1 INTRODUCTION One of the most intriguing phenomena in the late Neoproterozoic (~750 to 543 Ma) is the globa occurrence of thin carbonates that directly overlie glacial deposits in almost every continent (Kennedy 1996; Hoffman et al., 1998; Hoffman and Schrag 2002; Brasier and Shields, 2000; James et al., 2001 Jiang et al., 2003; Nogueira et al., 2003). These “cap carbonates”, commonly several to tens of meters thick, have attracted enormous interests because o their unusually negati… 相似文献
Magnetostratigraphic dating of the fluvio-lacustrine sequence in the Nihewan Basin, North China, has permitted the precise timing of the basin infilling and associated Nihewan mammalian faunas. The combined evidence of new paleomagnetic findings from the Hongya and Huabaogou sections of the eastern Nihewan Basin and previously published magnetochronological data suggests that the Nihewan Formation records the tectono-sedimentary processes of the Plio-Pleistocene Nihewan Basin and that the Nihewan faunas can be placed between the Matuyama-Brunhes geomagnetic reversal and the onset of the Olduvai subchron (0.78-1.95 Ma). The onset and termination of the basin deposition occurred just prior to the Gauss-Matuyama geomagnetic reversal and during the period from the last interglaciation to the late last glaciation, respectively, suggesting that the Nihewan Formation is of Late Pliocene to late Pleistocene age. The Nihewan faunas, comprising a series of mammalian faunas (such as Maliang, Donggutuo, Xiaochangliang, Banshan, Majuangou, Huabaogou, Xiashagou, Danangou and Dongyaozitou), are suggested to span a time range of about 0.8-2.0 Ma. The combination of our new and previously published magnetostratigraphy has significantly refined the chronology of the terrestrial Nihewan Formation and faunas. 相似文献
The regionally extensive, coarse-grained Bakhtiyari Formation represents the youngest synorogenic fill in the Zagros foreland basin of Iran. The Bakhtiyari is present throughout the Zagros fold-thrust belt and consists of conglomerate with subordinate sandstone and marl. The formation is up to 3000 m thick and was deposited in foredeep and wedge-top depocenters flanked by fold-thrust structures. Although the Bakhtiyari concordantly overlies Miocene deposits in foreland regions, an angular unconformity above tilted Paleozoic to Miocene rocks is expressed in the hinterland (High Zagros).
The Bakhtiyari Formation has been widely considered to be a regional sheet of Pliocene–Pleistocene conglomerate deposited during and after major late Miocene–Pliocene shortening. It is further believed that rapid fold growth and Bakhtiyari deposition commenced simultaneously across the fold-thrust belt, with limited migration from hinterland (NE) to foreland (SW). Thus, the Bakhtiyari is generally interpreted as an unmistakable time indicator for shortening and surface uplift across the Zagros. However, new structural and stratigraphic data show that the most-proximal Bakhtiyari exposures, in the High Zagros south of Shahr-kord, were deposited during the early Miocene and probably Oligocene. In this locality, a coarse-grained Bakhtiyari succession several hundred meters thick contains gray marl, limestone, and sandstone with diagnostic marine pelecypod, gastropod, coral, and coralline algae fossils. Foraminiferal and palynological species indicate deposition during early Miocene time. However, the lower Miocene marine interval lies in angular unconformity above ~ 150 m of Bakhtiyari conglomerate that, in turn, unconformably caps an Oligocene marine sequence. These relationships attest to syndepositional deformation and suggest that the oldest Bakhtiyari conglomerate could be Oligocene in age.
The new age information constrains the timing of initial foreland-basin development and proximal Bakhtiyari deposition in the Zagros hinterland. These findings reveal that structural evolution of the High Zagros was underway by early Miocene and probably Oligocene time, earlier than commonly envisioned. The age of the Bakhtiyari Formation in the High Zagros contrasts significantly with the Pliocene–Quaternary Bakhtiyari deposits near the modern deformation front, suggesting a long-term (> 20 Myr) advance of deformation toward the foreland. 相似文献
In 2001 a partial skeleton of an Iguanodon was discovered in the Upper Weald Clay (Barremian, Early Cretaceous) at Smokejacks Brickworks near Ockley, Surrey, UK. When the dinosaur was excavated, a detailed stratigraphic section was logged and 25 samples taken for palynological and micropalaeontological (ostracod and megaspore) analysis, including a detailed sample set of the dinosaur bed itself. Qualitative and quantitative analysis of the palynoflora revealed rich and well-preserved non-marine assemblages of pollen and spores, including early angiosperms, and freshwater green algae. Four types of angiosperm pollen are described and assigned to the genus Retimonocolpites Pierce, 1961, but left in open nomenclature. Some marine elements such as dinoflagellate cysts are identified as the result of reworking of Middle and Upper Jurassic sediments. The pollen/spore assemblages depict a vegetational change from principally gymnosperm-dominated assemblages at the base to principally pteridophyte-dominated assemblages at the top of the section. The dinosaur bed shows a pteridophyte-dominated assemblage, with a significantly high amount of the freshwater green alga Scenedesmus novilunaris He Cheng-quan et al., 1992. Samples close to the dinosaur bed yielded the first useful ostracod finds from Smokejacks Brickworks: well-preserved assemblages containing Cypridea clavata (Anderson, 1939), Damonella cf. pygmaea (Anderson, 1941), Stenestroemia cf. cressida Anderson, 1971 and Stenestroemia sp. A, and fragments and damaged valves of a thin-shelled ostracod, possibly belonging to Mantelliana Anderson, 1966. Those identified as Cypridea clavata show a wide range of morphological variety and in opposition to Anderson's (1967, 1985) taxonomic scheme, which would assign them to up to five different taxa, they are considered to be intraspecific variants of a single species. The possibilities and limitations of age determination of the Wealden sediments using palynomorphs and ostracods are discussed; distinct forms of early angiosperm pollen, together with the ostracod fauna, are consistent with an early Barremian age. Pollen and spores are discussed in terms of their parent plants and the reconstruction of vegetation and palaeoclimate. Palynology and ostracods give evidence for temporary freshwater conditions at the time when the Iguanodon died and the carcase was buried. 相似文献
This is a critical assessment of the paper by Oszczypko et al. (2004: Cretaceous Research 25, 89–113), in which they tried to prove a mid-Cretaceous age for the Szlachtowa (“black flysch”) and Opaleniec Formations, in the Pieniny Klippen Belt, West Carpathians, both of which had previously been shown to be of Jurassic age. We argue that the mid-Cretaceous age assignment is a misinterpretation, primarily resulting from their field samples having been collected from some Cretaceous lithostratigraphic units, tectonically associated with the Jurassic formations, and/or from tectonic contact-breccias involving Jurassic and Cretaceous strata. In addition, we suggest that they have overlooked a number of significant palaeontological papers, published since 1962, which record the presence of in situ ammonites, aptychi, belemnites, thin-shelled bivalves (Bositra), gryphaeids, foraminifera, and ostracod assemblages, all indicating a Jurassic (mainly Aalenian), and not a Cretaceous, age for the Szlachtowa Formation, and also the in situ Jurassic (Bajocian) ammonites and thin-shelled bivalves (Bositra), Bositra-microfacies, and age-diagnostic foraminiferal assemblages of the Opaleniec Formation.Our presentation here of recently published dinocyst data from well-preserved assemblages further supports the Jurassic ages for the Szlachtowa (“black flysch”) and Opaleniec Formations. 相似文献
A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuquén Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuquén Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France. 相似文献