia and mainly distributes on the W, S and NE flank of the volcanic cone.3. The volcanic conduit with its diameter more than one hundred meters is located about 100 meters south of the main peak of the Daliuchong volcano.4. The extrusive facies rock is only exposed near the peak of Daliuchong volcano.Therefore, the volcanism of Daliuchong volcano can be speculated as:Large-scale lava overflowing occurred in the early eruption period; then explosive eruptions happened; at last, the volcanisms ceased marked with magma extrusion as lava dome and plug. 相似文献
18.
Michael J. Grubensky Gary A. Smith John W. Geissman 《Journal of Volcanology and Geothermal Research》1998,83(1-2)
Cirque-wall exposures of cone-forming deposits of Pleistocene Broken Top volcano, Oregon Cascade Range, reveal that the volcano is composed of unconformity-bounded constructional units of coherent lava (lava-flow cores) and breccia. Coarse-grained autoclastic breccias are found above and below lava-flow cores and may extend downslope from coherent lava outcrops where they may or may not be associated with thin lava stringers. Mantle-bedded scoria-fall breccias are recognized by generally good sorting, mantle bedding, and presence of aerodynamically shaped bombs. These breccias vary considerably in thermal oxidation coloration (black, red, orange, purple). Many breccia layers are unsorted mixtures of scoria and lithic (nonvesicular) fragments that grade laterally to unambiguous autoclastic breccia or lava-flow cores. These layers are interpreted as hybrid pyroclastic–autoclastic deposits produced by incorporation of falling or fallen tephra into advancing lava-flow fronts. This latter breccia type is common at Broken Top and offers particular challenges for clast or deposit classification.Progressive thermal demagnetization results for selected examples of different breccia types show that most scoria-fall and autoclastic breccias are emplaced at elevated temperatures (averaging 100–300°C). Clasts within single deposits record different emplacement temperatures ranging, in some cases, from 100 to over 580°C indicating a lack of thermal equilibration within deposits. Magnetization directions for single breccia deposits are more dispersed than data typically reported for lava flows. Settling and rotation of clasts after cooling or incorporation of colder clasts that are not significantly reheated probably accounts for the relatively high dispersion and suggests that paleomagnetic studies demanding low within-site dispersion (e.g., for determining paleomagnetic poles or evaluating tectonic rotation) should avoid volcanic breccias. 相似文献
19.
The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member,Wido Island,Korea 下载免费PDF全文
下载免费PDF全文 The 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. 相似文献
20.
Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand 总被引：1，自引：1，他引：0
Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced
by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle
Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation
permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest
that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine,
unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational
with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580 °C and are interpreted as aa flow
breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche
bedding beneath flows, were emplaced at varying temperatures between 200 and 550 °C and are interpreted as forming part of
block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor,
well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises
constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic
properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after
acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550 °C.
These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia
formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor
and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting
of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature,
although some deposits contain clasts possibly emplaced at temperatures as high as 525 °C. These breccias are interpreted
as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic
and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite
volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather
than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent
lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely
many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described
as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic
beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences.
Received: 1 October 1998 / Accepted: 28 December 1998 相似文献