首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
A first palynostratigraphic scheme of Upper Triassic deposits in northern Switzerland was established based on spore-pollen associations and dinoflagellate cyst records from the upper part of the Upper Triassic Klettgau Formation and the lower part of the Lower Jurassic Staffelegg Formation. Drill cores from the Adlerberg region (Basel Tabular Jura) and from Weiach (northern part of Canton Zurich) as well as from an outcrop at the Chilchzimmersattel (Basel Folded Jura) were studied and five informal palynological associations are distinguished. These palynological associations correlate with palynological association of the Central European Epicontinental Basin and the Tethyan realm and provide a stratigraphic framework for the uppermost Triassic sediments in northern Switzerland. Throughout the uppermost Triassic to Jurassic palynological succession a remarkable prominence of Classopollis spp. is observed. Besides Classopollis spp. the three Rhaetian palynological associations A to C from the Upper Triassic Belchen Member include typical Rhaetian spore-pollen and dinoflagellate taxa (e.g., Rhaetipollis germanicus, Geopollis zwolinskae, Rhaetogonyaulax rhaetica, and Dapcodinium priscum). Association B differs from association A in a higher relative abundance of the sporomorph taxa Perinopollenites spp. and the consistent occurrence of Granuloperculatipollis rudis and Ricciisporites tuberculatus. Spore diversity is highest in the late Rhaetian palynological association C and includes Polypodiisporites polymicroforatus. A Rhaetian age for the Belchen Member is confirmed by palynological associations A–C, but there is no record of the latest Rhaetian and the earliest Jurassic. In contrast to the Rhaetian palynological associations the Early Jurassic associations W and D include Pinuspollenites spp., Trachysporites fuscus (in association W), and Ischyosporites variegatus. In the view of the end-Triassic mass extinction and contemporaneous environmental changes the described palynofloral succession represents the pre-extinction phase (associations A and B) including a distinct transgression, the extinction phase (association C) associated with a regression, and the post-extinction phase (association W).  相似文献   

2.
Quantified organic-walled dinoflagellate cyst (dinocyst) assemblages are presented for two sedimentary successions deposited in neritic environments of the Tethys Ocean during the Barremian and Aptian in an attempt to reconcile established dinocyst biostratigraphic schemes for Tethyan and Austral regions. One section is at Angles, southeast France (the Barremian stratotype section); the other is at Deep Sea Drilling Project Site 263, off northwest Australia. We also construct a carbon isotope record for Site 263 using bulk organic carbon.Both sections contain abundant, well-preserved dinocyst assemblages. These are diverse, with 89 taxa identified at Angles and 103 taxa identified at Site 263. Of these, more than 93% are cosmopolitan. When combined with other work at Angles and Site 263, we found that nine dinocysts have their first occurrence (FO) or last occurrence (LO) at both locations. These dinocyst events are, in alphabetical order: LO of Cassiculosphaeridia magna, FO of Criboperidinium? tenuiceras, LO of Kleithriasphaeridium fasciatum, LO of Muderongia staurota, FO of Odontochitina operculata, LO of Phoberocysta neocomica, FO of Prolixosphaeridium parvispinum, FO of Pseudoceratium retusum var. securigerum, and FO of Tehamadinium sousense. Although these events support a Barremian–Aptian age for both sections, their stratigraphic order is not the same in the sections. The δ13Corg record at Site 263 displays a characteristic series of changes that have also been recorded in other carbon isotope curves spanning the Late Barremian–Early Aptian. Such independent dating (along with ammonite zones at Angles) suggests that three of the nine dinocyst events are approximately isochronous at Angles and Site 263: the LO of K. fasciatum in the mid Barremian, the FO of P. retusum var. securigerum and the FO of C.? tenuiceras in the earliest Aptian; the other six dinocyst events are diachronous. Dinocyst assemblages at Site 263 can be loosely placed within existing Australian zonation schemes, providing much-needed calibration. Our data suggest that the Muderongia testudinaria Zone ends in sediments of mid Barremian age, the succeeding Muderongia australis Zone extends into the Early Aptian, and the younger Odontochitina operculata Zone begins in Early Aptian deposits. The boundary between the M. australis and O. operculata zones, and the Ovoidinium cinctum (as Ascodinium) Subzone, positioned at the top of the M. australis Zone when present, could not be recognized incontrovertibly. Interestingly, however, this horizon broadly correlates with the onset and extent of the Selli Event, a time of major biogeochemical change.  相似文献   

3.
Data on spores, pollen, and dinoflagellate cysts studied in composite section of Oligocene-Miocene deposits in southern part of West Siberia are presented. Eleven biostratigraphic units distinguished in the section are ranked as palynozones and beds with palynological assemblages. Palynological data substantiate age of deposits and specify ranges and boundaries of palynozones. Based on dinocyst assemblages first studied in sediments of the Zhuravka and Abrosimovo horizons (upper Oligocene, lower Miocene), the Pseudokomewuia Beds are included into local stratigraphic scheme. According to results of comparative analysis, similar and distinctive features of Oligocene-Miocene dinocyst assemblages from West Siberia, China and North America are elucidated. Based on palynological data, the local stratigraphic scheme of higher resolution is suggested for subdivision of Oligocene and Miocene deposits in southern part of West Siberia (Baraba and Kulunda lithofacies regions).  相似文献   

4.
Benthonic foraminifera indicate that part of the Middle Miocene Brasso Formation, central Trinidad (planktonic foraminiferal Zones N11–N12), was deposited during a regression. A stratigraphic sequence of five foraminiferal assemblages reflects changes in paleodepth and dissolved oxygen concentration and indicates that the relative fall in sea level brought the seabed into contact with an oxygen minimum zone (OMZ): (1) Assemblage 1 (Uvigerina quesqueyana, Siphonina pulchra) lived in upper bathyal, moderately oxygenated water beneath the OMZ; (2) Assemblage 2 (S. pulchra, Cassidulina laevigata, lesser Globocassidulina subglobosa) lived in outer neritic, moderately oxygenated water below the OMZ; (3) Assemblage 3 (U. subperegrina) occupied the outer neritic, lower margin of the OMZ; (4) Assemblage 4 (Brizalina subaenariensis, U. subperegrina) lived at the core of the OMZ and rates the lowest on the Benthonic Foraminiferal Oxygen Index; and (5) Assemblage 5 (middle-neritic species with few Uvigerina spp. and Brizalina spp.) lived in well-oxygenated water above the OMZ. The onset of the severest oxygen depletion was abrupt and occurred shortly after the N11–N12 boundary. Previous work on the Brasso Formation has reported a similar sequence of benthonic assemblages in planktonic foraminiferal Zones N8–N10. These assemblages may be useful for local correlation of the Brasso Formation.  相似文献   

5.
Organic-walled phytoplankton assemblages from Eocene and Oligocene reference sections of western Kamchatka are analyzed. They are close in taxonomic composition to coeval assemblages of northern Japan and Sakhalin that is a good opportunity to correlate regional units and verify their age. The euryhaline species Trinovantedinium boreale that is associated with Paralecaniella indentata and Micrhystridium preferring shallow low-salinity waters, on the one hand, and taxa dwelling in normal-salinity open sea environments, on the other, are dominant in most assemblages, which are examined. The suggested variant of bed succession with dinocyst assemblages may be valid for Japan, Sakhalin, Kamchatka, and the Bering Sea.  相似文献   

6.
This study of the upper Maastrichtian to Danian sedimentary succession from the northern part of the Romanian Eastern Carpathians (Varniţa section) aims to establish an integrated biostratigraphy based on calcareous nannofossils, organic-walled dinoflagellate cysts (dinocysts) and foraminiferal assemblages, and to reconstruct the depositional environments of the interval. The stratigraphic record across the studied section is incomplete, considering that an approximately 16 m thick strata interval from the top of the Maastrichtian to lowermost Danian cannot be analyzed due to a landslide covering the outcrop. The upper Maastrichtian is marked by a succession of biostratigraphic events, such as the First Appearance Datum (FAD) of the nannoplankton taxon Nephrolithus frequens and FAD of the dinocyst species Deflandrea galeata and Disphaerogena carposphaeropsis, and the Last Appearance Datum (LAD) of Isabelidinium cooksoniae in the lower part of the section. These bioevents are followed by the LAD of the Dinogymnium spp. and Palynodinium grallator dinocyst markers in the top of the Maastrichtian deposits analyzed. In terms of foraminiferal biostratigraphy, the upper Maastrichtian Abathomphalus mayaroensis Zone is documented in the lower part of the studied section. Some bioevents, such as the bloom of the calcareous dinoflagellate genus Thoracosphaera and the FAD of the organic-walled dinocysts Damassadinium californicum, Senoniasphaera inornata, Xenicodinium lubricum and X. reticulatum suggest an early Danian age for the middle part of the section. From the Danian deposits in the Varniţa section, we describe a new organic-walled dinocyst species, Pentadinium darmirae sp. nov., which is until now the only species of the Pentadinium genus discovered in the Paleocene. The occurrence of the global Danian dinocyst marker Senoniasphaera inornata in the top of the section, suggests an age not younger than middle Danian (62.6 Ma) for the analyzed deposits.The palynofacies constituents, as well as the agglutinated foraminiferal morphogroups, used to reconstruct the depositional environments, show that the late Maastrichtian sediments were deposited in an outer shelf to distal (bathyal) environment, followed by a marine transgression during the Danian.  相似文献   

7.
《China Geology》2022,5(3):439-456
This study identified two palynological assemblages, namely Bayanhuasporites-Cycadopites-Protoconiferus and Cicatricosisporites-Cedripites-Perinopollenites, in the Tongbomiao Formation in the Hongqi Sag in the Hailar Basin, Inner Mongolia, China for the first time. The former is distributed in the lower part of the Tongbomiao Formation and is characterized by abundant gymnosperm pollen and diverse fern spores. Among them, the gymnosperm pollen is dominated by Paleoconifer (4.98%–31.62%) and Cycadopite (8.55%–25.23%) pollen grains and also includes other pollen grains such as Classopollis, Parcisporites, Erlianpollis, Callialasporites, and Jiaohepollis. The fern spores in the former palynological assemblage contain Bayanhuasporite (0–8.96%), Granulatisporites (0.93%–6.97%), and some important Cretaceous genera, such as Cicatricosisporites, Concavissimisporites, Densoisporites, Hsuisporites, Foraminisporis, and Leptolepidites. The Cicatricosisporites-Cedripites-Perinopollenites palynological assemblage is distributed in the upper part of the Tongbomiao Formation. Gymnosperm (77.30%), Pinaceae (31.9%), and Paleoconiferus (19.02%) pollen predominate this palynological assemblage, and Quadraeculina, Erlianpollis, and Jiaohepollis pollen are also common in this assemblage. The fern spores in this palynological assemblage include abundant Cicatricosisporites (4.29%). Besides, Concavissimisporites, Aequitriradites, and Leptolepidites are also common in this palynological assemblage. No angiosperm pollen has been found in both palynological assemblages. The identification of both palynological assemblages provides important evidence for the biostratigraphic correlation between the Hailar Basin and its adjacent areas. It also enables the reconstructions of the Berriasian-Valanginian (Early Cretaceous) vegetation and the paleoclimate on the eastern Mongolian Plateau during 141–132 Ma. The vegetation reconstructed on the palynological data of the represented by Hailar Basin in eastern Mongolian Plateau (141.6–141.4 Ma), form conifer forest or conifer broad-leaved mixed forest to conifer forest with shrubs and grassland, the climate belongs to warm temperate and warm-subtropicalt, the highest temperature is estimated to reach 35–38°C. Form 132.3 Ma, the vegetation type is conifer forest, and its paleoclimate is sub-humid warm temperate, the highest temperature is estimated to reach 24–29°C.©2022 China Geology Editorial Office.  相似文献   

8.
The Paleocene-Eocene transition is one of the most remarkable Cenozoic periods coinciding with the global thermal maximum (PETM). Based on the complex biostratigraphic analysis of diatoms, silicoflagellates, and dinocysts, this global event is revealed in three sections of the Middle Trans-Urals region (Kamyshlov, Korkino, Chumlyak) represented by marine biosiliceous sediments of the Serov and Irbit formations. The interval of the Trinacra ventriculosa-Hemiaulus proteus-Coscinodiscus uralensis diatom zones is marked by the appearance of new genera Moisseevia, Solium, Fenestrella, Craspedodiscus, Podosira, Pseudotriceratium, intense radiations of Grunoweiella and Coscinodiscus, and development of extreme morphotypes among silicoflagellates. The defined diatom assemblages differ slightly from their coeval counterparts from the Middle Volga region, which casts doubt upon the assumed stable water exchange between these basins. On the contrary, the dinocyst assemblages are lacking zonal index species (Appectodinium homomorphum, A. augustum) and Apectodinium acme characteristic of the transitional Paleocene-Eocene strata in many worldwide localities. The facies settings of his period with intense vertical mixing and relatively low temperatures and salinity are characterized by the dominant role of dinocysts belonging to the genera Areoligera, Deflandrea, Spiniferites, and Operculodinium.  相似文献   

9.
The Berriasian-Barremian biostratigraphy based on spores and pollen of terrestrial plants found in sections from northern Siberia shows a succession of eight units. The ages of the units are proved by calibration against the Boreal zonal standard and their stratigraphic position is controlled by faunal and dinocyst records. The sections of the study are depth-stacked over one another, with partial overlap, and include several isochronous levels marked by the same palynological features. The levels are defined by changes in the taxonomic composition of spore-pollen assemblages found in the sections and recognized using published evidence from northern Siberia. Most of the identified boundaries of the palynostratigraphic units provide a good potential for northern Siberian regional correlation.  相似文献   

10.
The study of nannofossils and dinoflagelate cysts from the Paleocene-Eocene transition in the Nasypnoe section, Eastern Crimea identified the bed corresponding to the global event referred as the Paleocene-Eocene Thermal Maximum (PETM). The assemblages of both groups of microphytoplankton display significant changes including the appearance of Rhomboaster spp., Discoaster anartios and D. araneus nannofossils and Apectodinium augustum and Wilsonidium pechoricum dinocysts featured for this event and major variations in the ratio of taxa resulted in domination of eutrophic and warm-water species. The paleoecological interpretation of nannofossil and dinocyst distribution suggests a drastic sea-level fall preceded the PETM and occurrence of two transgressive episodes during it.  相似文献   

11.
The Dalichai Formation with an age of Late Bajocian-Late Callovian was sampled in Central Alborz Mountains of northern Iran and studied for palynological, palaeobiogeographical and palynocorrelation purposes. Palynological studies revealed diverse and well-preserved dinoflagellate cyst assemblages and lead to identification of three zones i.e., Cribroperidiniumcrispum (Late Bajocian), Dichadogonyaulaxsellwoodii (Bathonian to Early Callovian) and Ctenidodiniumcontinuum (Early to Middle Callovian) Zones. Subzone a of the D. sellwoodii Zone (Early to Middle Bathonian) was also differentiated. This biozonation corresponds to those recognised in Northwest Europe. Furthermore, the ammonoid families recorded including Phylloceratidae, Oppeliidae, Reineckeiidae, Perisphinctidae, Haploceratidae, Parkinsoniidae and Sphaeroceratidae, which confirm the Late Bajocian to Late Callovian age, are quite similar to those of Northwest Europe and the northwestern Tethys. The close similarities of the dinoflagellate cyst assemblages and ammonite fauna of northern Iran with those of Northwest Europe and the northwestern Tethys during the Middle Jurassic indicate direct but episodic marine connection and faunal exchange between the two areas.  相似文献   

12.
Diverse radiolarians (over 70 species) are detected in cherty rocks above the bituminous shale horizon, the marker of anoxic event OAE-2 recorded across the Cenomanian-Turonian boundary in the upper part of the Ananuri Formation of flyschoid deposits, the Lazarevskoe area of the western Caucasus. The radiolarian assemblages studied are comparable in composition with radiolarians from concurrent Cenomanian-Turonian boundary strata in other Mediterranean regions (e.g., in the Crimea and Turkey). The lower radiolarian assemblage includes index species Dactyliosphaera silviae of synonymous Cenomanian zone. Alievium superbum present in the upper assemblage is index species of the relevant Turonian zone. Within the studied flyschoid sequence, sediments indicative of the above event (bituminous shales and cherts) are confined to upper elements of flysch rhythms.  相似文献   

13.
Shallow-water carbonates are invaluable archives of past global change. They hold the record of how neritic biologic communities reacted to palaeoenvironmental changes. However, attempts to decipher these geological archives are often severely hampered by the low stratigraphic resolution attained by biostratigraphy. This is particularly the case for the Upper Cretaceous carbonate platforms of the central Tethyan realm: their biostratigraphy suffers from very low resolution and poor correlation with the standard biochronologic scales based on ammonites, planktic foraminifers and calcareous nannoplankton.In this paper we show how this problem can be tackled by integrating biostratigraphy with isotope stratigraphy. We present a detailed record of the benthic foraminiferal biostratigraphy and carbon and strontium isotope stratigraphy of three upper Cenomanian-middle Campanian sections belonging to the Apennine Carbonate Platform of southern Italy. For the upper Cenomanian-Turonian interval, the carbon isotope curves of the studied sections are easily correlated to the reference curve of the English Chalk. The correlation is facilitated by the matching of the prominent positive excursion corresponding to the Oceanic Anoxic Event 2. For the Coniacian-middle Campanian interval, the correlation is mainly based on strontium isotope stratigraphy. We use the 87Sr/86Sr ratios of the low-Mg calcite of well preserved rudist shells to obtain accurate chronostratigraphic ages for many levels of the three studied sections. The ages obtained by Sr isotope stratigraphy are then used to better constrain the matching of the carbon isotope curves.From the high-resolution chronostratigraphic age-model stablished by isotope stratigraphy, we derive the chronostratigraphic calibration of benthic foraminiferal biostratigraphic events. For the first time the benthic foraminiferal biozones of the Apennine Carbonate Platform can be accurately correlated to the standard ammonite biozonation. This result is of great relevance because the biostratigraphic schemes of other carbonate platforms in the central and southern Tethyan realm are largely based on the same biostratigraphic events.  相似文献   

14.
Upper Maastrichtian deposits formed in a nearshore subtidal environment within the Valdenoceda Formation (Castilian Ramp, North Iberian margin) are described together with two recently found selachian assemblages. Rare earth element concentrations (REE) have been used to assess the degree of taphonomic mixing and reworking, observing that it is minor or non-existent, and differences in degree of preservation and ecologic mixing can be explained by biostratinomic processes. The patterns of REE also helped to obtain a better understanding of the depositional environment, including the diagenetic history from burial to final degree of bone preservation.The fossil assemblages here described are close to that of the late Maastrichtian of Albaina (in the enclave of Condado de Treviño, Burgos), both in the Basque-Cantabrian Region, but their age may be slightly older (early late Maastrichtian). In total, the new assemblages consist of 17 taxa, assigned to 11 genera of shallow-water dwellers combined with individuals from the outer shelf. They represent cosmopolitan taxa (Squalicorax pristodontus, Serratolamna serrata and Rhombodus binkhorsti) together with local species (Rhinobatos echavei, Rhinobatos ibericus). Although there are not significant differences between Albaina and Quintanilla la Ojada faunas, the new assemblages add interesting taphonomic and geochemical information to the few existing uppermost Cretaceous deposits with fossil sharks in southwestern Europe.  相似文献   

15.
Two outcrop sections spanning the Paleocene–early Eocene boundary in the Sidi Nasseur–Wadi Mezaz area in northwest Tunisia provided rich ostracode assemblages, yielding 26 species of which three are newly described: Reymenticosta bassiounii, Reymenticosta nasseurensis and Buntonia? tunisiensis. The recorded ostracode fauna and associated foraminifera reflect deposition in a coastal to inner neritic environment. Many of the recorded taxa have a wide geographic distribution throughout the Middle East and North Africa. A correspondence is also observed with West African faunas, especially in the early Eocene fauna. These taxa seem to have originated in West Africa during the Paleocene and migrated northwards during the late Paleocene to early Eocene. Sea-level change and decrease in oxygenation associated with the Paleocene–Eocene thermal maximum (PETM) caused the local disappearance of the South Tethyan Paleocene fauna represented by Paracosta kefensis (morphotype-A), Paracosta aff. paleomokattamensis, Paracypris sp. B Esker, Loxoconcha saharaensis, Buntonia sp. 3 Donze et al., Protobuntonia nakkadii, and probably Reymenticosta bassiounii and R. nasseurensis. Simultaneously, a new but poorly diverse Afro-Tethyan fauna, mainly represented by Alocopocythere attitogonensis and Buntonia? tunisiensis, settled in the studied part of the basin. After the PETM, diversity increased again as various taxa (e.g. Bairdia aegyptiaca, Reticulina lamellata and Aegyptiana duwiensis) (re)appeared. Although detailed records across the P/E boundary are still sparse, it appears that the PETM exerted significant influence on the paleobiogeography and composition of Tethyan ostracode faunas.  相似文献   

16.
This paper presents the integrated results of palynofacies and geochemical analyses in Turonian carbonate strata from the Sergipe Basin, Brazil. Forty-three outcrop samples were analyzed from sections of the Cotinguiba Formation in the Votorantim and Rita Cacete quarries. Our results allow for the characterization of the organic matter and interpretation of the environments of deposition. Together with the existing geochemical data, the results allow for recognition of oxic and dysoxic intervals and inferences pertaining to local oceanographic conditions to explain these environmental changes. The palynofacies groups are dominated by marine palynomorphs and amorphous organic matter (AOM), with minor amounts of terrigenous palynomorphs and phytoclasts. The total organic carbon (TOC) and δ13C ratio are directly correlated with the dominant palynofacies elements, particularly AOM. The upper section in Rita Cacete quarry has the most complete data set, and the TOC, δ13C and marine-derived AOM in this section tend to decrease upsection and serve to differentiate the oxic and dysoxic intervals. The sections also contain abundant, low-diversity dinoflagellate assemblages composed primarily of Trichodinium boltenhagenii, Cribroperidinium? muderongense, Canningia reticulata and Xenascus plotei. The palynofacies and dinoflagellate assemblages indicate deposition in shallow marine to outer neritic environments. The dysoxic events are recognized by an increase in TOC and δ13C, high amounts of AOM, an abundance of the low-diversity dinoflagellate assemblages and the lowest amounts of terrestrial components. The changes in marine productivity seem to be related to periods of low-intensity upwelling, which likely decreased the levels of dissolved oxygen in the basin.  相似文献   

17.
This study is based on calcerous nannofossil assemblages changes and fluctutions of stable carbon and oxygen isotopes was collected clayey limestones, limestones, and marls in the Maastrichtian to Selandian from Akveren Formation (Western Black Sea). As the relative abundances of species of Micula spp, Watznauera barnesiae, and Arkhangelskiella cymbiformis, which tolerated changes of temperature and nutrition, carbon and oxygen isotopes compositions, and low species richness imply strong diagenesis effect at the Maastrichtian, there is no important diagenesis effect at Paleocene. Just after the Cretaceous/Tertiary (K-Pg) boundary, Thoracosphaera spp. and Braarudosphaera bigelowi were dominant species; Danian is characterized by Thoracosphaera, Ericsonia ovalis, Cruciplacolithus spp., Coccolithus pelagicus, and Ericsonia subpertusa. Generally, the nutrition productivity is good–moderate in Lower Maastrichtian, and decreasing carbon isotope values during the Uppermost Maastrichtian shows the presence of oligotrophic environmental conditions suitable with global nutrition crises before the K-Pg boundary and diagenesis in study area. Throughout the Danian, mesotrophic–oligotrophic environmental conditions dominate; however, the decrease in nutrition before Selandian represents oligotrophic environmental conditions. The increasing nutrition at Selandian is related to the change in the environmental conditions.  相似文献   

18.
Measured lithostratigraphic sections of the classic Permian–Triassic non-marine transitional sequences covering the upper Quanzijie, Wutonggou, Guodikeng and lower Jiucaiyuan Formations at Dalongkou and Lucaogou, Xinjiang Province, China are presented. These measured sections form the framework and reference sections for a range of multi-disciplinary studies of the P–T transition in this large ancient lake basin, including palynostratigraphy, vertebrate biostratigraphy, chemostratigraphy and magnetostratigraphy. The 121 m thick Wutonggou Formation at Dalongkou includes 12 sandstone units ranging in thickness from 0.5 to 10.5 m that represent cyclical coarse terrigenous input to the lake basin during the Late Permian. The rhythmically-bedded, mudstone-dominated Guodikeng Formation is 197 m and 209 m thick on the north and south limbs of the Dalongkou anticline, respectively, and 129 m thick at Lucaogou. Based on limited palynological data, the Permian–Triassic boundary was previously placed approximately 50 m below the top of this formation at Dalongkou. This boundary does not coincide with any mappable lithologic unit, such as the basal sandstones of the overlying Jiucaiyuan Formation, assigned to the Early Triassic. The presence of multiple organic δ13C-isotope excursions, mutant pollen, and multiple algal and conchostracan blooms in this formation, together with Late Permian palynomorphs, suggests that the Guodikeng Formation records multiple climatic perturbation signals representing environmental stress during the late Permian mass extinction interval. The overlap between the vertebrates Dicynodon and Lystrosaurus in the upper part of this formation, and the occurrence of late Permian spores and the latest Permian to earliest Triassic megaspore Otynisporites eotriassicus is consistent with a latest Permian age for at least part of the Guodikeng Formation. Palynostratigrahic placement of the Permian–Triassic boundary in the Junggar Basin remains problematic because key miospore taxa, such as Aratrisporites spp. are not present. Palynomorphs from the Guodikeng are assigned to two assemblages; the youngest, from the upper 100 m of the formation (and the overlying Jiucaiyuan Formation), contains both typical Permian elements and distinctive taxa that elsewhere are known from the Early Triassic of Canada, Greenland, Norway, and Russia. The latter include spores assigned to Pechorosporites disertus, Lundbladispora foveota, Naumovaspora striata, Decussatisporites mulstrigatus and Leptolepidites jonkerii. While the presence of Devonian and Carboniferous spores and Early Permian pollen demonstrate reworking is occurring in the Guodikeng assemblages, the sometimes common occurrence of Scutasporites sp. cf. Scutasporites unicus, and other pollen, suggests that the Late Permian elements are in place, and that the upper assemblage derives from a genuine transitional flora of Early Triassic aspect. In the Junggar Basin, biostratigraphic data and magnetostratigraphic data indicate that the Permian–Triassic boundary (GSSP Level) is in the middle to upper Guodikeng Formation and perhaps as high as the formational contact with the overlying Jiucaiyuan Formation.  相似文献   

19.
Benthic foraminiferal fauna are analyzed quantitatively and qualitatively at three stratigraphic sections in Eastern Desert of Egypt (Serai, Duwi, and Um El Huetat). These sections embrace the Paleocene–Eocene Thermal Maximum (PETM) interval which is represented by the occurrence of five distinctive beds. These beds constitute the Dababiya Quarry Member at the lower part of Esna Formation. The occurrence of them indicates an expanded and relatively continuous record across the P/E boundary. The organic-rich clay layer (bed no. 1 of the Dababiya Quarry Member) marks the start of the PETM event. This bed is characterized by the extinction of all benthic foraminiferal fauna except for the occurrence of rare agglutinated foraminiferal species. The presence of these species indicates an oceanic anoxic event at the sea floor. High concentration of phosphatic contents including fish remains occurred in the middle part of the PETM (bed nos. 2 and 3 of the Dababiya Quarry Member) with the continuous absence of benthic foraminiferal fauna except for few specimens at the top of bed 3. Bed nos. 4 and 5 of the Dababiya Quarry Member represent the upper part of the PETM and the initial stage of sea floor recovery. Low diversity and abundance of benthic foraminiferal taxa occurred within these beds, represented by Valvulineria scrobiculata, Lenticulina midwayensis, Loxostomoides appliane, and Siphogenerinoides eleganta. This phenomenon continues upward during the post-PETM event. The Paleocene velasco-type benthic foraminifera Angulogavelinella avnimelechi and Coryphostoma midwayensis species are extinct within the advent of the PETM event. The benthic foraminiferal assemblages at the studied sections are dominated by midway-type fauna with little representative of velasco-type fauna. The velasco-type species are represented with high abundance at Serai section and with low densities at Um El Huetat section, while at Duwi section, they rarely occurred. This suggests outer neritic-upper bathyal (150–400 m) setting at Serai section and mostly middle-outer neritic (50–150 m) setting at Um El Huetat and Duwi sections.  相似文献   

20.
The heteromorph ammonite Pravitoceras sigmoidale Yabe, of the family Nostoceratidae, is the zonal marker of the upper Campanian P. sigmoidale Zone in southwest Japan, and is the main component of ammonite assemblages in this zone. We explain the taphonomic processes underlying the occurrence of P. sigmoidale in the Izumi Group, specifically in the Minato (Awaji Island), Anaga (Awaji Island), Koike–Omoizaki (Shikoku), and Hidonodani sections (Shikoku). The first two sections consist mainly of the non-turbiditic Northern Marginal Facies (NMF), while the latter two sections comprise the Main Facies (MF), a turbiditic facies deposited in waters deeper than those of the NMF. We recognise three modes of occurrence of P. sigmoidale, as follows: (1) In nodules crowded with P. sigmoidale (NCP); this mode occurs only in the NMF, and includes juvenile and adult specimens, together with other ammonites such as Solenoceras (Oxybeloceras) aff. humei (Douvillé), (2) In mudstone with isolated P. sigmoidale (MIP), which is found in all of the sections studied, (3) In sandstone with isolated P. sigmoidale (SIP), which occurs only in the MF. Adult individuals of P. sigmoidale are dominant in most of the sections, while juveniles were observed only in the Minato section, which originally was closer to land areas. The number of specimens of P. sigmoidale and Solenoceras spp. tends to increase in sections representing depositional environments proximal to terrestrial areas. Ammonite assemblages could have been transported from shallow- to deep-water settings by turbidity currents. The sorting of components by transport processes likely contributed to the formation of these fossil assemblages in different areas. This is an important clue to understanding the habitat of heteromorph ammonite life assemblages consisting mainly of P. sigmoidale.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号