Lower Triassic Sedimentary Rocks Research Articles

Provenance of upper Permian–Triassic sediments in the south of North China: Implications for the Qinling orogeny and basin evolution

The Triassic Qinling Orogenic Belt was formed during the closure of the Paleo-Tethys Ocean, and its evolution can be traced by provenance analysis of sediments in the south of North China. We present detrital zircon ages, sandstone petrographic observations, paleocurrent data, and paleogeographic reconstructions that allow the evolution of this basin–mountain system to be reconstructed. Detrital zircons from upper Permian sedimentary rocks yield two major age peaks at 1876 and 259 Ma, and two minor age peaks at 2526 and 363 Ma. These zircons were likely sourced from recycled sediments in the Southern North China Block. Detrital zircons from Lower Triassic sedimentary rocks have multiple age peaks at ca. 2500, 1850, 950, 745, 450–430, and 310–250 Ma, reflecting a mixed provenance from the Qinling Orogenic Belt and Southern North China Block. Zircons from a Middle Triassic sandstone have a concentrated age peak at ca. 250 Ma and two subordinate age peaks at ca. 1800 and 2500 Ma. Hafnium isotope data for the late Paleozoic detrital zircons are characterized by negative εHf(t) values of −23.11 to −0.11, and only two grains have positive εHf(t) values of +1.45 and +2.10. We infer that these zircons were derived from recycled sediments in the Southern North China Block. The provenance data show that the North Qinling Belt and Southern North China Block had been uplifted prior to the Early Triassic. The uplift was mainly related to the continental collision between the South China and North China blocks. These and previously published data indicate the Southern North China Block experienced exhumation during the Middle−Late Triassic, firstly involving Permian cover rocks, followed by Cambrian and Neoproterozoic strata, and finally Mesoproterozoic basement rocks. The fold-and-thrust belt in the Southern North China Block probably formed during the Middle Triassic. Uplift and denudation of the Qinling Orogenic Belt controlled Triassic basin evolution in the south of North China.

Scarcity of chert in upper Lower Triassic Panthalassic deep-sea successions of Japan records elevated clastic inputs rather than depressed biogenic silica burial flux following the end-Permian extinction

Lower Triassic sedimentary rocks deposited in pelagic regions of Panthalassa show anomalously clay-rich and silica-poor lithologies compared with the deep marine chert-dominated successions that characterise older and younger strata. The end-Permian mass extinction, which impacted radiolarians, has been proposed as an explanation for this so-called “chert gap” through its inferred impact on biogenic silica production. To test this hypothesis, we evaluated linear sedimentation rates (LSRs) for an upper Olenekian (upper Lower Triassic) siliceous claystone succession in Tsukumi, Oita Prefecture, Southwest Japan. Correlation of conodont biohorizons allows extrapolation of absolute ages obtained by previous studies in South China onto our studied sections. Results show that LSR is higher in the Lower Triassic claystone than in typical Mesozoic radiolarian bedded chert. This implies that increased burial flux of clastic material, not decreased biogenic silica burial flux, drove deposition of clay-rich sediments in pelagic Panthalassa during the late Olenekian. • Lithostratigraphy of upper Lower Triassic pelagic Panthalassic deep-sea claystone. • Most detailed conodont biostratigraphy from Olenekian Panthalassic deep-sea strata. • Much of the Lower Triassic claystone was formed by high inputs of clastic material. • Collapse in radiolarian deposition during the Early Triassic needs reconsideration.

Paleomagnetic Constraints of the Lower Triassic Strata in South Qinling Belt: Evidence for a Discrete Terrane Between the North and South China Blocks

AbstractThe paleoposition of the South Qinling Belt is a crucial key to understanding the collisional process between the North and South China blocks. In order to identify the paleogeography of the South Qinling Belt during the Early Triassic and its correlation to the North and the South China blocks, combined paleomagnetic studies have been conducted on the Lower Triassic sedimentary rocks in the South Qinling Belt. After systematic thermal demagnetization, a high‐temperature characteristic remanence was isolated from 133 specimen embracing 22 sites with a mean direction ofD= 330.5°,I= 27.7° (α95= 2.5°). The positive fold and reversal tests suggest that the characteristic remanence was acquired during deposition. This characteristic remanence corresponds to a mean paleopole atφp= 7.2°E,λp= 62.5°N (A95= 2.4°), indicating a paleolatitude of ~23.6°N for the South Qinling Belt. These new data are in concordance with the coeval paleomagnetic pole of the North China Block but are distinctly different from that of the South China Block with ~10.0° paleolatitude and ~73.6°declination differences. Together with the regional geology, our new data suggest that the South Qinling Belt had amalgamated with the North China Block along the Shangdan Suture by the Early Triassic times; however, it was still separated from the South China Block by the Mianlue Ocean up to the Early Triassic. The final collision between the North and South China blocks resulted from a ~73.6° clockwise rotation of the South China Block relative to the South Qinling Belt along the Mianlue Suture during the Late Triassic.

Gondwana fragments in the Eastern Alps: A travel story from U/Pb zircon data

We compare detrital U/Pb zircon age spectra of Carboniferous and Permian / Lower Triassic sedimentary rocks from different structural positions within the Austroalpine nappe pile with published ages of magmatic and metamorphic events in the Eastern Alps and the West Carpathians. Similarities between sink and possible sources are used to derive provenance of sediments and distinct frequency peaks in sink and source age pattern are used for paleogeographic plate tectonic reconstructions. From this, travel paths of Austroalpine and West Carpathian basement units are traced from the Late Neoproterozoic to the Jurassic. We place the ancestry of basement units on the northeastern Gondwana margin, next to Anatolia and the Iranian Luth-Tabas blocks. Late Cambrian rifting by retreat of the Cadomian Arc failed and continental slivers re-attached to Gondwana during a late Cambrian / early Ordovician orogenic event. In the Upper Ordovician crustal fragments of the Galatian superterrane rifted off Gondwana through retreat of the Rheic subduction. An Eo-Variscan orogenic event at ~390 Ma in the Austroalpine developed on the northern rim of Galatia, simultaneously with a passive margin evolution to the south of it. The climax of Variscan orogeny occurred already during a Meso-Variscan phase at ~350 Ma by double-sided subduction beneath Galatia fragments. The Neo-Variscan event at ~330 Ma was mild in eastern Austroalpine units. This orogenic phase was hot enough to deliver detrital white mica into adjacent basins but too cold to create significant volumes of magmatic or metamorphic zircon. Finally, the different zircon age spectra in today's adjacent Carboniferous to Lower Triassic sediments disprove original neighbourhood of basins. We propose lateral displacement of major Austroalpine and West-Carpathian units along transform faults transecting Apulia. The intracontinental transform system was released by opening of the Penninic Ocean and simultaneous closure of the Meliata Hallstatt Ocean as part of the Tethys.

Late Cretaceous exhumation and uplift of the Harz Mountains, Germany: a multi-method thermochronological approach

The Harz Mountains represent one of the most prominent surface expressions of Late Cretaceous intraplate shortening in Central Europe. We present a comprehensive low-temperature thermochronological data set (zircon and apatite, fission track and [U–Th]/He) covering the exhumed Paleozoic basement of the Harz Mountains and the adjacent Kyffhauser block, as well as Lower Triassic sedimentary rocks of the western and southern rim of the Harz Mountains. Integration of results with sedimentological data from the syntectonic Late Cretaceous Subhercynian Basin allows for a detailed reconstruction of the timing of uplift and erosion of the Harz Mountains. The data reveal that (i) tectonic reorganization and initial exhumation started at around 90 Ma, (ii) uplift and emergence caused erosion of the Mesozoic sedimentary cover between 86–85 Ma and 83–82 Ma, and (iii) erosion of at least 3–4 km of underlying Paleozoic rocks followed and continued into the Paleogene. The thickness of removed overburden amounts to at least 6 km, and most erosion occurred in Santonian to Campanian time at minimum rates of ~ 0.5 km/Myr. The southwestern rim of the Harz has exhumed slower over a longer period of time, and may record a phase of Late Cretaceous, syntectonic sediment accumulation.

Integrated rock magnetic and mineralogical study of the Lower Triassic sedimentary rocks from Spitsbergen

A rock magnetic, mineralogical and Mössbauer spectroscopy study was undertaken on the Lower Triassic sediments from the West Spitsbergen Fold and Thrust Belt (WSFTB) in Svalbard as well as the foreland of this orogen for a thorough recognition of magnetic mineralogy, necessary for a better-constrained interpretation of paleomagnetic results. SEM and Mössbauer spectroscopy indicate the presence of magnetite and pyrrhotite, both most probably of secondary origin but altered clasts of volcanic origin and grains of ilmenite and detrital magnetite are also observed. Thermomagnetic curves of whole-rock samples exhibit paramagnetic behavior whereas ferromagnetic separates indicate the presence of magnetite. IRM component analysis shows three major contributors – magnetite, hematite, and pyrrhotite to the remanence, with ferromagnetic iron oxides the most dominant. Identified ferromagnetic minerals display a wide range of grain-size and magnetic behavior, with the domination of PSD particles. Rock magnetic characteristics of both areas are similar, but sites in the fold belt were most likely remagnetized, whereas those in the foreland carry the primary magnetization. The age of remagnetization was estimated as pre-Eurekan related to rift-controlled Mesozoic extensional phases of the Barents Sea Shelf (Clark et al., 2014) and/or Cretaceous injections of dolerites in the Svalbard Archipelago (Nejbert et al., 2011) as well as early stages of the WSFTB formation. However, Triassic remanence might persist in volcanic lithoclasts and detrital magnetite indicated along the WSFTB. The multicomponent nature of the NRM, displaying a broad range of grain size, might impede the paleomagnetic interpretation of Triassic rocks in Spitsbergen.

A Detailed Study on the Magnetic Mineralogy of the Lower Triassic Sedimentary Rocks from Spitsbergen

A Detailed Study on the Magnetic Mineralogy of the Lower Triassic Sedimentary Rocks from Spitsbergen

The Strandja Massif and the İstanbul Zone were once parts of the same palaeotectonic unit: new data from Triassic detrital zircons

ABSTRACTSpatially continuous rock assemblages that share similar environmental evolution or structural features can be classified as a single tectonic unit. This approach enables to link dispersed units or massifs with each other and sometimes can be subjective, depending on the classification criteria. The relationship and the nature of the contact between the Strandja Massif and the İstanbul Zone have been controversial due to the Cainozoic cover. Amalgamation of these units was claimed as early as the Aptian-Albian.Lower Triassic sedimentary rocks, which are overlain by the Carboniferous flysch with a N-verging thrust fault are exposed NW of the İstanbul Zone. This study reveals the spatial relationship between the Strandja Massif and the İstanbul Zone deduced from the U-Pb dating and Lu-Hf isotopes of the detrital zircons from these Lower Triassic clastics. Our results show that the early Triassic basin was fed from a provenance that included arc-related Upper Carboniferous-Lower Permian magmatic rocks which is much more likely to be the Strandja Massif than the İstanbul Zone. The second outcome of this study is that a unit that previously assigned to Palaeozoic turned out to be Triassic, which brings the Strandja Massif farther to the east, into the northern İstanbul Zone.

Tentative identification of diagenetic products of cyclic biphytanes in sedimentary rocks from the uppermost Permian and Lower Triassic

Biphytanes, which are derived from glycerol dialkyl glycerol tetraether (GDGT) lipids, are biomarkers for the presence of Archaea in geological samples. They (especially cyclic structures) are rarely reported from sediments or oils that are older than the Triassic period, where their first record may be traced back to the Late Archean. There have been only two studies that have reported diagenetic products of cyclic biphytanes using two dimensional gas chromatography. In this study, we used full scan and multiple reaction monitoring (MRM) mass spectrometry (MS) to tentatively assign a series of diagenetic products of cyclic biphytanes within uppermost Permian and Lower Triassic sedimentary rocks. We observed three distinct groups derived from mono-, bi-, or tricyclic biphytanes, echoing previous studies. Each group could be further divided into three discrete series according to structure and composition. Using MRM also revealed that numerous sedimentary rocks contained such diagenetic products, but at trace concentration below the limit of detection of full scan MS. This indicates that the presence of Archaea-containing GDGT lipids has been underestimated within paleoenvironments. Episodic occurrences of biphytanes in the Permian-Triassic transition also provide additional evidence for the distribution of Archaea in the geological past.

Predominance of archaea-derived hydrocarbons in an Early Triassic microbialite

We investigated the distribution of lipids in Lower Triassic sedimentary rocks (252–247myr) from South China, including a shallow water microbialite in the uppermost section of the outcrop. Archaeal derived hydrocarbons were the major constituents of the microbialite from the latest Early Triassic. Among these, we detected (i) abundant C40 acyclic and monocyclic biphytanes (possibly derived from glycerol dialkyl glycerol tetraether lipids) and their degradation products, C30–39 pseudohomologues and (ii) a C25 head-to-tail linked (regular) isoprenoid hydrocarbon [possibly derived from dialkyl glycerol diether lipids (DGDs)] and its degradation products, C21–24 pseudohomologues and abundant pristane and phytane. Through combination of compound-specific stable carbon isotope analysis of isoprenoid hydrocarbons, which had average δ13C values of −35‰ to −30‰, and their molecular distribution, it was not possible to unambiguously define the archaeal source for the biphytanes in the microbialite. The δ13C values for pristane and phytane were similar to those for head-to-tail linked C21–25 isoprenoids; potential source organisms for these compounds were halophilic archaea. Except for methane seep microbialites, no other ancient or recent phototrophic microbialites have been reported to contain predominantly archaeal isoprenoid hydrocarbons. Our findings suggest the presence of a new type of microbialite.

Evolution of depositional settings in the Torrey area during the Smithian (Early Triassic, Utah, USA) and their significance for the biotic recovery

This work focuses on well‐exposed Lower Triassic sedimentary rocks in the area of Torrey (south‐central Utah, USA). The studied Smithian deposits record a large‐scale third‐order sea‐level cycle, which permits a detailed reconstruction of the evolution of depositional settings. During the middle Smithian, peritidal microbial limestones associated with a rather low‐diversity benthic fauna were deposited seaward of the tidal flat siliciclastic red beds. Associated with siliceous sponges, microbial limestones formed small m‐scale patch reefs. During the late middle to late Smithian interval, the sedimentary system is characterized by tidal flat dolostones of an interior platform, ooid‐bioclastic deposits of a tide‐dominated shoal complex, and mid‐shelf bioclastic limestones. Microbial deposits, corresponding to sparse stromatolites formed in the interior platform, are contemporaneous with a well‐diversified marine fauna living in a seaward shoal complex and mid‐shelf area. The nature and distribution of these Smithian microbial deposits are not related to any particular deleterious environmental condition, highlighting that observed patterns of biotic recovery after the end‐Permian mass extinction were directly influenced by depositional settings. Facies evolution and stratal stacking patterns allow us to identify large, medium and small‐scale, as well as elementary depositional sequences. Large‐ and medium‐scale sequences are consistent with sea‐level changes, whereas small‐scale and elementary sequences are better explained by autocyclic processes. Copyright © 2015 John Wiley & Sons, Ltd.

A sedimentary archive of tectonic switching from Emeishan Plume to Indosinian orogenic sources in SW China

Upper Permian to Middle Triassic sedimentary rocks in the Youjiang Basin, SW China, record a change from a within-plate mafic-dominated source to the NW, to a mixed source involving magmatic arc and recycled orogenic detritus that lay to the west and east. Upper Permian and some Lower Triassic sedimentary rocks are characterized by relatively high contents of TiO 2 and Nb, and low ratios of Al 2 O 3 /TiO 2 and Th/Nb. Detrital zircons yield U–Pb ages of c . 260 Ma and have geochemical affinities to those crystallized from the within-plate type magmas. These features, combined with extensive basaltic lithic fragments and plagioclase grains in the sandstones, imply a sedimentary source dominated by the Emeishan Large Igneous Province exposed to the NW of the basin. The Early and Middle Triassic marked a significant change in sediment provenance, with rocks displaying geochemical affinity to upper continental crust. U–Pb ages and trace elements of detrital zircons from this material show input from multiple sedimentary sources including the subduction–collision rocks related to the Indosinian Orogeny to the west of the basin and the recycled Precambrian–early Palaeozoic sedimentary and granitic rocks in the South China hinterland to the east of the basin. The Early Triassic change in the nature of the source supplying detritus to the Youjiang Basin, together with regional sedimentary and tectonothermal events, suggests a tectonic transition from the Emeishan Plume to the Indosinian Orogeny in SW China. Regional stratigraphic correlation and provenance data suggest that the Emeishan basalts provided a large sediment influx for the riverine–littoral–marine systems in the Late Permian. This basaltic particulate and dissolved input may have contributed significant unradiogenic Sr to the Late Permian seawater. Supplementary material: Details of sample preparation, analytical procedure and results, and additional figures are available at www.geolsoc.org.uk/SUP18698 .

Microbial ooids and cortoids from the Lower Triassic (Spathian) Virgin Limestone, Nevada, USA: Evidence for an Early Triassic microbial bloom in shallow depositional environments

Lower Triassic sedimentary rocks contain a variety of unusual facies and fabrics, with microbialites being a distinctive component of many carbonates deposited following the Permian–Triassic mass extinction. Coated grains are common in shallow water facies from the upper Lower Triassic (Spathian) Virgin Limestone (Moenkopi Formation) in southern Nevada, and were investigated in order to determine their origin. Petrographic analysis reveals that the majority of the coated grains found within the Virgin Limestone are micritic ooids with a concentric fabric, or with a homogenous fabric composed of dense, often cloudy micrite. In addition, asymmetric ooids, aggregate grains, and distorted ooids are also locally common in some oolitic units; low-Mg calcite ooids and bimineralic ooids composed of low-Mg calcite and dense, cloudy micrite are less commonly found, but are also documented from the Virgin Limestone. Cortoids (i.e., grains that are coated with constructive micrite envelopes) are a minor component of oolitic grainstones and packstones (typically 10–15% of the grains), although they may also comprise entire beds. The cortoids are coated with micrite similar to that which comprises the ooid cortices, and may be finely laminated or dense and cloudy in nature. The micrite ooids and constructive micrite envelopes are interpreted as microbial in origin based on the finely laminated or cloudy, dense nature of the micrite, as well as coatings that are uneven, or often of greater thickness on one side of elongate nuclei, such as bivalve shells or phylliod algae blades. The origin of the low-Mg calcite ooids and layers is less certain, but may also be microbial. The results of this study suggest that a microbial bloom occurred in shallow water environments, which was the result of 3 factors: (1) the unusual chemistry of Early Triassic oceans; (2) runoff of nutrient-rich waters, which enhanced microbialite growth; and, (3) wave agitation and warm waters that led to CO2 degassing and further supersaturation of shallow waters with respect to calcium carbonate.

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.

Evidence for right-lateral transfer faulting accommodating en echelon Miocene extension, Mojave Desert, California

In the southwestern United States, two early Miocene extensional terranes, one along the Colorado River trough and the other in the central Mojave Desert, have been inferred to be part of an en echelon rift system linked by a ductile detachment at depth. Evidence from pre-Tertiary surface geology in the central and western Mojave Desert suggests that a zone of east- to northeast-trending right-lateral strike-slip faulting may have transferred extension between these two areas. These data include the apparent offsets of the Late Jurassic age Independence dike swarm, a Middle to Late Jurassic age deformation belt, Triassic(?)-Jurassic volcanic rocks, Lower Triassic sedimentary rocks, and late Paleozoic age stratigraphic and tectonic elements. This inferred fault zone probably served as the main shallow-crustal kinematic link between extension in the central Mojave Desert and in the Colorado River trough.

Widespread late Mesozoic to Recent remagnetization of Paleozoic and lower Triassic sedimentary rocks from South China

Paleomagnetic results are described from eighteen formations sampled in the Nanjing (Jiangsu Province) and Yichang (Hubei Province) areas of South China. The marine platform carbonates and elastics range in age from early Cambrian to early Triassic. Progressive alternating field and/or thermal demagnetization analyses reveal predominantly single component magnetizations, sometimes of both normal and reverse polarity. These magnetizations, isolated in 457 of the 549 samples studied, typically have directions in geographic coordinates coincident with late Mesozoic to Recent fields. A secondary origin for the magnetizations is supported by negative fold tests in several units. A lower Carboniferous and possibly a lower Triassic limestone may have escaped the pervasive remagnetization; if the remanences isolated are indeed prefolding, a large counterclockwise tectonic rotation of the Nanjing area is implied by comparison with available Carboniferous and Triassic paleopoles reported from elsewhere in South China. This rotation may have been associated with Mesozoic left-lateral motion on the Tancheng-Lujiang fault zone.