Radiolarian Zones Research Articles

Late Miocene Radiolarian Biostratigraphy and Paleoceanography of Sawai Bay Formation, Neill Island, Andamans, India

Stichocorys peregrina, Didymocyrtis penultima and Didymocyrtis antepenultima Late Miocene radiolarian zones are encountered from mudstone strata of Sawai Bay Formation, Neill Island, Andamans. Percentage data of forty-five coarser taxonomic groups of radiolarians were subjected to Q-mode cluster analysis. Cluster A comprises Stichocorys, Phormospyris, Eucyrtidium, Lamprocyclas, Acrosphaera and Cycladophora groups. Cluster B was divided into subclusters at 0.83 level of clustering. Subcluster B 1 comprised Didymocyrtis-Diartus, Pyloniids, Collosphaera, Phacodiscids, Pterocorythids, Pterocanium and Spongodiscids, whereas subcluster B 2 comprised Porodiscus, Euchitonids, Stylodictya-Stylochlamydium, Spongopyle, Phormostichoartus, Lophophaenids and Lithelius groups. Based on the ecology of the modern homeomorphs of the dominant radiolarian groups, it is suggested that cluster A and cluster B indicate colder and warmer periods, respectively. SubclusterB 1 indicates surface-water fauna and subcluster B 2 represents subsurface water fauna. Dominance of subcluster B 2 in Didymocyrtis antepenultima zone (8.5-7.2 Ma) suggests that subsurface fauna was thriving more probably due to the monsoonal upwelling during warmer periods. This finding is also substantiated with diatom/radiolaria ratio. Presence and absence of deep (1200-2000m) and intermediate (700-1200m) water dwelling radiolarians like the Plectopyramids, Botryostrobus and Sethoperinids groups indicate basinal shallowing during Late Miocene. It may be due to subduction of the Indian plate below the Asian plate, coupled with huge sediment discharged from the Irrawaddy River of Burma during monsoon dominated warmer periods (5.0-6.3 and 8.5-7.7 Ma) in Late Miocene.

Abyssal sediment erosion in the Central Indian Basin: Evidence from radiochemical and radiolarian studies

Radiochemical analyses of three spade cores collected around 78°E between 10°S and 12°S from the abyssal depths in the Central Indian Basin yield an average accumulation rate of 2 mm/ka. The ratios of 230Th flux in the sediments to its production rate in the overlying water column ( Fa Fp ) are extremely low (∼0.2). This fact, and the absence of Collosphaera invaginata (first appearance datum, 150–200 ka), an index radiolarian species of Neogene Radiolarian Zone 1, indicate intense erosion and lateral transport of younger sediments from this region. The radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent Antarctic Bottom Water entering the Central Indian Basin through the northern saddles of the Ninety East Ridge.

Pliocene-Pleistocene radiolarian events and magnetostratigraphic calibrations for the tropical Indian Ocean

A composite of four piston cores from the Central Indian Basin and adjacent Ninetyeast Ridge has yielded a continuous magnetobiostratigraphic reference section for most of the Pliocene and the Pleistocene (0.0–4.5 Ma). We identified thirty-three radiolarian events (first- or last-occurrences), and precisely correlated each event to the Neogene geomagnetic polarity time scale. Thirteen of these events are based on revised taxonomic studies of the genera Anthocyrtidium and Pterocorys. Some events show significant departures from synchroneity: five of the radiolarian first-appearances and seven of the last-appearances are time-transgressive by 0.4 m.y. or greater. We here propose a revised, eleven-fold radiolarian zonation for the Pliocene-Pleistocene, using zonal boundaries defined by events which are easily recognized and are demonstrably synchronous in the tropical Indo-Pacific. The sequence of faunal and floral events reported in this paper will allow high-resolution biostratigraphic correlations within the tropical Indian Ocean; however, the same sequence of events is not necessarily applicable to other tropical or extratropical regions.

Upper Tithonian Vallupinae (Radiolaria) from the Taman Formation, East-Central Mexico

The pantanelliid subfamily Vallupinae Pessagno and MacLeod reached its acme of development during the late Tithonian. Two new genera (Neovallupus and Supervallupus) and seven new species are described from the upper Tithonian portion of the Taman Formation, east-central Mexico; new species are Bivallupus oppositus, B. primigenus, Neovallupus dumitricai, N. modestus, Protovallupus excellens, Supervallupus haeckeli, and Vallupus laxus. In addition, the definition of the subfamily Vallupinae is emended. Many taxa belonging to the Vallupinae are short-ranging, distinctive, and cosmopolitan-occurring in both Boreal and Tethyan strata. Vallupinid taxa have proven to be useful in developing a meaningful radiolarian zonation for the upper Tithonian of North America.

Radiolarian zonation and volcanic ash layers in two Quaternary sediment cores from the central Indian Ocean Basin

Radiolarian zonation and volcanic ash layers in two Quaternary sediment cores from the central Indian Ocean Basin

A newly proposed radiolarian zonation for the Jurassic of Japan

Eight radiolarian zones are established on the basis of biostratigraphic investigations on the Japanese Jurassic. These are the Parahsuum sp. C, Archicapsa pachyderma, Laxtorum(?) jurassicum, Tricolocapsa plicarum, Tricolocapsa conexa, Stylocapsa(?) spiralis, Cinguloturris carpatica and Pseudodictyomitra primitiva Zones in ascending order. Biohorizons reflecting evolutionary lineages are utilized, where possible, in defining the zonal boundaries. In the case of absence of adequate lineages, biohorizons of the first or last appearance of characteristic species are applied. The zones are compared with the zones previously proposed by several workers in Japan.

Middle to Late Cretaceous radiolarian zonation of the Bering region, U.S.S.R.

The Middle-Upper Cretaceous volcanogenic-siliceous geological sections of the Koryak highland, the submerged Shirshov Ridge, and northern, central and eastern Kamchatka are investigated micropaleontologically. Radiolarians were extracted from siliceous rocks by means of HF. Their preservation is satisfactory. As a result of biostratigraphical research, six radiolarian assemblages are distinguished. The oldest is the Albian-Turonian assemblage with Pseudodictyomitra pseudomacrocephala. The age of this complex is controlled by planktonic foraminifers co-occurring with radiolarians. The following is the Coniacian-Early Santonian assemblage with Archaeospongoprunum bipartitum, which is accompanied by Early Santonian planktonic foraminifers. The Late Santonian-Early Campanian age of the assemblage with Pseudoaulophacus floresensis is confirmed by macrofossils. A Middle-Late Campanian age of the Amphipyndax enesseffi assemblage is proved by the widespread Inoceramus schmidti. A Late Campanian-Early Maastrichtian age of the radiolarian assemblage with Clathrocyclas diceros is in good agreement with diatoms and silicoflagellates. A Late Maastrichtian-Danian age of the assemblage with Bothryopyramis sanjoaquinensis is proved by foraminifers. These radiolarian assemblages can be considered as zonal for the boreal area of the northeast of the U.S.S.R. and as a key for further biostratigraphic research.

Miocene Radiolarian Biostratigraphy, U.S. Mid-Atlantic Coastal Plain

Cenozoic radiolarian biostratigraphy was developed in low-latitude pelagic sections where these microfossils are abundant, diverse and well preserved. Radiolarian faunas, ifpreserved, are usually less abundant and diverse in shelf sediments than in deep-sea sediments. Such an assemblage occurs in the neritic Calvert, Choptank, and Eastover formations (Chesapeake Group) of the mid-Atlantic Coastal Plain. Although many biostratigraphically important forms are sparse or absent in these middle-latitude, shallow shelf sediments, the standard radiolarian biostratigraphic zones can be recognized. The Calvert Formation is of early to middle Miocene age (Stichocorys wolffii to Dorcadospyris alata zones), the Choptank Formation of middle Miocene age (Diartus petterssoni Zone), and the Eastover Formation of latest Miocene or early Pliocene age (Stichocorys peregrina or Spongaster pentas zones). These results suggest that radiolarians may be valuable for biostratigraphic purposes over a wider spectrum of facies than was previously recognized.

Neogene Deep-Living Radiolarian Zonation and Its Uses: ABSTRACT

A Neogene radiolarian zonation using deep-living radiolarians has been developed. This zonation consists of 6 radiolarian zones with the nominant taxa of each zone first occurring at the base of its zone, and that nominant taxa running throughout its zone. These zones are the Eucyrtidium cienkowskii (22.5-19 Ma), Siphocampe arachnea (19-14 Ma), Oroscena with digitate spines (14-11 Ma), Botryostrobus bramlettei (11-6 Ma), Clathrocyclas bicornis (6-2.5 Ma), and Cycladophora davisiana davisiana (2.5-0 Ma) Zones. These zones were developed by using Antarctic, equatorial, and North Pacific Deep Sea Drilling Project holes, and are believed to be cosmopolitan zones due to their being based on deep-living radiolarians. These zones are dated by correlating the first occurrences of the nominant taxa with warm-water radiolarian datums that have been paleomagnetically dated. These zones are not designed for routine use, but rather for tying together other more provincial zonations, and for comparing oceanographic phenomena on global datum planes. To date, this zonation has been used to correlate Antarctic zonations intra- and inter-regionally and to study the evolution of radiolarian lineages. End_of_Article - Last_Page 243------------

A comparison of radiolarian and foraminiferal paleoecology in the Southern Indian Ocean: New evidence for the interhemispheric timing of climatic change

The deglacial transition between oxygen-isotope Stages 6 and 5e (about 127,000 yr B.P.) is marked by both oxygen isotopic depletion and estimated sea-surface temperature (SST) increase in two subantarctic Indian Ocean cores. The data show synchroneity between warming of foraminifera-based SST estimates and depletion of δ 18O, but an earlier warming trend on the basis of radiolarian SST estimates. These data have been previously interpreted to indicate that the high-latitude Southern Ocean warms prior to significant melting of glacial-age ice sheets. Comparison of core-top assemblages with surface and subsurface conditions in the Southern Indian Ocean reveals that (1) a three-part foraminiferal zonation reflects the surface hydrographic regime, with abrupt faunal transitions at two major fronts: the Subtropical Convergence (STC) and the Antarctic Polar Front (APF); and (2) a two-part radiolarian zonation coincides with a two-part subsurface hydrographic regime, with an abrupt faunal transition corresponding to the southern terminus of subtropical lower water (STLW) between the STC and the APF. It is suggested that shifts of these surface and subsurface regimes are recorded by these foraminiferal and radiolarian assemblages. In this interpretation, the observed lead of radiolarian SST with respect to δ 18O indicates an early response to a southward shift of STLW, while the later foraminiferal SST warming indicates a southward shift of the STC. Thus, the origin of the Southern Hemisphere SST lead may be related to STLW, which emanates from the subtropical gyres, rather than the polar regions.

Stratigraphy of the Late Middle Eocene to Early Oligocene in the Bath Cliff Section, Barbados, West Indies

Changes in fauna and flora across the Eocene/ Oligocene boundary are documented in an expanded section of indurated deep sea oozes and ash beds deposited close to the CCD and now exposed at Bath Cliff, Barbados. A sedimentation rate of 27 m/m.y. for the Late Eocene is calculated. The spread of events through time does not suggest any catastrophe at the boundary. The last occurrence of discshaped discoasters (CP 15/CP 16 Zone boundary) occurs before the last occurrence of Hantkenina, which is taken to represent the epoch boundary. The last occurrence of Turborotalia cerroazulensis s.I. occurs slightly before the boundary, whereas the Cryptoprora omata/Theocyrtis tuberosa radiolarian Zone boundary occurs slightly after it. The Thyrsocyrtis bromia radiolarian Zone of the Late Eocene has been replaced by three new zones-from top to bottom: Cryptoprora omata Zone, Calocyclas bandyca Zone, and Carpocanistrum azyx Zone. A microtektite horizon occurs in the Cryptoprora ornata Zone slightly less than one million years before the epoch boundary. In the basal Oligocene a sudden enrichment of 180 in Oridorsalis umbonatus suggests a drop in bottom water temperature of 5-6?C. Stratigraphy of the late Middle Eocene to Early Oligocene in the Bath Cliff section, Barbados, West Indies

Lower Jurassic Radiolaria, San Hipolito, Formation, Vizcaino Peninsula, Baja California Sur: ABSTRACT

End_Page 568------------------------------Lower Jurassic radiolarian faunas recovered from the sandstone member of the San Hipolito Formation, Baja California Sur, indicate a younger age for much of this member than previously assumed. The San Hipolito Formation, exposed at Punta San Hipolito on the Vizcaino Peninsula, consists of 2,400 m (7,875 ft) of marine sedimentary rocks. The oldest member, a Halobia-bearing green chert, lies on pillow basalts and is successively overlain by limestone, breccia, and sandstone members. Upper Triassic (upper Karnian? and/or lower Norian to lower upper Norian) radiolarians have been described from the chert member. The hemipelagic pelecypod Monotis cf. M. subcircularis (Gabb) of Late Triassic (late Norian) age is found in the upper part of the limestone member. The uppermost member of the San Hipolito Formation, a volcaniclastic sandstone 1,840 m (6,035 ft) thick, previously yielded only poorly preserved radiolarians, and was inferred to be of Late Triassic age. In this study, well-preserved radiolarians were not recovered from the sandstone and tuff beds. Thin limestone beds at the very base of the sandstone member contain a unique radiolarian fauna with some forms similar to Triassic radiolarians described from Austria and Italy. Discoidal limestone concretions and thin limestone beds from the top of the lower part of the sandstone member yielded abundant, well-preserved Lower Jurassic radiolarians. No ammonites are found in association with the San Hipolito Lower Jurassic radiolarians, but the radiolarians are similar to Lower Jurassic radiolarian faunas from British Columbia and Oregon, for which excellent ammonite biostratigraphy is available. Based on the occurrence of the radiolarian genera Canoptum, Droltus, Bagotum, Canutus, Hsuum, Lupherium and Pseudoheliodiscus, a Pliensbachian and/or Toarcian age is assigned to the top of the lower part of the sandstone member of the San Hipolito Formation. The continuation of radiolarian studies in British Columbia, Oregon, and Baja California will provide a basis for further elaboration and refinement of a Lower Jurassic radiolarian zonation. End_of_Article - Last_Page 569------------

Upper Eocene North American Microtektite Layer: Associated Radiolarian Extinctions, Climatic Change, and Iridium Anomaly: ABSTRACT

Tektites are glass objects believed by many authors to have been formed by meteorite (or cometary) impact. The areas on the earth's surface where tektites are found are called strewn fields. Thus, tektites found in Texas and Georgia belong to the North American strewn field. Microtektites (< 1-mm diameter tektites) have been found in upper Eocene sediments from one piston core and in cores from nine Deep Sea Drilling Project sites in the Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern Indian Ocean. Based on their fission-track age (34.6 ± 4.2 m.y.), geologic age, geographic location, and chemistry, these microtekties are thought to be part of the North American strewn field. The North American tektites have fission-track, K-Ar, and 40Ar-39Ar ages of ~ 34 m.y. The North American microtektite layer End_Page 470------------------------------ occurs in the Thyrsocyrtis bromia radiolarian Zone, the P15 foraminiferal Zone, and the Discoaster barbadiensis calcareous nannoplankton Zone. At each site, the layer is associated with the last appearance of several species of Radiolaria (e.g., Thyrsocyrtis bromia, t. tetracantha, t. rhizodon, Calocyclas turris). Previously published oxygen isotope data indicate a drop in temperature at about this time, which may be related to the tektite event and may have been responsible for the radiolarian extinctions. An iridium anomaly has recently been found associated with the microtektite layer that supports an impact origin for the tektites. The extent of the strewn field and the calculated mass of microtektite material (~ 1012 kg) indicate that the North American tektite event w s a relatively large event. End_of_Article - Last_Page 471------------

Stratigraphic and Environmental Interpretation of Miocene Radiolarian Assemblages of Maria Madre and Maria Cleofas Islands and Baja California, Mexico: ABSTRACT

Miocene radiolarian assemblages were collected from Maria Madre and Maria Cleofas islands and five localities along Baja California (Tortugas, Bahia de Asuncion, Cien, Cabo San Lucas-La Paz, and San Felipe). Most of these sequences are composed of diatomaceous and/or diatomaceous shales or very fine sands. Stratigraphic correlation with previous work on the Monterey Formation, Experimental Mohole, and DSDP Legs 5, 7, 18, 32, and 66 indicate that the radiolarian zones Diartus petterssoni, Didymocyrtis antepenultima, Didymocyrtis penultima, and Stichocorys peregrina are present in these newly worked sections. The oldest dates obtained were from the Tortugas Formation (middle Miocene); the youngest was uppermost Miocene from the section at San Felipe. Paleoenvironmental interpretations were based on quantitative analyses on warm, cold, deep, and intermediate water forms, as well as abundances of diatoms and silicoflagellates. The Tortugas Formation (middle to upper Miocene) represents a slope biofacies characterized by relatively high percentages of cold and intermediate water forms, and the absence of deepwater radiolarians. Diatomaceous layers increase upsection and show an increase in radiolarian abundance and diversity (especially of cold water forms and silicoflagellates). These conditions reflect an enhancement of upwelling areas and low oxygen depositional facies. Previous radiolarian and diatom studies in Maria Madre and Bahia de Asuncion as well as Maria Cleofas, indicate that these environments of deposition are analogous to the Monterey Formation. Radiolarian faunas from these three sections are correlated to the Didymocyrtis antepenultima Zone and reflect an intensification of upwelling, perhaps as a result of the buildup of the Antarctic ice cap. The El Cien section and the section collected between La Paz and Cabo San Lucas are representative of the Didymocyrtis penultima Zone. The El Cien sequence is underlain by pillow basalts that may represent the contact between the East Pacific Rise and North America. Radiolarian faunas in both sections represent a mixture of warm and cold water forms with considerable dilution by terrigeneous sediments. San Felipe, the only section located on the gulf side, contains a unique radiolarian/fauna. This unique fauna may have been the result of basin isolation during the opening of the Gulf of California. End_of_Article - Last_Page 531------------

Preliminary radiolarian zonation for Late Devonian through Permian time

Nine time-successive radiolarian assemblages of wide geographic distribution are recognizable in Upper Devonian through Permian rocks. Identification of assemblages in bedded chert terranes provides a means of dating where other evidence is lacking. Changes in nature of these radiolarian populations with time possibly coincided with inferred fluctuations of world sea level.

Deep Sea Carbonates: Acoustic, Physical, and Stratigraphic Properties

ABSTRACT An examination of the relationship between acoustic, stratigraphic and physical properties of deep-sea carbonates from the eastern equatorial Pacific revealed that variations in acoustic impedance are almost entirely controlled by changes in saturated bulk density (or its inverse, porosity). Velocity changes are very small. Saturated bulk density changes are highly correlated with variations in percent calcium carbonate. High carbonate samples are dominated by high density platy carbonate material while low carbonate material is dominated by low density spiny siliceous microfossils. Stratigraphic control was achieved through: (1) comparison of the downcore variations in percent carbonate with the carbonate curves of well-dated cores (e.g., RC11-209), (2) stratigraphy of Pulleniatin coiling directions, (3) radiolarian zonation, (4) the extinction of P. lacunosa, and (5) last appearance of discoasters. Sedimentation rates in the area studied are found as 6.17 meters per million years for the last 700,000 years and 4.15 meters per million years before that time. The oxygen isotope record provided the final evidence linking the acoustic properties to the paleo-oceanographic record. Variations in saturated bulk density (or its inverse, porosity) that account for the changes in acoustic impedance are ultimately caused by climatic changes. Warm periods were times of enhanced dissolution which caused low percent carbonate and low saturated bulk density (and therefore low impedance) values. The opposite is true for cold periods. These climatic cycles can be matc ed with glacials and interglacials for the past one million years, but have also occurred before the advent of Pleistocene glaciation. Thus the acoustic record of equatorial carbonates contains a strong paleo-oceanographic signal.

Upper Triassic Radiolaria from Eastern Oregon and Queen Charlotte Islands, British Columbia: ABSTRACT

Radiolarian-bearing rocks have been collected from eastern Oregon (Rail Cabin Formation) and the Queen Charlotte Islands, British Columbia (middle member of the Kunga Formation). The Rail Cabin Formation consists predominantly of 365 m of thin-bedded, manganiferous argillite. The middle or black limestone member of the Kunga Formation consists of 210 to 280 m of thin-bedded, black carbonaceous limestone with minor amounts of black argillite. The Rail Cabin Formation can be subdivided into two zonal units based on radiolarian biostratigraphy: a lower Mostlerium Zone (?Karnian, early Norian to late middle Norian) and an upper Pantanellium silberlingi Zone (late middle to late Norian). Samples from the middle member of the Kunga Formation contain radiolarians which are indicative of the Pantanellium silberlingi Zone. Norian ages for both radiolarian zones have been confirmed primarily by associated pectinacid bivalves. End_of_Article - Last_Page 421------------

Absolute Ages of Quaternary Radiolarian Datum Levels in the Equatorial Pacific

AbstractRadiolarian assemblages were examined in two Quaternary cores (V24-58; RC11-209) from the tropical Padific Ocean. Eight radiolarian datum levels were identified in each core, and “absolute” ages were estimated for these levels by interpolation between paleomagnetic reversal boundaries previously established for the cores. The tropical radiolarian zonation for the Quaternary proposed by Nigrini (1971) appears to be most useful in terms of the reliability and ease of identification of the proposed zonal boundaries. Our estimated ages for the base of each of these zones are:Buccinosphaera invaginataZone (Zone 1): 210,000 yr BP;Collosphaera tuberosaZone (Zone 2): 370,000 yr BP;Amphirhopalum ypsilonZone (Zone 3): 940,000 yr BP;Anthocyrtidium angulareZone (Zone 4): 1,700,000 yr BP.A comparison of our age estimates with those of Quaternary radiolarian datum levels in cores from other regions suggests that significant diachroneity on a scale of up to several hundred thousand years may exist for some (and perhaps all) of these “events.” Diachroneity is most readily studied and documented in late Neogene cores where the absolute ages of the magnetic polarity reversals are known most precisely, but may also exist (though difficult to recolve) in earlier Cenozoic sediments. The existence of such diachroneity, if demonstrated through further studies, would have significant implications for our understanding of evolutionary patterns of planktonic communities in different biogeographic regions.

Some New Tertiary Radiolaria from Antarctic Deep-Sea Sediments

Systematic descriptions of fifteen new species and one new subspecies of Tertiary Antarctic Radiolaria are presented in this paper. These Radiolaria, ranging in age from Oligocene to Pleistocene, are constituents of the biosiliceous sediments collected from 16 holes drilled in the ocean floor during Leg 28 of the Glomar Challenger. In addition, several are designated as index fossils in a proposed radiolarian zonation. Some new Tertiary Radiolaria from Antarctic deep-sea sediments INTRODUCTION Systematic descriptions of fifteen new species and one new subspecies of Tertiary Radiolaria are presented in this paper. These Radiolaria are constituents of the southern high-latitude fauna extracted from the sediments recovered by Leg 28 of the Deep Sea Drilling Project. This cruise represents the first of five expeditions by the Glomar Challenger into Antarctic waters. The pre-Pliocene sediments recovered during this leg have not previously been collected by other coring techniques. Therefore some Radiolaria in the drilling cores have been made available for examination for the first time. It is anticipated that subsequent legs of the Glomar Challenger may recover siliceous sediments of similar ages. Consequently, it is desirable to establish the taxonomy of previously undescribed Radiolaria discovered in the recovery from the drill holes of Leg 28 at the earliest possible time. A more detailed treatment of the total radiolarian assemblages observed in these sediments will be presented in the initial reports of the Deep Sea Drilling Project for Leg 28. Sixteen holes at eleven sites were drilled in the Australian sector of the Antarctic during Leg 28 (text-figure 1 and table 1). All of the drilling sites yielded Radiolaria of varying abundance and preservation. Much of the recovery from holes drilled on or adjacent to the Antarctic continental rise (Holes 268, 269 and 269A) and on the Ross Sea shelf (Holes 270, 271, 272 and 273) contained only sparse quantities of Radiolaria because of the masking effects of terrigenous components. Other intervals in these holes were barren of Radiolaria. Moderately well-preserved Radiolaria were found only in restricted stratigraphic levels on the Naturaliste Plateau (Holes 264 and 264A), where calcareous sediments dominate. Holes drilled on the flanks of the Southeast Indian Ridge or in deep basins far from the Antarctic continent (Holes 265, 266, 267, 267A and 267B) generally had substantial amounts of well-preserved Radiolaria. Radiolaria in Leg-28 sediments range in age from Eocene to Pleistocene. Eocene radiolarian-bearing sediments are present at Site 264, the only site drilled north of the Antarctic Polar Front. This radiolarian assemblage is similar to but less diverse than Eocene low-latitude assemblages, yet it is approximately correlatable with the zonation of Riedel and Sanfilippo (1970). None of these Eocene Radiolaria is treated in this paper. Sediments ranging in age from Oligocene to Pleistocene were recovered from sites drilled south of the Antarctic Polar Front (Sites 265 to 274). The radiolarian biostratigraphy established by Hays and Opdyke (1967) has been micropaleontology, vol. 20, no. 4, pp. 480-492, pis. 1-4, october, 1974 480 This content downloaded from 157.55.39.17 on Thu, 01 Sep 2016 06:08:07 UTC All use subject to http://about.jstor.org/terms

Paleomagnetic polarity sequence and radiolarian zones, brunhes to polarity epoch 20

Superposition of paleomagnetic polarity logs of seven chronologically overlapping piston cores from the central equatorial Pacific, using the established tropical radiolarian zonation as a stratigraphic reference, produced a nearly continuous correlation of magnetic and radiolarian events ranging from late Pleistocene to earliest Miocene. Twenty magnetic polarity epochs, and possibly as many as 30 polarity events, occur during this time span. Epoch 16 (reversed polarity) appears to be the longest interval ( ∼ 14.8–17.6m.y. B.P.) among these Neogene magnetostratigraphic units. The middle/late Miocene boundary is shown to fall within latest Epoch 11 (normal) and its approximate age is between 10.5 and 11 m.y. B.P. The early/middle Miocene boundary occurs within the top of Epoch 16 at a suggested age of about 15 m.y. B.P.