Stratigraphy and volcanology of the Türkbükü volcanics: products of a stratovolcano in the Bodrum Peninsula,SW Anatolia

The Middle‐Upper Miocene Bodrum magmatic complex of the Aegean region, southwestern Turkey, is mainly represented by intermediate stocks, lavas, pyroclastic and volcaniclastic deposits. Monzonitic stocks and connected porphyry intrusions and extrusions are the first products of the magmatism. These are followed by a volcanic succession consisting of andesitic‐latitic lavas, autobrecciated lavas, pyroclastic and volcaniclastic deposits. The final stage is represented by basaltic and basaltic andesitic flows and dykes intruded into previous units. The volcanic succession crops out in the northern part of the Bodrum peninsula. In the lower part of this succession are widespread pyroclastic deposits, composed of pyroclastic fall and flow units, alternating with epiclastic deposits. Grain size, volume and thickness of the pyroclastic deposits were mainly controlled by the type, magnitude and intensity of the eruption. Further up the section, there are two horizons of debris avalanche deposits forming the coarsest and thickest deposits of the volcaniclastic succession. The debris avalanche deposits indicate at least two different flank collapses coeval with the volcanism. The stratigraphy and map pattern of these volcanic units imply that the northern part of the Bodrum peninsula was the north‐facing flank of a stratovolcano during the mid‐Late Miocene. Copyright © 2006 John Wiley & Sons, Ltd.     

The Ediacaran volcanosedimentary succession of Gabal Abu Had,North Eastern Desert,Egypt: geological study,facies analyses,and depositional setting

Gabal Abu Had is an exposure of a volcanosedimentary succession in the North Eastern Desert Basement Complex. This succession includes intercalation of two major rock units, which are Dokhan Volcanics and Hammamat Group with different styles of formation, deposition environments, and genesis. Gabal Abu Had succession (GHS) is a northward dipping, c. 700-m-thick volcanosedimentary succession that rests on metavolcanic and old granitoid rocks with erosion unconformity. The lower part of GHS is dominated by volcaniclastic mass flow deposits and andesitic lava with interbedded gravely sandstone, whereas the upper sequence is composed of pyroclastic flow deposits including welded to no welded ignimbrite intercalated with gravely sandstone and massive clast-support conglomerate toward the top. Facies analysis study of GHS presented eight lithofacies types, which grouped into five lithofacies associations. The GHS basin started with effusive eruption of silica-poor volcanic center, which produced andesitic lava. A part of lava underwent hyaloclastic fragmentation due to the presence of fluvial water in places producing the volcaniclastic mass flow deposits. Later, an explosive silica-rich volcanic center affected the GHS basin and created the pyroclastic plain deposits (ignimbrite and bedded tuff). The fluvial braided river is still in action since the first eruption, producing gravely sandstone, which is intercalated with the volcanic sequence. The upper GHS is characterized by thick, massive, and clast-supported conglomerate (well rounded clasts up to 100 cm) of alluvial fan facies. Several silica-rich and silica-poor subvolcanic intrusions were emplaced in the GHS. The GHS development displays a cycle from low- to high-energy sedimentation under humid climatic conditions, in addition to extension and down faulting of basin shoulders. In comparison with Gabal El Urf, located to the north of GHS and was studied by El-Gameel (2010), the GHS is a lava-rich succession rather than Gabal El Urf succession which is mainly pyroclastic rich.     

Syndepositional volcanism in the Rietgat and arenaceous Bothaville Formations,Ventersdorp Supergroup (late Archaean—early Proterozoic), in South Africa

The volcanic-sedimentary succession of the Ventersdorp Supergroup which is virtually undisturbed tectonically and of low-grade (greenschist facies) metamorphism, affords a unique opportunity for studying the interplay between volcanic and sedimentary processes. The transitional sequence between the Rietgat and Bothaville Formations consists of a number of lithofacies. These are a basal breccia representing pyroclastic and laharic deposits, an overlying breccia—arenite—conglomerate (BAC) which formed by debris flow and fluvial processes, an arenite deposited offshore during a transgression, and an upper conglomerate laid down on a beach. In the volcaniclastic BAC and arenite lithofacies the presence of thin tuff beds, deformed acid lava fragments (bombs?) and glass shards in the arenaceous matrix suggest syndepositional volcanism.Sedimentation took place along the flanks of an asymmetrical, actively volcanic, domal structure which consisted partly of unstable pyroclastic deposits in the east. Resedimentation of the pyroclastic debris by subaerial debris flows and braided streams built a volcaniclastic fan lobe at the foot of the domal structure. As volcanic activity subsided, sands derived from a granitic terrain, mixed with minor air-fall debris to subsequently cover the fan lobe during a regional transgression.     

Late-orogenic basins in the Archaean Superior Province, Canada: characteristics and inferences

The late-orogenic Archaean Duparquet, Kirkland and Stormy basins of the Canadian Superior Province are characterized by bounding crustal-scale faults and abundant porphyry stock emplacement. Lava flows and pyroclastic deposits are restricted to the Kirkland and Stormy basins, and coarse clastic detritus characterizes the Duparquet basin. Seven distinct lithofacies are identified: (1) mafic volcanic, (2) felsic volcanic, (3) pyroclastic, (4) volcaniclastic, (5) conglomerate-sandstone, (6) sandstone-argillite  conglomerate), and (7) argillite-sandstone  tuffaceous sandstone). The mafic and felsic volcanic lithofacies represent effusive lava flows, the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris. The conglomerate-sandstone lithofacies is interpreted as alluvial fan, fan delta or proximal braided stream deposits, whereas the sandstone-argillite lithofacies is consistent with sandy-dominated flood- or braidplain deposits. A dominantly shallow-water lacustrine setting is inferred for the argillite-sandstone lithofacies. These different lithofacies record the basin history and can be used to identify basin-forming processes. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected by volcanic input. Catastrophic influx of pyroclastic material facilitated mass-wasting processes and formation of non-confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns. Confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift. In addition, climate which controls the weathering processes, and vegetation which stabilizes unconsolidated material, affects the transport and depositional process. A CO₂-rich aggressive weathering, humid Archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation. Although tectonism is the prevalent long-term controlling factor in restricted basins, the effects of volcanism, climate and lack of vegetation can also be detected.     

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.     

Sedimentological and petrographic evolution of a fluvio-lacustrine environment during the onset of volcanism: Volcanically-induced forcing of sedimentation and environmental responses

This work focuses on the transition from the Las Leoneras to Lonco Trapial Formations, in the lower part of the Early Jurassic succession of the incipient rift phase of the Cañadon Asfalto Basin (western part of Chubut Province – Patagonia, Argentina). Twenty lithofacies have been identified and grouped into seven facies associations on the basis of field characteristics (sedimentological and lithological) and optical microscope analysis, from two localities representing proximal and distal locations in the basin. The spatial relationship between all the lithofacies provided a four-dimensional reconstruction of the palaeoenvironmental evolution, showing how the original, clastic sedimentation in alluvial/fluvial and lake environments was modified by short-lived volcanic events during three volcanic cycles, and how the environment reacted after the input of huge amounts of volcaniclastics. Progradation of small deltas and subaqueous lobes, retrogradation caused by rising lake levels, and frequent erosion of valleys were typical processes in this environment. When explosive volcanism began, the original tectono-climatic control on sedimentation was replaced by the volcanic control, and the volcanically-forced sedimentation broke the equilibrium among production, delivery and accumulation of sediments. The nature of the volcanic eruptions and the different propensity of volcanic lithofacies to produce particles of different size and types (lithics, crystals and glass) are also analyzed. The role of volcanism in the production and transport of great volumes of sediments across sedimentary systems needs to be carefully re-examined, and the analysis on the variability in the composition of volcaniclastic deposits must take into account that volcaniclastic particle types may not simply reflect a linear deepening in the dissection of magmatic arcs through time but are often controlled by the style of the eruptions and the lithological variation of the volcanic products.     

北大巴山志留系滔河口组火山碎屑岩相序、组构特征及古火山作用环境分析

志留系滔河口组是一套发育于北大巴山地区的火山岩-火山碎屑岩-沉积岩组合。在1:10000地质填图和大比例地质剖面实测基础上,通过详细的火山碎屑岩相序和组构分析,本文在滔河口组火山-地层中共识别和划分出22个岩相,5个相组合类型。区域岩相测量与对比揭示,滔河口组火山-沉积宏观序列自下而上由玄武岩相(一般下部为块状粗粒/细粒玄武岩相、上部为枕状玄武岩相)、凝灰角砾岩相、再沉积富辉石火山碎屑砾岩相、无结构或叠瓦状凝灰质粗砾岩相、凝灰质砂岩相、生物灰岩相或泥岩相构成。岩相组合横向变化显示滔河口组古火山活动西强东弱,火山活动类型为斯托柏林型(Strombolian-type eruption)喷发。相对地,研究区西部火山-沉积序列发育较为完整。滔河口组的地层序列与岩石组合与板内火山活动产物相似,本文认为北大巴山地区志留系滔河口组火山岩-火山碎屑岩-沉积岩组合形成于洋岛或海山构造环境,是板内拉伸作用的产物。     

The fluvial and pyroclastic deposits of the Cagayan Basin, Northern Luzon, Philippines—an example of non-marine volcaniclastic sedimentation in an interarc basin

ABSTRACT The Cagayan basin of Northern Luzon, an interarc basin 250 km long and 80 km wide, contains a 900 m thick sequence of Plio-Pleistocene fluvial and pyroclastic deposits. These deposits are divided into two formations, the Ilagan and Awidon Mesa, and three lithofacies associations. The facies, which are interpreted as meandering stream, braided stream, lahar, and pyroclastic flow and fall deposits, occur in a coarsening upward sequence. Meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits; lahars and ignimbrites. The coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent Cordillera Central volcanic arc. Uplift of the arc resulted in the progradation of coarser clastics further into the basin, the development of an alluvial fan, and migration of the basin depocentre away from the arc. The coarsening of the pyroclastic deposits reflects the development of a more proximal calc-alkaline volcanic belt in the maturing volcanic arc. The Cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins. This is because the tectonic and volcanic processes which controlled sedimentation in the Cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins.     

Volcanic evolution of a long-lived Ordovician island-arc province in the Parkes region of the Lachlan Fold Belt,southeastern Australia

The Ordovician mafic volcanic rocks in the Parkes region of New South Wales occur as three distinct packages of volcaniclastic and coherent volcanic rocks and minor limestone that formed part of an oceanic island arc succession. The oldest package is the Early Ordovician Nelungaloo Volcanics and overlying Yarrimbah Formation. These formations consist of volcanic siltstone, sandstone, polymictic breccia, conglomerate facies interpreted as moderately deep-water turbidites and coarser grained debris-flow deposits emplaced in the medial to distal part of a subaqueous volcaniclastic apron flanking an active volcanic centre(s). Broadly conformable massive to brecciated andesites in the apron deposits are interpreted as synsedimentary sills and/or lava flows. A hiatus in volcanism occurred between the Bendigonian and early Darriwilian (ca 476 – 466 Ma). Deposition of the second package, which produced the Middle to Late Ordovician Goonumbla Volcanics, Billabong Creek Limestone and Gunningbland Formation, commenced with shallow-water limestones and minor volcaniclastic rocks. During an approximately 15 million years period, a thick sequence of bedded volcanic sandstone, limestone and minor siltstone and volcanic breccia were deposited in very shallow to moderate water depths. The top of this package is marked by thick volcanic conglomerate and sandstone mass-flow deposits and approximately coeval basaltic andesite lavas and sills sourced from a nearby volcano. The upper age limit of this package is constrained as approximately 450 Ma by Ea3/4 fossils and monzodiorite that intrudes the Goonumbla Volcanics. The lower limit of the third package, which constitutes the Wombin Volcanics, is poorly constrained and the duration of the hiatus that separates the Goonumbla and Wombin Volcanics is unknown but may be as long as 10 million years. The Wombin Volcanics record development of a thick, proximal volcaniclastic apron flanking a compositionally more evolved volcanic edifice in the immediate Parkes area. Thick crystal-rich turbiditic sandstones of mafic provenance are intercalated with polymictic volcanic breccias and megablock breccias that are interpreted as proximal subaqueous debris-flow and debris-avalanche deposits, respectively. The sequence also includes numerous trachyandesite bodies, many of which were emplaced within the volcaniclastic apron as synsedimentary sills. No evidence was found at Parkes to support the existence of a previously proposed 22 km diameter collapse caldera and the source volcanoes for the Ordovician are envisaged as complex stratovolcanoes.     

Petrology and provenance of the Toro Negro Formation (Neogene) of the Vinchina broken-foreland basin (Central Andes of Argentina)

Detrital modes of sandstones and conglomerates of the Toro Negro Formation (Late Miocene-early Pliocene) were used to analyze the evolution of the broken-foreland stage of the Vinchina Basin (28°30′–29°00′ S and 68°30′–68°20′ W) of NW Argentina. This basin located in the Western Sierras Pampeanas is bounded to the west by the Precordillera and to the east by the Famatina System. Three sandstone petrofacies: plutonic-metamorphic, volcanic and mixed petrofacies and three conglomerate lithic associations: basement, sedimentary and volcanic lithic associations were recognized, allowing to establish three source areas: Western Sierras Pampeanas (Toro Negro and Umango Ranges), Cordillera Frontal and Precordillera.During the Late Miocene, the Toro Negro Range (to the north) together with the Cordillera Frontal and Precordillera (to the west) were the main sources for depositional sequences I and II (lower member of the Toro Negro Formation). On the contrary, during the latest Miocene-early Pliocene, Depositional Sequence III (upper member) exhibited a progressive increase in the supply from the eastern Precordillera (to the west) with additional material from the Umango Range to the south. Besides, evidence of synchronic volcanism is recorded in the upper part of Depositional Sequence II and the lower part of Depositional Sequence III.The coexistence of the three source areas and the changing distribution patterns due to re-accommodation of sediment dispersal routes demonstrate that the evolution of this type of basin is much more complex than previously envisaged. Therefore, an integrated analysis using different tools (sedimentary facies, paleocurrent measurements, sandstone petrography and conglomerate composition) is needed for a clearer understanding of broken-foreland basins.     

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt,Quadrilátero Ferrífero,Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures.Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.     

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

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

A medial to distal volcaniclastic record of an andesite stratovolcano: detailed stratigraphy of the ring-plain succession of south-west Taranaki,New Zealand

The >25 ka volcaniclastic ring-plain succession in south-west Taranaki has been remapped to establish a much more detailed understanding of the older stratigraphic record of Mt. Taranaki. Coastal cliff exposures show a range of volcaniclastic lithofacies, including debris-avalanche and lahar deposits, and allow a detailed chronological reconstruction of past volcanic and sedimentary events. Five new debris-avalanche deposits were identified, and their distribution in coastal cross-sections mapped. In addition, four previously described units were renamed and their stratigraphic position and lateral extent redefined. Chronostratigraphic control of the younger (<50 ka) sequence was obtained by radiocarbon dating of wood found within, or peat interbedded with, the deposits. Emplacement ages of the older units were estimated from their stratigraphic position and underlying marine wave-cut surfaces. Overall, at least 14 widespread debris-avalanche deposits occur within the <200 ka ring-plain record of Mt. Taranaki, suggesting one major edifice failure on average every 14,000 years, with an increase in frequency since 40 ka. The stratigraphic reconstruction of the ring-plain succession showed that the same pattern of deposition was repeatedly produced throughout the existence of Mt. Taranaki. Depending on their sedimentological characteristics, the different volcanic and sedimentary lithofacies can be related to phases of edifice-construction or collapse events. Based on the identified cyclic sedimentation pattern, we present a new episodic stratigraphy that integrates existing and new lithostratigraphic units into a coherent chronostratigraphic framework that can be applied to the entire volcanic and volcaniclastic succession at Mt. Taranaki. This model takes into account the complex geological processes that have taken place on the volcano and provides a more uniform stratigraphic terminology that could be applied to repeatedly collapsing stratovolcanoes elsewhere.     

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures.Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.     

Magnetostratigraphy and paleomagnetism of early and middle Miocene synorogenic strata: basement partitioning and minor block rotation in Argentine broken foreland

Magnetostratigraphic and paleomagnetic studies on early Andean synorogenic strata (Del Crestón Fm.), in the Famatina Belt (28.7°S, 67.5°W) clarify details of chronology that permit calculation of sedimentation rates within the broken foreland of west Argentina. The Del Crestón Fm represents the first record of broken foreland sedimentation within the southern Central Andean belt and the earliest retroarc volcanic rocks exposed several hundred kilometers from the trench. Twenty-five out of 49 sites collected along the succession presented a primary remanence, as determined through positive fold and reversal tests. Correlation of the local magnetic polarity section with the global polarity time scale indicates that the sedimentation of Del Crestón Fm started at ~16.7 Ma and continued until ~14.5 Ma. The youngest strata are represented by conglomerates bearing abundant Lower Paleozoic granite boulders indicating unroofing of the crystalline basement within the NW Sierras Pampeanas. This result supports the hypothesis of an early broken foreland stage at these latitudes of the Andes, with involvement of the basement in deformation and coeval retroarc volcanism, common attributes of flat-subduction regimes. A mean site paleomagnetic direction of Dec: 6.3°, Inc: ?43.6° (α95: 8.0°, N = 24) confirm our earlier intrepretation that the central part of the Famatina Belt within the Sierras Pampeanas did not undergo large vertical axes rotations since the Middle Miocene.     

The Early Palaeozoic Break-up of Northern Gondwana: Sedimentology, Physical Volcanology and Geochemistry of a Submarine Volcanic Complex in the Bavarian Facies Association, Saxothuringian Basin, Germany

Ordovician volcano-sedimentary successions of the Bavarian facies association in the Saxothuringian basin record the continental rift phase of the separation of the Saxothuringian Terrane from Gondwana. An 80 m succession from the Vogtendorf beds and Randschiefer Series (Arenig-Middle Ordovician), exposed along the northern margin of the Münchberg Gneiss Massif in northeast Bavaria, were subjected to a study of their sedimentology, physical volcanology and geochemistry. The Randschiefer series previously has been interpreted as lavas, tuffs, sandstones and turbidites, but the studied Ordovician units include four main lithological associations: mature sandstones and slates, pillowed alkali-basalts and derivative mass flow deposits, trachyandesitic lavas and submarine pyroclastic flow deposits interbedded with turbidites. Eight lithofacies have been distinguished based on relict sedimentary structures and textures, which indicate deposition on a continental shelf below wave base. The explosive phase that generated the pyroclastic succession was associated with the intrusion of dykes and sills, and was succeeded by the eruption of pillowed basalts. Debris flow deposits overlie the basalts. Ordovician volcanism in this region, therefore, alternated between effusive and explosive phases of submarine intermediate to mafic volcanism.

Based on geochemical data, the volcanic and pyroclastic rocks are classified as basalts and trachyandesites. According to their geochemical characteristics, especially to their variable concentrations of incompatible elements such as the High Field Strength Elements (HFSE), they can be divided into three groups. Group I, which is formed by massive lavas at the base of the succession, has extraordinarily high contents of HFSE. The magmas of this group were probably derived from a mantle source in the garnet stability field by low (ca. 1%) degrees of partial melting and subsequent fractionation. Group II, which comprises the pillow lavas at the top of the sequence, displays moderate enrichment of HFSE. This can be explained by a slightly higher degree of melting (ca. 1.6%) for the primary magma. Group I and II melts fractionated from their parental magmas in different magma chambers. The eruption centres of Groups I and II, therefore, cannot be the same, and the volcanic rocks must have originated from different vents. The sills and pyroclastic flow deposits of Group III stem at least partly from the same source as Group I. Rocks of Group I most likely mixed together with Group II components during the formation of the Group III flows, which became hybridised during eruption, transportation and emplacement.

The sedimentological and geochemical data best support a rift as the tectonic setting of this volcanism, analogous to modern continental rift zones. Hence, the rift-associated volcanic activity preserved in the Vogtendorf beds and Randschiefer Series represents an early Ordovician stage of rift volcanism when the separation of the Saxothuringian Terrane from Gondwana had just commenced.     

Problems of Stratigraphic Correlation and New K-Ar Data for Ignimbrites from Cappadocia,Central Turkey

Extensive calc-alkaline ignimbrite volcanism occurred in Central Anatolia since the late Miocene. At least eight major and two minor rhyolitic nonwelded and welded deposits can be identified within the Ürg¨p Basin and on the adjacent Nevsehir Plateau in the Central volcanic province. They are separated from one another by Plinian pumice, surge deposits of individual eruptions, and extensive secondary volcaniclastic sediments. Although exposures in the study area between Nevsehir and the Sultansazligi depression are excellent, disconnected outcrops resulting from recent and paleotopographic as well as young volcanic and sedimentary cover have led to controversial interpretations of the stratigraphic succession, especially in the upper part.

The stratigraphic succession presented here modifies previous views, also based on the ignimbrites. Furthermore, an attempt is made to further subdivide the Kavak stratigraphic unit. The ignimbrites of the Ürg¨p Basin/Nevsehir Plateau are, from bottom to top, Lower Göreme-Upper Göreme-Akdag-Zelve-Sarimaden-Cemilköy-Tahar-Gördeles-Kizilkaya-Incesu-Valibaba-Sofular. Lower and Upper Göreme ignimbrites, separated from each other by the Bahceli surge deposits and the surges of Sulusaray, represent part of the former Kavak unit. Some K-Ar dating performed on ignimbrite samples contribute to the completion of the scale of absolute ages of the tephra deposits, whose formation age was found to be roughly between 11 and 1.1 Ma.     

Discussion: History of coastal dunes at triangle Cliff,Fraser Island,Queensland

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

Sedimentology of an ancient erg margin: the Lower Jurassic Aztec Sandstone, southern Nevada and southern California

The Lower Jurassic Aztec Sandstone is an aeolian-deposited quartzose sandstone that represents the western margin of the southerly-migrating Navajo-Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments. In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue-shaped aeolian sand bodies interstratified with fluvially-deposited coarse sandstone and mudstone. Above, evaporite-rich fine sandstone and mudstone are overlain by thick, cross-stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west. Thin aeolian sequences are interbedded with volcanic flow rocks, ash-flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian-deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc-derived sediment. A two-part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross-strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore-erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian-deposited sediment encased in fluvial and sabkha deposits. Continued sand-sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central-erg sediments buried the fore-erg deposits. The trailing, upwind margin of the erg is represented by back-erg deposits in northern Utah and Wyoming.     

An approach of understanding acid volcanics and tuffaceous volcaniclastics from field studies: A case from Tadpatri Formation,Proterozoic Cuddapah basin,Andhra Pradesh,India

The lower stratigraphic part of the Cuddapah basin is marked by mafic and felsic volcanism. Tadpatri Formation consists of a greater variety of rock types due to bimodal volcanism in the upper part. Presence of bimodal volcanism is an indication of continental rift setting. Various genetic processes involved in the formation of such volcanic sequence result in original textures which are classified into volcaniclastic and coherent categories. Detailed and systematic field works in Tadpatri–Tonduru transect of SW Cuddapah basin have provided information on the physical processes producing this diversity of rock types. Felsic volcanism is manifested here with features as finger print of past rhyolite-dacite eruptions. Acid volcanics, tuffs and associated shale of Tadpatri Formation are studied and mapped in the field. With supporting subordinate studies on geochemistry, mineralogy and petrogenesis of the volcanics to validate field features accurately, it is understood that volcanism was associated with rifting and shallow marine environmental condition. Four facies (i.e., surge, flow, fall and resedimented volcaniclastic) are demarcated to describe stratigraphic units and volcanic history of the mapped area. The present contribution focuses on the fundamental characterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation.