Geochemistry and origin of a black mudstone in a volcaniclastic environment, Ordovician Lower Rhyolitic Tuff Formation, North Wales, UK

Black mudstones in marine volcaniclastic environments have been interpreted both as non-volcanic ‘background’sedimentation and as fine grained vitric dust from the waning stages of primary volcanism. Although difficult to distinguish by standard petrographic techniques, differentiation between the two is crucial when attempting to determine sedimentation rates or to infer periods of volcanic quiescence. In the Ordovician Lower Rhyolitic Tuff Formation of North Wales such a fine grained black unit at Cwm Idwal is geochemically similar to an underlying rhyolite ash flow tuff. Its chemical index of alteration (CIA) is identical to that of the tuff. These data suggest that the black mudstone unit is a vitric tuff related to the underlying ash flow tuff. Use of a CIA in addition to trace element geochemistry should, in most cases, serve to distinguish tuffs from silicified mudstones.     

The age,origin, and volcanological significance of the Y-5 ash layer in the Mediterranean

The Y-5 ash is the most widespread layer in deep-sea sediments from the eastern Mediterranean. This ash layer was previously correlated with the Citara-Serrara tuff on Ischia Island and dated at approximately 25,000 yr B.P. New data on the glass chemistry of the Y-5 ash and pyroclastic deposits from the Neopolitan volcanic province suggest that the layer is correlative with the large-volume Campanian ignimbrite and not with the deposit from Ischia Island. The volume of the Y-5 ash is approximately 65 km³ which is comparable in magnitude to the volume of the Campanian ignimbrite. An interpolated age of approximately 38,000 yr B.P. is estimated based on sedimentation rates derived from δ¹⁸O stratigraphy. There is a discrepancy between this estimate and previously reported radiocarbon ages which range from 24,000 to 35,000 yr B.P. We propose that the “Campanian tuff ash layer” should be adopted as the full stratigraphic name for the Y-5 ash. The deep-sea ash layer is divisible into two units in proximal localities, probably correlating with two major phases of the eruption: plinian and ignimbrite.     

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

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

The Hianana Volcanics: Remnants of a Late Permian tuff ring and lava flow,Coombadjha Volcanic Complex,northeastern New South Wales

The Hianana Volcanics consist of bedded tuff and dacitic lava that form a locally mappable unit within the extensive, Late Permian silicic volcanic sequence of northeastern New South Wales. Principal components of the bedded tuff are crystal and volcanic lithic fragments ranging from coarse ash to lapilli, accompanied by variable amounts of fine ash matrix. Well denned plane parallel thin bedding is characteristic. Sandwave bed forms, including low‐angle cross‐beds and wavy beds, are confined to an area of 2–3 km² coinciding with the thickest sections (70 m) of bedded tuff. A high‐aspect ratio flow of porphyritic dacitic lava overlies the bedded tuff in the same area. The setting, lithofacies, extent and geometry of the bedded tuffs of the Hianana Volcanics are comparable with modern tuff rings which are composed of the deposits from base surges generated by explosive phreatomagmatic eruptions at primary volcanic vents. Many of these have also discharged lava late in their activity. Proximal parts of the Hianana tuff ring were buried by the porphyritic lava after the phreatomagmatic eruptions had ceased. In more distal sections, the bedded tuff is less than 10 m thick and dominantly comprises fine grained, plane parallel, very thin beds and laminae; these features suggest an origin by fallout from ash clouds that accompanied the phreatomagmatic eruptions. The distal ash was covered and preserved from erosion by a layer of welded ignimbrite, the source of which is unknown.     

Eruption and emplacement of a laterally extensive, crystal-rich, and pumice-free ignimbrite (the Cretaceous Kusandong Tuff, Korea)

The Cretaceous Kusandong Tuff, Korea, is a thin (1–5 m thick) but laterally extensive (~ 200 km) silicic ignimbrite emplaced in a fluviolacustrine basin adjacent to a continental volcanic arc. The tuff has been used as an excellent key bed because of its great lateral continuity and unique lithology, characterized by the virtual absence of juvenile clasts and an abundance of quartz and feldspar crystals (up to 55–73 vol.%). The tuff is mostly massive and ungraded and locally shows crude internal layering, basal inverse grading and near-top normal grading of crystals, either erosional or non-erosional lower surfaces, and flat-lying to imbricated grain fabrics. Fragile intraformational clasts of mudstone and tuff are also included. These features provide only ambiguous information on the properties of the responsible pyroclastic density currents: i.e. whether they were dense and laminar or dilute and turbulent. The overall lateral continuity and sheet-like geometry of the tuff suggests, however, that the transport system of the currents was highly expanded, dilute, and turbulent. A plug-flow or slab-flow model cannot explain the origin of crude internal layering, imbricated grain fabrics, and the high crystal content, which is most likely the result of vigorous sorting processes within a dilute and turbulent current. Features indicative of deposition from a dense and laminar transporting medium are locally present, suggesting that a dense and laminar depositional system could develop locally at the base of the dilute and turbulent transport system. The virtual absence of juvenile clasts in the tuff is interpreted to be due to rapid ascent, sudden decompression, and full fragmentation of silicic magma into fine glass shards and crystals. Scarcity of basement-derived accidental components together with the absence of pumiceous fallout deposits beneath the tuff is interpreted to be due to shallow-level fragmentation of magma followed by immediate generation of pyroclastic density currents from shallow-level blasts at the onset of eruption. The eruption occurred through multiple vent sites in a short period of time, producing a seemingly single but actually composite ignimbrite unit. Such an eruption was probably possible because of a regional tectonic event within the basin or in its vicinity. It is proposed that a composite ignimbrite with the characteristics of the Kusandong Tuff can be an exemplary product of syntectonic volcanism that can provide an insight into the interpretation of structural and stratigraphic evolution of a sedimentary basin.     

Evolution of the early devonian bindook volcanic complex,wollondilly basin,eastern lachlan fold belt

The Early Devonian Bindook Volcanic Complex consists of a thick silicic volcanic and associated sedimentary succession filling the extensional Wollondilly Basin in the northeastern Lachlan Fold Belt. The basal part of the succession (Tangerang Formation) is exposed in the central and southeastern Wollondilly Basin where it unconformably overlies Ordovician rocks or conformably overlies the Late Silurian to Early Devonian Bungonia Limestone. Six volcanic members, including three new members, are now recognised in the Tangerang Formation and three major facies have been delineated in the associated sedimentary sequence. The oldest part of the sequence near Windellama consists of a quartz turbidite facies deposited at moderate water depths together with the shallow‐marine shelf Windellama Limestone and Brooklyn Conglomerate Members deposited close to the eastern margin of the basin. Farther north the shelf facies consists of marine shale and sandstone which become progressively more tuffaceous northwards towards Marulan. The Devils Pulpit Member (new unit) is a shallow‐marine volcaniclastic unit marking the first major volcanic eruptions in the region. The overlying shallow‐marine sedimentary facies is tuffaceous in the north, contains a central Ordovician‐derived quartzose (?deltaic) facies and a predominantly mixed facies farther south. The initial volcanism occurred in an undefined area north of Marulan. A period of non‐marine exposure, erosion and later deposition of quartzose rocks marked a considerable break in volcanic activity. Volcanism recommenced with the widespread emplacement of the Kerillon Tuff Member (new unit), a thick, non‐welded rhyolitic ignimbrite followed by dacitic welded ignimbrite and air‐fall tuff produced by a large magnitude eruption leading to caldera collapse in the central part of the Bindook Volcanic Complex, together with an additional small eruptive centre near Lumley Park. The overlying Kerrawarra Dacite Member (new unit) is lava‐like in character but it also has the dimensions of an ignimbrite and covers a large part of the central Bindook Volcanic Complex. The Carne Dacite Member is interpreted as a series of subvolcanic intrusions including laccoliths, cryptodomes and sills. The Tangerang Formation is overlain by the extensive crystal‐rich Joaramin Ignimbrite (new unit) that was erupted from an undefined centre in the central or northern Bindook Volcanic Complex. The volcanic units at Wombeyan and the Kowmung Volcaniclastics in the northwestern part of the complex are probably lateral time‐equivalents of the Tangerang Formation and Joaramin Ignimbrite. All three successions pre‐date the major subaerial volcanic plateau‐forming eruptions represented by the Barrallier Ignimbrite (new unit). The latter post‐dated folding and an extensive erosional phase, and unconformably overlies many of the older units in the Bindook Volcanic Complex. This ignimbrite was probably erupted from a large caldera in the northern part of the complex and probably represents surface expressions of part of the intruding Marulan Batholith. The final volcanic episode is represented by the volcanic units at Yerranderie which formed around a crater at the northern end of the exposed Bindook Volcanic Complex.     

Crystal chemistry of diagenetic zeolites in volcanoclastic deposits of Italy

Zeolites from the most important volcanoclastic deposits of Italy include: (1) phillipsite and heulandite from the cinerite of the central northern Apennines; (2) chabazite and phillipsite from the phonolitic tephritic ignimbrite with black pumices; (3) phillipsite from the “tufo lionato” of Vulcano Laziale; (4) chabazite and phillipsite from the Campanian ignimbrite; (5) phillipsite from the Neapolitan yellow tuff; and (6) chabazite and phillipsite from the pyroclastics of Monte Vulture. Compared with sedimentary phillipsites and chabazites described in the literature, the chabazites and phillipsites studied here have lower Si/Al ratios and higher K contents. These chemical peculiarities are correlated with both the K-rich vesuvitic-leucititic, latitic-phonolitic, and potassic alkali-trachytic chemistry of the ash from which they were derived and, very likely, with the character of the hydrologically open system environment in which they formed. The zeolite of the heulandite-clinoptilolite group from the cinerite of the central northern Apennines is classified as a true heulandite on the basis of its chemical composition and thermal behavior.     

Geology of the Mwadui kimberlite, Shinyanga district, Tanzania

The Mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in-fill, albeit without the tuff ring.

The geology of Mwadui can be subdivided into five geological units, viz. the primary pyroclastic kimberlite (PK), re-sedimented volcaniclastic kimberlite deposits (RVK), granite breccias (subdivided into two units), the turbidite deposits, and the yellow shales listed in approximate order of formation. The PK can be further subdivided into two units—lithic-rich ash and lapilli tuffs which dominate the succession, and lithic-poor juvenile-rich ash and lapilli tuffs. The lower crater is well bedded down to at least 684 m from present surface (extent of current drill data). The bedding is defined by the presence of juvenile-rich lapilli tuffs vs. lithic-rich lapilli tuffs, and the systematic variation in granite content and clast size within much of the lithic-rich lapilli tuffs. Four distinct types of bedding have been identified in the pyroclastic deposits. Diffuse zones characterised by increased granite abundance and size, and upward-fining units, represent the dominant types throughout the deposit.

Lateral heterogeneity was observed, in addition to the vertical changes, suggesting that the eruption was quite heterogeneous, or that more than one vent may have been present. The continuous nature of the bedding in the pyroclastic material and the lack of ash-partings suggest deposition from a high concentration (ejecta), sustained eruption column at times, e.g. the massive, very diffusely stratified deposits. The paucity of tractional bed forms suggest near vertical particle trajectories, i.e. a clear air-fall component, but the poorly sorted, matrix-supported nature of the deposits suggest that pyroclastic flow and/or surge processes may also have been active during the eruption.

Available diamond sampling data were examined and correlated with the geology. Data derive from the old 120 (37 m), 200 (61 m), 300 (92 m) and 1200 ft (366 m) levels, pits sunk during historical mining operations, drill logs, as well as more recent bench mapping. Correlating macro-diamond sample data and geology shows a clear relationship between diamond grade and lithology. Localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post-eruptive sedimentary in-fill processes. Clear distinction can be drawn between upper (re-sedimented) and lower (pyroclastic) crater deposits at Mwadui, both from a geological and diamond grade perspective.

Finally, an emplacement model for the Mwadui kimberlite is proposed. Geological evidence suggests that little or no sedimentary cover existed at the time of emplacement. The nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous, but that the eruption column may have been heterogeneous, both petrologically as well as geometrically. Volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re-sedimented volcaniclastic material derived from the tuff ring.

The Mwadui kimberlite exhibits marked similarities compared to the Orapa kimberlite in Botswana.     

Volcanological study of the middle Miocene Okiduse Volcanic Group,Woodlark Island (Muyuw), eastern Papua

Woodlark Island (Muyuw) is located in a tectonically complex region, one of the few places on Earth where continental breakup is occurring ahead of seafloor spreading. Rifting commenced in the late Miocene (8.8–6 Ma) and is associated with the westward-propagating Woodlark Basin Spreading Centre. The island comprises approximately 850 km² of raised Pleistocene coral reef and associated sediments with a central, moderately elevated range underlain by the middle Miocene calc-alkaline to shoshonitic Okiduse Volcanic Group (new name). It provides an exposure of upper Cenozoic geology in close proximity to the spreading centre. The Okiduse Volcanic Group is host to most of the island's historical gold and silver production and recently defined mineral resources totalling 1.75 Moz gold. This study uses facies analysis of pyroclastic deposits to develop a detailed geological map of the Okiduse Volcanic Group, with a revision and reinterpretation of the unit. Facies associations suggest that two major volcanic centres erupted synchronously during the middle Miocene (14–12 Ma), referred to as the Watou Mountain Eruptive Centre (new name) and the Uvarakoi Caldera (new name). The mafic–intermediate Watou Mountain Eruptive Centre formed during frequent small eruptions of widely varying style. Strombolian, subplinian, vulcanian and dome-related explosive eruptions occurred, alternating with extrusion of block and ash flow deposits and lava domes. Pyroclastic deposits were rapidly reworked from the steep cone, and were redeposited in a series of coalescing aprons surrounding the volcano. The felsic Uvarakoi Caldera formed during a series of violent explosive eruptions by rapid removal of magma from the underlying chamber, followed by collapse. Plinian and possibly phreatoplinian eruptions, as a result of magma–water mixing in the surface environment, resulted in widely dispersed, highly fragmented tuff deposits. The caldera was modified by widespread erosion following eruptions, resulting in fluvial, laharic and slope-wash deposits. This study highlights lithological controls (porosity and permeability) by various units within the Okiduse Volcanic Group on ore deposition.     

Comment on ' ... Magmatic Conditions of the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado ' by Whitney & Stormer (1985)

Magmatic conditi ons inferred from the composition of the FishCanyon Tuff by Whitney &Stormer (J. Petrology, 26, 726–62(1985)) and Stormer & Whitney (Am. Miner. 70, 52–64(1985)) differ from studies of other ash-flow tuff and calderasystems, in that they consider the magma to have been unzonedand to have resided at {small tilde}9 kb pressure just priorto eruption. We find these conclusions unconvincing becauseof incomplete sampling of the Fish Canyon Tuff, alteration ofthe tuff after emplacement, and, in particular, serious limitationsof coupled Fe-Ti oxide and two-feldspar geothermobarometry.     

松辽盆地东缘下白垩统营城组二段火山碎屑岩的发育特征

通过精细的野外剖面测量,发现松辽盆地东缘营城组二段中发育熔岩、凝灰岩和凝灰质砂岩。凝灰岩包括熔结凝灰岩、岩屑晶屑凝灰岩、灰球泥粒凝灰岩和角砾凝灰岩。这表明作为火山活动间歇期的营城组二段沉积期依然存在一定规模的火山活动,其沉积作用具有独特的火山和沉积双重控制的特点,区别于正常的沉积作用。营城组二段是一套介于火山岩和陆源碎屑岩之间的过渡岩性,物源既有来自同期火山喷发,也有来自营城组一段和营城组下段以及更老的地层的风化剥蚀。由于存在火山物质和陆源剥蚀物质的双重物源及存在火山物质堆积和沉积作用的双重机理,这套岩石在类型上具有特殊性。存在特殊的火山-沉积作用类型,主要为冲积平原上热碎屑流河道沉积、冲积平原泛滥盆地上热基浪沉积、冲积平原泛滥盆地上空落火山灰云沉积。     

论火山碎屑流和涌浪堆积的特征和成因模式：以浙东南沿海地区为例

本文根据野外地质特征、岩相学特征以及岩石化学和地球化学特征,将火山碎屑流和涌浪堆积归纳为三种不同的岩相组分,即次火山型熔结凝灰岩组合、涌浪型熔结凝灰岩组合和灰流型熔结凝灰岩组合。对这三种不同的岩相组合,特别是其中的熔结凝灰岩的各类特征分别进行了详细论述和相互对比,并在此基础上提出了一个火山碎屑流和涌浪堆积的综合成因模式。     

Mineralogy, Petrology, and Magmatic Conditions from the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado

The Fish Canyon Tuff is one of the largest currently recognizedash-flow tuffs (> 3000 km³). It is a crystal-rich quartzlatite containing about 40 per cent phenocrysts of plagioclase,sanidine, biotite, hornblende, quartz, magnetite, sphene, andilmenite. Pyrrhotite occurs as inclusions in magnetite, sphene,and hornblende. The consistency of mineralogy and whole rockchemistry confirms that the Fish Canyon tuff is remarkably homogeneous.Most chemical variations can be accounted for by phenocryst-matrixfractionation, probably due to glass winnowing during eruptionand emplacement. The composition of the parent magma, correctedfor such winnowing, is very similar to that of calc-alkalinebatholiths such as the Boulder and the Sierra Nevada batholiths. Fe-Ti oxide geothermometers indicate temperatures of 800 ? 30?C for most of the outflow tuff. No evidence for a regular thermalgradient in the magma chamber could be detected. Two feldsparand Fe-Ti oxide equilibria indicate that the magma developedat depths of 25 to 30 km (about 9 kb pressure), and was eruptedwithout time for phenocryst re-equilibration. The reconstructedcomposition of the liquid in equilibrium with the phenocrystsalso suggests a deep source for this ash flow. A late, upperpackage of flow units have mineralogical characteristics whichmay reflect partial re-equilibration in a shallower environment. Oxygen fugacities are moderately high (log fO₂ = — 11.5?0.3) but are similar to those obtained from other continentalcalc-alkaline ash-flow tuffs. The water fugacity is limitedby calculations using biotite equilibria and experimental workrelating to the stability of the phenocryst assemblage. Bestestimates are that water fugacity was 2000 ? 1000 bars. Theactivities of sulphurous gases are estimated at fSO₂ = 2 to4 bars, fso₂ = 150 to 200 bars, fH₂S = 70 to 80 bars. The Fish Canyon Tuff therefore came from a deep, homogeneous,granitic magma body of batholithic proportions. Calculationsof its probable viscosity, density, and size indicate that thesystem should convect with any reasonable thermal gradient.Convective mixing may account for the homogeneity of the parentmagma body.     

Caldera-related volcanic rocks in the Shammar Group, northern Arabian Shield

Volcanic formations of the ca 630-620 Ma old Shammar Group in the Tuluhah area in the northern Arabian Shield occupy an oval area some 8×12 km. They overlie sedimentary rift-fill of the Kuara Formation and are interpreted as related to the formation of a caldera, here named the Awad Caldera. The earliest of the volcanic formations, the Dabsah Tuff, is more than 450 m thick in the south and wedges out in the north. It is composed of silicic, medial to proximal pyroclastic flow rocks that record an eruption during which an initial caldera is interpreted to have formed by probably trapdoor-style collapse. The Nijab Basalt, more than 200 m thick and present as flows overlying the Kuara Formation to the north of the caldera, is presumed to have originated outside the study area during an interval between periods of silicic volcanic activity, and to have flowed onto the Dabsah Tuff in the first-stage caldera. The succeeding Mindassa Megabreccia contains large rafts of the older Shammar rocks, mainly Nijab Basalt, in a tuff matrix, and is regarded as probably a caldera collapse and fallback megabreccia formed during a silicic eruption that led to the second stage of caldera development. The megabreccia is overlain by the post-collapse Sutayih Tuff, more than 450 m thick, composed of proximal pyroclastic flow units.     

松辽盆地庆深气田深层火山岩储层储集性控制因素研究

松辽盆地庆深气田深层火山岩主要岩石类型为流纹岩、熔结凝灰岩、凝灰岩、火山集块岩、安山岩、英安岩和粗面岩,以中酸性火山岩为主,其主要储气岩为流纹岩和凝灰岩,与国内其他含油气盆地的火山岩储层明显不同。其原生孔隙受岩性、岩相控制,后期的成岩改造作用,包括构造活动、风化淋滤、溶蚀作用、脱玻化作用决定了火山岩次生孔隙发育程度,尤其是溶蚀作用,它是决定原生孔隙不发育火山岩能否成为有效储层的关键因素。不同地区火山岩储层的发育状况与火山岩所处的位置有关。火山机构多沿断裂带分布,火山口附近多种岩相叠合区和近火山口爆发相和喷溢相叠合区的储层物性较好,是天然气勘探的有利目标。     

安徽巢县南陵湖组火山碎屑流沉积物的发现

在安徽巢县下三叠统南陵湖组上部，发育着一套火山碎屑沉积物，主要由英安质角砾岩、英安质晶屑、玻屑凝灰角砾岩和英安质玻屑、晶屑凝灰岩等组成。它厚１．８ｍ，层序清楚，分为２个布玛旋回。研究表明，它属于海底喷发、水流搬运的火山碎清流沉积（火山浊流沉积），并且可能是印支早期华北板块与扬子板块相互碰撞的一种反映。     

鄂尔多斯盆地延长组凝灰岩夹层展布特征及其地质意义

鄂尔多斯盆地延长组凝灰岩夹层分布广泛,通过众多凝灰岩夹层的实际岩心观察,归纳总结出凝灰岩夹层的常规测井曲线表现为具有相对低电位、高自然伽马、高电阻率、高声波时差值的特征,并且高值常略低于致密泥岩段;单井分析凝灰岩夹层与上、下部岩石整合接触,较少发育同沉积构造,以典型的火山尘灰大气降落沉积产物为主,但也偶见凝灰岩与砂泥岩一起发育扰动构造,明显经受了水动力改造;连井剖面分析说明延长组长1—长9皆有凝灰岩分布,横向上可对比性凝灰岩主要有4套,分别为长91、长73底、长72、长71期,以盆地西南部长73期最发育;凝灰岩平面展布特征整体呈北西向展布,由南西—北东向凝灰岩厚度逐渐变薄,并且与烃源岩、放射性异常展布形态一致,因此认为凝灰岩的沉积作用所引起的元素迁移和古环境变化,可能对延长组优质烃源岩的发育具有重要意义。     

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.