Journal of Palaeogeography最新文献

The shallow marine carbonates of the upper Oligocene Qom Formation yielded several occurrences of the foraminiferan genus Neoplanorbulinella Matsumaru. Neoplanorbulinella saipanensis Matsumaru has so far been recorded from the late Eocene–early Miocene of Japan and late Oligocene of NE Italy whereas N. malatyaensis Gedik is only reported from the Oligocene from its type locality in the Malatya Basin, eastern Turkey. The new records reported here from the Qom Formation indicate that both species occur in the Dobaradar section, ca. 10 km south of the city of Qom in North–Central Iran. The palaeogeographic distribution of these two species therefore has to be extended as far as the palaeolongitude of current Central Iran. The accompanying larger benthic foraminifers, including Miogypsinoides complanatus (Schlumberger), M. formosensis Yabe and Hanzawa, Spiroclypeus margaritatus (Schlumberger), Operculina complanata (Defrance), and Risananeiza pustulosa Boukhary et al., indicate the upper Chattian SBZ 23 Zone. The co-existence of the N. saipanensis and N. malatyaensis points to suitable palaeobiogeographic conditions of Central Iran to host Western and Eastern Tethyan taxa.

After the extinction of Archaeocyatha (sponges), microbial bioherms were well developed in the lower Cambrian of the Yangtze cratonic basin, especially in the Qingxudong Formation (Cambrian (Stage 4)) of the Huayuan area, northwestern Hunan Province, southern China. Herein, four sections from this area were chosen for investigating and analyzing their microfacies and depositional environment. Twelve microfacies types were recognized through petrographic analysis of textural attributes and calcimicrobes (including Epiphyton, Renalcis, Girvanella and Kenella), respectively as: laminated calcareous mudstone (MF1), algal wackestone (MF2), intraclastic grainstone (MF3), algal intrasparrudite (MF4), oolitic algal intrasparrudite (MF5), sparry oolitic grainstone (MF6), Epiphyton framestone (MF7), Renalcis bafflestone (MF8), Kenella bafflestone (MF9), Girvanella boundstone (MF10), thrombolitic boundstone (MF11), and dolomite (MF12). These microfacies represent four major depositional environments: carbonate ramp, carbonate platform, slope and tidal flat. Calcimicrobes played an important role in the transition from carbonate ramp to platform, while the evolution from ramp to tidal flat must be ascribed to sea-level fall during the Cambrian Age 4.

For more than 150 years, geologic characteristics claimed to be evidence for pre-Pleistocene glaciations have been debated. Advancements in recent decades, in understanding features generated by mainly glacial and mass flow processes, are here reviewed. Detailed studies of data offered in support of pre-Pleistocene glaciations have led to revisions that involve environments of mass movements. Similarities and differences between Quaternary glaciogenic and mass movement features are examined, to provide a more systematic methodology for analysing the origins of more ancient deposits. Analyses and evaluation of data are from a) Quaternary glaciogenic sediments, b) formations which have been assigned to pre-Pleistocene glaciations, and c) formations with comparable features associated with mass movements (and occasionally tectonics). Multiple proxies are assembled to develop correct interpretations of ancient strata. The aim is not per se to reinterpret specific formations and past climate changes, but to enable data to be evaluated using a broader and more inclusive conceptual framework.

Regularly occurring pre-Pleistocene features interpreted to be glaciogenic, have often been shown to have few or no Quaternary glaciogenic equivalents. These same features commonly form by sediment gravity flows or other non-glacial processes, which may have led to misinterpretations of ancient deposits. These features include, for example, environmental affinity of fossils, grading, bedding, fabrics, size and appearance of erratics, polished and striated clasts and surfaces (“pavements”), dropstones, and surface microtextures. Recent decades of progress in research relating to glacial and sediment gravity flow processes have resulted in proposals by geologists, based on more detailed field data, more often of an origin by mass movements and tectonism than glaciation.

The most coherent data of this review, i.e., appearances of features produced by glaciation, sediment gravity flows and a few other geological processes, are summarized in a Diamict Origin Table.

The stratigraphic surface represented by a major contact between the Archean Bundelkhand Granite and the extensive Proterozoic Vindhyan sediments is a regional basement cover unconformity. This crystalline–sedimentary interface reflects an intense weathering of continental crust during marine transgression. Three time-transgressive sand deposition events viz. Kaimur, Rewa and Bhander are mainly witnessed during the entire Vindhyan sedimentation. Stratigraphically, the Upper Rewa Sandstone comprises one of these events in the Vindhyan Basin. Considerable progress has been made in interpreting these sandstones as a function of entirely marine process to a combination of tidal–fluvio–eolian activities. All the results have so far been attained on the basis of sedimentary facies, provenance, palaeocurrent analysis, and some of petrography. A distinct differentiation between marine and fluvial components of the Upper Rewa Sandstone still remains uncertain. Here, we use diagenesis as a parameter for the first time along with facies and palaeocurrent analysis to acquire a clear comprehension of marine- and fluvial-dominated processes. The present study spans 27 square kilometer area covering 15 stratigraphic sections with a collection of 571 directional data from the facies specific sedimentary structures, and 28 samples obtained for the thin section analysis.

The lower unit of the Upper Rewa Sandstone shows facies association typical of tidal environment, along with polymodal palaeocurrent. These rocks are well sorted and dominantly lithified by authigenic cement. Profuse development of a complex of syntaxial, passive pore fills, and grain replacive cements reflect normally pressured nature of the lower unit sandstones. The upper unit of the Upper Rewa Sandstone however, reveals a facies pattern resembling fluvial processes and unimodal palaeocurrent plot. They exhibit poor sorting, siliceous, and ferruginous matrix, and development of intense stylolites. Since, pressure solution is a dominant mode of lithification; the upper unit rocks therefore, are overpressured. Sand deposition event pertaining to the Upper Rewa Sandstone is found to comprise both marine and fluvial processes. Such a diagenesis based approach can be applied for marking a possible facies based unconformity between genetically different depositional units.

Early Cretaceous succession of the Oued Fodda Formation in the Ouarsenis Mountains (northwestern Algeria) is mainly composed of marl–limestone alternations, which are subdivided into four informal units (Units 1 to 4), based on distinct lithological, stratonomical, and ichnological features. The ichnological analysis reveals a low diversity of the trace-fossil assemblage, which is exclusively reported from Units 2 and 3. The ichnoassemblage contains six ichnotaxa (Chondrites intricatus, Ophiomorpha isp., Planolites isp., Thalassinoides isp., Zoophycos brianteus, and Zoophycos cauda-galli), among which Zoophycos and Chondrites are the most common elements of the assemblage and occur in distinct mud-rich substrates showing different bioturbation intensities. The development of Zoophycos in the middle part of Unit 2 shows a high degree of bioturbation (bioturbation index (BI) = 4). Zoophycos specimens are of large size, between 45 cm and 75 cm in width, which were interpreted to have formed in a lower offshore environment where the oxygenation amount was optimal, the sedimentation rate was low, and the benthic food was abundant on the seafloor. Toward the upper part of Unit 2, Zoophycos-bearing levels exhibit a less intense degree of bioturbation (BI between 1 and 2) in contrast to Planolites- and Chondrites-bearing levels which have a bioturbation index (BI) between 3 and 4. At these levels, Zoophycos displays relatively small, coiled to U-shaped spreiten, probably in response to stressful and dysoxic conditions prevailing in the water bottom. With improved oxygenation in a quiet lower offshore to shelf margin environment in Unit 3, the benthic organisms recovered, as represented by medium to large size Zoophycos in association with Ophiomorpha and scarce Chondrites burrows, even if the overall bioturbation intensity is very low. The combination of trace-fossil assemblage and lithofacies of the Oued Fodda Formation indicates relatively stable outer shelf environments below the storm wave base, which corresponds classically to the lower offshore to shelf edge environments, and the prevailing palaeoecological conditions are optimal and stressful for the benthic organisms.

The paleoclimate change impacts the sedimentary environment and process, which in turn control the accumulation of organic carbon. Numerous studies have shown that the paleoclimate is controlled by astronomical cycles. Hence, understanding how these cycles impact the accumulation of organic carbon is a critical question that requires in-depth discussion. Previous studies have shown that Milankovitch cycle can be revealed from the sediments of the 7^th Oil Member (Chang 7 Member for short) of the Triassic Yanchang Formation in the southern Ordos Basin, suggesting that the deposition was controlled by astronomically-forced climate changes. Building on previous research, this study collected natural gamma (GR) data of Chang 7 Member from the N36 Well to further analyze astronomical cycles, combined with X-ray fluorescence (XRF) analysis and total organic carbon (TOC) tests, to reconstruct the paleoenvironment and analyze the organic matter enrichment characteristics. The results of this contribution show that, paleoclimate, paleo-redox conditions, paleo-water level, paleo-productivity and sediment accumulation rate (SAR) collectively controlled the enrichment of organic matter. Notably, this study identified the presence of eccentricity, obliquity, precession, and the 1.2 Myr long obliquity cycle in the Chang 7 Member. These cycles controlled the paleoenvironmental changes at different timescales and influenced the enrichment of organic matter, which has implications for subsequent energy exploration.

We present a detailed pollen record and interpretations of late Pleistocene climatic change at the Chongphadae Cave Site, Democratic People's Republic of Korea. The mean annual paleotemperature and mean annual paleoprecipitation of the site were calculated using the temperature index and precipitation index based on ecological features and geographical distribution of each taxon. Temperature index and precipitation index range from 8.8 °C to 10.4 °C and from 805.0 mm to 963.1 mm, respectively. Four dates (radiocarbon, uranium series, fission track, and paleomagnetic excursion dating) of the deposit profile investigated yield a range of ∼21.3 ka BP to ∼117 ka BP, geochronologically corresponding to the late Pleistocene, and extend from the last interglacial highstand through the Last Glacial Maximum. Our results are thus consistent with the climatic shift from interglacial to glacial conditions, provide evidence that the environments of the region, which was reconstructed from the paleoclimatic index, changed from a mild and humid to a cool and dry climate during the late Pleistocene, and suggest trends similar to those of several parts of the Northern Hemisphere which lie in the same latitudinal zone as our study area.

Two deltas developed simultaneously during the Eocene on the eastern side of a large lake that existed in the Dongying Sub-basin, which forms part of the Bohai Bay Basin in eastern China. The rivers that built the deltas had different catchment areas, which resulted in sediments with different permeability and porosity, due to differences in sorting and mud content. Both deltas prograded, and mass flows that originated frequently on their fronts formed lobes that expanded laterally. This eventually led to merging of both deltas, a feature that has rarely been described from ancient deposits thus far. Core analysis and seismic reflection data show that the merging of the two deltas took place in nine phases, determined by phases of slower progradation or even temporary retrogradation in between. The alternation of sediments from both deltas and their eventual mixing makes the architecture of the merged deltas much more complex than that of ‘classical’ single deltas. This affects the predictability of the spatial distribution of possible reservoir characteristics significantly, but detailed core analysis shows that the best hydrocarbon reservoirs consist of sand bodies formed in distributary channels on the delta plains, and of sandy turbidites formed in the deep-lacustrine environment in front of the merged deltas.