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21.
The Tiribí Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibás Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribí Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tárcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10−3) covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 (15 km3 DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new 40Ar/39Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribí Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments. 相似文献
22.
Resat Ulusay Candan Gokceoglu Tamer Topal Harun Sonmez Ergün Tuncay Zeynal Abiddin Erguler Ozgu Kasmer 《Environmental Geology》2006,50(4):473-494
The Cappadocia Region of Central Anatolia having a very distinct culture is one of the attractive touristic sites of Turkey due to its spectacular and unique landforms and historical heritages. In this region, the structures carved into thick to massive tuffs survived and kept their original integrity for a number of centuries. Environmental and anthropological factors at the Cappadocia Region have been the main reasons for extensive subsurface and multi-purpose use in the past and present. In addition, thermal insulation properties of the tuffs make these rocks suitable for use in underground openings. The Kayakapi Neighborhood, located in the town of Urgüp, is one of the famous historical sites. This site is situated within the “Göreme National Park and the Rock Sites of Cappadocia” which was inscribed on the World Heritage List in 1985. In order to develop the tourism potential of this abandoned site while preserving its cultural and natural values, a natural environmental conservation and revitalization project was initiated. As an integral part of this project, in this study, environmental and engineering geological problems, including rock fall potential and stability of about 1,200 rock-hewn structures, were investigated and an inventory was prepared for the possible re-use of the underground openings and other structures at the site, and remedial measures were recommended. The assessments based on observations and experimental studies indicated that the tuffs do not show significant changes both perpendicular and parallel to layering, and discontinuities and rock weathering seem to be more important factors controlling the stability of rock-hewn structures. The major stability problems threatening the re-use of the openings are structurally-controlled block instabilities, overbreaks, and erosion and shearing of the pillars made of tuff. On the other hand, the area at the entrance of the site requires some protection measures such as the construction of a retaining wall, systematic bolting of rock slabs and removal of some blocks having rock fall potential. The inventory for the openings suggests that there are a number of openings that can be re-used after necessary remedial measures. 相似文献
23.
Tereza Cristina Junqueira-Brod Jos Affonso Brod Jos Carlos Gaspar Hardy Jost 《Lithos》2004,76(1-4):261-282
This paper describes the internal organisation of two diatremes (Águas Emendadas and Neuzinha) and one small breccia-filled conduit (Tigre) in the central portion of the Late Cretaceous Goiás Alkaline Province (GAP), central Brazil, and explores the criteria for facies recognition. The GAP kamafugitic diatremes are emplaced into Carboniferous sandstones of the Aquidauana Formation, at the northern margin of the Paraná Basin. They are usually elliptical structures, not longer than 900 m, filled with breccia and partially covered by thin kamafugitic to basanitic lava flows. The breccias are dominated by juvenile pyroclasts, with subordinate amounts of cognate fragments and xenoliths. In addition to variations in ash and lapilli proportions, juvenile fragment types may be used to discriminate genetic processes and the corresponding pyroclastic deposits.
An extensive field, textural and compositional dataset was analysed by multivariate statistical techniques. Combined with field observations, this allowed us to define a set of facies for kamafugitic diatremes, and, more importantly, to understand the internal structure of the studied bodies and to cross-correlate them. Seven distinct facies were recognised. The Fluidised Conduit Facies (FCF) represents high-energy, strongly fluidised but only moderately fragmented systems. It occurs in a confined environment, and is typical of deeper parts of the conduit, before the actual diatreme level is reached by the ascending fluidised magma. Large amounts of spinning droplets are formed within this region. The Fluidised Conduit–Diatreme Facies (FCDF) is characteristic of intermediate depths in the conduit, where highly fluidised and highly fragmented systems produce large amounts of ash. Spinning droplets decrease in abundance, ordinary juvenile fragments become very common, and xenoliths from the country rock in the immediate vicinity of the diatreme are present. The Fluidised Fragmented Facies (FFF) and the Magmatic Fluidised Facies (MFF) produce very heterogeneous deposits that dominate the shallower part of the system, making up most of the diatreme-filling materials. The Fluidised Fragmented Facies can be distinguished by much higher degrees of fluidisation, fragmentation and system energy. It occupies the internal part of the diatreme and is characterised by the common presence of tuff pockets, tuff fragments, and accretionary and armoured lapilli. The Magmatic Fluidised Facies typically occupies the outer portion of the diatreme and can be distinguished from the Fluidised Fragmented Facies by the dominance of lapilli over ash and by the presence of abundant wrapped fragments. The Magmatic Facies (MF) and the Coherent Magmatic Facies (CMF) are volumetrically subordinate and represent late stages, when less fluidised and less fragmented material, or even coherent magma erupts relatively passively, in the aftermath of the main explosive stage that generated the diatreme. The Border Facies is defined by the increased abundance of material from the immediate country rock. At Águas Emendadas and Neuzinha this facies is marked by the presence of fragments of peperite-like rock, formed by the interaction of the fluidised magma with friable sandstone. 相似文献
24.
Abstract. A series of mudstones corresponding to N.8 to N.17 of Blow from the Uyashinai Mudstone Member, the Onnagawa Formation and the Funakawa Formation in the Taiheizan area of the central Green Tuff region, northeast Japan, was examined by chemical analysis. According to conventional chemical systematics, the mudstones are analyzed in terms of detrital, biogenic-A (siliceous), biogenic-B (calcareous) and hydrogenous components. The relative contribution of the biogenic-A (siliceous) component increases in upward succession in the Uyashinai and Onnagawa mudstones, whereas the contribution of the biogenic-B (calcareous) component is restricted to the lower Uyashinai mudstones. The contribution of hydrogenous components Zn, Pb, Cu and Ba tends to increase during the Nishikurosawa stage, and decreases near the Nishikurosawa/Onnagawa boundary (Pb and Ba) or in the Onnagawa stage (Zn and Cu). The observed enrichment of heavy metals in the Uyashinai mudstones is attributed to the onset of vigorous upwelling of deep water with higher nutrient and metal content associated with a contemporaneous change in global deep-water circulation. 相似文献
25.
26.
A new micro-fluidic method, which is known as the Micro-Reactor Simulated-Channel (MRSC) method, has been employed to rapidly determine the effective diffusion coefficients of lithium in three important representative low permeability lithologies including: Melechov granite (Czech Republic), Borrowdale tuff, and Land's End Cornish granite (both UK). The concept of MRSC is similar to the micro chemical reactor which enables fast measurements to be done on a small intact sample. The effective diffusion coefficients were measured and comparisons between the MRSC results and conventional column methods showed excellent agreement. Our measured effective diffusion coefficient for Melechov granite is 1.7 × 10−12 m2/s, directly comparable to previous conventional measurements. However the measurement time of the MRSC method is at least one order of magnitude faster than the conventional method and only requires small reaction volumes (as small as 10 ml). In addition, by exploiting the advantages of the MRSC method, the effects of velocity and concentration on diffusive transport for the two different UK rock types have also been investigated. Depending on flow rate and inlet tracer concentration, the effective diffusion coefficient for lithium in the Cornish granite ranges between 0.9 and 1.5 × 10−11 m2/s while that measured for the Borrowdale tuff varies between 1.2 and 1.6 × 10−11 m2/s. 相似文献
27.
Age and significance of the Platypus Tuff Bed,a regional reference horizon in the Upper Permian Moranbah Coal Measures,north Bowen Basin 总被引：1，自引：1，他引：0
P. Michaelsen R. A. Henderson P. J. Crosdale C. M. Fanning 《Australian Journal of Earth Sciences》2013,60(2):183-192
The Platypus Tuff Bed in the Permian Moranbah Coal Measures provides a basin‐wide marker horizon traceable for over 300 km along strike. The bed is a tephra event unit, the product of a large‐scale volcanic eruptive episode involving a pyroclastic volume > 10 km3. The relatively even thickness (～1–1.5 m) of the tuff across the entire northern Bowen Basin (～10 000 km2) implies a distant source. The tuff is ash‐rich and its original geochemistry has been compromised by diagenetic alteration. Crystal content (10–15%) is dominated by quartz, suggesting a rhyolitic association. SHRIMP U–Pb analysis of zircons indicates an age of 258.9 ± 2.7 Ma for the Platypus Tuff Bed, confirming the Late Permian age that has generally been assigned to the Blackwater Group. The age framework now apparent for the coal‐bearing Blackwater Group suggests an average depositional rate ranging from ～133 m/106 years for its eastern depocentre in the northern Bowen Basin to ～70 m/106 years in more marginal settings to the west. 相似文献
28.
Xin Bushe Yang Hua Wang Duoyun Fu Jinhua Yao Jingli Luo Anxiang Zhang Yu 《地球科学进展》2013,28(9):1043-1048
Using zircon SHRIMP U-Pb dating method in this paper, we conduct the SHRIMP U Pb ages of tuff at the bottom of the Nanying’er Group, Wangjiashan area, Jingyuan County, Gansu Province and acquire a set of weighted average age of (244.9±2.8)Ma. Combined with the feature of Cathodoluminescence images and Th/U specific value of zircon, we believe that the sedimentary age of Wangjiashan tuff is (244.9±2.8)Ma. High precision and high confidence isotopic age data fully demonstrate that the bottom of Nanying’er Group exists with the early of middle Triassic strata. The author proposes that the Wangjiashan tuff and its hereinafter strata should be incorporated in Dingjiayao Formation. Meanwhile, it can be confirmed that the Wangjiashan Tuff and the Baojishan tuff are the deposition product of the same volcanic eruption. It can be used as a marker bed of the regional stratigraphic division and correlation. The above research results are beneficial for the upper and middle Triassic stratigraphic division and correlation of Hexi corridor region, and its stratigraphic significance for the final establishment of Triassic chronostratigraphic division and correlation framework of China’s northern continental strata. 相似文献
29.
火山喷发的多期性是导致凝灰岩潜山纵向多期风化壳(带)发育的重要原因。从地球化学、测井响应特征和岩石学 证实了二连盆地凝灰岩潜山多期风化壳(带)的存在及其特征:(1)哈31井凝灰岩地层在1062~1080 m和1165~1179 m两 个井段表现为 K,Na,Ca 等元素淋滤,CIA 指数较大,而 Al,Fe,Mn,P 等元素相对富集,S/A 指数变小;(2)这两个层段 测井响应表现为风化壳的特征,而且其测井曲线在纵向上具旋回特征并有连井上的可对比性;(3)不同喷发期次形成的凝 灰岩结构、组分以及粒度不同。凝灰岩潜山多期风化壳(带)的存在对潜山油气藏的形成具有重要作用,成藏效应主要表 现在储集层垂向分带性、风化壳油藏(晚期风化壳中)和内幕油藏(早期风化壳中)纵向叠置和复式油气聚集成藏模式等 3 个方面。文中对多期凝灰岩古风化壳的认识扩展了火山岩潜山的勘探深度,为火山岩潜山内幕油气勘探提供了理论依据。 相似文献
30.
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. 相似文献