6. EOCENE-OLIGOCENE RADIOLARIANS FROM LEG ... 6. EOCENE-OLIGOCENE RADIOLARIANS FROM LEG 145, NORTH PACIFIC1

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  • Rea, D.K., Basov, I.A., Scholl, D.W., and Allan, J.F. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 145

    6. EOCENE-OLIGOCENE RADIOLARIANS FROM LEG 145, NORTH PACIFIC1

    V.V. Shilov2

    ABSTRACT

    During the Leg 145 cruise in the North Pacific Ocean, well-preserved Paleogene radiolarians were obtained from Holes 883B, 883E, and 884B. Their distribution pattern in these sections has been revealed and high-latitude radiolarian assemblages in the North Pacific are described. Biostratigraphic schemes proposed in low latitudes and other Northern Hemisphere regions are not applicable in the subarctic Pacific, because most zone marker species of other regions are absent here. Two new species, Dictyomitra amygdala and Lithomitra micropore are described.

    INTRODUCTION

    At the present, the Eocene-Oligocene radiolarians of the Northern Pacific have not been adequately analyzed because of the absence of well-studied and continuous geological sections that contain them. Many of the earlier studied sections contain breaks or do not correlate reliably with any scales (paleomagnetic, scales based on nannoplank- ton or foraminifers). Among the earlier studies on the Eocene-Oli- gocene radiolarians in the Northern high latitudes, the works by Clark and Campbell (1942, 1945) and Blueford (1988) on California and Vitukhin (1992) and Runeva (1984) on far eastern of Russia should be noted. Eocene radiolarians of the Boreal Realm have been studied by Kozlova (1983, 1984). Lipman (1987) described radiolarians in sections of the western Siberian Lowland and distinguished zonal units. Eocene-Oligocene radiolarians from Norwegian Sea deposits have been studied by Petrushevskaya and Kozlova (1979) and Bj0rk- lund(1976).

    During Leg 145 of the Ocean Drilling Program (ODP) in the North Pacific (North Pacific Transect) several holes were drilled that penetrated Eocene-Oligocene deposits on the Emperor Seamounts at the latitude of 51°N (Sites 883 and 884), and yielded well-preserved radiolarians. Nannofossil chalks, calcareous chalks and claystones were the enclosing deposits for these radiolarian assemblages.

    The materials obtained in the course of drilling appear to be unique for the Eocene-Oligocene biostratigraphy of the Boreal Realm of the Pacific for several reasons: (1) at present, they contain the most complete information on the middle Eocene-Oligocene of the Boreal Realm of the Pacific; (2) they contain well-preserved ra- diolarians that are sufficiently representative for use in biostrati- graphic correlations; (3) they can be correlated with biostratigraphic zones based on the nannoplankton observed (Holes 883B and 883E).

    PROCEDURES

    Preparation of radiolarian samples for microscopic examination during Leg 145 and next period followed the methods described by Sanfilippo et al (1985) and Petrushevskaya (1986). Between 3 and 5

    1 Rea, D.K., Basov, LA., Scholl, D.W., and Allan, J.F. (Eds.), 1995. Proc. ODP, Sci. Results, 145: College Station, TX (Ocean Drilling Program).

    institute of Geology and Mineral Resources of the World Ocean, Maklina Avenue 1, St. Petersburg, 190121, Russia.

    cm3 of sediments was disaggregated and oxidized in a 10% hydrogen peroxide solution. In addition, those samples containing carbonate were treated with a solution of hydrochloric acid to dissolve all cal- careous microfossils. Brief treatment of most samples in an ultrason- ic bath was followed by washing in a 63-µm mesh sieve. Strewn slides were prepared from the residue for examination of radiolari- ans.

    Radiolarian assemblage abundance was assessed as follows:

    A = abundant (>500 specimens on a 22 × 50 mm slide), C = common (100-500 specimens on a slide), F = few (50-100 specimens on a slide), and R = rare (20), C = common (10-20), F = few (5-10), and R = rare (

  • V.V. SHILOV

    Vitukhin, in his thesis on materials from geological sections of far eastern Russia (1992), suggested that beds with fauna should be dis- tinguished in the Eocene-Oligocene deposits with radiolarian assem- blages. Beds with Theocotyle (l)bicornis-Buryella tetradica, Theocyrtis litos, and Theocyrtis litos-Ceratospyris tons have been distinguished for the Eocene. In the Oligocene, beds with Lithomitrissa conica- Theocotyle multichornia and Lithomitrissa conica-Haliomma (?)ex- tima have been distinguished. It was also impossible to apply this scheme of distinguishing beds with fauna for subdivision of the sed- imentary succession from Sites 883 and 884 because of the lack of zonal species.

    Some Eocene-Oligocene radiolarian species discovered in the de- posits of Sites 883 and 884 (Cenosphaera aspera, Spongodiscus os- culosus, and Clathrocyclas extensa) were revealed earlier in samples taken from holes drilled in the Norwegian Sea during Deep Sea Drill- ing Project Leg 38 (Petrushevskaya and Kozlova, 1979).

    Application of the zonal biostratigraphic scale for the Eocene- Oligocene based on radiolarians and developed for the tropical part of the World Ocean (Riedel and Sanfilippo, 1970, 1978; Foreman, 1973; Sanfilippo et al., 1985) appeared to be impossible because of the lack of zone marker species. Among the events that could be re- vealed in the sedimentary succession of Hole 883B are the following: (1) the presence of Eusyringium striata Brandt (one specimen in Sample 145-883B-79X-CC) and (2) the appearance of Dictyoprora mongolfieri Ehrenberg. Therefore, there were no grounds for distin- guishing biostratigraphic zones based on radiolarians, applied for the tropical area.

    The materials obtained during Leg 145 enabled us to determine the specific distribution features of the Eocene-Oligocene radiolari- ans in Sites 883 and 884 and to propose a new biostratigraphic zona- tion (for discussion) correlated with nannoplankton zones for the middle-late Eocene.

    The middle-late Eocene geological sections yield well-preserved radiolarian assemblages.

    The following zonation is proposed (Hollis, 1976) for the middle- late Eocene:

    1. Podocyrtis mitrella Range Zone (middle Eocene; NP 12-14). Base: First Occurrence (F0) of Podocyrtis mitrella. Top: Last Occurrence (FO) of Podocyrtis mitrella. Radiolarian assemblages from this zone comprise the fol-

    lowing species: Podocyrtis mitrella, Dictyoprora amphora, Dictyomitra amygdala, and Lithomitra micropore.

    2. Dictyomitra amygdala Interval Zone (middle Eocene: NP 14). Base: LO of Podocyrtis mitrella. Top: F0 of Dictyoprora mongolfieri. Radiolarian assemblages from this zone comprise the fol-

    lowing species: Dictyoprora amphora and Dictyomitra amygdala.

    3. Dictyoprora amphora Interval Zone (middle-late Eocene; NP 14-18).

    Base: F0 of Dictyoprora mongolfieri. Top: LO of Dictyoprora amphora. Radiolarian assemblages from this zone comprise the fol-

    lowing species: Dictyoprora amphora, Phormocyrtis embol- um gr. (the appearance of this species is close to the base of the zone), Dictyomitra amygdala, Clathrocyclas extensa, Clath- rocyclas universa gr., Artobotrys auriculaleporis, and Lych- nocanella babylonis.

    Several events can be recorded in the Oligocene part of the sec- tion: (1) appearance and disappearance of Calocyclas asperum, (2) abundance of Axoprunum losbanosensis, (3) appearance of Sty- losphaera angelina, (4) appearance of Lithocarpium titan, (5) appear- ance of Spongodiscus osculosus, (6) disappearance of Cenosphaera aspera, and (7) appearance of Axoprunum bispiculum.

    RADIOLARIANS IN EACH HOLE

    Hole 883B

    Hole 883B (Table 1) is located at 51°11.908'N, 167°46.128'E, in a water depth of 2384 m. There are no radiolarians within the interval between Samples 145-883B-87X-CC and -88X-CC. In the interval between Samples 145-883B-85X-CC and -86X-CC, radiolarians have been replaced by ore substance. In addition, single impressions of the Jurassic-Cretaceous forms Stichocapsa spp. occur in this inter- val. In the interval between Samples 145-883B-84X-CC and -84X-1, 38-39 cm, single specimens of Cenosphaera spp. occur. In the inter- val between Samples 145-883B-83X-CC and -82X-CC, single, strongly altered radiolarians occur, indefinable as to genus. Section 145-883B-81X-CC yielded a radiolarian assemblage from the Podocyr- tis mitrella Zone. The first and the last appearance of the species Podocyrtis mitrella is recorded. The radiolarian assemblage is repre- sented by the following species: Dictyoprora amphora, Dictyomitra amygdala, Lithomitra micropore, Gorgospyris hemisphaerica, and Stylosphaera coronata. Sample 145-883B-80X-CC contains radi- olarians Dictyoprora amphora and Dictyoprora amygdala. Within the interval between Samples 145-883B-79X-CC and -75X-CC, a ra- diolarian assemblage from the Dictyoprora amphora Zone was dis- covered, comprising the following species: Dictyoprora amphora, Dictyoprora mongolfieri, Phormocyrtis embolum group?, Dictyomi- tra amygdala, Clathrocyclas extensa, Clathrocyclas universa, Arto- botrus auriculaleporis, Lychnocanella babylonis, and Stylatractus ostracion. Sample 145-883B-74X-CC contains no radiolarians. In the interval between Samples 145-883B-73X-CC and -69X-1, 90-92 cm (Oligocene), a radiolarian assemblage occurs that contains the following species: Cenosphaera aspera, Cenosphaera eocenica, Spongodiscus osculosa, Axoprunum bispiculum, Axoprunum los- banosensis, Stylosphera angelina, Lithocarpium titan, and Calocy- clas asperum. Main radiolarian events of the Oligocene are associated with samples from the following cores: (1) appearance and disappearance of Calocyclas asperum in Sample 145-883B-71X-CC, (2) common occurrence (CO) of Axoprunum losbanosensis in Sam- ple 145-883B-71X-5, 90-91 cm, (3) appearance of Stylosphaera an- gelina in Sample 145-883B-71X-5, 90-91 cm, (4) appearance of Lithocarpium titan in S