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Western Margin (western + margin)

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Earth and Environmental Science	96%

Selected Abstracts

Tectonic Evolution of the Tianhuan Depression and the Western Margin of the Late Triassic Ordos

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2009
LI Xiangbo
Abstract: The Ordos Basin is one of the most important oil and gas basins in China. Based on surface outcrop, key exploratory wells and seismic reflection data and by using the technology of "prototype basin recovery", seismic profile "layer flattening" and "restoration of balanced section", and other methods, the sedimentary boundary, structure and the evolution history of the Tianhuan depression on the western margin of the Ordos Basin are reestablished. The following results have been obtained. (1) The west boundary of the Late Triassic Ordos Basin was far beyond the scope of the current basin. The basin is connected with the Late Triassic Hexi Corridor Basin, and its western margin did not have tectonic characteristics of a foreland basin. (2) The Tianhuan depression was first formed in the Late Jurassic. At the late stage it was impacted by the late Yanshanian and Himalayan tectonic movement and the depression axis gradually moved eastwards to the present location with a cumulative migration distance of ,30 km. (3) Eastward migration of the depression axis caused adjustment and even destruction of the originally formed oil and gas reservoirs, so that oil and gas remigrated and aggregated, resulting in secondary structural reservoirs formed at high positions on the western flank of the depression. [source]

Tilting neotectonics of the Guadiamar drainage basin, SW Spain

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2004
Josep M. SalvanyArticle first published online: 23 DEC 200
Abstract The Guadiamar river ,ows from the southern Iberian Massif to the Guadalquivir foreland basin, SW Spain. Its drainage basin displays asymmetries in the stream network, the arrangement of alluvial terraces and the con,guration of the trunk river valley. The stream network asymmetry was studied using morphometric measures of transverse topographic sym-metry, asymmetry factor and drainage basin shape. The alluvial terraces were studied through the lithologic logs of more than a hundred boreholes and ,eld mapping. The morphometric methods demonstrate a regional tectonic tilting toward the SSE, causing both the migration of the Guadiamar river toward the east and the migration of the Guadiamar tributaries toward the southwest. As a consequence of the Guadiamar river migration, an asymmetric valley developed, with a steep eastern margin caused by river dissection, and a gentle western margin where the main alluvial deposits are found. The ages obtained using the 14C analysis of samples from several alluvial deposits show that the river migration, and thus tilting, has occurred during the Holocene as well as earlier in the Quaternary. This interpretation revises the Guadiamar longitudinal fault assumed by previous studies. Copyright © 2003 John Wiley & Sons, Ltd. [source]

The nature of calcareous deposits along pan margins in eastern central Namibia

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2002
Florias MeesArticle first published online: 13 JUN 200
Abstract In a region along the western margin of the Kalahari in eastern Namibia and western Botswana, many pan basins have mainly calcareous deposits along part of their margins. These are typically lined by low vertical scarps. In Namibia, these pans are mainly located in dry river beds. The petrographical study of these deposits demonstrates that they consist of lacustrine sediments that have to a varying extent been affected by early-diagenetic processes and by the formation of late-diagenetic features. The original composition of the deposits ranges from highly calcareous sediments, typically with ostracod, diatom and charophyte remains, to entirely non-calcareous diatomites. The deposits generally show an upward increase in total carbonate content, which is mainly a synsedimentary feature. The early-diagenetic processes that affected the deposits include the formation of orthic siliceous nodules. At a later stage, secondary calcite enrichment occurred, leaving only the silica-impregnated sections unaffected. This enrichment partly accounts for the upward increase in total carbonate content in some profiles and often resulted in the development of a highly calcareous surface horizon. Sepiolite and amorphous silica that are part of the groundmass of the deposits may also partly have formed at this stage. Late-diagenetic features include various forms of secondary calcite and silica. This study of pan basins in eastern central Namibia indicates that a lacustrine rather than purely pedogenic origin should also be considered for calcareous deposits that commonly occur along pan margins in other parts of southern Africa. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Quaternary paleoenvironments and potential for human exploitation of the Jordan plateau desert interior

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 4 2005
Caroline P. Davies
The physical, chemical, numerical, and radiometric analyses of a 31-m sediment core from the Qa'el-Jafr basin provide an important record of Quaternary paleoenvironments for the Jordan Plateau and evidence for several significant changes in climate regime. Cluster and PCA analyses of the geochemical data support the designation of major sedimentation regimes identified by stratigraphic and sediment analyses. Multiple cycles of alluvial deposition, lacustrine units, and erosional unconformities characterize the deepest sediments, followed by a period(s) of intense evaporation. Radiocarbon ages of charcoal in the uppermost 7 m place the aeolian/alluvial phase between 16,030 ± 140 yr B.P. and 24,470 ± 240 yr B.P. Deflation processes may explain the lack of a Holocene sequence. Despite lacking radiometric ages for the lower sediments, the thickness and degree of calcium-carbonate cementation suggest considerable age for the basal sediments, which suggests that a very long terrestrial record of Quaternary climate changes has been preserved in the Jafr basin. This new record of paleoenvironments provides important context to the archaeological record of the Jordan Plateau during the Quaternary. Several archaeological surveys demonstrate extensive human exploitation of lakes and springs of the major wadis along the western margin of the Rift Valley. However, little is known of human exploitation of the desert interiors. Qa'el-Jafr sediments demonstrate significant lacustrine and high moisture phases sufficient for human exploitation of the eastern desert during the Pleistocene. © 2005 Wiley Periodicals, Inc. [source]

Tectono-sedimentary evolution of the northernmost margin of the NE German Basin between uppermost Carboniferous and Late Permian (Rotliegend)

GEOLOGICAL JOURNAL, Issue 1 2001
H. Rieke
Abstract The tectono-sedimentary evolution of the Rotliegend deposits of the northernmost margin of NE German Basin (NEGB) has been analysed on the basis of detailed sedimentary logs of 300,m of core material together with the re-evaluation of 600,km of seismic lines. Three distinct phases were recognized. During the initial Phase I, basin geometry was largely controlled by normal faulting related to deep-seated ductile shearing leading to a strong asymmetric shape, with a steep fault-controlled eastern margin and a gently, dipping western margin. The results of forward modelling along a cross-section fit the basin geometry in width and depth and reveal a footwall uplift of c. 1000,m. Adjacent to the steep faults, local sedimentation of Lithofacies Type I was confined to non-cohesive debris flow-dominated alluvial fans, whereas the gently dipping western margin was dominated by alluvial-cone sedimentation. During the post-extensional period (Phase II), cooling of the lithosphere generated additional accommodation space. The sediments of Lithofacies Type II, comprising mainly clast-supported conglomerates, are interpreted as braided ephemeral stream flow-surge deposits. Tectonic quiescence and an increase in flood events resulting from wetter climate led to progradation of this facies over the entire region. At the end of this period, the accommodation space was almost completely filled resulting in a level topography. Phase III was controlled by the thermal-induced subsidence of the southerly located NEGB in post-Illawarra times. The formerly isolated region tilted towards the SW, thus forming the northern margin of the NEGB during uppermost Havel and Elbe Subgroup times. The sediments of Lithofacies Type III were divided into a marginal sandstone-dominated environment and a finer-grained facies towards the SW. The former consists of poorly-sorted coarse-grained sandstones of a proximal and medial ephemeral stream floodplain facies. The latter comprise mud flat fines and fine-grained distal ephemeral stream deposits. The end of the tectono-sedimentary evolution is marked by the basinwide Zechstein transgression. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Mechanical deformation model of the western United States instantaneous strain-rate field

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2006
Fred F. Pollitz
SUMMARY We present a relationship between the long-term fault slip rates and instantaneous velocities as measured by Global Positioning System (GPS) or other geodetic measurements over a short time span. The main elements are the secularly increasing forces imposed by the bounding Pacific and Juan de Fuca (JdF) plates on the North American plate, viscoelastic relaxation following selected large earthquakes occurring on faults that are locked during their respective interseismic periods, and steady slip along creeping portions of faults in the context of a thin-plate system. In detail, the physical model allows separate treatments of faults with known geometry and slip history, faults with incomplete characterization (i.e. fault geometry but not necessarily slip history is available), creeping faults, and dislocation sources distributed between the faults. We model the western United States strain-rate field, derived from 746 GPS velocity vectors, in order to test the importance of the relaxation from historic events and characterize the tectonic forces imposed by the bounding Pacific and JdF plates. Relaxation following major earthquakes (M, 8.0) strongly shapes the present strain-rate field over most of the plate boundary zone. Equally important are lateral shear transmitted across the Pacific,North America plate boundary along ,1000 km of the continental shelf, downdip forces distributed along the Cascadia subduction interface, and distributed slip in the lower lithosphere. Post-earthquake relaxation and tectonic forcing, combined with distributed deep slip, constructively interfere near the western margin of the plate boundary zone, producing locally large strain accumulation along the San Andreas fault (SAF) system. However, they destructively interfere further into the plate interior, resulting in smaller and more variable strain accumulation patterns in the eastern part of the plate boundary zone. Much of the right-lateral strain accumulation along the SAF system is systematically underpredicted by models which account only for relaxation from known large earthquakes. This strongly suggests that in addition to viscoelastic-cycle effects, steady deep slip in the lower lithosphere is needed to explain the observed strain-rate field. [source]

BARGEN continuous GPS data across the eastern Basin and Range province, and implications for fault system dynamics

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004
Nathan A. Niemi
SUMMARY We collected data from a transect of continuous Global Positioning System (GPS) sites across the eastern Basin and Range province at latitude 39°N from 1997,2000. Intersite velocities define a region ,350 km wide of broadly distributed strain accumulation at ,10 nstr yr,1. On the western margin of the region, site EGAN, ,10 km north of Ely, Nevada, moved at a rate of 3.9 ± 0.2 mm yr,1 to the west relative to site CAST, which is on the Colorado Plateau. Velocities of most sites to the west of Ely moved at an average rate of ,3 mm yr,1 relative to CAST, defining an area across central Nevada that does not appear to be extending significantly. The late Quaternary geological velocity field, derived using seismic reflection and neotectonic data, indicates a maximum velocity of EGAN with respect to the Colorado Plateau of ,4 mm yr,1, also distributed relatively evenly across the region. The geodetic and late Quaternary geological velocity fields, therefore, are consistent, but strain release on the Sevier Desert detachment and the Wasatch fault appears to have been anomalously high in the Holocene. Previous models suggesting horizontal displacement rates in the eastern Basin and Range near 3 mm yr,1, which focused mainly along the Wasatch zone and Intermountain seismic belt, may overestimate the Holocene Wasatch rate by at least 50 per cent and the Quaternary rate by nearly an order of magnitude, while ignoring potentially major seismogenic faults further to the west. [source]

Structure of Sumatra and its implications for the tectonic assembly of Southeast Asia and the destruction of Paleotethys

ISLAND ARC, Issue 1 2009
Anthony J. Barber
Abstract It is now generally accepted that Southeast Asia is composed of continental blocks which separated from Gondwana with the formation of oceanic crust during the Paleozoic, and were accreted to Asia in the Late Paleozoic or Early Mesozoic, with the subduction of the intervening oceanic crust. From east to west the Malay peninsula and Sumatra are composed of three continental blocks: East Malaya with a Cathaysian Permian flora and fauna; Sibumasu, including the western part of the Malay peninsula and East Sumatra, with Late Carboniferous,Early Permian ,pebbly mudstones' interpreted as glaciogenic diamictites; and West Sumatra, again with Cathaysian fauna and flora. A further unit, the Woyla nappe, is interpreted as an intraoceanic arc thrust over the West Sumatra block in the mid Cretaceous. There are varied opinions concerning the age of collision of Sibumasu with East Malaya and the destruction of Paleotethys. In Thailand, radiolarites have been used as evidence that Paleotethys survived until after the Middle Triassic. In the Malay peninsula, structural evidence and the ages of granitic intrusions are used to support a Middle Permian to Early Triassic age for the destruction of Paleotethys. It is suggested that the West Sumatra block was derived from Cathaysia and emplaced against the western margin of Sibumasu by dextral transcurrent faulting along a zone of high deformation, the Medial Sumatra Tectonic Zone. These structural units can be traced northwards in Southeast Asia. The East Malaya block is considered to be part of the Indochina block, Sibumasu can be traced through Thailand into southern China, the Medial Sumatra Tectonic Zone is correlated with the Mogok Belt of Myanmar, the West Burma block is the extension of the West Sumatra block, from which it was separated by the formation of the Andaman Sea in the Miocene, and the Woyla nappe is correlated with the Mawgyi nappe of Myanmar. [source]

Sandstone diagenesis of the Lower Cretaceous Sindong Group, Gyeongsang Basin, southeastern Korea: Implications for compositional and paleoenvironmental controls

ISLAND ARC, Issue 1 2008
Yong Il Lee
Abstract The Gyeongsang Basin is a non-marine sedimentary basin formed by extensional tectonism during the Early Cretaceous in the southeastern Korean Peninsula. The sediment fill starts with the Sindong Group distributed along the western margin of the basin. It consists of three lithostratigraphic units: the Nakdong (alluvial fan), Hasandong (fluvial) and Jinju (lacustrine) formations with decreasing age. Sindong Group sandstones are classified into four petrofacies (PF) based on their detrital composition: PF-A consists of the lower Nakdong Formation with average Q73F12R15; PF-B the upper Nakdong and lower Hasandong formations with Q66F15R18; PF-C the middle Hasandong to middle Jinju formations with Q49F29R22; and PF-D the upper Jinju Formation with Q26F34R41. The variations of detrital composition influenced the diagenetic mineral assemblage in the Sindong Group sandstones. Illite and dolomite/ankerite are important diagenetic minerals in PF-A and PF-B, whereas calcite and chlorite are dominant diagenetic minerals in PF-C and PF-D. Most of the diagenetic minerals can be divided into early and late diagenetic stages of formation. Early diagenetic calcites occur mostly in PF-C, probably controlled by arid to semiarid climatic conditions during the sandstone deposition, no early calcite being found in PF-A and PF-B. Late-stage calcites are present in all Sindong Group sandstones. The calcium ions may have been derived from shale diagenesis and dissolution of early stage calcites in the Hasandong and Jinju sandstones. Illite, the only diagenetic clay mineral in PF-A and lower PF-B, is inferred to be a product of kaolinite transformation during deep burial, and the former presence of kaolinite is inferred from the humid paleoclimatic conditions during the deposition of the Nakdong Formation. Chlorites in PF-C and PF-D are interpreted to be the products of transformation of smectitic clay or of precipitation from alkaline pore water under arid to semiarid climatic conditions. The occurrence of late-stage diagenetic minerals largely depended on the distribution of early diagenetic minerals, which was controlled initially by the sediment composition and paleoclimate. [source]

Variation of crustal thickness in the Philippine Sea deduced from three-dimensional gravity modeling

ISLAND ARC, Issue 3 2007
Takemi Ishihara
Abstract Crustal thickness of the northern to central Philippine Sea was gravimetrically determined on the simple assumption of four layers: seawater, sediments, crust and lithospheric mantle, with densities of 1030, 2300, 2800 and 3300 kg/m3, respectively. As for the correction of the regional gravity variation, a 15 km difference of the lithospheric thickness with a density difference of 50 kg/m3 against the asthenosphere below between both sides of the Kyushu-Palau Ridge was taken into consideration. Mantle Bouguer anomalies were calculated on the assumption of constant crustal thickness of 6 km, and then the crustal thickness was obtained by three-dimensional gravity inversion method. The results show occurrence of thin crust areas with a thickness of approximately 5 km in the southern part and at the western margin of the Shikoku Basin and also of thick crust areas in the northwestern and northeastern parts of the Parece Vela Basin. We suggest that these are because of the variation of magma supply at the time of sea floor spreading in the Shikoku and Parece Vela Basins, which is possibly related to the variation of spreading rate and enhanced magmatism near the past arc volcanic fronts. The results further show the occurrence of crust thinner than 5 km in the northeastern part of the West Philippine Basin, of crust thicker than 15 km in the Amami Plateau, the Daito and Oki-Daito Ridges, and also in the northern part of Kyushu-Palau Ridge, whereas the southern part of the Kyushu-Palau Ridge the crust is thicker than 10 km. It was also inferred that small basins in the Daito Ridge province have the thinnest oceanic crust of less than 5 km in the Kita-Daito Basin. [source]

Fault configuration produced by initial arc rifting in the Parece Vela Basin as deduced from seismic reflection data

ISLAND ARC, Issue 3 2007
Mikiya Yamashita
Abstract The Parece Vela Basin (PVB), which is a currently inactive back-arc basin of the Philippine Sea Plate, was formed by separation between the Izu-Ogasawara Arc (IOA) and the Kyushu-Palau Ridge (KPR). Elucidating the marks of the past back-arc opening and rifting is important for investigation of its crustal structure. To image its fault configurations and crustal deformation, pre-stack depth migration to multichannel seismic reflection was applied and data obtained by the Japan Agency for Marine-Earth Science and Technology and Metal Mining Agency of Japan and Japan National Oil Corporation (Japan Oil, Gas and Metals National Corporation). Salient results for the pre-stack depth-migrated sections are: (i) deep reflectors exist around the eastern margin of KPR and at the western margin of IOA down to 8 km depth; and (ii) normal fault zones distributed at the eastern margin of the KPR (Fault zone A) and the western margin of the IOA (Fault zone B) have a total displacement of greater than 500 m associated with synrift sediments. Additional normal faults (Fault zone C) exist 20 km east of the Fault zone B. They are covered with sediment, which indicates deposition of recent volcanic products in the IOA. According to those results: (i) the fault displacement of more than 500 m with respect to initial rifting was approximately asymmetric at 25 Ma based on PSDM profiles; and (ii) the faults had reactivated after 23 Ma, based on the age of deformed sediments obtained from past ocean drillings. The age of the base sediments corresponds to those of spreading and rotation after rifting in the PVB. Fault zone C is covered with thick and not deformed volcanogenic sediments from the IOA, which suggests that the fault is inactive. [source]

Timing of high-grade metamorphism: Early Palaeozoic U,Pb formation ages of titanite indicate long-standing high- T conditions at the western margin of Gondwana (Argentina, 26,29°S)

JOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2003
F. Lucassen
Abstract Concordant U,Pb ages of c. 530,510 Ma and c. 470,420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high- T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26,29°S). The U,Pb ages document the time of titanite formation at high- T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were > c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U,Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U,Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U,Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation-enhanced recrystallization of titanite and/or different thermal peaks during a long-standing, geographically fixed, high- T regime in the mid-crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region. [source]

Phylogeography and population structure of an ecotonal marsupial, Bettongia tropica, determined using mtDNA and microsatellites

MOLECULAR ECOLOGY, Issue 12 2000
L. C. Pope
Abstract The northern bettong, Bettongia tropica, is an endangered species of Potoroidae with a restricted distribution in the wet tropics of north Queensland, Australia. The species is only found within a thin strip of sclerophyll forest along the western margin of rainforest. This tight association with rainforest boundaries is predicted to have resulted in population isolation as rainforest contracted during the Pleistocene, though some have proposed that the northern bettong was not present in the wet tropics until the late Pleistocene. The dispersal ability of the species, and of the family, is not known. This study examined gene flow among populations within areas of continuous habitat complemented by a broader analysis of phylogeography. Individuals trapped at each of the four known regions (one region was subsampled at three different sites), were sequenced for 547 base pairs of the mitochondrial DNA (mtDNA) control region and typed for seven microsatellite loci. The mtDNA phylogeny showed congruence with a biogeographical hypothesis, a relatively deep split suggesting historical isolation in separate northern and southern refugia. The two divergent clades were both present within the Lamb Range, indicating an expansion from these refuges and subsequent admixture at one site. mtDNA allele frequencies indicated relatively limited gene flow within the Lamb Range over distances as short as nine km. Tests of population divergence using microsatellites (FST and assignment tests) strongly supported this result. A molecular signal indicative of a recent bottleneck was unexpectedly detected in one of the Lamb Range subpopulations. This lead us to examine the behaviour of the statistics used in this bottleneck test under a linear stepping-stone model with varying migration rates. We found that it may be more difficult to detect molecular signatures for recent bottlenecks under conditions of very low migration rates than for isolated populations and, conversely, that ,false' bottleneck signatures may be observed at higher migration rates. The Lamb Range FST estimate clearly fell within the category of potentially ,false' bottleneck signals. Despite relatively limited gene flow, evidence for asymmetric dispersal suggests more complicated population dynamics than a simple linear stepping-stone model. [source]

A new cardiid bivalve from the Pliocene Baklan Basin (Turkey) and the origin of modern Ponto-Caspian taxa

PALAEONTOLOGY, Issue 4 2010
FRANK P. WESSELINGH
Abstract:, We present the first record of the cardiid genus Monodacna from the Pliocene of Anatolia, Turkey. Monodacna imrei sp. nov. was found in the Pliocene Killik Formation from the western margin of the Baklan Basin, in very marginal brackish to freshwater lacustrine deposits. The new record extends the stratigraphic range of the modern Ponto-Caspian genus back into the Pliocene and adds to earlier evidence that modern Ponto-Caspian taxa originated in the Pliocene of south-western Turkey. [source]

Carbonatitic Volcanic Genesis of Hetaoqing Fe-Cu Deposit in Central Yunnan, China

RESOURCE GEOLOGY, Issue 4 2003
Yongbei Zhang
Abstract. The Kunyang rift on western margin of Yangtze Platform is a continental rift, and also a rare Precambrian Fe-Cu mineralization zone in China. The Wuding-Lufeng mineralization area in the middle section of the rift is an important part of the zone, and an elliptic-shaped volcanic collapsed basin, controlled by a ring fracture system with carbonatitic volcanic rocks mainly occurring along the northwestern edge of the basin. The Hetaoqing Fe-Cu ore deposit at the western side of the basin is hosted in carbonatitic volcanic rocks and pyroclastic sedimentary rocks. The original ore bodies occur as layers, bands and lenses conformable to the host carbonatitic rocks. The ores usually appear as massive, impregnated and granular in carbonatitic rocks, and as brecciform and sandy in pyroclastic sedimentary rocks. Ore-forming minerals are magnetite, hematite, chalcopy-rite, bornite, pyrite, carrollite, molybdenite, cobaltite and skinnerite, and secondary minerals limonite, chalcocite, azurite, malachite and tenorite. Gangue minerals are calcite, dolomite, ankerite, common hornblende, arfvedsonite, augite, aegirine-augite, albite, phlogopite, biotite, chlorite and apatite. Evidences of mineral chemistry, trace elements and isotopic ratios of ores, as well as geological features, suggest that the original ores are igneous in origin. Chemical features of magnetites in the deposit belong to carbonatite type, and are similar to those from the Bayan Obo carbonatites. The ores are rich in iron, titanium, rare earth elements, niobium, tantalum, gold, silver, phosphor and sulfur. These features indicate that the Fe-Cu deposit associated with volcanic activity in the Wuding-Lufeng basin is alkali-carbonatite volcanic type. [source]

Dashuigou Tellurium Deposit in Sichuan Province, China: S, C, O, and H Isotope Data and Their Implications on Hydrothermal Mineralization

RESOURCE GEOLOGY, Issue 1 2002
Jingwen MAO
Abstract: Dashuigou, a unique tellurium-dominated deposit over the world, is located in the western margin of the Yangtze cra-ton in southwestern China. It is characterized by high-grade tellurium accompanied by bismuth, gold, silver, and sulfur, and occurs in the area of less than one km2. The mineralization is divided into three stages, i.e. (1) tellurium-bearing pyrrhotite,pyrite stage, (2) tetradymite stage, and (3) auriferous quartz veins stage. Tellurium mineralization coexisting with bismuth, silver, selenium, and gold predominantly develops in the stage 2, while the stage 1 is enriched only in sulfur and iron, and the stage 3 is very weakly mineralized with gold. The ,34S values of sulfides in the ore of the deposit vary in a narrow range of ,3.1 - +2.8 per mil with ,3.1 - +2.8 per mil for the stage 1 and ,0.5 - +2.1 for the stage 2, showing the isotopic characteristics of mantle derived sulfur. The ,13C values of vein dolomites vary from ,5.3 to ,7.2 per mil, with ,5.3 - ,6.6 per mil for the stage 1 and ,5.3 - ,7.2 per mil for the stage 2, which are significantly different from those of surrounding Triassic marble with ,13C values of ,0.3 - +2.8 per mil, and show characteristics of mantle derived carbon. The ,18O values of vein dolomites range from +10.2 to +13.1 per mil, which are higher than those of carbonatite, but lower than those of the marble. Their corresponding ,18Owater values are +0.6 - +3.9 per mil, with +2.7 - +3.8 per mil for the stage 1 and +0.6 - +3.9 per mil for the stage 2. The data implies that these vein carbonates were formed by the mixing fluids of magmatic or mantle source with meteoric or formation water. The ,18O values of ore-forming fluids responsible for the formation of vein quartz are estimated to be +3.2 to +6.8, the ,D values of inclusion fluids of the quartz are measured to be ,54 to ,82 per mil. All those stable isotopic data suggest the involvement of the fluids from mantle and/or mantle-derived magmas through fault system in the forming process of the Dashuigou tellurium deposit. [source]

Erosional vs. accretionary shelf margins: the influence of margin type on deepwater sedimentation: an example from the Porcupine Basin, offshore western Ireland

BASIN RESEARCH, Issue 5 2009
M. C. Ryan
ABSTRACT A 1000 km2 three-dimensional (3D) seismic data survey that extends out from the western margin of the Porcupine Basin, offshore western Ireland reveals the internal geometry and depositional history of a large Palaeogene (Palaeocene,Early Eocene) shelf-margin. Two wells intersect the margin thereby constraining the depositional environments. The 34/19-1 well (landward end) intersects slope, shelf, marginal marine to coastal plain facies. The 35/21-1 well (basinward end) intersects seismically imaged shelf-margin clinoforms where base of slope back up to coastal plain deposits (source-to-sink) are represented. The basin-fill stratal architecture of the Palaeogene succession reveals sediment deposition under two end member, basin physiographic styles: (1) an erosional margin style and (2) an accretionary or progradational margin style. Uplift of the western margin of the basin is suggested as the major cause of the initially oversteepened shelf-slope erosional profile. Key characteristics of an erosional margin include sediment bypass of the shelf, canyon formation, and the development of significant onlapping submarine fan deposits on the lower slope. Failure on the slope is also revealed by several mass,transport complexes (MTCs) that carve out major erosive features across the slope. Three-dimensional seismic analysis illustrates variations in size, geometry and depositional trend and transport mechanisms of the MTCs. Confined, thick chaotic seismic facies, erosional basal scours and syn-depositional thrusting (pressure ridges) at terminus as opposed to thin, high-amplitude discontinuous facies with an unconfined lobate terminus are interpreted to indicate slump- and slide-dominated vs. debris flow-dominated MTCs, respectively. The erosional margin was transformed into an accretionary margin when the gradient of the shelf-slope to basin-floor profile was sufficiently lowered through the infilling and healing of the topographic lows by the onlapping submarine-fan deposits. This shallowing of the basin allowed nearshore systems to prograde across the deepwater systems. The accretionary margin was characterised by a thick sediment prism composed of clinoforms both at the shoreface/delta (tens of metres) and shelf-margin (hundreds of metres) scales. Shelf-margin clinoforms, the focus of this study, are the fundamental regressive to transgressive building blocks (duration 10,100 kyr) of the stratigraphic succession and can be observed on a larger scale (,1 Myr) through the migration and trajectory patterns of the shelf-edge. Trajectory pathways in the accretionary margin are accretionary in a descending or ascending manner. The descending style was characterised by a shelf-slope break that migrated seawards and obliquely downwards as a result of a relative sea-level fall. The descending trajectory geometry is lobate along strike suggestive of a point source progradation. Internally, the descending trajectory consists of downward stepping, steeply dipping shelf-margin clinoforms that display extensive slumping and deposition of sediment on the lower slope indicative of rapid deposition. Furthermore, basin-floor fans and associated ,feeder' channels extend basinwards beyond toe of slope. The ascending trajectory reflects a shelf-slope break that is interpreted to have migrated seawards during steady or rising relative sea level. The ascending trajectory geometry is associated with significant lateral sediment dispersal along the shelf-edge, reflecting distributary systems that were less ,fixed' or a greater reworking and longshore drift of sediment. Accretion involving the ascending shelf-edge trajectory characteristically lacked significant basin-floor deposits. Variable ascending trajectories are recognised in this study, as read from the angle at which the shelf-slope break migrates. Horizontal to high angle ascending trajectories correspond to dominantly progradational and dominantly aggradational shelf-edge trajectories, respectively. The sequence stratigraphic analysis of the Porcupine deltaic complex reveals a long-term relative sea-level rise. [source]

Clinoform nucleation and growth in coarse-grained deltas, Loreto basin, Baja California Sur, Mexico: a response to episodic accelerations in fault displacement

BASIN RESEARCH, Issue 3 2005
Estelle Mortimer
We investigate the controls on the architecture of coarse-grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half-graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault-controlled subsidence along the basin-bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well-exposed, vertically arranged fluvio-deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo-seaward into distal pro-delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert-delta facies associations as they are traced palaeo-seaward. This transition is characterised by down-transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset,foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault-controlled subsidence during each PU. During episodes of low slip rate, shoal-water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long-term (>100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault. [source]

Rethinking Late Weichselian ice-sheet dynamics in coastal NW Svalbard

BOREAS, Issue 1 2005
JON Y. LANDVIK
New marine geological evidence provides a better understanding of ice-sheet dynamics along the western margin of the last Svalbard/Barents Sea Ice Sheet. A suite of glacial sediments in the Kongsfjordrenna cross-shelf trough can be traced southwards to the shelf west of Prins Karls Forland. A prominent moraine system on the shelf shows minimum Late Weichselian ice extent, indicating that glacial ice also covered the coastal lowlands of northwest Svalbard. Our results suggest that the cross-shelf trough was filled by a fast-flowing ice stream, with sharp boundaries to dynamically less active ice on the adjacent shelves and strandflats. The latter glacial mode favoured the preservation of older geological records adjacent to the main pathway of the Kongsfjorden glacial system. We suggest that the same model may apply to the Late Weichselian glacier drainage along other fjords of northwest Svalbard, as well as the western margin of the Barents Ice Sheet. Such differences in glacier regime may explain the apparent contradictions between the marine and land geological record, and may also serve as a model for glaciation dynamics in other fjord regions. [source]

Tectonic Evolution of the Tianhuan Depression and the Western Margin of the Late Triassic Ordos

Guandishan Granitoids of the Paleoproterozoic Lüliang Metamorphic Complex in the Trans-North China Orogen: SHRIMP Zircon Ages, Petrogenesis and Tectonic Implications

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009
Shuwen LIU
Abstract: The Paleoproterozoic Lüliang Metamorphic Complex (PLMC) is situated in the middle segment of the western margin of the Trans-North China Orogen (TNCO), North China Craton (NCC). As the most important lithological assemblages in the southern part of the PLMC, Guandishan granitoids consist of early gneissic tonalities, granodiorites and gneissic monzogranites, and younger gneissic to massive monzogranites. Petrochemical features reveal that the early gneissic tonalities and granodiorites belong to the medium-K calc-alkaline series; the early gneissic monzogranites are transitional from high-K calc-alkaline to the shoshonite series; the younger gneissic to massive monzogranites belong to the high-k calc-alkaline series, and all rocks are characterized by right-declined REE patterns and negative Nb, Ta, Sr, P, and Ti anomalies in the primitive mantle normalized spidergrams. SHRIMP zircon U,Pb isotopic dating reveals that the early gneissic tonalities and granodiorites formed at ,2.17 Ga, the early gneissic monzogranites at ,2.06 Ga, and the younger gneissic to massive monzogranites at ,1.84 Ga. Sm,Nd isotopic data show that the early gneissic tonalities and granodiorites have ,Nd(t) values of +0.48 to ,3.19 with Nd-depleted mantle model ages (TDM) of 2.76,2.47 Ga, and early gneissic monzogranites have ,Nd(t) values of ,0.53 to ,2.51 with TDM of 2.61,2.43 Ga, and the younger gneissic monzogranites have ,Nd(t) values of ,6.41 to ,2.78 with a TDM of 2.69,2.52 Ga. These geochemical and isotopic data indicate that the early gneissic tonalities, granodiorites, and monzogranites were derived from the partial melting of metamorphosed basaltic and pelitic rocks, respectively, in a continental arc setting. The younger gneissic to massive monzogranites were derived by partial melting of metamorphosed greywackes within the continental crust. Combined with previously regional data, we suggest that the Paleoproterozoic granitoid magmatism in the Guandishan granitoids of the PLMC may provide the best geological signature for the complete spectrum of Paleoproterozoic geodynamic processes in the Trans-North China Orogen from oceanic subduction, through collisional orogenesis, to post-orogenic extension and uplift. [source]

What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma?

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2006
Guochun ZHAO
Abstract: The Trans-North China Orogen (TNCO) was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greenstone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic (TTG) magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, ,2250 Ma 2110,21760 Ma and ,2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-arc basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks. [source]

Lower Jurassic Foraminifera and Calcified Microflora from Gibraltar, Western Mediterranean

PALAEONTOLOGY, Issue 4 2001
M. K. Boudagher-Fadel
Benthic foraminifera are described for the first time from the Gibraltar Limestone Formation of the Rock of Gibraltar. The new species Siphovalvulina colomiS. gibraltarensisRiyadhella praeregularis occur with Duotaxis metula Kristan, Everticyclammina praevirguliana Fugagnoli, Siphovalvulina sp.,an atypically early example of Textulariopsis sp., and Nodosaria sp. Microflora are present as the probable cyanobacterium Cayeuxia ?piae Frollo, the alga Palaeodasycladus ?mediterraneus (Pia), and the disputed alga Thaumatoporella ?parvovesiculifera (Raineri). The foraminifera compare most closely with poorly-known taxa from Italy, Spain and Morocco, and are consistent with an Early Jurassic (Sinemurian) age for the upper part of the &62;460-m-thick Gibraltar Limestone. Most are textulariids and more primitive than species well known from the later Early Jurassic (Pliensbachian) of the Mediterranean region, especially Morocco and Italy. The biota as a whole is characteristic of inner carbonate platform environments widespread along the rifted western margins of the Early Jurassic Tethys, notably those recorded from Morocco, Italy and Greece as well as southern Spain. [source]

Testing the case for a Middle Pleistocene Scandinavian glaciation in Eastern England: evidence for a Scottish ice source for tills within the Corton Formation of East Anglia, UK

BOREAS, Issue 4 2002
JONATHAN R. LEE
The provenance of the Happisburgh Till and Corton Till of the Corton Formation is investigated using erratic clast lithologie s and allochthonou s palynomorph s to test whether the long held assumptio n that they were deposited by ice that originated in Scandinavia is valid. The results show a wide range of lithologie s including Carboniferous Limestone and Coal Measures, and Permian Magnesian Limestone that are not found in Scandinavia, and an absence of distinctive Scandinavian material such as rhomb porphyry and larvikite. Lithologies found indicate deposition by an ice sheet which flowed southwards into north-east East Anglia from central and southern Scotland eroding and transporting materials derived from outcrops in these areas and from eastern England and the western margins of the southern North Sea Basin. It is concluded that the long held assumption that the Happisburgh Till and Corton Till of the Corton Formation were deposited by a Scandinavian ice sheet is erroneous and that they were instead deposited by Scottish ice. [source]