查看更多>>摘要：The stratigraphic records of sedimentary basins situated on the Lhasa block provide crucial evidence to reconstruct the early history of the Tibetan Plateau associated with the successive closure of oceanic branches and related magmatic activities. The Upper Jurassic to Lower Cretaceous Lagongtang Formation of the Nagqu Basin located in the northern part of the Lhasa block chiefly consists of fine-grained quartzose to litho-quartzose sandstones, siltstones and shales deposited in continental shelf to bathyal environments. Provenance data, including sandstone petrography, U-Pb ages and in situ Hf isotopes of detrital zircon, together with paleocurrent indicators, suggest that the Lagongtang Formation was mainly fed by recycling of quartz-rich siliciclastic strata of the central-southern Lhasa block. Fresh felsitic volcanic rock fragments, plagioclase prevailing among detrital feldspars, and zircon grains yielding the same age as that indicated by biostratigraphic analysis testify to penecontemporaneous magmatic activity. Volcanic detritus may have been derived from the Middle Triassic Lower Cretaceous Yeba and/or Sangri volcanics exposed in the southern Lhasa block to the south as well as from Middle Triassic-Lower Cretaceous magmatic rocks of the central Lhasa block. Although the Nagqu Basin is adjacent to the Bangong-Nujiang suture, no debris from Bangong-Nujiang suture and Qiangtang was found in the Lagongtang Formation here. Our provenance data rule out the hypothesis that the Lagongtang Formation could have been deposited in a forearc basin associated with the southward subduction of the Bangong-Nujiang Ocean or in a basin related to the supposed Shiquanhe-Namco Ocean.
查看更多>>摘要：The Cenozoic crustal deformation of the Asian continent is constrained by the location of its southern margin in the Late Cretaceous, which is, however, affected by potential remagnetization and inclination shallowing of redbeds in the Linzhou Basin. Our paleomagnetic results from the Shexing Formation redbeds in the Late Cretaceous yield a mean paleo-inclination of similar to 21.5 degrees, consistent with the recalculation of previous data. Rock-magnetic, paleomagnetic and petrographic analyses indicate that the redbeds have recorded the original paleomagnetic field and have not suffered from significant inclination shallowing. The paleopole from 70 red beds sites is at 71.8 degrees N, 281.5 degrees E with A(95) = 2.6 degrees, yielding a paleolatitude of similar to 12.0 +/- 2.6 degrees N at -70-91 Ma for the Linzhou Basin. This result indicates a latitudinal convergence of 1264 +/- 477 km north of the sampling area within Asia since the Late Cretaceous. Combined with previous data, our results show that the southern margin of Asia was at similar to 11-15 degrees N in the Late Cretaceous, aligning in a WNW-ESE direction, which is similar to its presentday orientation. A review of available data shows that the sharp bending of the southern margin of Asia around the eastern Himalayan syntaxis (EHS) is probably due to different crustal movement patterns around the EHS. The shape of the southern margin of Asia determines the arc-shaped orientation of Himalayan orogen.
查看更多>>摘要：The Silurian conodont biozonation has been well established in many areas of the world, but not yet in China. The Nyalam region of southern Xizang (Tibet) is one of the best locations for studying Silurian conodonts in China. In this paper, we establish five conodont biozonal units from the Pulu Formation: the Ozarkodina sagitta sagitta Biozone, the Kockelella crassa Biozone, the Kockelella variabilis variabilis-Ancoradella ploeckensis Zonal Group, the Polygnathoides siluricus Biozone, and the "Ozarkodina" eosteinhornensis s.l. interval Biozone. We assign the Pulu Formation to the Llandovery (upper Telychian) to Pridoli stages based on the conodonts as well as the graptolites from underlying strata. The Silurian conodont species in the Nyalam region are widely distributed in the strata from the Laurussia and Gondwana, which indicates the cosmopolitan characteristic of the conodont fauna from the Wenlock to Pridoli epochs was caused by the circulation of ocean currents.
Laura Musotto, LorenaMaria Borromei, AnaSoledad Candel, MariaMehl, Adriana...
15页
查看更多>>摘要：In the present study, we carried out palynological (pollen, spores, fungal remains, and freshwater algae) and pedological analyses from three paleosols (Ps6, Ps7, and Ps8) that developed in the upper part of the Arturo perched dune deposits, in the periphery of the homonymous lake, and compared the results with modern palynological assemblages from the Fuegian steppe in southern Argentina. These proxies offer an exceptional opportunity to enhance our knowledge of environmental conditions during the Holocene in the southern high latitudes. Pollen spectra from the paleosols Ps6 (middle Holocene), Ps7 and Ps8 (late Holocene) were dominated by Asteraceae subf. Asteroideae undifferentiated, which suggests that herbaceous and/or shrub vegetation probably developed locally on the dune. The existence of herbivorous grazers is indicated by the record of ascospores of several coprophilous fungi (Coniochaeta cf. lignaria, cf. Delitschia pachylospora, cf. Schizothecium sp., Sordaria-type, Sporormiella-type). Most other fungal remains (Glomus, cf. Cryptendoxyla hypophloia, Microthyriaceae, Alternaria sp., Dictyosporium sp.) are likely associated with the presence of steppe vegetation communities. In general, the paleosols displayed a low degree of pedological development and in one of them (Ps7) there is evidence of the re-activation of eolian processes. Taken together, our palynological and pedological results, as well as sedimentological and geomorphological data, indicate that Ps6 developed under less humid conditions than the overlying paleosols, Ps7 and Ps8. The variations in moisture availability during soil-forming intervals could be related to shifts in the latitudinal position and/or strength of westerlies throughout the middle to late Holocene.
查看更多>>摘要：A middle Coniacian-middle Santonian interval has been studied in the Kopet-Dagh Basin (NE Iran) to reconstruct the paleoenvironmental conditions in the eastern part of Tethyan realm at the time of the last Cretaceous Oceanic Anoxic Event (OAE3). This succession is composed of a chalky limestone unit, sandwiched by light gray calcareous mudstone and marlstone beds. The delta C-13 curve indicates several minor positive shifts up to 1 parts per thousand in the studied section that is correlative to other carbon isotope curves from the Tethys and Boreal realms. A prominent upward increase of P/B ratio in the studied section suggests a relative sea-level rise toward the Santonian. Lithology (low carbonate contents) and faunal contents (high %benthic and higher infaunal benthic foraminifera) reveal that the lower part of the studied section was deposited in a neritic environment influenced by continental-derived nutrients and mesotrophic to eutrophic conditions. Enhanced carbonate contents, higher diversity of benthic foraminifera and dominance of epifaunal taxa suggest more oligotrophic conditions and/or higher oxygen concentrations in the interval equivalent with a 813C positive shift, caused by a transgression phase at this time. The overlying deposits are characterized by the maximum flooding surface and the highstand phase. The stratification of water column and more oxygen consumption in this interval resulted in higher abundance of infaunal benthic foraminifera and decline of carbonate production in a bathyal environment.
查看更多>>摘要：Paleozoic sediment provenance of the Permian Basin remains controversial. This study presents new detrital zircon morphology, U-Pb ages, and Hf isotope data of eight Paleozoic sandstone samples, and integrates published zircon U-Pb data in the basin to illustrate the spatial and temporal changes of sediment provenance. The Cambrian sample has predominantly Mesoproterozoic (1500-1300 Ma) grains because of fluvial transport of basement detritus from the local Texas Arch. The Ordovician shoreface sandstone contains mostly rounded grains of Paleoproterozoic (1800-1600 Ma) age, mostly likely by longshore drift reworking detritus from the Trans-continental Arch. The Pennsylvanian sediment provenance shows heterogeneity. While samples from the Northwestern Shelf have both the Mesoproterozoic and Paleoproterozoic groups and low grain roundness, reflecting sediment delivery from basement rocks in the Ancestral Rocky Mountains, a sample in the Midland Basin contains a broad range of ages, including a major Neoproterozoic (800-500 Ma) group that were primarily from peri-Gondwana terranes. The difference suggests that there was limited sediment mixing in the Permian Basin. The similarity of zircon age distributions between the Midland Basin and Fort Worth Basin confirms synorogenic progradation of detritus from the Fort Worth Basin to the Eastern Shelf during the Pennsylvanian. During the early Permian Cisuralian epoch, the compiled U-Pb data show that the southern and eastern basin contains a significant fraction of Gondwanan detritus, and the amount reduced by the latest Cisuralian as the orogenesis waned. The middle Permian Guadalupian samples on the Northwestern Shelf contain well-rounded grains that are of mainly Grenvillian and Paleozoic ages, suggesting sediment recycling from Laurentia by wind and likely by river as well. The change of sediment provenance from the early to the middle Permian suggests that after the continental collision, Laurentian sediments dominated detritus in the Permian Basin.
查看更多>>摘要：To better understand the breakup mechanism of eastern Gondwana, U-Pb Zircon geochronological and wholerock geochemical studies were performed on the Zhela and Weimei formations volcanic rocks in the Zhuode area of the eastern Tethyan Himalaya. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons yielded 206Pb/238U ages of ~135-130 Ma, indicating that the volcanic rocks from Zhela and Weimei formations erupted during the Early Cretaceous. Whole rock analyses of major and trace elements show that the volcanic rocks are characterized by high content of TiO2 (1.84 wt%-4.03 wt%) and P2O5 (0.29 wt%-0.61 wt%), highly fractioned in LREE/HREE [(La/Yb)N = 9.12-14.53], and most of them have no obvious anomaly of Eu. The characteristic, highly enriched HFSE with no obvious anomaly of Nb and Ta, is similar to those of alkali basalts originated from the Kerguelen mantle plume. Our geochemical and geochronological results, combined with recent paleomagnetic results show that the original erupted position of the Zhela and Weimei formations volcanic rocks was just located in the center of the presently Kerguelen mantle plume, and support that the Kerguelen mantle plume led to the breakup of eastern Gondwana.
Yuzhanina, E. D.Ivanov, S. N.Afonin, A. S.Kostomarov, V. M....
14页
查看更多>>摘要：Lacustrine-peat sediments from the Oskino-09 borehole in Western Siberia, which span the last 7.5 cal ka BP, were sampled for pollen and plant macro-remains to better understand regional climatic changes and the position of the forest-steppe border. Analysis of palynological assemblages indicate that meadow-steppe landscapes occupied the largest area in the middle of the Holocene (7.5-4.7 cal ka BP) due to reduced effective moisture during a warm climate interval. A subsequent gradual decrease in temperatures and evaporation led to an increase in effective moisture and emergence of birch groves during 4.7-2.0 cal ka BP, with short interruptions at ca. 3.3 and 2.5 cal ka BP. The expansion of pine forests and the advance of taiga to the south were recorded at ca. 2.0 cal ka BP. There is little evidence for significant human impact on forest-steppe belt evolution, and specifically evidence for anthropogenic deforestation on the southern border of the taiga is lacking. Human activities did not start to impact forests until the emergence of semi-nomadic cattle breeding in the Iron Age. A methodological finding of this paper is that we show that the Landscape Reconstruction Algorithm (LRA) is not always reliable and often overestimates forested areas.
Bialik, O. M.Reolid, J.Kulhanek, D. K.Hincke, C....
17页
查看更多>>摘要：During the early and middle Miocene, the Mediterranean had become a restricted marginal marine sea with diminishing and ultimate loss of connectivity to the Indian Ocean. This dramatically changed the heat, energy, freshwater and nutrient budgets across the Mediterranean and most notably in its eastern basin. While one of the most prominent lines of evidence of this change in the Eastern Mediterranean is the onset of sapropel formation, many other aspects of the sedimentary system changed in response to this rearrangement. Here we present a detailed analysis of a hemipelagic succession from southwestern Cyprus dated to the late Aquitanian to the early Serravallian (22.5-14.5 Ma). This sequence is carbonate-dominated and formed during the decoupling of the Mediterranean Sea and the Indian Ocean. It exhibits sedimentation with mass transport contribution from shallow water carbonates to deeper facies with phosphatization and bottom current (at intermediate depth) interactions. This succession traces both local subsidence and loss of a local carbonate factory. Additionally, it records a shift in bottom current energy and seafloor ventilation, which are an expected outcome of connectivity loss with the Indian Ocean.
Giraldo-Gomez, Victor M.Petrizzo, Maria RoseErba, ElisabettaBottini, Cinzia...
19页
查看更多>>摘要：The paleoenvironmental impact of the early Aptian Ocean Anoxic Event 1a (OAE 1a, ca. 121 Ma) has been investigated in detail in the Cismon Core (Lombardy Basin, western Tethys) by using different geochemical and micropaleontological proxies. We provide the first high-resolution data of benthic foraminiferal assemblages through the upper Barremian -lower upper Aptian stratigraphic interval. Benthic foraminifera data are integrated with calcareous nannofossil and planktonic foraminifera records to create a comprehensive characterization of bottom and surface waters across OAE 1a. Benthic foraminiferal communities are indicative of a marked change in bottom-waters around the "nannoconid decline" (latest Barremian) due to increased flux of organic matter to the seafloor and intermittent dysoxic conditions probably promoted by pulses of higher productivity during the initial Greater Ontong Java Event (GOJE). Benthic foraminifera experienced a marked crisis in abundance ("benthic foraminiferal crisis" BFC) ca. 35 kyr before the OAE 1a, in correspondence with the "nannoconid crisis" and the onset of the most intense GOJE phase. The literature survey shows that the BFC is commonly recorded before the OAE 1a onset in several stratigraphic sections worldwide, and therefore, it is here proposed as a global event. At Cismon, deep-water anoxia was reached at the OAE 1a onset and lasted for ca. 300 kyr, promoted by higher productivity and eventually enhanced water stratification during the super-greenhouse climate. The continuation of OAE 1a was marked by a benthic foraminiferal repopulation event, probably resulting from the influx of relatively cooler and oxygenated waters. In turn, the OAE 1a was marked by intermittent anoxic to dysoxic conditions, likely in response to primary productivity sustained by N-fixing bacteria. The distribution and abundance of benthic foraminifera documented in other sections across the Selli Level equivalent show different features that point to local factors such as paleodepth and increased runoff. The post-OAE 1a was characterized by dysoxic conditions coupled with moderate organic matter flux to the seafloor. At the Cismon site and worldwide, the termination of OAE 1a coincided with the return of relatively more abundant benthic taxa in response to the restoration of favorable conditions allowing the development of diversified benthic foraminiferal communities.
NSTL
Elsevier