Clastic Volcanic Rocks Research Articles

Formation and Evolution of Supradetachment Basins During Continental Extension: Insights From the Fuxin Basin in NE China

Supradetachment basins can record the stratigraphy and development of metamorphic core complexes (MCCs). The Fuxin supradetachment basin, which lies immediately to the west of the Yiwulvshan MCC in NE China, provides an excellent opportunity to establish the relationship between supradetachment basins and MCCs. In this study, we conducted field investigations, sedimentary facies analysis, and seismic profile interpretation to decipher the sedimentary processes and structural evolution of the basin and link them to the development of the Yiwulvshan MCC. The Fuxin Basin is filled predominantly by syn-rift volcanic sedimentary and post-rift clastic rocks, which developed in four stages, namely, proto-rift, fault subsidence, transition, and compression. The Fuxin Basin developed simultaneously with the two stages (earlier faulting-dominated and later exhumation stages) of development of the MCC. Erosion of the core of the Yiwulvshan MCC provided clastic material to the Fuxin Basin. Based on multi-stage reconstruction of the formation and evolution of the Fuxin Basin and Yiwulvshan MCC, we propose that rollback of the Paleo-Pacific Plate and retreat of the subduction trench provided the geodynamic setting for the crustal extension that formed the basin and MCC.

Structure Boundary Separating using Gravity Data for Central and Southern Parts of the East Java Basin Beneath the Quaternary Volcanic Deposits

Stratigraphic interpretation shows that the relationship between Cenozoic volcanic rocks deposited in the Southern Mountains and marine clastic volcanic rocks in the Kendeng Basin possibly forms a stratigraphic transition bordered by the major fault that is yet to be defined. The thick Quaternary volcanic deposits obstruct field observation on the surface. Miocene outcrops in limited presence are found on the volcanic slope, positioned relatively more to the north than the same outcrop. This feature indicates major thrusting in southern Java that moved it to the north. The gravity data analysis was used to obtain the field observation and identify the subsurface geology. Total Horizontal Derivative analysis of the gravity data showed a major fault boundary with a relative west-east direction separating the high Bouguer anomaly area in the south from a large negative area of about -50 mGal in the north. Second Vertical Derivative analysis reconstructed the fault slope direction and showed a south-facing slope with a large angle. Based on the combination of the surface data and the subsurface analysis, the boundary is a major fault with a west-east direction on the south side of the Quaternary volcanic range and a south-facing slope. It is interpreted as the result of the compressive tectonics that formed the fold-thrust belt system.

Study on lithology identification method of carboniferous in the west wing chepaizi salient in western margin of junggar basin

Reservoir characteristics are important indexes for oil and gas exploration and development. Therefore, the identification of igneous lithology of target formation is carried out by using intersection diagram and mechanical parameter method according to the Carboniferous logging data of the west wing chepaizi salient in western margin of junggar basin. The results show that the study area is dominated by volcanic clastic rocks and sedimentary rocks. The cross-section and mechanical parameters method can be used to identify the lithology and obtain the corresponding physical parameters. Finally, this work has guiding significance for the exploration and development of Carboniferous reservoirs in the west wing chepaizi salient in western margin of junggar basin.

Characteristics of Carboniferous volcanic reservoirs in the Chun-feng Oilfield of the Junggar basin, China

As a major petroliferous layer, Carboniferous volcanic reservoirs have developed widely in the Chun-feng Oilfield in the Junggar basin. Volcanic reservoir rocks have been the principal target for exploration and development in the study area, having relatively high porosity and permeability which is favorable for hydrocarbon accumulation. According to observation of thin sections of core samples and logging lithology, the volcanic reservoir rocks of the Chun-feng Oilfield can be divided into 5 types according to their attitudes and composition. These are volcanic fulgurite rocks, clastic volcanic rocks, volcanic clastic rocks, sedimentary volcanic rocks, and volcanic sedimentary rocks. Lithofacies has also been studied according to their lithological distribution and associations. According to observation of thin sections and SEM images of core samples, the pore system of the volcanic reservoir is mainly composed of primary pores, secondary pores, and fractures. Statistical analysis of measured data shows that volcanic reservoirs have poor reservoir properties, with generally low porosity and permeability. Reformed by late diagenesis and tectogenesis, a weathered crust structure has developed in the Carboniferous volcanic reservoirs in the study area, which is composed of five zones. These are soil zone, hydrolytic zone, leaching zone, disintegrating zone, and original zone. Of these, the leaching zone has developed the best reservoirs, with relatively high porosity, followed by the disintegration zone. With low porosity, the reservoirs of the soil and hydrolytic zones act generally as cover layers. There are three main factors which determine the physical properties of volcanic reservoirs. The principal factor is that lithofacies control the primary pore types, which have an influence on subsequent reformation. Faults tend to promote the development of secondary porosity, and paleogeography restricts the distribution of favorable volcanic reservoirs in the area.

Study on the relationship between lithology and reservoir characteristics of Yingcheng Formation in the southern Songliao Basin

In order to deeply understand the relationship between the microstructural characteristics of volcanic reservoirs and the lithology of volcanic rocks, it provides important theoretical significance for the effective development of volcanic reservoir oil and gas fields. The microscopic characteristics of the volcanic reservoirs of the Yingcheng Formation in the Songliao Basin were analyzed by means of rock section determination, pore structure parameters analysis and casting thin section analysis. The results show that the target layer has low porosity and permeability, and the volcanic pore structure of different lithologies is different. Volcanic lava generally develops original gas pores, phenocryst dissolved pores, matrix dissolved pores, and tectoclase, while volcanic clastic rocks generally have developmental dissolution pores as the main feature. The micro-heterogeneity of volcanic reservoirs is generally strong, and the development of tectonic fractures is an important factor in changing the reservoir performance of volcanic rocks.

Palaeomagnetism of Late Triassic volcanic rocks from the western margin of Khorat Basin, Thailand and its implication for ambiguous inclination shallowing in Mesozoic sediments of Indochina

SUMMARY Palaeomagnetic constraints are essential factors in the reconstruction of the Mesozoic convergence of Eastern Asia blocks. As one of the key blocks, Indochina was constrained only by sedimentary-rocks-derived palaeomagnetic data. To evaluate whether the palaeomagnetic data used to restore the Late Triassic position of Indochina suffered inclination shallowing effects, we conducted a palaeomagnetic and geochronologic study on a coeval volcanic clastic rocks sequence in the western margin of the Khorat Basin, Thailand. The U-Pb SIMS dating on zircons indicates the age of the sampling section is between 205.1 ± 1.5 and 204.7 ± 1.4 Ma. Site mean directions are Dg/Ig = 217.2°/−39.4° (κg = 45.1, α95g = 10.1°) before and Ds/Is = 209.2°/−44.5° (κs = 43.8, α95s = 10.2°) after tilt correction. The new data set indicates a positive reversal test result at ‘Category C’ level. The characteristic remanent magnetization recorded by the coexistent magnetite and hematite is interpreted to be primary remanence acquired during the initial cooling of the volcanic clastic rocks. The consistence of the corresponding palaeolatitudes derived from the volcanic clastic rocks and the former reported sedimentary rocks suggests that there is probably no significant inclination shallowing bias in the sedimentary-rocks-derived palaeomagnetic data. Therefore, the estimates of the Late Triassic position of Indochina are confirmed to be reliable. The Indochina Block had collided to the southern margin of Eurasia by the Late Triassic and played an important role in the Mesozoic convergence of the Eastern Asia blocks.

Distribution and gas-bearing properties of Permian igneous rocks in Sichuan Basin, SW China

Abstract Based on the analysis of outcrop, seismic, logging and drilling data, combined with exploration practice, the characteristics, distribution, reservoir performance and gas-bearing properties of Permian igneous rocks in Sichuan Basin are studied. The study shows that central volcanic eruptive facies are developed in Sichuan Basin, and their lithological assemblages and distribution characteristics show obvious differences. The igneous rocks are mainly distributed in three regions: the southwestern part of the basin has dominantly large- scale overflow facies basalts; the central and western part of the basin, Jianyang-Santai area, develop intrusive rocks, volcanic lavas (basalts) and pyroclastic rocks; and the eastern part of Sichuan, Dazhou-Liangping area, only develop diabase and basalts. Five aspects of understandings are achieved: (1) The Upper Permian igneous rocks can be divided into intrusive rocks and extrusive rocks, with the extrusive rocks as the main body. The chemical compositions of the extrusive rocks are characterized by both alkaline basalt and tholeiitic basalt, and belong to the subalkaline type of transitional basalt magma eruption. (2) There are obvious rhythmic structures vertically among overflow facies basalt, and the single rhythmic layer consists of, from bottom up, pyroclastic rocks (undeveloped), gray and dark gray porphyritic basalts (unstable), dark gray and purple microcrystalline-cryptocrystalline basalts, dark greyish green porous and amygdaloid basalts; the central volcanic eruption shows the rhythm and the vertical sequence of volcanic clastic rocks (agglomerates and breccias), volcanic lava, tuffaceous lava from bottom to top. (3) The pore types of basalt and pyroclastic rocks are diverse, mainly dissolution pore and de-vitrification micropore, but their physical properties are different. Basalt is characterized by ultra-low pore permeability, small reservoir thickness, and reservoirs are distributed in the upper and middle parts of the cycle, with poor lateral comparability. Volcanic clastic rocks are medium to high porous reservoirs (Well YT1: porosity: 8.66%−16.48%, average 13.76%) with large thickness and good reservoir quality. (4) Natural gas in basalts in southwestern basin mainly comes from Middle Permian, and natural gas in volcanic clastic rocks in central and western basin comes from Cambrian Qiongzhusi Formation. (5) Analysis of igneous reservoir-forming conditions in different areas shows that there are relatively insufficient gas sources and great differences in preservation conditions in southwestern basin. Reservoirs are poorly developed and gas-bearing is complex. The Jianyang-Santai area in the central and western part of Sichuan Basin has abundant hydrocarbon sources, developed reservoir, favorable preservation conditions and favorable gas geological conditions, and it is a favorable area for gas exploration.

Geology at Beruang Kanan, Central Kalimantan, Indonesia

The samples are collected during our field investigations both from out crops and from core drillings. They will be selected from rock units within the Beruang area, particularly from the pre_and all syn mineralization rocks. The Beruang area is situated within a mid-tertiary age magmatic arc that host a number of epithermal gold deposits (e.g., Kelian, Indon, Muro) and significant prospects such as Muyup, Masupa ria, Gunung Mas dan Mirah. Mineralization in the Beruang is associated with a number of intrusions that have been emplaced at shallow crustal levels at the junction between Mesozoic metamorphic rocks to the south and accreted Lower Tertiary sediments to the north. These intrusions are interpreted to be part of the Oligocene central Kalimantan Arc. The Cretaceous rock that found in Beruang Kanan is older intrusions wich associated with volcanic and volcanic clastic rocks. The Beruang Kanan mineralization is hosted in a sequence of dacite tuff of probable Oligocene age that overly lower Tertiary volcanicclastic siltstone and sandstones in the eastern prospect area. Premiered andesite, dacite to basalt-gabbro dykes are intruded into the tuffs and sediments zinc-arsenic- antimony in soils is hosted in intensely chlorite-pyrite altered tuff. In the West Zone were scattered the zones of phyllic alteration. Subsequent shearing of quatz veins was accompanied by wall rock alteration and vein deposition of the quatz veins was accompanied by wall rock alteration and vein deposition of mineral assemblages that are zone temporally from early advanced argilic, through intermediete ohyllic to late stage argilic and sub-propylitic. The alteration asseblages and associated mineralization are also spatially zoned

Decoding magma storage and pre-eruptive processes in the plumbing system beneath early Carboniferous arc volcanoes of southwestern Tianshan, Northwest China

The volcanic rocks, consisting of basalt, basaltic andesite, andesite, and tuff with volcanic clastic sedimentary rocks in the Dahalajunshan region represent an early Carboniferous continental arc formed during subduction of the Paleo-southern Tianshan ocean. The basalt is characterized by a phenocryst assemblage of clinopyroxene and plagioclase, while basaltic andesite and andesite contain variable amounts of clinopyroxene, orthopyroxene, plagioclase, and FeTi oxides phenocrysts. Clinopyroxene, the most common phenocryst in the studied rocks, has a large variation of compositions and displays multiple zoning patterns, suggesting complex magmatic processes in different magma storages beneath the Dahalajunshan region. A deep reservoir at depths of >28 km is suggested by clinopyroxene with Mg# values of 84–88 and an intermediate reservoir is suggested by clinopyroxene with Mg# values of 75–79. The shallow reservoir is crystal-rich as indicated by the presence of crystal clots consisting of Fe-rich clinopyroxene (Mg# < 75), orthopyroxene (Mg# = 65–76), plagioclase, and FeTi oxides. Petrological observations, combined with the whole-rock geochemical and SrNd isotopic data, suggest that the studied basalt mainly formed by fractional crystallization in both deep and intermediate magma reservoirs, while basaltic andesite and andesite might be produced via open-system processes of crystal fractionation, recycling of early-formed crystals, interaction of colder magmas with hotter, recharging magmas, and crustal contamination in the shallow reservoir. This study suggests the importance of textures and compositions of erupted minerals to decode the pre-eruptive histories of magmatic plumbing systems beneath fossil arc volcanoes.

Characteristics of Ductile Shear Zone and Its Ore-controlling Effect in Dongtian Polymetallic Mining Area

The genesis of the Red Pacific polymetallic deposit in Yanbian area has long been regarded as a volcanic hydrothermal deposit or a volcanic jet deposit. In the field investigation and comprehensive study, authors found that the so-called 'metamorphic volcanic rocks and volcanic clastic rocks' is actually a set of highly deformed terrigenous clastic rocks. The main rock types are metamorphic conglomerate, sericite schist, sericite to form a structural deformation belt having a width of 50-400 m. The deformed conglomerate rock is the conglomerate of the valley flag, and the latter two are the former siltstone, the silty mudstone and the tectonic deformation zone are basically the same as those of the strata and ore body. The red and Taiping polymetallic ore bodies are produced in the Miaopun Formation in the deformation zone. The ore and the surrounding rock also have certain deformation structures, such as pleated structure, shear page and so on. Therefore, the cause of this deposit is ductile shear zone type, layer structure ore control.

Joint Inversion of the 3D P Wave Velocity Structure of the Crust and Upper Mantle under the Southeastern Margin of the Tibetan Plateau Using Regional Earthquake and Teleseismic Data

AbstractThe special seismic tectonic environment and frequent seismicity in the southeastern margin of the Qinghai–Tibet Plateau show that this area is an ideal location to study the present tectonic movement and background of strong earthquakes in mainland China and to predict future strong earthquake risk zones. Studies of the structural environment and physical characteristics of the deep structure in this area are helpful to explore deep dynamic effects and deformation field characteristics, to strengthen our understanding of the roles of anisotropy and tectonic deformation and to study the deep tectonic background of the seismic origin of the block's interior. In this paper, the three‐dimensional (3D) P‐wave velocity structure of the crust and upper mantle under the southeastern margin of the Qinghai–Tibet Plateau is obtained via observational data from 224 permanent seismic stations in the regional digital seismic network of Yunnan and Sichuan Provinces and from 356 mobile China seismic arrays in the southern section of the north–south seismic belt using a joint inversion method of the regional earthquake and teleseismic data. The results indicate that the spatial distribution of the P‐wave velocity anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and lithology. Baoxing and Kangding, with their basic volcanic rocks and volcanic clastic rocks, present obvious high‐velocity anomalies. The Chengdu Basin shows low‐velocity anomalies associated with the Quaternary sediments. The Xichang Mesozoic Basin and the Butuo Basin are characterised by low‐velocity anomalies related to very thick sedimentary layers. The upper and middle crust beneath the Chuan–Dian and Songpan–Ganzi Blocks has apparent lateral heterogeneities, including low‐velocity zones of different sizes. There is a large range of low‐velocity layers in the Songpan–Ganzi Block and the sub–block northwest of Sichuan Province, showing that the middle and lower crust is relatively weak. The Sichuan Basin, which is located in the western margin of the Yangtze platform, shows high‐velocity characteristics. The results also reveal that there are continuous low‐velocity layer distributions in the middle and lower crust of the Daliangshan Block and that the distribution direction of the low‐velocity anomaly is nearly SN, which is consistent with the trend of the Daliangshan fault. The existence of the low‐velocity layer in the crust also provides a deep source for the deep dynamic deformation and seismic activity of the Daliangshan Block and its boundary faults. The results of the 3D P‐wave velocity structure show that an anomalous distribution of high‐density, strong‐magnetic and high‐wave velocity exists inside the crust in the Panxi region. This is likely related to late Paleozoic mantle plume activity that led to a large number of mafic and ultra‐mafic intrusions into the crust. In the crustal doming process, the massive intrusion of mantle‐derived material enhanced the mechanical strength of the crustal medium. The P‐wave velocity structure also revealed that the upper mantle contains a low‐velocity layer at a depth of 80–120 km in the Panxi region. The existence of deep faults in the Panxi region, which provide conditions for transporting mantle thermal material into the crust, is the deep tectonic background for the area's strong earthquake activity.

Timing and implications for the late Mesozoic geodynamic settings of eastern North China Craton: Evidences from K–Ar dating age and sedimentary–structural characteristics records of Lingshan Island, Shandong Province

The Lingshan Island in Shandong Province in the eastern North China Craton, well known for the Late Mesozoic multi-scale slide-slump structures is related to paleo-earthquake. Terrigenous clastic rocks, volcanic clastic rocks and volcanic lavas are extensively exposed in the Lingshan Island and its adjacent regions of the Shandong Province, which led to fierce debates on their ages, sedimentary characteristics and tectono-sedimentary evolution. In this contribution, we present the characteristics of the Late Mesozoic stratigraphy in the Lingshan Island. Whole-rock K–Ar dating of dyke at Beilaishi and rhyolites at Laohuzui of the Lingshan Island yielded ages of 159 Ma and \(106\hbox {–}92\hbox { Ma}\) which coincides with the Laiyang Period rifting and the Qingshan Period rifting in the Jiaolai Basin, respectively. On the basis of the analysis to the Late Mesozoic sedimentary environment of ‘flysch’ and ‘molasse’-like formations as well as tectonic stress fields reconstruction, four episodes of the tectono-sedimentary evolution were established in the Lingshan Island and its adjacent regions in the eastern North China Craton. They consist of two episodes of extensional events for the syn-rift, and two episodes of compression events for the inversion of the post-rift. The entire episodes can be summarized as follows: (1) the first syn-rift NW–SE extension in Laiyang Period can be identified by the ‘flysch’ formation (Unit 1) and by emplacement of the NE-trending dyke in the Laiyang Group. This syn-rift episode can be related to the NW–SE post-orogenic extension resulted from the gravity collapse of the thickened lithosphere along the Sulu Orogen. (2) The first post-rift NW–SE inversion, which was caused by the NW-directed subduction of Izanaqi Plate, can be well documented by the ‘X’ type conjugate joints as well as slide slump folds in Unit 1. (3) The second syn-rift NW–SE extension in Qingshan Period is characterized by rhyolite rocks (Unit 2). This syn-rift episode can be considered to be associated with lithospheric delamination of the thickened lithosphere in the eastern North China Craton. And finally, (4) the second post-rift NW–SE inversion which resulted from the subduction of the Pacific Plate under the eastern North China Craton in the NW direction at the end of the Qingshan Period is recorded by ‘molasse’-like formation (Unit 3).

Architecture of volcanic sequence and its structural control of Yingcheng Formation in Songliao Basin

Yingcheng Formation is a set of volcanic strata composed of lava rocks, volcanic clastic rocks and sedimentary rocks, filled in some fault depressions in Songliao Basin, early Cretaceous. The study about litho-facies succession of Yingcheng Formation in the outcropped area of the southeast margin and in Xujiaweizi fault depression and its distribution based on analysis of seismic data, shows that the sequence of volcanic strata is quite different from the clastic sedimentary sequence. To study the architecture of volcanic sequence and its structural control of Yingcheng Formation in Songliao Basin, in this work, dividing of the volcanic sequence and dating of the sequence boundaries were finished firstly, then displacement and displacement rate of faults were calculated. The results show that, sample ages of top of the first member, the seconde member, and the third member are 127 Ma, 115 Ma, 110.7 Ma, respectively and sample age of the bottom of the third member is 114.7 Ma. The maximum displacement and displacement rate of the fault 1 are 3 km and 300 m/Ma, respectively, and those of the fault 2 are 3 km and 1000 m/Ma. Studies suggest that, the cooling unit of lava rock or pyroclastic rock is a basic genetic stratigraphic unit in volcanic sequence stratigraphy. Cooling units can construct a parasequence reflecting a volcanic eruption stage. A sequence was superimposed by some parasequences, responding to a volcanic active cycle. There are three types of volcanic sequences in Yingcheng Formation: type of explosion, type of effusion and type of mixed explosion-effusion. The surface of the volcanic sequence, an unconformity surface widely spread and traced in seismic profiles, is a base for analysis of volcanic sequence. The development of volcanic sequence was controlled by faulting, and the curves of fault displacement (rate) can reflect this control. The preservation of volcanic sequence was controlled by the type of volcanic structure and the regional subsidence, also different from that of the sedimentary. The type of volcanic structure of Xujiaweizi was a volcanic depression during the forming of Yingcheng Formation, and the breakdowns of volcanoes and structural subsidence were key factors in the volcanic sequences preservation.

Distribution of Carboniferous source rocks and petroleum systems in the Junggar Basin

With pre-Cambrian continental nuclei and Early Paleozoic accretionary folded belts as the collaged basement, the Junggar Basin underwent an evolution of intra-cratonic basin after the transitional period of Carboniferous to Permian. The Carboniferous Period was a critical phase for the cratonization of Junggar Terrane, with two continental passive margins developed on both the southern and northern margins, and with rifts and upswells in the basin interior. The rifts were full-filled with volcanic clastic rocks up to 2 000 to 5 000 m in thickness. Influenced by the marine transgression, the regional extended source-rocks of the Lower Carboniferous Dishuiquan Formation(C1d) and the Upper Carboniferous Batamayneishan Formation(C2b) occurred within or around the basin, with the former one as oil and gas source rocks, while the latter one as effective gas source rocks. Volcanic rocks in the Batamayneishan Formation are the main reservoirs. Two petroleum systems were formed: C1d-C (.) and C2b-C2b (!). The tectonic events in the Late Jurassic and Neogene gave rise to the formation, modification, and finalizing of the Carboniferous prospects. The difference between the evolutionary process of each structural unit resulted in the differentiation of the petroleum systems horizontally. A series of oil and gas fields have been discovered in Carboniferous, such as the Wucaiwan, Shixi, Kelameili, and Chepaizi oil and gas fields as well as the oil-bearing Block-2, 4, and 6 in the Karamay Oilfield. The exploration has proven that the Carboniferous has great potential for oil and gas discovery. 准噶尔盆地是在前寒武纪陆核和早古生代增生褶皱带的拼合基底之上经历石炭纪—二叠纪过渡阶段后形成的克拉通内坳陷盆地，石炭纪是准噶尔及邻区克拉通化过渡发展的关键时期，在准噶尔地块南、北缘发育了被动大陆边缘，地块内部断陷与凸起相间，断陷内沉积了厚度达2 000～5 000 m的火山碎屑岩系。受海侵范围控制，盆地内部及周缘发育区域展布的石炭系下统滴水泉组和上统巴塔玛依内山组两套有效烃源岩，前者可能是油、气烃源岩，后者是有效的气源岩，上石炭统巴塔玛依内山组火山岩是主要的储集体，由此形成了C1d—C(.)和C2b—C2b(!)两个含油气系统。中侏罗世晚期—晚侏罗世和新近纪以来的构造活动导致了石炭系圈闭的形成、改造与最终定型。不同构造单元后期演化历史的差异导致石炭系含油气系统在平面上产生分异。迄今在石炭系已发现了五彩湾、石西、克拉美丽、克拉玛依二/四/六井区、车排子等多个油气田，油气勘探实践表明准噶尔盆地石炭系具有较好的勘探前景。图8表3参41

Sedimentary geochemistry of chert from the Middle-Upper Ordovician in Shihuigou area, North Qilian orogenic belt and its tectonic implication

The North Qilian orogenic belt is an elongate tectonic unit that lies between the North China plate to the north and the Middle Qilian microplate to the south, and is formed by a collision of the two plates in the Caledonian. The Shihuigou Section from Yongdeng County, Gansu Province, is in the eastern sector of the North Qilian Mountains, spanning the Ordovician island-arc zones. The Zhongpu Group is distributed in the Shihuigou area and composed of medium-basic volcanic rocks and volcanic clastic rocks interspersed with cherts, limestones, slates, and metamorphic sandstones. The geochemistry of chert from the Zhongpu Group reveals that all cherts coexisting with island-arc volcanic rocks formed in a continental margin basin environment. Research results of the rare earth elements reveal that these cherts formed in a relatively deep-water basin with no significant terrestrial interference. Therefore, it is inferred that the North Qilian orogenic belt was previously an archipelagic ocean in the Ordovician.

Carboniferous biogenic sedimentary structures on the northwestern margin of Jungar Basin

Volcanic clastic rocks of 2 300 m thick are developed in the Carboniferous of the Halaalate Mountains in the northwestern margin of the Jungar Basin. The research of sedimentology indicates that they were formed by the sedimentation of deep-water environnent. They could be divided into three ichnofacies from the lower part to the upper part, i.e. deep-sea ichnofacies, Nereites ichnofacies, andZoophycos ichnofacies. From the vertical evolution of biogenic structures and ichnofacies it could be considered that during the Carboniferious a bathayl basin was gradually changed into a semideep sea basin where progradational sediments in the sea regression were deposited. The Carboniferous represents the sedimentary characteristics during a sea-level falling cycle.

Regional depth-controlled hydrothermal metamorphism in the Zymoetz River area, British Columbia

An estimated 6 km of basic to silicic volcanic flows and clastic rocks of the Early Jurassic Telkwa Formation is exposed in moderately east-dipping fault blocks along the Zymoetz River, British Columbia. Extensive wholesale zeolitic replacement of porous tuff beds suggests widespread hydrothermal activity. Metamorphic grade increases regionally from laumontite–albite facies to prehnite–pumpellyite facies with increasing stratigraphic depth. Telkwa Formation strata and the imparted metamorphic zonation are cut and tilted by rotational block faulting, and are repeated in each of the upturned blocks. Late Mesozoic to Tertiary plutonism locally thermally overprinted the regional facies, particularly in the western part of the area. Fluid-inclusion isochores, combined with calculated mineral equilibria, suggest that metamorphism took place at fluid pressures of 2 kbar (1 kbar = 100 MPa) or less, consistent with estimates of stratigraphic burial. Metamorphic fluids were H2O rich and low in dissolved salts. Maximum temperatures during regional depth-controlled hydrothermal metamorphism, based upon the widespread presence of laumontite and the lack of wairakite in the middle to upper parts of the Telkwa Formation, probably did not exceed about 250 °C at H2O pressures of 2 kbar. Mineral zones, estimated paleotemperatures, and geothermal gradients are comparable to regional hydrothermal metamorphism in active volcanic settings such as Iceland.

Fractal pattern integration for mineral potential estimation

Concepts of fractal/multifractal dimensions and fractal measure were used to derive the prior and posterior probabilities that a small unit cell on a geological map contains one or more mineral deposits. This has led to a new version of the weights of evidence technique which is proposed for integrating spatial datasets that exhibit nonfractal and fractal patterns to predict mineral potential. The method is demonstrated with a case study of gold mineral potential estimation in the Iskut River area, northwestern British Columbia. Several geological, geophysical, and geochemical patterns (Paleozoic-Mesozoic sedimentary and volcanic clastic rocks; buffer zones around the contacts between sedimentary rocks and Mesozoic intrusive rocks; a linear magnetic anomaly; and geochemical anomalies for Au and associated elements in stream sediments) were integrated with the gold mineral occurrences which have fractal and multifractal properties with a box-counting dimension of 1.335±0.077 and cluster dimension of 1.219±0.037.

Stratigraphy and tectonic setting of the upper part of the Cadwallader terrane, southwestern British Columbia

The Upper Triassic to Middle Jurassic Cadwallader terrane lies on the northeastern edge of the Coast Plutonic Complex in southwestern British Columbia. Previous work on the Cadwallader Group, the basal unit of the terrane, suggested it was an Upper Triassic (Carnian to middle Norian) volcanic arc and related clastic rocks. Volcanism ceased in early Norian time. A detailed study of the upper part of the Cadwallader terrane (Tyaughton Group and overlying Last Creek formation) shows that it is a sedimentary sequence deposited on the fringe of the inactive Cadwallader magmatic arc. The Upper Triassic (middle to upper Norian) Tyaughton Group consists of nonmarine to shallow-marine clastic rocks and limestones that show sudden changes in depositional setting. The Lower to Middle Jurassic Last Creek formation, a transgressive sequence of clastic rocks, disconformably overlies the Tyaughton Group. The clastic rocks in the two units were derived from a mixed volcanic and plutonic source region that also included a minor metamorphic component and local lower Norian limestones. The stratigraphy of the upper part of the Cadwallader terrane records long-term thermal subsidence of the basin caused by cooling of the magmatic arc after volcanism ceased in the early Norian. The detailed stratigraphy of the upper Cadwallader terrane supports correlation of the Cadwallader with the Stikine terrane, along which it is currently structurally juxtaposed.

Correlation and Implications of Results from Recent Wildcat Wells in Mid-Continent Rift in Northeastern Kansas: ABSTRACT

Three recent wildcat wells, the Texaco 1 Poersch (Sec. 31, T5S, R5E), the Producers Engineering et al 1 Finn (Sec. 4, T4S, R7E), and 1 Freiderich (Sec. 8, T7S, R5E), respectively, penetrated 8,455, 1,848, and 1,781 ft of Precambrian rocks in the Mid-Continent rift system (MRS) in Kansas. The Producers Engineering et al 1 Finn, which is 18 mi northeast of the Texaco 1 Poersch, encountered mostly arkosic sandstones and shales. The Producers Engineering et al 1 Freiderich, 7 mi south of the Texaco 1 Poersch, penetrated mafic igneous and volcanic clastic rocks. In contrast, the Poersch well drilled approximately 90% mafic igneous rocks and 10% arkosic sandstones in the upper 4,584 ft of the Precambrian section. In the lower 3,871 ft of the well, the proportions of these rock-types were reversed. No Precambrian petroleum source rocks were found in the Poersch and Freiderich wells, but 300 ft of gray siltstone (approximately 0.5% TOC) were encountered directly below the Precambrian unconformity in the Finn well. The Finn well also has significantly more red siltstones than the Poersch well, thereby possibly indicating more distal lacustrine or low-energy marine environments of deposition. Alluvial fans and playas are probably responsible for most of more » the sedimentary deposition in the Poersch well. Geophysical and well data indicate the MRS in northeastern Kansas is divided into small subbasins that probably are only a few tens of square miles in areal extent. Abrupt facies changes and complicated structure make lithostratigraphic correlations difficult. « less