Triassic Sedimentary Rocks Research Articles

Age and composition of Antarctic bedrock reflected by detrital zircons, erratics, and recycled microfossils in the Prydz Bay–Wilkes Land–Ross Sea–Marie Byrd Land sector (70°–240°E)

The age and composition of the 14 × 10 6 km 2 of Antarctica's surface obscured by ice is unknown except for some dates on detrital minerals. In remedy, we bring together proxies of Antarctic bedrock in the form of (1) detrital zircons analysed for U–Pb age, T DM C, εHf, and rock type, including five new analyses of Neogene turbidites, (2) erratics that reflect age, composition, and metamorphism, and (3) recycled microfossils that reflect age, facies, and metamorphism. Each sample is located in its ice-drainage basin for backtracking to the potential provenance. Gaps in age between sample and upslope exposure are specifically attributable to the provenance. This work indicates that the central Antarctic provenance about a core of the Gamburtsev Subglacial Mountains (GSM) and Vostok Subglacial Highlands (VSH) contains a basement that includes igneous (mafic granitoids) and metamorphic rocks with peak U–Pb ages of 0.5–0.7, 0.9–1.3, 1.4–1.7, 1.9–2.1, 2.2–2.3, 2.6–2.8, and 3.15–3.35 Ga, T DM C 1.3–3.6 Ga, and εHf + 12 to − 40. Other modelled cratons with similar ages are set in a matrix of foldbelts of 0.5–0.7 Ga age. The basement in the core is surmounted by Permian red beds, at the periphery by Permian and Triassic sedimentary rocks unaffected by igneous heating or load metamorphism, and west of the Transantarctic Mountains (TAM) in the Wilkes Basin arguably by Late Cretaceous through Pliocene marine sediments. Erratics of undated red sandstone along the coast of Wilkes Land and George V Land indicate a red-bed provenance in the interior. The Prince Charles Mountains (PCM) provide an exposed example of a crust of Precambrian igneous and metamorphic rocks and Permian and Triassic sedimentary rocks.

A comparison of thermal maturity parameters between freely extracted hydrocarbons (Bitumen I) and a second extract (Bitumen II) from within the kerogen matrix of Permian and Triassic sedimentary rocks

In this study we compared various maturity dependent aromatic, steroid and triterpenoid hydrocarbon ratios in bitumens that are freely extractable from sedimentary rocks (Bitumen I) with those in second extracts that comprised hydrocarbons closely associated with the kerogen/mineral matrix (Bitumen II). Bitumen II was released through kerogen isolation and demineralisation using HCl and HF/H 3BO 3. The samples studied, although of similar age, come from different localities. They represent a range of facies and two kerogen types (II and III), and all were deposited under marine conditions. The results show that the more stable β isomers of methylnaphthalene (MN) and methylphenanthrene (MP) are relatively more abundant in Bitumen II. The difference between the methylnaphthalene ratio (MNR) of Bitumens I and II, when plotted against the ratio of clay to total organic carbon (clay/TOC), gives an excellent correlation ( R 2 = 0.88). The highest clay/TOC ratio corresponds with the biggest difference in MNR for the respective bitumens, consistent with the more stable β isomers being preferentially retained within the clay/kerogen matrix. Since the β MP isomers are higher in Bitumen II than in Bitumen I, it was anticipated that their methylphenanthrene index (MPI-1) values, a commonly used measure of thermal maturity, would also be different. For most samples, the measured MPI-1 in Bitumen I is generally higher than that in Bitumen II (except for three samples), thus showing a bias towards a lower thermal maturity for the second extract. The opposite is true of the β/α MP ratio and MNR. This may be attributed to the fact that phenanthrene (P), which is part of the denominator in MPI-1, is more stable than its methylated counterparts. Steroid and triterpenoid maturity parameters also showed differences between the two extracts, with C 27 diasterane/sterane and Ts/(Ts + Tm) being higher in Bitumen I. Only the former parameter is positively correlated with the clay/TOC ratio, suggesting that clay content probably influences the diagenetic transformation of steroid precursors to diasteranes. The observed differences between the maturity parameters of Bitumen I and Bitumen II may be especially important in sedimentary rocks that contain high percentages of clay minerals.

Structural features of the northern West Siberian geosyneclise and new exploration targets

The northern part of the West Siberian geosyneclise is characterized by a thick sedimentary cover and widespread Triassic sedimentary and volcanosedimentary rocks and Paleozoic platform structures. New targets have been recognized in the basement and deeply buried horizons of the geosyneclise cover. Reservoirs might be found in the following formations: Paleozoic cover deposits, weathering crusts, zones of Paleozoic rock deconsolidation, Triassic sedimentary and volcanosedimentary deposits, buried structures in the lower part of the cover, Lower and Middle Jurassic basal layers, pinch-outs of Jurassic horizons, Upper Jurassic bituminous shales and cavernous carbonates. Exploration of these potential structures will change the structure of the existing resource base toward the long-term replenishment of hydrocarbon resources and a stable rate of production replacement.

Tectonic rotations in the Deseado Massif, southern Patagonia, during the breakup of Western Gondwana

Paleomagnetic investigation in the Deseado Massif, southern Patagonia, suggests that Triassic sedimentary rocks carry a latest Triassic to Jurassic remagnetization and that earliest Jurassic plutonic complexes carry a reversed polarity magnetization of thermoremanent origin. Despite uncertainties in the timing of the observed remanence in the Triassic rocks and the lack of paleohorizontal control on the plutonic complexes, comparison of the derived pole positions with the most reliable Late Triassic–Jurassic apparent polar wander paths indicates that the study areas underwent significant clockwise vertical-axis rotation. In contrast, paleomagnetic results from mid-Cretaceous rocks in the region indicate no rotation. The observed crustal rotations in the Deseado Massif are thus bracketed to have occurred between Jurassic and Early Cretaceous times, documenting southern Patagonian deformation during the breakup of Western Gondwana and then enlarging the regional record of clockwise rotations associated with this event. These results suggest a more complex than previously supposed tectonic evolution of this part of South America.

Characteristics of the Middle-Late Triassic sedimentary facies assemblages in the Songpan-Ruoergai area

Abstract The Middle-Late Triassic sedimentary rocks in the Songpan-Ruoergai area mainly consist of calcareous siltstone, muddy limestone, lithic arkose, feldspathic litharenite, mudstone, wormkslk, oolithic limestone, and conglcrnerate. Except for limestone bed increasing eastward longitudinally and vertidly, the calcareous component of the sandstones increases obviously. Abundant benthic and plant fossils and their clasts occur thin these rocks. The sedimentary stnjctures predominately contain flaser, parallel, tabular, wavy, and herringbone cross beddings. These data coevsJly imply that the Middle-Late Triassic sediments deposited in the fluvial, lake and tide environments. Additionally, the rocks display graded, parallel, hummocky beddings, and sandy lamination, a feature characteristic of storm deposits. *Supported by National Natural Science Foundation of China (Grant No. 40334044) and Geological Surveying Project of China (Project No. 1212010611807)

Characteristics of deep groundwater flow in a basin marginal setting at Sellafield, Northwest England: 36Cl and halide evidence

A number of chemical and physical processes inside and outside a sedimentary basin (e.g. evaporite dissolution and topographic drive, respectively) affect groundwater flow near the basin’s margin. Contrasting formations at the margin, typically basinal sedimentary rocks and basement, are host to the interplay between these processes so that groundwater flows and compositions change within a relatively small volume. To interpret how groundwater flow and geochemistry have evolved, interactions between these processes must be understood. Such interactions were investigated near the margin of the East Irish Sea Basin in NW England, by sampling deep groundwaters (to 1500 m below sea level) from Ordovician volcanic basement rocks and Carboniferous to Triassic sedimentary cover rocks. Variable Br/Cl ratios and Cl concentrations in deep saline waters and brines indicate mixing patterns. Variations in 36Cl/Cl constrain the timing of mixing. Relatively low Br/Cl ratios (ca. 1 × 10 −3 by mass) characterise brine from the western sedimentary cover and reflect halite dissolution further west. Saline water with relatively high Br/Cl ratios (ca. 2 × 10 −3 by mass) of uncertain origin occupies the eastern basement. These two waters mix across the area. However, mixing alone cannot explain variable 36Cl/Cl ratios, which partly reflect differing in situ 36Cl production rates in different rock formations. Most 36Cl/Cl ratios in groundwater sampled from the eastern metavolcanic basement (mean = 25 × 10 −15) and western sedimentary cover (mean = 10 × 10 −15) are at or close to equilibrium with in situ 36Cl production. These variations in 36Cl/Cl across the site possibly took >1.5 Ma to be attained, implying that deep groundwater flow responded only slowly to the Quaternary glaciation of the site. Interplay between varied processes in basin marginal settings does not necessarily imply flow instability.

Imbricate structure of the Luobusa Ophiolite and surrounding rock units, southern Tibet

The Cretaceous Luobusa Ophiolite is a tectonic slice less than 1.2 km thick. The structurally underlying unit is the Tertiary Luobusa Formation and the overlying unit is composed of feebly metamorphosed Triassic sedimentary rocks. The top and bottom contacts of the ophiolite dip gently to the south. The major part of the Luobusa Ophiolite is mantle peridotite, including podiform chromitite. This chromitite has received much attention because it contains an ‘unusual mineral assemblage’ which includes diamond and moissanite. A serpentinite mélange zone, including clasts of basaltic and sedimentary rocks, occurs underneath the mantle peridotite. Mesoscopic–microscopic structures were observed in the sheared rocks. Shear-indicating structures (C′-type shear bands and σ-type porphyroclasts) in the rocks near the top and bottom boundaries of the Luobusa Ophiolite show consistent top-to-the-north (or northeast) reverse displacement. The results reveal that the Luobusa Ophiolite was overturned and intercalated into an imbricate structure. The thrust faults on the top and bottom of the ophiolite can be correlated with north-vergent back-thrusting which was associated with crustal shortening along the Main Central Thrust due to the continued northward movement of India, after the welding of India to Asia.

Geochemistry and evolution of ore-forming fluids of the Yueshan Cu–Au skarn- and vein-type deposits, Anhui Province, South China

The Yueshan mineral belt is geotectonically located at the centre of the Changjiang deep fracture zone or depression of the lower Yangtze platform. Two main types of ore deposits occur in the Yueshan orefield: Cu–Au–(Fe) skarn deposits and Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits. Almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the Yueshan dioritic intrusion. In the vicinity of the Zongpu and Wuhen intrusions, there are many Cu–Pb–Zn–Au–(S) vein-type and a few Cu–Fe–(Au) skarn-type occurrences. Fluid inclusion studies show that the ore-forming fluids are characterised by a Cl −(S)–Na +–K + chemical association. Hydrothermal activity associated with the above two deposit types was related to the Yueshan intrusion. The fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water. In comparison, the hydrothermal activity related to the Zongpu and Wuhen intrusions was characterised by low salinity fluids. Chlorine and sulphur species played an important role in the transport of ore-forming components. Hydrogen- and oxygen-isotope data also suggest that the ore-forming fluids in the Yueshan mineral belt consisted of magmatic water, mixed in various proportions with meteoric water. The enrichment of ore-forming components in the magmatic waters resulted from fluid–melt partitioning. The ore fluids of magmatic origin formed large Cu–Au deposits, whereas ore fluids of mixed magmatic-meteoric origin formed small- to medium-sized deposits. The sulphur isotopic composition of the skarn- and vein-type deposits varies from − 11.3‰ to + 19.2‰ and from + 4.2‰ to + 10.0‰, respectively. These variations do not appear to have been resulted from changes of physicochemical conditions, rather due to compositional variation of sulphur at the source(s) and by water–rock interaction. Complex water–rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing. Lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore-forming fluids. Hydrodynamic calculations show that magmatic ore-forming fluids were channelled upwards into faults, fractures and porous media with velocities of 1.4 m/s, 9.8 × 10 − 1 to 9.8 × 10 − 7 m/s and 3.6 × 10 − 7 to 4.6 × 10 − 7 m/s, respectively. A decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the Triassic sedimentary rocks led to the deposition of the ore-forming components. The major species responsible for Cu transport are deduced to have been CuCl, CuCl 2 −, CuCl 3 2− and CuClOH, whereas Au was transported as Au 2(HS) 2S 2−, Au(HS) 2 −, AuHS and AuH 3SiO 4 complexes. Cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of Cu deposition. Gold deposition was related to decrease of pH, total sulphur concentration and fO 2, which resulted from fluid boiling and mixing. Geological and geochemical characteristics of the two deposit types in the Yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism. Geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 Ma and represent a continuous process during the Yanshanian time. The cooling of the intrusions and the mineralisation event might have lasted about 6 Ma. The cooling rate of the magmatic intrusions was 80 to 120 °C my − 1 , which permitted sufficient heat supply by magma to the ore-forming system.

Gold Mineralization And Zonation In The State Of Kelantan

Gold mineralization in Kelantan is mainly distributed in the central part of the state, bounded by Stong Igneous Complex and Senting Granite on the west, Kemahang Granite in the north and Boundary Range Granite in the east. The gold mineralization is more common in Triassic sedimentary rocks followed by Permian metasedimentary rocks. The oldest Silurian-Ordovician metasedimentary rocks only contain some insignificant gold mineralization, whilst the youngest Cretaceous-Jurassic sedimentary rocks are devoid of gold mineralization. Basically, the gold mineralization are identified as volcanogenic massive sulphides, skarns and hydrothermal quartz vein deposits. Six types of hydrothermal quartz veins can be observed, viz. low sulphide quartz veins, high sulphide quartz veins, quartz veins in sheared granite zones, quartz veins at the boundary of sedimentary rocks, structurally controlled quartz veins in volcanic-sedimentary rocks and metamorphic segregation quartz veins. The main factors contributing succession of gold mineralization are source rocks, heating chamber as well as depositional structures. The principal source rocks are Permian-Triassic volcanic rocks that are associated with sedimentary rocks. The heating chamber that induced the hydrothermal fluids is the granitoind bodies that intruded under the volcanic-sedimentary rocks, whilst the structures which allow the infiltration and deposition of gold are shear zones and fault zones originating from depth. Based on the type of ore deposits, geochemical data and geological setting, the study area can be divided into five zones, namely gold zone (hydrothermal veins), gold-base metal zone (volcanic exhalative), gold-silver­ mercury zone (hydrothermal veins), base metal-gold zone (massive sulphides) and silver-lead-zinc zone (skarn and massive sulphides).

Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field, west of Ireland

Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from − 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ 13C increases from about − 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ 13C then decreases to about − 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ 13C values decreasing to about − 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ 13C correlates with indicators of aridity and biological stress such that the highest δ 13C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ 13C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance.

Geology, mineralogy and fluid inclusion data from the Arapucan Pb–Zn–Cu–Ag deposit, Canakkale, Turkey

The Arapucan Pb–Zn–Cu–Ag deposit occurs as hydrothermal veins in diabase and altered Triassic metasandstones adjacent to calc-alkaline intrusive igneous rocks. The deposit is an important commercial source of base metal in northwestern Turkey. Potential by-products are silver and gold. The geology of the area includes a Paleozoic metamorphic basement, Triassic sedimentary rocks with carbonate blocks, Tertiary granitoids and Neogene volcanics. The mineral assemblage includes galena, quartz, calcite, sphalerite, chalcopyrite and pyrite as well as minor bismuthinite, tetrahedrite, pyrolusite, hematite, scheelite, malachite, magnetite, limonite and rutile. Silver is associated with tetrahedrite. Early hydrothermal activity was responsible for the formation of three hypogene alteration types of decreasing intensity: silicification, sericitization and argillic alteration. These alteration styles show a rough spatial zonation. The ore stage clearly postdates hydrothermal alteration, as indicated by the occurrence of ore minerals in vuggy cavities and fractures in silica bodies. The deposit contains evidence of at least two periods of hypogene mineralization separated by a period of faulting. In addition to Pb, Zn, Cu, Ag and Au, the ores contain substantial quantities of W, Bi, Sb and Te. Average δ 34S values for galena and pyrite are −3.95 and −2.24‰, respectively, suggesting an igneous source for both the sulphur and metals. However, geological and geochemical interpretations suggest that at least some of the metals were leached from the metasandstones and diabases. Fluid inclusions in main-stage sphalerite homogenize at 229–384 °C with salinities ranging from 1.7 to 18.5 eq.wt% NaCl. The deposits formed as the result of the interaction of two aqueous fluids: a higher-salinity fluid (probably magmatic) and a dilute meteoric fluid. The narrow range of δ 34S (galena and pyrite) values (−5.2 to −1.2‰ CDT) suggests that the sulphur source of the hydrothermal fluids was the same during deposition of the metals.

UPPER TRIASSIC‐MIDDLE JURASSIC STRATIGRAPHY AND SEDIMENTOLOGY IN THE NE QAIDAM BASIN, NW CHINA: PETROLEUM GEOLOGICAL SIGNIFICANCE OF NEW OUTCROP AND SUBSURFACE DATA

Although Mesozoic source and reservoir rocks are known to occur at oilfields in the northern Qaidam Basin (NW China), the precise identification and distribution of Mesozoic rocks in the subsurface are outstanding problems. The Dameigou locality has in the past been considered as the type section for Lower‐Middle Jurassic strata in northern Qaidam. Previous studies have concluded that the onset of non‐marine sedimentation here took place in the Early Jurassic; and that Mesozoic strata penetrated by wells in the Lenghu structural zone are Middle Jurassic.In this paper, we present new data from the Lengke‐1 well, drilled in the Lenghu structural zone in 1997. This data indicates the existence of a more extensive pre‐Middle Jurassic stratigraphy than has previously been recognized. Biostratigraphic data together with regional seismic mapping suggest that the pre‐Middle Jurassic succession at Lengke‐1 includes both Late Triassic and Early Jurassic deposits. The Late Triassic sedimentary rocks appear to have been deposited in local half graben, some of which were later inverted during Jurassic, Cretaceous and Cenozoic tectonism.Lower and Middle Jurassic strata (lacustrine and fluvial deposits) are present in the SW and NE parts of the Lenghu structural zone, respectively. Extensive organic‐rich intervals are present in both successions. Lower Jurassic lacustrine mudstones may represent a previously under‐appreciated, and potentially large, source rock sequence.

Geochemical and rock magnetic records from sediments of the Cenozoic Pagodroma Group, Prince Charles Mountains, East Antarctica: implications for provenance and weathering

Geochemical and magnetic data from glacigenic sediments can be useful indicators of past environments and climate. In this paper, the results of X-ray fluorescence (XRF) and rock magnetic analyses are presented for samples from the Pagodroma Group, an assemblage of pre-Quaternary glacimarine sediments preserved as remnants along the raised flanks of the Lambert Graben, Prince Charles Mountains, East Antarctica. Samples were obtained from four formations: Mount Johnston, Battye Glacier, Fisher Bench and Bardin Bluffs, which range in age from late Oligocene or early Miocene through to Pliocene or early Pleistocene. Principal component analysis of the XRF data indicates the occurrence of two main element assemblages, which we infer are determined by the presence in the Bardin Bluffs formation of a significant component derived from Permo–Triassic sedimentary rocks of the Amery Group. Calculation of chemical indices of alteration suggests that the values reflect provenance differences rather than the syndepositional weathering environment. Multidomain ferrimagnetic grains mainly dominate the magnetic mineral assemblages of the samples, probably magnetite, with varying concentrations of high coercivity material, probably hematite.

Stable isotope (C, O, S) systematics of the mercury mineralization at Idrija, Slovenia: constraints on fluid source and alteration processes

The world-class Idrija mercury deposit (western Slovenia) is hosted by highly deformed Permocarboniferous to Middle Triassic sedimentary rocks within a complex tectonic structure at the transition between the External Dinarides and the Southern Alps. Concordant and discordant mineralization formed concomitant with Middle Triassic bimodal volcanism in an aborted rift. A multiple isotopic (C, O, S) investigation of host rocks and ore minerals was performed to put constraints on the source and composition of the fluid, and the hydrothermal alteration. The distributions of the δ13C and δ18O values of host and gangue carbonates are indicative of a fracture-controlled hydrothermal system, with locally high fluid-rock ratios. Quantitative modeling of the δ13C and δ18O covariation for host carbonates during temperature dependent fluid-rock interaction, and concomitant precipitation of void-filling dolomites points to a slightly acidic hydrothermal fluid (δ13C≈−4‰ and δ18O≈+10‰), which most likely evolved during isotopic exchange with carbonates under low fluid/rock ratios. The δ34S values of hydrothermal and sedimentary sulfur minerals were used to re-evaluate the previously proposed magmatic and evaporitic sulfur sources for the mineralization, and to assess the importance of other possible sulfur sources such as the contemporaneous seawater sulfate, sedimentary pyrite, and organic sulfur compounds. The δ34S values of the sulfides show a large variation at deposit down to hand-specimen scale. They range for cinnabar and pyrite from −19.1 to +22.8‰, and from −22.4 to +59.6‰, respectively, suggesting mixing of sulfur from different sources. The peak of δ34S values of cinnabar and pyrite close to 0‰ is compatible with ore sulfur derived dominantly from a magmatic fluid and/or from hydrothermal leaching of basement rocks. The similar stratigraphic trends of the δ34S values of both cinnabar and pyrite suggest a minor contribution of sedimentary sulfur (pyrite and organic sulfur) to the ore formation. Some of the positive δ34S values are probably derived from thermochemical reduction of evaporitic and contemporaneous seawater sulfates.

Paleoweathered surfaces on granitoids of southern Nova Scotia: paleoenvironmental implications of saprolites

In addition to minor postglacial weathering effects, a complex history of three distinct weathering events has been identified within the granitoids of the South Mountain Batholith of southwestern Nova Scotia. Weathering prior to deposition of Carboniferous strata produced a well-developed saprolite, an in situ weathering profile. Feldspars and micas are altered to clay minerals, and only weak preservation of granitic texture is evident. Subsequent burial and relithification have masked the original weathering mineralogy in this horizon. Beneath Triassic clastic sedimentary rocks is a thick horizon (30 m) of weathered granitoid. This argillaceous saprolite with relict biotite exhibits an intensifying-upward weathering zonation typical of weathered horizons, with increased proportions of clay minerals at the upper levels. A younger arenaceous saprolite of pre-Pleistocene age can be found beneath glacial till at a number of locations throughout the batholith. These paleoweathered horizons attest to relatively warmer climates at several times in the geologic past. Their patterns of preservation imply some were much thicker and more widely distributed before partial erosion. The eroded materials may have formed the sediment for quartz sand in the Carboniferous sequence, clay and silica sand deposits of Cretaceous age, and a sandy till of Pleistocene age. The remnants preserved in situ today serve as significant conduits for water, and the geochemistry, mineralogy, and textures of these weathered profiles may enhance mobility and migration of U and Rn. These weathered horizons are mechanically weak, and the possibility of their presence should be considered when selecting construction and waste-disposal sites.

Timing of granite emplacement and cooling in the Songpan–Garzê Fold Belt (eastern Tibetan Plateau) with tectonic implications

New U–Pb and Rb–Sr geochronology on syn- and post-orogenic granites provide constraints on the timing of major tectonic events in the Songpan–Garzê fold belt, west Sichuan, China. The Ma Nai granite was probably syn-kinematic with the main deformation and yields an age of 197±6 Ma that is interpreted as an upper age limit of the Indosinian event. Zircons and apatites from the post-kinematic Rilonguan granite also yield Jurassic ages (195±6 and 181±4 Ma). The post-orogenic Markam massif gives two ages of 188±1 and 153±3 Ma. Both granites are undeformed and cut structures in the Triassic sedimentary rocks. These results demonstrate that the major deformation and décollement tectonics in the Songpan–Garzê fold belt occurred prior to the Early Jurassic. The wide range of ages obtained for post-kinematic granites (from Early Jurassic to Late Jurassic) suggests that, locally, magmatic activity persisted for a long time (at least 50 Ma) after the Indosinian compressional tectonism. No Tertiary ages have been obtained, suggesting that these granites were not affected strongly by the India–Asia collision.

Geology, distribution, and classification of gold deposits in the western Qinling belt, central China

Gold deposits of the western Qinling belt occur within the western part of the Qinling–Dabie–Sulu orogen, which is located between the Precambrian North China and Yangtze cratons and east of the Songpan–Ganzi basin. The early Paleozoic to early Mesozoic orogen can be divided into northern, central, and southern zones, separated by the Shangdan and Lixian–Shanyang thrust fault systems. The northern zone consists of an early Paleozoic arc accreted to the North China craton by ca. 450 Ma. The central zone, which contains numerous orogenic gold deposits, is dominated by clastic rocks formed in a late Paleozoic basin between the converging cratonic blocks. The southern zone is characterized by the easternmost exposure of Triassic sedimentary rocks of the Songpan–Ganzi basin. These Early to Late Triassic turbidities, in part calcareous, of the immense Songpan–Ganzi basin also border the western Qinling belt to the west. Carlin-like gold deposits are abundant (1) along a westward extension of the southern zone defined by a window of early Paleozoic clastic rocks extending into the basin, and (2) within the easternmost margin of the basinal rocks to the south of the extension, and in adjacent cover rocks of the Yangtze craton. Triassic and Early Jurassic synkinematic granitoids are widespread across the western Qinling belt, as well as in the Songpan–Ganzi basin. Orogenic lode gold deposits along brittle–ductile shear zones occur within greenschist-facies, highly-deformed, Devonian and younger clastic rocks of the central zone. Mainly coarse-grained gold, along with pyrite, pyrrhotite, arsenopyrite, and minor base metal sulfides, occur in networks of quartz veinlets, brecciated wall rock, and are disseminated in altered wall rock. Isotopic dates suggest that the deposits formed during the Late Triassic to Middle Jurassic as the leading edge of the Yangtze craton was thrust beneath rocks of the western Qinling belt. Many gold-bearing placers are distributed along the river systems that flow south from the lode-bearing central zone. Carlin-like gold deposits have only been identified during the last decade in the southern zone of the western Qinling and in the northeastern corner of the Songpan–Ganzi basin. The deposits mainly contain micron-diameter gold in arsenical pyrite; are characterized by the common occurrence of cinnabar, stibnite, realgar, and orpiment; exhibit strong silicification, carbonatization, pyritization, and decalcification dissolution textures; and are structurally controlled. The lack of reactive host lithologies may have prevented development of large (>100 tonnes of gold), stratigraphically-controlled orebodies, which are typical of the Carlin deposits in the western USA. These deposits are hosted by Triassic turbidities and shallow-water carbonates, and an early Paleozoic inlier in the Songpan–Ganzi basin that extends in an east–west belt for about 300 km. Rather than true "Carlin" deposits, these Carlin-like deposits may be some type of shallow-crustal (i.e., epithermal) hybrid with features intermediate to Nevada-style Carlin deposits and the orogenic gold deposits to the immediate north. These Carlin-like deposits also overlap in age with the early Mesozoic orogenic gold deposits and, therefore, also formed during the final stages of collision between the cratons and intermediate basin closure.

The Cretaceous Igneous Province of Madagascar: Geochemistry and Petrogenesis of Lavas and Dykes from the Central–Western Sector

, 1999). Similarly, the & Buchan, 1997), crops out in the area between thecontributions of hotspot (usually taken to be plume) Mailaka limestones and the contact between the Permo-mantle and non-hotspot asthenosphere to flood basalt Triassic sedimentary rocks of the Morondava basin andmagmas appear to vary greatly from province to province the Precambrian basement (Fig. 1b). The dykes reach(e.g. Kent

Cenozoic tectonics and landform evolution of the coast and adjacent highlands of southeast New South Wales

This paper has been written firstly to dispel a widely held notion that the structure and stratigraphy of Permian and Triassic sedimentary rocks of the southern Sydney Basin precludes post‐Permian tectonic lowering of the coastal zone of southeast New South Wales with respect to the highlands; and secondly to present evidence of Cenozoic tectonic lowering of the coastal zone by gentle tilting and, locally, by faulting. The apparent horizontal attitude of Triassic sandstone in an east‐west section inland from Nowra precludes later faulting or steep monoclinal folding in this immediate area, but does not preclude geomorphically significant tilting or gentle warping; and it places no constraints on the tectonic and geomorphic history in areas south of the sandstone outcrop. The form of an Oligocene erosion surface around Ulladulla, and the distribution of older rock units below this surface are also not incompatible with significant Cenozoic tectonism. Near Ulladulla, Mogo, Moruya, and Merimbula, mid‐Cenozoic non‐marine sediments and basalts in north‐south‐trending palaeovalleys dip gently seaward at angles around 1°—about the same as the eastward slope of the offshore seismic basement and also similar to the average slope between the coastal zone and inland plateaux; indicating that the coastal zone and continental shelf have been downwarped relative to the inland plateaux in these areas. The area between Brooman, on the Clyde River, and the coast between Termeil and Batemaʼns Bay, has been lowered by Cenozoic faulting. The steep slope on the west side of the Clyde River is a dissected 200–300 m fault scarp, and there are two other east‐facing fault scarps between the Clyde River and the coast. Fault blocks between the Clyde River and the coast are gently backtilted to the west. Further south, there may be more faults and tilt blocks between Batemaʼns Bay and Moruya. Tectonic lowering of the coastal zone and continental shelf appears to be mostly post‐mid‐Cenozoic. This suggests that marine shelf sediments are likely to be not older than Miocene. There could also be older Cenozoic non‐marine sediments and perhaps basalts in palaeovalleys at the base of the section.

Granites of Straža and their sedimentary roof (SW Serbia)

In many localities on N, NE and SE hillslopes of Murtenica Mt. several varieties of red granites are exposed. The roof rock of granites consists of different Lower Triassic sedimentary rocks which discordantly overlie the granites. They are the Kladnica clastites Fm., Seisian clastites, Bioturbate Fm. and Conglomeratic sandstones and conglomerates.