Radiolarian Fauna Research Articles

Early to Middle Pleistocene paleoceanographic history of southern Japan based on radiolarian data from IODP Exp. 314/315 Sites C0001 and C0002

During the mid-Pleistocene Transition (MPT), spanning from 0.8 to 1.2 Ma, the prevailing 41 kyr glacial cycles were replaced by longer cycles with a period of ca. 100 kyr. This event was also accompanied by important climate changes, such as the expansion of the Northern Hemisphere Ice Sheet. The impact of the MPT on the Kuroshio Current off southern Japan remains unknown, however, in the absence of a long core covering the period of the MPT. In this area, preservation of calcareous microfossils is poor, but polycystine radiolarians are abundant and well preserved. Therefore, they are an excellent alternative proxy for reconstructing past climate change. Indeed, because numerous studies have examined the geographic distribution of modern radiolarian species, the relationships between several radiolarian assemblage groups (e.g., equatorial to sub-arctic assemblage groups) and climatic conditions are well understood. We examined the impact of the MPT on the Kuroshio Current off the Kii Peninsula, southern Japan, by investigating radiolarian assemblages in deep-sea cores collected during Integrated Ocean Drilling Program Expedition 314/315. We studied the radiolarian assemblages in 293 samples from cores obtained at Site C0001 Holes E and F and Site C0002 Hole D. The age models for these cores, which are based on biostratigraphy and chemostratigraphy, showed that sediments from the period between Marine Isotope Stage (MIS) 15 and MIS 8, which includes the Mid-Brunhes Event, are missing from these cores. However, continuous data are available between MIS 63 (1.82 Ma) and MIS 15 (0.5 Ma). Therefore, using the ecological properties of 11 key radiolarian species, we reconstructed regional paleoceanographic changes between 1.82 and 0.5 Ma. Radiolarian fauna revealed that the relative abundance of Kuroshio and the Central Pacific water taxa (e.g., Didymocyrtis tetrathalamus and the Polysolenia spinosa/lappacea group, respectively) increased drastically during the MPT, possibly indicating a northward shift of the Kuroshio at that time. The relative abundance of Rhizosphaera mediana, an Oyashio/Kuroshio mixed-water species, also increased during this interval. Thus, a southward shift of the Oyashio Current may have occurred synchronously with a northward shift of the Kuroshio during the MPT.

Flux variations and vertical distributions of siliceous Rhizaria (Radiolaria and Phaeodaria) in the western Arctic Ocean: indices of environmental changes

Abstract. The vertical distribution of radiolarians was investigated using a vertical multiple plankton sampler (100–0, 250–100, 500–250, and 1000–500 m water depths, 62 μm mesh size) at the Northwind Abyssal Plain and southwestern Canada Basin in September 2013. To investigate seasonal variations in the flux of radiolarians in relation to sea ice and water masses, a time-series sediment trap system was moored at Station NAP (75°00´ N, 162°00´ W; bottom depth 1975 m) in the western Arctic Ocean during October 2010–September 2012. The radiolarian flux was comparable to that in the North Pacific Ocean. Amphimelissa setosa was dominant during the season with open water as well as at the beginning and end of the seasons with sea-ice cover. During the sea-ice-cover season, however, oligotrophic and cold-water-tolerant actinommids were dominant, productivity of Radiolaria was lower, and species diversity was greater. These suggest that the dynamics of sea ice are a major factor affecting the productivity, distribution, and composition of the radiolarian fauna.

New high latitude Capnuchosphaera species (Triassic Radiolaria) from Waipapa Terrane, New Zealand

Two new species and one new subspecies of genus Capnuchosphaera, (Capnuchosphaera tumida nov. sp., C. waihekeensis nov. sp. and C. texensis australis nov. ssp.) are described herein from phosphatic nodules included in mudstone and sandstone beds of the Waipapa Terrane, Waiheke Island, New Zealand. The phosphatic nodules yielded a rich Late Triassic (Carnian-Norian) radiolarian fauna, with a high abundance of spumellarian taxa including numerous species of the genera Capnuchosphaera, Vinassaspongus, Kahlerosphaera, Sarla and Dumitricasphaera. Waiheke Island Capnuchosphaera are characterized by a large cortical shell and a distinctively low ratio of spine length to cortical shell diameter. These features differ significantly from those of Capnuchosphaera in the Tethyan Realm and are considered to be the result of adaptation to an Austral-New Zealand peripheral ocean of Gondwanaland in the Mesozoic Southern Hemisphere.

Revised radiolarian zonation of the Upper Cretaceous Izumi inter-arc basin (SW Japan)

The radiolarian zonation of the Izumi Group is revised for an integrated biostratigraphy across the Campanian/Maastrichtian boundary, together with the ammonite occurrence, fission track and magnetostratigraphic calibration of the acidic tuff key beds. The significance of direct correlation in the same sections, between macro- and micro-fossils (i.e. between relatively near-shore nektonic and turbiditic planktonic-fauna) is discussed in relation to the litho- and bio-facies of the Izumi group. Late Cretaceous paleoceanographic changes inferred from the studied radiolarian fauna represent a regional marker event for the chronostratigraphic correlation of continental shelf to turbidite deep-sea sequences, which accumulated on a mid latitudinal margin facing towards the NW paleo-Pacific. The proposed Campanian/Maastrichtian boundary is at the limit between Pachydiscus awajiensis and Nostoceras hetonaiense ammonite zones. This boundary corresponds to the radiolarian last occurrence of Amphipyndax pseudoconulus (= A. enesseffi), Archaeodictyomitra sliteri, Dictyomitra formosa, D. koslovae and Rhopalosyringium magnificum at the top of the Myllocercion acineton Assemblage Zone (uppermost Campanian), and the first occurrence of Clathrocyclas gravis and Dorcadospyris sp. A, at the base of the Maastrichtian C. gravis Assemblage Zone. The new data settle partly a chronostratigraphic discrepancy between radiolarian and ammonite zonations in the Izumi Group, which was pointed out previously by Ishida et al. (2010).

Conodonts, radiolarians and ostracodes in the Permian E-Lert Formation, Loei Fold Belt, Indochina Terrane, Thailand

AbstractConodonts are rare in the Permian carbonates of Indochina but abundant conodonts and ostracodes have been obtained from turbiditic limestones of the Permian E-Lert Formation along with radiolarians from overlying cherts, all deposited on the margins of the interplatform Nam Duk Basin. Conodonts are typically Tethyan and are very similar to faunas from Sicily and south China. They includeHindeodus gulloides,Pseudohindeodus oertlii,Mesogondolella siciliensisandSweetognathus subsymmetricuswhich indicate a probable late Kungurian – Roadian age range although a Wordian age cannot be excluded.M. siciliensis, which has a high blade and small cusp supposedly typical of warm-water conodonts, is found in deep (<500 m) carbonate turbidites in Thailand and in very deep deposits in Oman and Sicily, but generally not in shallow-water tropical limestones in Thailand, Oman and Sicily. The chert sequence yields a radiolarian fauna consisting of 11 confidently assigned species of whichAlbaillella asymmetrica,A. sinuata,Tormentum delicatumandLatentifistula patagilateratasuggest a latest Kungurian – earliest Roadian age, deposited at a palaeodepth ofc.500 m. Ostracodes consist of 16 genera and 23 species which belong toShivaella,Paraberounella,Carinaknightina,Paraparchites,Shemonaella,Bairdia,Cryptobairdia,Bairdiacypris?,Spinocypris,Pseudobythocypris,Baschkirina,Microcheilinella,Basslerella,PolycopeandCyathus, of whichShivaella elertensis sp. nov.Chitnarin is newly described. The ostracodes are a palaeoecologically mixed assemblage comprising external platform and deeper-water forms, suggesting that the limestone turbidites were deposited on the proximal part of the slope.Cyathus caperataandC. ellipticashow a palaeobiogeographic link to south China.

New data on the stratigraphy of the landward slope of the Japanese Trench

Results of the ecostratigraphic analysis using statistical data handling of radiolarian fauna from the sections of holes 434, 434A, 434B firstly allow us: 1) to identify the stratigraphic succession and some hiatuses in the hemipelagic sedimentation, 2) to determine the age of basal layers of the sedimentary cover as an Oligocene-Early Miocene boundary. These results present a new data on the age and environmental conditions of the formation of the sedimentary cover on the landward slope of the Japanese Trench. The “layers with radiolarians” correspond to previously established areas. The boundaries of “layers with radiolarians” marked by radiometric dates obtained on sections of the Japan Sea on transect: holes 436, 439 and 584. These results revealed the periodicity of the sedimentation process.

Paleoceanographic history of the Northwest Pacific Ocean over the past 740 kyr, discerned from radiolarian fauna

The Northwest Pacific Ocean is characterized by a strong mixing of water masses, due to the interplay of three distinct currents, i.e., the Kuroshio, the Tsugaru, and the Oyashio. The C9001C drill core site, located east of the Shimokita Peninsula and directly influenced by the Tsugaru warm current and the Oyashio subarctic current was used here to reconstruct the paleoceanographical history of this region, especially focusing on the Mid-Brunhes Event (MBE) and its consequences. This core provides a continuous record from marine isotope stage (MIS) 18 (740ka) to the present. Polycystine radiolarian assemblages were analyzed to highlight paleoceanographic and sea surface temperature changes at this site. Based on the radiolarian fauna, seven time periods are defined, which coincide with changes in the dynamics of the Tsugaru and Oyashio currents, respectively. The oldest time interval covered by our analysis (i.e., VII: 740–621ka) was marked by generally sluggish ocean circulation. The Tsugaru Current influence increased during the following Interval VI (621–478ka) which encompasses interglacials MIS 15 and 13, while the Oyashio Current strengthened during the Interval V (478–337), i.e. from glacials MIS 12 to 10. These latter intervals (VI to V) constitute a long climatic transitional period where Tsugaru Current and Oyashio Current influences are strengthened. In the time period from Interval IV to Interval I (0–337ka), the warming intensity of interglacials (MIS 9, MIS 5) appears to be close to the modern one. However, several unusually warm glacials, associated with a relatively strong Tsugaru Current flow, were identified during this interval (e.g., MIS 8 and MIS 6). Radiolarian productivity data suggest that during Intervals VII to V, deep water masses are rich in nutrient content. However, a high vertical mixing event is recorded after Interval IV, when high nutrient concentrations appear to shift to the surface layer.

Cycladophora davisiana abundances as a paleoceanographic and stratigraphic tool in high latitude siliciceous sediments

Abstract Like the oxygen isotope curve, the relative abundance curve for Cycladophora davisiana (hereafter, c -curve) exhibits synchronous fluctuations between high-latitude regions. It was also thought that high abundances of C. davisiana indicated glacial periods. Although numerous short c -curves have been calibrated to oxygen isotope curves (spanning the last 100 kyr), only two longer c -curves have been associated with oxygen isotopes (spanning the last 0.425 Myr in the Southern Ocean and 1.1 Myr in the Sea of Okhotsk). This lack of well-established long c -curve calibrations to the oxygen isotope curve decreases the reliability of the c -curve for periods beyond 100 ka. Furthermore, recent work indicates that high abundances of C. davisiana do not necessary indicate glacial conditions. This study sought, to establish a long C. davisiana curve by directly calibrating the c -curve to oxygen isotope records measured from benthic foraminifera in the North Pacific over the last 740 kyr, to discuss the paleoceanographic significance of the c -curve and to check the suitability of the c -curve as a stratigraphic tool. Samples were obtained from Core C9001C, which was recovered at a water depth of 1180 m, east of the Shimokita Peninsula in the Tohoku region of Japan. The core contained a continuous stratigraphic succession down to marine isotope stage 18 (740 ka). Abundance peaks exceeding 5% of the total radiolarian fauna were defined as Davisiana event (DAE), and a total of 20 DAEs were observed over the last 740 kyr. The DAEs were generally near interglacial/glacial period boundary whereas peaks in the radiolarian total abundance, reflecting warm surface radiolarian productivity, occur mainly within the interglacial intervals. The DAEs appear to reflect hydrological and productivity events in the intermediate water masses of the studied site during climatic transition. We correlated the occurrence of 20 DAEs in Shimokita area with c -curve Davisiana events identified in previous studies. The c -curve fluctuations appear to be generally synchronized in the North Pacific, Sea of Okhotsk and Southern Ocean, supporting the interpretation that DAEs are intermediate water changes possibly caused by ice cover fluctuations in high latitude regions on the background of the glacial/interglacial cycling.

Radiolarian diversity changes during the Late Cambrian–Early Ordovician transition as recorded in the Cow Head Group of Newfoundland (Canada)

Abstract Upper Cambrian to Lower Ordovician pelagic carbonate rocks of the Cow Head Group in western Newfoundland (Canada) were sampled for the study of their radiolarian fauna. Well-preserved and diverse faunal assemblages were obtained from a number of levels of the Shallow Bay and Green Point formations. Three different assemblages, including a total of five families, eight genera and 20 species, are recognized in three different sections situated on a palaeobathymetric profile. The various recognized assemblages are compared with previous studies conducted on the Cow Head Group; they improve considerably our understanding of the pattern of changes in radiolarian diversity during the Cambrian–Ordovician transition. The Radiolarian biotic record of western Newfoundland unveils a two step faunal change across the Cambrian–Ordovician transitional interval: one situated at the base of the uppermost Cambrian Stage 10 and the second at a poorly constrained interval situated towards the top of the same Stage 10. These biotic changes consist of genus-level selective extinctions, as well as within-clade species faunal turnover (i.e. Echidnina, Paleospiculum), decrease of species abundance, species extinction and within-clade episodes of species diversification (i.e. Protoentactinia, Parechidnina).

Radiolarian Kalimnasphaera from the Cambrian Shuijingtuo Formation in South China

The origin of radiolarians and their early evolution are of great interest for paleontologists and biologists. The needle-like or spicular construction of radiolarians represented by Order Archaeospicularia has been considered as an ancestor of radiolarians until Braun et al. (2007) discovered some radiolarians with latticed shells from the Early Cambrian in South China. The two radiolarian tests reported here were collected from the Shuijingtuo Formation corresponding to the basal part of Cambrian Series II, in South China. One of them is spherical, latticed, and similar to the radiolarians described by Braun et al. (2007). The other is grouped within the genus Kalimnasphaera formerly reported only from the Ordovician. The result indicates that the oldest known radiolarian fauna does not belong to Order Archaeospicularia. The hypothesis that Archaeospicularia is the representative of the oldest radiolarian group needs to be reexamined. Further investigation of early forms of radiolarians in the Early Cambrian or older strata is warranted.

Middle Jurassic Radiolaria from a siliceous argillite block in a structural melange zone near Viqueque, Timor Leste: Paleogeographic implications

Thin-bedded siliceous argillite forming a large block within a structural melange zone at Viqueque, Timor Leste, has yielded a Middle Jurassic (late Bathonian–early Callovian) radiolarian assemblage belonging to Unitary Association Zone 7. Fifty-five species are recognized and illustrated, forming the most diverse radiolarian fauna yet documented from the Jurassic of Timor. The fauna shows little similarity in species content to the few other assemblages previously listed from the Middle or Late Jurassic of Timor, and also has few species in common with faunas known elsewhere in the region from Rotti, Sumatra, South Kalimantan, and Sula. Based on lithofacies similarities and age, the siliceous argillite succession in the melange block at Viqueque is included in the Noni Group originally described as the lower part of the Palelo Series in West Timor. In terms of lithofacies, the Noni Group is distinct from other stratigraphic units known in Timor. It may be associated with volcanic rocks but age relationships are uncertain, although some of the radiolarian cherts in the Noni Group in West Timor have been reported to include tuffaceous sediment. The deep-water character of the siliceous hemipelagite-pelagite facies, the probable volcanic association, and an age close to that of continental breakup in the region suggest deposition in a newly rifted Indian Ocean. In Timor’s tectonostratigraphic classification scheme, the Noni Group is here placed in the “Indian Ocean Megasequence”.

Changhsingian radiolarian fauna from Anshun of Guizhou, and its relationship to TOC and paleo-productivity

Abundant siliceous micro-fossils were found from the Dalong Formation, Xinmin section, Guizhou, which include nine species of radiolarians belonging to seven genera as well as nine types of siliceous spongy spicules. The radiolarian fauna consists of all spherical radiolarians, without any species of Albaillellaria and Latentifistularia, indicating a relative shallow water sedimentary environment. The high relative co-efficient between the radiolarian abundance and total organic carbon (TOC) shows that the radiolarian assemblage represented by Copicyntra and Paracopicyntra can be used as a proxy to the paleo-productivity. However, the relative co-efficient between biogenic silica and the radiolarian and sponge spicule abundances is low. A possible explanation is that an oceanic circulation has changed the biogenic silica content after radiolarian test and siliceous spongy spicule were dissolved.

The radiolarian evidence for the accretion of the Fu-saki Formation with the inferred oceanic plate stratigraphy: A case of weakly-metamorphosed accretionary complex in Ishigaki Jima, southern Ryukyu Arc, Japan

The island of Ishigaki Jima, located in the western part of the southern Ryukyu Arc, Japan, is underlain by a basement comprising the Tumuru and Fu-saki formations. The former is a pelitic glaucophane schist with a metamorphic age of 220–190Ma, and the latter is a weakly metamorphosed accretionary complex, composed mainly of chert, mudstone and sandstone with minor amounts of limestone and mafic rocks. The Fu-saki Formation was weakly metamorphosed at ∼140Ma. Latest Carboniferous–Early Jurassic microfossils have been obtained from the limestones, cherts and siliceous mudstones of this formation, but no fossils have been collected from the phyllitic mudstones. The radiolarian fauna of the phyllitic mudstones described herein indicates a late Pliensbachian–early Toarcian (Early Jurassic) age. This result, when combined with existing data, enables the reconstruction of an oceanic plate stratigraphy, showing a succession of (in ascending order) Upper Carboniferous–Triassic cherts, Sinemurian–lower Pliensbachian siliceous mudstones and upper Pliensbachian–lower Toarcian phyllitic mudstones and sandstones. The radiolarians from the phyllitic mudstones are important in constraining the timing of the accretion of the Fu-saki Formation to the base of the Tumuru Formation.

Guadalupian (Middle Permian) radiolarian and sponge spicule faunas from the Bancheng Formation of the Qinzhou allochthon, South China

The Gujingling (古井岭) Section in Xiaodong (小董), Guangxi (广西) Zhuang Autonomous Region, China, consists of cherts and siliceous siltstones, which belongs to the Bancheng (板城) Formation of the Qinzhou (钦州) allochthon. Three successive radiolarian assemblage-zones related to the Guadalupian (Middle Permian) appear in the section in the following ascending order: Pseudoalbaillella sp. aff. Pseudoalbaillella longicornis-Pseudoalbaillella fusiformis, Follicucullus monacanthus, and Follicucullus scholasticus-Follicucullus porrectus. The radiolarian content in this section is generally greater than that of sponge spicules. The radiolarian fauna commonly consists of abundant Albaillellaria and spherical radiolaria with minor Latentifistularia and Entactinaria. The sponge fauna is composed mainly of hexactinellids with minor demosponges. These siliceous fossil faunal features, comparable with those in a deep basin of the western belt of the Phosphoria Basin in the western United States, indicate that the Bancheng Formation in the Gujingling Section was deposited in a basin deeper than 1 000 m. The siliceous siltstones in the section are characterized by inclusion of silt-sized quartz and no inclusion of sand-sized materials, suggesting that the Gujingling Section was located at least a few hundred kilometers from the South China Block in the Guadalupian.

Late Maastrichtian Radiolaria from ODP Leg 165 — Site 999B, Colombian Basin

ABSTRACT This work presents a taxonomic study on the Late Maastrichtian radiolarian fauna from ODP Leg 165, Hole 999B, Colombian Basin. The studied fauna is composed by 24 species, being the family Archaeodictyomitridae Pessagno, 1976 the most abundant and diverse. Original opaline skeletons are replaced by calcium carbonate and precise identifications are possible only for those specimens with sturdy skeletons, what strongly suggests a preservational bias. The studied radiolarian fauna was attributed to the Amphipyndax tylotus zone, due to the occurrence of that species. Besides, the fauna described herein presents a low to intermediate latitude paleobiogeographic affinity and falls within the designation of a typical lower bathyal to abyssal one. Finally, the data presented herein are compared to those reported from the closely related ODP Hole 1001B.

Systematic paleontology and biostratigraphy of Paleocene to late Oligocene Radiolaria from DSDP Site 329, Falkland Plateau, South Atlantic Ocean

A systematic study was carried out on the relatively rich and abundant Paleocene to late Oligocene radiolarian fauna from DSDP Site 329 (Leg 36), located on the Falkland Plateau. Among the 78 taxa identified in the studied interval, a new species (Siphocampe ewingensis n. sp.) and a new combination (Eucyrtidium amygdala (Shilov 1995) n. comb. are proprosed. The high-latitude radiolarian interval zones Eucyrtidium antiquum, Lychnocanoma conica and Clinorhabdus robusta, Oligocene in age, were identified in the upper part of the interval. The lowermost studied sample, within core 33, was assigned to an interval of high-latitude radiolarian zones (RP2-RP5; Paleocene in age), mainly due to the co-occurrence of Amphisphaera kina and Protoxiphotractus wilsoni. These biostratigraphic results improve resolution in the early and late Oligocene, whichwas previously undifferentiated asOligocene, based on calcareous nannofossil biostratigraphy.

Chapter 25 Palaeogeographical distribution of Ordovician Radiolarian occurrences: patterns, significance and limitations

Abstract Over the past 15 years, significant progress has been achieved in our understanding of Lower Palaeozoic radiolarian faunas. However, description of biogeographical patterns of Ordovician Radiolaria is hampered by the paucity of known occurrences as well as possible taphonomic influences. The distribution of all known assemblages is analysed in time and space. Lower Ordovician (especially Tremadocian) Radiolaria are known from two distinct tropical localities of Laurentia. Geographical coverage is much better for the Middle Ordovician (Darriwilian). However, data are concentrated in tropical palaeolatitudes (between 30°N and 30°S). The absence of data from mid/high-latitude localities limits any biogeographical insights. In addition to this there are taphonomic and taxonomic biases. Data are also sparse for the Upper Ordovician. However, comparison between Australian and Nevadan material of Katian age shows strong similarities, suggesting the presence of a coherent tropical radiolarian bioprovince, as in the modern ocean.

EARLY LADINIAN RADIOLARIAN FAUNA FROM THE MONTE SAN GIORGIO (SOUTHERN ALPS, SWITZERLAND): SYSTEMATICS, BIOSTRATIGRAPHY AND PALEO(BIO)GEOGRAPHIC IMPLICATIONS

A well-preserved radiolarian fauna was recovered from the lower part of the San Giorgio Dolomite (Monte San Giorgio, UNESCO WHL, Switzerland), belonging to the Eoprotrachyceras curionii Ammonoid Zone (earliest Ladinian). The depositional environment, consisting of an intraplatform basin with restricted circulation, seemingly resulted in an unusual radiolarian assemblage displaying neglegible percentages and very low diversity of Nassellaria. Seventy-three species have been identified and seven new species have been described: Eptingium danieli n. sp., Eptingium neriae n. sp., Parentactinosphaera eoladinica n. sp., Sepsagon ticinensis n. sp., Sepsagon ? valporinae n. sp., Novamuria wirzi n. sp. and Pessagnollum hexaspinosum n. sp. The taxonomic status of the recovered species has been reassessed and four new genera have been introduced: Bernoulliella gen. n., Eohexastylus gen. n., Ticinosphaera gen. n. and Lahmosphaera gen. n. The new radiolarian assemblages, the first so far described from the Monte San Giorgio, supplement further information to the taxa occurring in the uppermost Ladinocampe multiperforata Radiolarian Zone, as yet poorly defined, also confirming the absence of major changes in the radiolarian fauna across the Anisian/Ladinian boundary. The widespread co-occurrence of taxa doubtlessly testifies to the existence, in early Ladinian times, of open-marine connections between the basin of the Monte San Giorgio and the pelagic “Buchenstein” basins of the central and eastern South-Alpine domain and farther afield across the Tethys.

Late Paleocene radiolarian fauna from Tibet and its geological implications

A diverse, abundant, and well-preserved radiolarian fauna in Jiazhu, Zhongba County of Tibet, in the western sector of Yarlung Zangbo Suture Zone, is assigned to a late Paleocene radiolarian zone, the Buryella pentadica interval zone, spanning 59–56.5 Ma. Regionally, a late Paleocene basalt block in the bathyal–abyssal siliceous mudstone and graywacke yielded an age of 59.1 Ma (zircon SHRIMP U–Pb). The late Paleocene radiolarian fauna, the tectonic attribution of the radiolarian cherts and the basalt block indicate that oceanic crust persisted in the Zhongba area until the late Paleocene and the initial collision between the Indian and Eurasian plates post-dates the late Paleocene. It is inferred that the Neo-Tethys transformed into a remnant oceanic basin in the late Paleocene, at the terminal stage of the oceanic crust subduction, and the closure of the remnant oceanic basin in the studied region took place after the late Paleocene. In contrast to the previous investigations, we suggest that there was a remnant oceanic basin to the west of the Saga area and a foreland basin to the east of Saga in southern Tibet during the late Paleocene. We argue that the closure of the Neo-Tethys progressed from east to west.

徳島県北川地域の四万十帯付加コンプレックスから産出した白亜紀放散虫化石

Cretaceous radiolarians were recovered from three mudstone samples within the Shimanto accretionary complex in the Kitagawa district, Tokushima Prefecture, Southwest Japan. Two of them, red mudstone and gray mudstone, in the Osodani Unit yield radiolarian faunas indicating the ages of late Albian to Cenomanian and late Cenomanian, respectively. Black mudstone within the Taniyama Unit includes a representative radiolarian fauna presenting a Turonian age. Based on the age determination by our present data together with the radiolarian evidences reported from surrounding area, the accretionary age of the Osodani Unit is constrained to be Turonian to early Coniacian, and those of the Taniyama Unit is reinterpreted as the age of Turonian to Santonian. New findings of radiolarian fossils in the Kitagawa district help establish the age constraint for the Cretaceous Shimanto accretionary complex in eastern Shikoku.