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 328 毫秒
1.
Recognition of the Campanian stage on the Brazilian Continental Margin, using calcareous nannofossils, has been historically problematic. This paper constitutes an overview of earlier works, showing how nannofossil biostratigraphic ideas have evolved since Troelsen & Quadros provided the first biozonation of this region in 1971. Recent studied have provided data which have helped to clarify these apparent biostratigraphic problems, and allows this region to be placed in a global biostratigraphic context.The earliest researchers identified the Santonian/Campanian boundary by the last occurrences of ‘Lithastrinus grillii’ andPetrobrasiella venata. P. venatawas later abandoned as an index species due to its rarity and, instead, the last occurrences ofMarthasterites furcatusand ‘Lithastrinus grillii’ became the most-used markers. However, the stratigraphic age of these biohorizons diverged from those quoted in the literature. In the Brazilian basins, these extinctions, rather than having occurred in the Campanian as was recorded elsewhere, were considered to mark the top of the Santonian, as suggested by correlations with other microfossil groups (primarily foraminifera and palynomorphs). To explain this phenomenon, the existence of a condensed sequence was postulated for most of the Brazilian marginal basins, where the uppermost Santonian deposits were apparently indistinguishable from those of the lowermost Campanian. In line with current correlations presented in the nannofossil literature, and with new information obtained from core and side-wall samples, it is now believed that the extinction of these speciesdidoccur in the Campanian in the Brazilian basins, whilst the last occurrence ofLithastrinus moratus(previously misidentified asLithastrinus grillii) has become a useful Santonian marker. Thus the Santonian/Campanian boundary in Brazil lies in a stratigraphic position similar to elsewhere in nannofossil terms, that is below CC18.The Campanian/Maastrichtian boundary was initially characterised in nannofossil terms in Brazil by the last occurrence ofBroinsonia parca constricta, and later by the last occurrence ofEiffellithus eximius. Recently acquired data has shown that the sequence of events in the Brazilian marginal basins is similar to that of the Sissingh/Perch-Nielsen standard biozonation scheme through this interval. Again, correlations in the literature with the recently defined boundary (in macrofossil terms) thus allow the boundary to be determined between the last occurrences ofBroinsonia parca constrictaandUniplanarius trifidus, that is, in CC23b.  相似文献   

2.
A previously uncollected fauna of ammonites, bivalves, and other molluscs, associated with radiolarian microfossils, has been newly recognized near Lawn Hill on the east coast of central Queen Charlotte Islands, British Columbia. The regional biostratigraphic zonation indicates that the Lawn Hill fauna is correlative with the Nostoceras hornbyense zonule of the Pachydiscus suciaensis ammonite biozone, recognized in the Nanaimo Group of southeast Vancouver Island. The Nostoceras hornbyense Zone (new) is herein proposed for strata of Pacific coast Canada containing the zonal index. Several molluscan taxa present in the Lawn Hill section are new to British Columbia and the ammonite fauna suggests that the Nostoceras hornbyense Zone is late Campanian in age, supported by radiolarian taxa present in the section. Strata sampled in the Lawn Hill section preserve reversed-polarity magnetization, considered likely correlative with Chron 32r. The presence of the Nostoceras hornbyense Zone on Queen Charlotte Islands is the first recognition of this zone in Canada north of central Vancouver Island and represents the youngest Cretaceous known in this region. Campanian radiolarians identified from the Lawn Hill section are also the first recognized from the Pacific coast of Canada.  相似文献   

3.
High-resolution carbon isotope stratigraphy of the upper Campanian-Maastrichtian is recorded in the Boreal Realm from a total of 1968 bulk chalk samples of the Stevns-1 core, eastern Denmark. Isotopic trends are calibrated by calcareous nannofossil bio-events and are correlated with a lower-resolution δ13C profile from Rørdal, northwestern Denmark. A quantitative approach is used to test the reliability of Upper Cretaceous nannofossil bio-events and provides accurate biohorizons for the correlation of δ13C profiles. The Campanian-Maastrichtian boundary (CMB) is identified through the correlation of dinoflagellate biostratigraphy and δ13C stratigraphy between Stevns-1 and the Global boundary Standard Stratotype-section and Point at Tercis les Bains (SW France), allowing the identification of new chemical and biostratigraphic markers that provide a precise placement of the stage boundary on a regional scale. The boundary interval corresponds to the third phase of a stepwise 0.8‰ negative δ13C excursion, lies in calcareous nannofossil subzone UC16dBP, and encompasses the last occurrence of nannofossil Tranolithus stemmerikii and first occurrence of nannofossil Prediscosphaera mgayae. Fifteen δ13C events are defined and correlated to sixteen reliable nannofossil biohorizons, thus providing a well-calibrated standard high-resolution δ13C curve for the Boreal Realm.  相似文献   

4.
Integration of calcareous nannofossil data, δ13C and δ18O values, and carbonate contents of the lower Paleocene–upper Paleocene sequence that crops out at the Misheiti section, East Central Sinai, Egypt, were used to denote the Danian/Selandian (D/S) and Selandian/Thanetian (S/T) stage boundaries. The study interval belongs to the Dakhla and Tarawan formations. Four calcareous nannofossil zones (NP4, NP5, NP6, and NP7/8) were recognized. The base of the Selandian Stage is tentatively placed at the lowest occurrences (LOs) of taxa ascribable to the second radiation of fasciculiths (i.e., Lithoptychius janii). This level is marked by a sudden drop of δ13C and δ18O values and carbonate content. No distinctive lithological changes were observed across the D/S boundary at the study section. A hiatus at the NP5/NP6 zonal boundary is indicated by the condensation of zones NP5 and NP6.The base of the Thanetian is placed at the base of Zone NP7/8 at the lithological change observed in correspondence to the boundary between the Dakhla and Tarawan formations. The δ13C and δ18O values abruptly decrease slightly above the base of Zone NP7/8. No consistent variations in the carbonate contents were recorded within Zone NP6 or across the NP6/NP7/8 zonal boundary.  相似文献   

5.
One hundred and thirty nine samples have been studied from the Late Campanian–Early Maastrichtian of three deep wells drilled in Jiza’-Qamar Basin, Eastern Yemen to determine the calcareous nannofossil zones and the age of the sediments. Forty-seven calcareous nannofossil species were identified and four biozones were determined in the present study (CC21–CC24). These biozones are assigned to the Late Campanian–Early Maastrichtian ages. Most of the studied species in this work refer to tropical–subtropical environment. The Campanian–Maastrichtian Boundary was determined in Al-Fatk well based on the last occurrence of Eiffelithus eximus and the last occurrences of Uniplanarius sissinghii and Uniplanarius trifidus.  相似文献   

6.
The late Turonian to early Campanian calcareous nannofossil biostratigraphy of the Austrian Gosau Group is correlated with ammonite and planktonic foraminiferal zones. The standard Tethyan zonations for nannofossils and planktonic foraminifers are applied with only minor modifications. The basal marine sediments of the Gosau Group, bearing late Turonian-early Coniacian macrofossils, belong to the Marthasterites furcatus nannofossil Zone (CC13). The Micula decussata Zone (middle Coniacian to early Santonian) is combined with the Reinhardtites anthophorus Zone because of the rare occurrence of Renhardtites cf. R. anthophorus already in the Coniacian and taxonomic problems concerning the correct identification of this species. The Santonian-Campanian boundary lies within the Calculites obscures Zone (CCl7).  相似文献   

7.
The Pol Dokhtar section of southern Lorestan, faulted Zagros range of southwestern Iran, contains one of the most complete Early Campanian to Danian sequences. The lack of a good fundamental paleontological study is a strong motivation for investigating calcareous nannofossils in southwestern Iran. The majority of the section is made of shale, marl, and partly of marly limestone and clay limestone, respectively. As a result of this study, 24 genera and 45 species of nannofossils have been identified and presented for the first time. This confirms the existence of biozone CC18 of zonation scheme of Sissingh (Geologie en Minjbouw 56:37–65, 1977) to NP1 of zonation of Martini, which suggests the age of Early Campanian to Danian. All Early Campanian to Danian calcareous nannofossil biozones from CC18 (equivalent to the Aspidolithus parcus zone) to NP1 (equivalent to the Markalius inversus zone) are discussed. Also, the zonal subdivision of this section based on calcareous nannofossils has shown continuity in Cretaceous/Paleocene boundary in south part of Lorestan Province. We can also learn about the predominant conditions of the studied sedimentary basin that was in fact part of the Neotethys basin with the existence of indexed species calcareous nannofossils that indicate warm climate and high water depths of the basin in low latitudes.  相似文献   

8.
New carbon (δ13C) isotope records calibrated by planktonic bioevents provide general support for a late Campanian age assignment of the Shiranish Formation (Fm.) and its boundaries in the Dokan section (NE Iraq). The Shiranish Fm. is characterised at the base by a mid-Campanian unconformity as can be interpreted by absences of nannofossil zones CC20-21. The Shiranish Fm. then spans nannofossil biozones CC22-CC23a (UC15d-eTP to UC16aTP). Results obtained on carbon isotopes suggest that diagenesis affected and compromised a few carbonate samples in the uppermost 50 m of the section. However, once these samples are discarded, pristine trends suggest that the top of the section records a negative carbon isotope excursion that is interpreted as CMBa-c events that straddle the Campanian–Maastrichtian boundary. This interpretation is supported by the lowermost occurrence of planktic foraminifers Rugoglobigerina scotti and Contusotruncana contusa some 30 m above the base of the negative excursion and 10 m below a positive excursion identified as the Maastrichtian M1+ event. Discrepancies in the stratigraphic range of several planktic foraminifer bioevents are highlighted and advocate for the need of many more integrated records of planktic foraminifer and nannofossil biostratigraphy alongside carbon isotope stratigraphy in the eastern Tethys in order to improve regional and global schemes.  相似文献   

9.
Fluctuation in calpionellid, foraminiferal, and nannofossil diversity and abundance are documented in two successions located in the eastern part of the Upper Jurassic–Lower Cretaceous carbonate platform of the Southern Carpathian area, Romania. The lower part of the studied sections consists of upper Tithonian–upper Berriasian bioclastic limestones. This age is supported by the presence of the calpionellid assemblages assigned to the Crassicollaria, Calpionella, and Calpionellopsis Zones. Based on biostratigraphical data, a gap was identified within the uppermost Berriasian–base of the upper Valanginian (the interval encompasses the Boissieri, Pertransiensis, Campylotoxum, and lower part of the Verrucosum ammonite Zones). Hence, the upper Tithonian–upper Berriasian bioclastic limestones are overlain by upper Valanginian–lower Hauterivian pelagic limestones (the interval covered by the NK3B and NC4A nannofossil Subzones). A detailed qualitative and semiquantitative analysis of the nannoflora was carried out over this interval. To estimate the surface water fertility conditions, the nannoplankton-based nutrient index (NI) was calculated. The fluctuation pattern of NI allow us to recognize four phases in the investigated interval, as follows: (1) phase I (covering the lower part of the NK3B nannofossil Subzone and the upper part of the Verrucosum ammonite Zone, respectively) is characterized by low values of the NI (below 20%), by the dominance of the genus Nannoconus in the nannofloral assemblages (between 60–70%), and moderate abundance of Watznaueria barnesae (up to 23%), while the high-fertility nannofossils constitute a minor component of the assemblages; (2) phase II (placed in the NK3B nannofossil Subzone, extending from the top of Verrucosum ammonite Zone, up to the lower part of the Furcillata ammonite Zone) is characterized by increase of NI above 30%, a decrease of nannoconids (up to 50% at the top), while Watznaueria barnesae increases in abundance up to 27%. The fertility proxies (Diazomatolithus lehmanii, Zeugrhabdotus erectus, Discorhabdus rotatorius, and Biscutum constans) represent again a minor component of the recorded nannofloras (less than 7% in both sections), but they have an ascending trend; (3) phase III (which encompasses the boundary interval of the NK3B and NC4A nannofossil Subzones, corresponding to the upper part of the Furcillata ammonite Zone) contains higher NI values (over 35%, and up 52% towards the base of this phase), an abrupt nannoconid decrease (down to 20%), higher abundance of Watznaueria barnesae (over 30%), while the fertility nannofossils became an important nannofloral component, jointly amounting to almost 20%; (4) phase IV (identified within the NC4A Nannofossil Zone and corresponding to the boundary interval of the Furcillata and Radiatus ammonite Zones) is characterized by a decrease of NI to 25%, a recovery of the nannoconids up to 40%, a decline in abundance of Watznaueria barnesae to 25%, together with a pronounced drop of fertility taxa, which make together no more than 8%. We assume that maximum of eutrophication in the studied interval from the Southern Carpathians was in the Furcillata ammonite Zone. Notably, within the phases 2 and 3, the morphological changes identified in the benthic foraminiferal assemblages (the predominance of flattened morphologies, together with the presence of conical and trochospiral inflated forms), as well as the occurrence of the Zoophycos trace fossils and pyrite framboids, indicate dysaerobic conditions. In the Southern Carpathians, the late Valanginian–early Hauterivian biogeographical changes are coeval with the initiation of the carbonate platform drowning.  相似文献   

10.
An integrated biostratigraphic (foraminifera, calcareous nannofossils, crinoids), chemostratigraphic (stable carbon isotopes) and magnetostratigraphic study of the Bocieniec section (southern Poland) is presented here. The section presents a continuous and lithologically monotonous sedimentary record across the Santonian–Campanian boundary transition. A large number of macrofossil, foraminiferal and calcareous nannofossil bioevents along with several well-identified carbon-isotope excursions of the upper Santonian and lowermost Campanian are documented. The base of the Campanian is well-constrained by the last occurrence (LO) of the crinoid Marsupites testudinarius, and correlates to the onset of the first δ13C positive peak of the Santonian–Campanian Boundary Event (SCBE peak a). A presumable primary Cretaceous paleomagnetic signal highlights the potential presence of the C34N/C33R magnetic reversal although its exact position remains uncertain between peaks a and b of the SCBE. The planktic foraminifer Dicarinella asymetrica is very rare at Bocieniec but a potential LO of this important marker may be recorded in coincidence with peak b of the SCBE. The first occurrence (FO) of calcareous nannofossil Broinsonia parca parca coincides with the lower part of chron C33R and with the early Campanian pilula zone event. A large set of additional nannofossil events and benthic foraminifer events further constrain the stratigraphy of the section and along with the carbon isotopes, allows for correlation with other important sections of the Boreal realm. Although the Bocieniec section is relatively thin and condensed (5.5 m), the successive order of events and presence of all past proposed stratigraphic criteria for the Santonian-Campanian boundary makes it the most complete reference section for this interval at the European and at the global scale. Moreover, this section allows for a precise correlation of the Tethyan and Boreal domains. The Bocieniec section fulfils the geological criteria to be a potential boundary stratotype candidate for the base of the Campanian Stage.  相似文献   

11.
The middle–late Campanian was marked by an increase in the bioprovinciality of calcareous microfossil assemblages into distinct Tethyan, Transitional, and Austral Provinces that persisted to the end of the Maastrichtian. The northwestern Australian margin belonged to the Transitional Province and the absence of key Tethyan marker species such as Radotruncana calcarata and Gansserina gansseri has led petroleum companies operating in the area to use the locally developed KCCM integrated calcareous microfossil zonation scheme. The KCCM zonation is a composite scheme comprising calcareous nannofossil (KCN), planktonic foraminiferal (KPF) and benthonic foraminiferal (KBF) zones. This paper presents the definitions and revisions of Zones KCCM8–19, from the highest occurrence (HO) of Aspidolithus parcus constrictus to the lowest occurrence (LO) of Ceratolithoides aculeus, and builds on our previous early–late Maastrichtian study. The presence of a middle–upper Campanian disconformity is confirmed by microfossil evidence from the Vulcan Sub-basin, Exmouth and Wombat plateaus, and the Southern Carnarvon Platform. In the Vulcan Sub-basin and on the Exmouth Plateau (ODP Hole 762C) the hiatus extends from slightly above the LO of common Rugoglobigerina rugosa to above the LO of Quadrum gothicum. On the Wombat Plateau (ODP Hole 761B) it spans from above the LO of Heterohelix semicostata to above the LO of Quadrum gothicum; and in the Southern Carnarvon Platform the disconformity has its longest duration from above the HO of Heterohelix semicostata to above the LO of Quadrum sissinghii. A significant revision of the events which define Zones KCCM18 and 19 was necessary owing to the observation that the LO of Ceratolithoides aculeus occurs below the HOs of Archaeoglobigerina cretacea and Stensioeina granulata incondita and the LO of common Rugoglobigerina rugosa. In the original zonation these events were considered to be coincident.  相似文献   

12.
Lithostratigraphy, physicochemical stratigraphy, biostratigraphy, and geochronology of the 77–70 Ma old series bracketing the Campanian–Maastrichtian boundary have been investigated by 70 experts. For the first time, direct relationships between macro- and microfossils have been established, as well as direct and indirect relationships between chemo-physical and biostratigraphical tools. A combination of criteria for selecting the boundary level, duration estimates, uncertainties on durations and on the location of biohorizons have been considered; new chronostratigraphic units are proposed. The geological site at Tercis is accepted by the Commission on Stratigraphy as the international reference for the stratigraphy of the studied interval. To cite this article: G.S. Odin, C. R. Geoscience 334 (2002) 409–414.  相似文献   

13.
In European Russia, the most complete succession of Boreal sediments of the terminal Bathonian and lower Callovian is exposed near the Prosek Settlement. After its revision, the infrazonal division of the upper Bathonian and lower Callovian and position of the Bathonian-Callovian boundary are difined more carefully. The Calyx Zone and bodylevskyi Biohorizon are established in the upper Bathonian. The base of the lower Callovian is defined at the first occurrence level of Macrocephalites jacquoti. Based on four successive ammonite assemblages occurring in lower part of the Elatmae Zone, the breve, frearsi, quenstedti, and elatmae biohorizons are identified. The joint occurrence of Boreal, Subboreal, and Tethyan ammonites in the section facilitate its correlation with the other sections of the Panboreal paleobiogeographic superrealm.  相似文献   

14.
Eight biohorizons, four of which were previously distinguished in Central Poland and four new (contradictionis, pommerania, kuteki, and pilicensis), were identified in the Dorsoplanites panderi zone of the Upper Jurassic Middle Volgian Substage of the European part of Russia on the basis of the succession of ammonites of the Zaraiskites genus. The peculiarities of variations of the ammonite complexes in space and time testify to the stepwise warming during the Panderi Chron and the occurrence of the significant latitudinal temperature gradient in the Middle Russian Sea. New species Zaraiskites kuteki is described.  相似文献   

15.
A detailed stratigraphic analysis was carried out on the Lower–Middle Cenomanian hemipelagic deposits of the Blieux section (Alpes-de-Haute-Provence; southeast France) in order to identify the Middle Cenomanian event I (MCE I) in the Vocontian Basin. These deposits are represented by five bundles composed of limestone–marl alternations that are separated by thick marly intervals. The Blieux section, which is well exposed, very thick, continuous and relatively rich in macrofauna, provides an ideal succession for an integrated approach. Biostratigraphy by ammonoids and sequence stratigraphy have been established for the whole succession whereas calcareous nannofossil and geochemical analyses have been carried out on a restricted interval across the Lower/Middle Cenomanian boundary. The uppermost part of the Mantelliceras mantelli Zone, the Mantelliceras dixoni Zone and the lower part of the Acanthoceras rhotomagense Zone have been recognized. The appearance of the genus Cunningtoniceras (C. inerme or C. cunningtoni) is used to place the base of the A. rhotomagense Zone and the Lower/Middle Cenomanian boundary. This boundary is also well characterized by the presence of nannofossil Subzone UC2C. Two orders of hierarchically stacked depositional sequences have been identified. Medium- and large-scale sequences correspond to 400 ky eccentricity cycles and to third-order cycles, respectively. The duration of the interval studied (from the uppermost part of the M. mantelli to the lower part of the A. rhotomagense zones) is estimated to be 2.8 my. Carbon-isotope values determined from bulk carbonate sediments show a first positive excursion (+0.6‰) corresponding to the MCE Ia, in the lower part of the A. rhotomagense Zone. A subsequent increase (+1.1‰) is recorded and could correspond to MCE Ib, but a sharp return to baseline values as expected in an excursion is not observed. The duration of the MCE I is estimated to be less than 400 ky. The Blieux section is correlated with some classical sections of the Anglo-Paris (Southerham, Folkestone, Cap Blanc-Nez) and Lower Saxony (Baddeckenstedt and Wunstorf) basins using ammonoid biostratigraphy, sequence stratigraphy, and chemostratigraphy. It is proposed as a candidate for the Middle Cenomanian GSSP (Global Boundary Stratotype Section and Point).  相似文献   

16.
The Cretaceous outcrop belt of the Mississippi Embayment in the Gulf Coastal Plain (GCP) spans the Cretaceous/Paleogene (K/Pg) boundary. A detailed reconstruction of this time interval is critical for understanding the nature of biotic and environmental changes preceding the end-Cretaceous Mass Extinction event and for deciphering the likely extinction mechanism (i.e., bolide impact versus volcanism). Eight sections encompassing the K/Pg succession across the Mississippi Embayment were analyzed using biostratigraphic sampling of ammonites, dinoflagellates, and nannofossils. An upper Maastrichtian ammonite zonation is proposed as follows, from oldest to youngest: Discoscaphites conradi Zone, D. minardi Zone, and D. iris Zone. Our study documents that the ammonite zonation established in the Atlantic Coastal Plain (ACP) extends to the GCP. This zonation is integrated with nannofossil and dinoflagellate biostratigraphy to provide a framework to more accurately determine the age relationships in this region. We demonstrate that ammonites and dinoflagellates are more reliable stratigraphic indicators in this area than nannofossils because age-diagnostic nannofossils are not consistently present within the upper Maastrichtian in the GCP. This biostratigraphic framework has the potential to become a useful tool for correlation of strata both within the GCP and between the GCP, Western Interior, and ACP. The presence of the uppermost Maastrichtian ammonite D. iris, calcareous nannofossil Micula prinsii, and dinoflagellates Palynodinium grallator and Disphaerogena carposphaeropsis suggests that the K/Pg succession in the GCP is nearly complete. Consequently, the GCP is an excellent setting for investigating fine scale temporal changes across the K/Pg boundary and ultimately elucidating the mechanisms causing extinction.  相似文献   

17.
The first occurrence (FO) of Marthasterites furcatus was correlated with the FOs of other nannofossils, inoceramid bivalves and foraminifers in the Bohemian Cretaceous Basin and Outer Flysch Carpathians. The correlation showed that the FO of M. furcatus was diachronous, becoming younger from east to west. In the Silesian Unit it appears in the lower Turonian in association with Eprolithus moratus (UC6b nannofossil Zone). In the Pavlovské vrchy klippes it appears in the upper middle Turonian together with Lithastrinus septenarius (UC9 Zone). In the Bohemian Cretaceous Basin, the FO of M. furcatus was observed in the lower upper Turonian just above the FO of Liliasterites angularis. The presence of M. furcatus in Turonian strata is scarce and discontinuous up to its sudden quantitative increase (represented by 5–27% in assemblages) below the FO of the inoceramid bivalve species Cremnoceramus waltersdorfensis and C. deformis erectus in the Turonian–Coniacian boundary interval. The top of the M. furcatus acme was recorded below the FO of Micula staurophora. The second quantitative rise of M. furcatus (12% in assemblage) was found in the lower lower Campanian of the Pavlovské vrchy klippes above the FO of Broinsonia parca parca in the UC14a Zone and the last occurrence of the planktonic foraminifer Whiteinella baltica. Above this second acme M. furcatus disappears. The significantly earlier appearance of M. furcatus in the Silesian Basin may be connected with a southeast-heading surface current from the North European epicontinental sea where the species appeared in the early Turonian too.  相似文献   

18.
The palaeogeographic setting of the studied Ain Medheker section represents an Early Campanian to Early Maastrichtian moderately deep carbonate shelf to distal ramp position with high rates of hemipelagic carbonate production, periodically triggered by mass-flow processes. Syndepositional extensional tectonic processes are confirmed to the Early Campanian. Planktonic foraminifera identified in thin sections and calcareous nannofossils allow the identification of the following biozones: Globotruncanita elevata, Contusotruncana plummerae (replacing former Globotruncana ventricosa Zone), Radotruncana calcarata, Globotruncana falsostuarti, and Gansserina gansseri. The following stable C-isotope events were identified: the Santonian/Campanian boundary Event, the Mid-Campanian Event, and the Late Campanian Event. Together with further four minor isotopic events, they allow for correlation between the western and eastern realms of Tunisia. Frequently occurring turbidites were studied in detail and discussed in comparison with contourites.  相似文献   

19.
Rare ammonite species are reported from the Inferior Oolite Formation of Dorset. In the Aalenian the ammonite Shahrudites is described from the Scissum Zone for the first time and Malladaites is recorded from the Murchisonae Zone. Tmetoceras regleyi occurs in the Scissum and Murchisonae zones and Tmetoceras henriquesae is described from the Concavum Zone. In the Lower Bajocian, specimens of Zurcheria are presented from the Discites Zone. A revised zonal scheme is used for the Aalenian Stage and two new biohorizons are introduced. The Leioceras subglabrum biohorizon is erected in the Opalinum Subzone and Leioceras evolutum in the Bifidatum Subzone.  相似文献   

20.
The Upper Cretaceous Toyajo Formation is distributed around the Mt. Toyajo in the Aridagawa area, Wakayama, southwestern Japan. The formation is subdivided into three newly defined members, the Nakaibara Siltstone Member, Hasegawa Muddy Sandstone Member, and Buyo Sandstone Member, in ascending order. Close field observation elucidated the detailed biostratigraphy of the Toyajo Formation, and high-precision biostratigraphic correlation was made with the Yezo Group in Hokkaido (northern Japan) and Sakhalin and the Izumi Group in southwestern Japan.The Toyajo Formation contains diversified lower Campanian to upper Campanian heteromorph ammonoid assemblages, including Eubostrychoceras and Scaphites. Discovery of the heteromorph fauna demonstrates that scaphitid ammonoids survived until Campanian time in the northwestern Pacific region. Although Eubostrychoceras elongatum has been known in the northeastern Pacific region, the occurrence of this species in the northwestern Pacific region has been uncertain before. The rich occurrence of E. elongatum in the Aridagawa area indicates that this species was distributed widely in the northern Pacific realm.The Toyajo Formation is similar to the Izumi Group in various geologic features, and may indicate that the Toyajo Formation was deposited in a strike-slip basin along the Chichibu Belt formed by the movement along the Kurosegawa Tectonic Zone in the latest Cretaceous, like the Izumi Group, along the Median Tectonic Line.  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