Neogene Planktonic Foraminiferal Biostratigraphy of the Japan Trench, ODP Leg 186
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本山（1999a， b）によるレビュー以降の本邦新第三系放散虫化石層序学における進展について考証を試みた．過去20年間を振り返ると，主要な進歩として，第一に，深海掘削コアに基づいて中部中新統にまで遡って古地磁気層序との直接対比がなされ，それによって化石帯と生層序イベントの数値年代が精度良く求められたことが挙げられる．第二に，北西太平洋，カリフォルニア沿岸海域，日本海などの異なる海域の地域的な放散虫化石層序の研究が進んだこと，そして，第三に，改訂された放散虫化石帯が陸上の新第三系セクションの対比に適用されるようになったことが挙げられる．これらの進歩とともに，分類学的研究により学名の整理や新種の記載がなされ，低次から高次分類群におよぶ分類体系の見直しも進められてきた．また，最近とくに日本周辺海域において第四紀放散虫化石帯の研究が活発化している点が注目される．
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Analysis of the microfauna of the Jealousy Formation and Kingshill Limestone from four cored wells, two water wells, and ten outcrops in the Kingshill basin of St. Croix allows a comprehensive reconstruction of the Neogene stratigraphic evolution of the island. This study results in the first precise biostratigraphic zonation and paleoenvironmental evaluation of these Neogene units in the subsurface as well as a re-evaluation of the age and correlation of outcropping strata. The Jealousy Formation is an entirely subsurface unit of dark, plankton-rich marls which contains planktonic foraminifera indicative of the lower part of the middle Miocene (Praeorbulina glomerosa Zone to the Globorotalia fohsi fohsi Zone) and a middle bathyal benthic microfauna. The Kingshill Limestone conformably and diachronously overlies the Jealousy Formation and is subdivided into two members, both of which occur in outcrops and the subsurface. The lower of these, the La Reine Member, is characterized by buff pelagic limestones and marls with an upward increasing proportion of intercalated shelf-derived debris. It ranges from the basal part of the middle Miocene (Praeorbulina glomerosa Zone) to the upper part of the uppermost Miocene (Globorotalia humerosa Zone), and exhibits a transition from middle bathyal (600-800m) to upper bathyal (200-300m) environments. The uppermost part of the Kingshill Limestone, the Manning's Bay Member, is made up of skeletal debris-rich carbonate slope deposits and is placed near the Miocene-Pliocene boundary. Biostratigraphic control indicates activation of the St. Croix graben fault system and formation of the Kingshill basin at the beginning of the late Miocene. The Neogene section records deposition in an open-marine setting at middle bathyal depths during the early middle Miocene, shoaling gradually to upper bathyal depths in the late middle and late Miocene and more rapidly to a shallow-marine setting at the beginning of the Pliocene. Shoaling was accompanied by an increasing proportion of transported shallow marine sediments. Subsidence analysis for the section indicates more than 400 m of tectonic uplift between 10.5 and 3.5 Ma. Two depositional shifts in the Kingshill Limestone may be tied chronostratigraphically to postulated major global sea-level falls. An interval of carbonate debris in the type section of the Kingshill Limestone may record a fall at 10.5 Ma, and a disconformity at the base of the Mannings Bay Member may be tied to a fall at 5.5 Ma. INTRODUCTION St. Croix is the southernmost of the U.S. Virgin Islands, located on the crest of an east-west oriented submarine platform near the eastern end of the northern Caribbean plate boundary zone. The central plain of the island is underlain by a northeast-southwest trending sedimentary trough (Gerhard et al. 1978; Whetten 1966), here referred to as the Kingshill basin (text-fig. 1), and bounded by the mountainous East End and Northside Ranges. Most studies of the Kingshill basin have been based on outcrop data and considered the Tertiary carbonate succession to record deposition in an isolated graben system (Whetten 1966; Multer et al. 1977; Gerhard et al. 1978; Lidz 1984). These strata have been assigned a wide range of ages in past studies, ranging from middle Oligocene (Vaughan 1923) to early Pliocene (Lidz 1982) (Table 1). However, exposures are limited in number and predominantly represent the upper part of the section. A drilling program undertaken in the mid-1980's has allowed us to conduct a more comprehensive study of the Neogene stratigraphy of the island and better constrain the nature and timing of the tectonic events affecting the evolution of the Kingshill basin. This study focuses on foraminiferal and ostracode faunas present in cores collected during this drilling program and in outcrop samples collected during field studies. The goals of this study are to: (1) to establish an accurate age and environmental framework for the Neogene strata of St. Croix; (2) to resolve surface-subsurface correlations of these strata; and (3) to outline the stratigraphic evolution of the Kingshill basin and its implications for the Neogene tectonics of the structurally complex region at the eastern end f the northern Caribbean plate boundary zone.
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Lithostratigraphy
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This paper is a synthesis of calcareous nannofossil biostratigraphy for the Lower and Middle Jurassic of the Mediterranean Province based on several sections from Northern and Central Italy. Nannofossil events were calibrated with ammonite biostratigraphy and, when necessary, ammonite-controlled sections in South East France were incorporated. Data derive from previously published biostratigraphies and unpublished data of the authors.The large data-set allowed estimates of reliability and reproducibility of single events. As a result, in the Hettangian-Bathonian interval we propose 47 main events based on diagenesis-resistant and common taxa, 17 events based on rare but ubiquitous taxa and 12 potential events requiring further investigations due to taxonomic problems and sporadic occurrence. A biostratigraphic scheme, consisting of 11 zones and 15 subzones, is proposed for the Tethyan Lower and Middle Jurassic. The proposed biostratigraphy is compared to recent schemes compiled for Portugal, Morocco, Switzerland and the Boreal Realm. Only 27 events are reproducible in various regions, but diachroneity of most events seems to derive from different ammonite biostratigraphies applied in different areas. A very high stratigraphic resolution is achieved in Italy/France for the Pliensbachian to Lower Bajocian interval. The Sinemurian and Bathonian are characterized by the lowest resolution, and very few sections with ammonite control and/or favourable lithologies are available for improvement of nannofossil biostratigraphy. This study confirms the potential of calcareous nannofossil biostratigraphy for dating Lower and Middle Jurassic successions as well as for intra- and inter-regional correlations.
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Trilobite
Paleoecology
Ichnofacies
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We have evaluated the planktonic foraminiferal biostratigraphy of several Gulf of Mexico Eureka boreholes, emphasizing the Miocene depositional history of the northeastern region (De Soto Canyon). The E68-136 borehole (De Soto Canyon; 1826', 557m present depth) provides the most continuous Miocene record, although deposition was punctuated by several short (<2.0 m.y.) hiatuses. The E66-73 borehole (De Soto Canyon; 2802', 854m present depth) provides an uppermost lower to upper Miocene and upper Pleistocene record. E68-151 (north of the Florida Escarpment; 4340', 1323m present depth) provides a discontinuous record of lower, upper middle, and upper upper Miocene sediments. By integrating planktonic foraminiferal biostratigraphy with nannoplankton biostratigraphy and Sr-isotope stratigraphy, we were able to estimate the ages of the hiatuses associated with six distinct unconformities at E68-136 (23.7-23.2 Ma; 18.0-16.5 Ma; 16.3-15.2 Ma; 14.1-13.6 Ma; -10.4-9.5 Ma, 5.0-1.9 Ma), three at E66-73 (-11.5-10.8 Ma, 10.3-9.2 Ma, 6.5-1.7 Ma), and two at E68-151A (19.0-12.6 Ma, 12.5-6.9 Ma). We correlate these unconformities with previously reported unconformities from other deep-sea locations, although their causes (i.e., whether related to sea-level or deep-water changes) remain uncertain. We relate a distinct Pliocene unconformity to intensification of the Loop Current associated with the closure of the Isthmus of Panama.
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The radiolarian assemblages recovered from Upper Miocene and Pliocene sequences at DSDP Sites 192, 302 and 438A and from an onshore section in northern Japan lack many of the species used as zonal markers in low and middle latitudes. A new high-latitude radiolarian zonation based on nine easily recognized events is proposed. Forty-five radiolarian events are identified and the sediment accumulation rate curve of Site 438A allows most of these events to be correlated with the geochronologic time scale. Five new species, Lithelius barbatus, Cycladophora nakasekoi, Cycladophora sakaii, Dictyophimus robustus and Lychnocanoma parallelipes, are described. INTRODUCTION Low-latitude Cenozoic zonations for calcareous and siliceous microfossils including foraminifera, radiolaria, calcareous nannofossils and diatoms have been greatly improved by the study of deep-sea sediments. The combination of these multiple zonations offers high stratigraphic resolution (Berggren et al. 1985). High-latitude biostratigraphy has remained poorly resolved because of the low diversity of calcareous plankton and little knowledge of siliceous microfossils except for diatoms whose zonations have been improved in middle and high latitudes (e.g. Koizumi 1973; Barron 1985). Radiolarians are diverse in high latitudes and are abundant and well-preserved in deep-sea sedimentary sequences which are poor in calcareous microfossils. Early biostratigraphic studies of radiolarians in midto highlatitude Northwest Pacific were based on piston cores that recovered mostly Pliocene and Pleistocene sediment. With the advent of the Deep Sea Drilling Project, much thicker Neogene sedimentary sequences became available for micropaleontological investigations. Radiolarians have contributed to biostratigraphic subdivision and correlation of Neogene sections drilled by the DSDP Legs 19 (Aleutian region), 31 (Japan Sea), 32 (Shatsky Rise, Hess Rise), 55 (Emperor Sea Mounts), 56, 57 (offshore Japan), 86 (Shatsky Rise) 127 and 128 (Japan Sea). However, radiolarian studies showed that low-latitude zonations could not be easily applied to mid to high latitudes. Alternative radiolarian zonations were developed for mid latitudes but not for high latitudes or the Japan Sea. Neogene radiolarian studies of Japanese land-based sections generally focused on contributing to oil-research, i.e. correlation of Neogene strata in the oilbearing field. Hays (1970) first developed a zonation from piston core samples in the northern North Pacific. He established four zones for the late Pliocene to Quaternary and correlated them with the paleomagnetic time scale. Ling (1973) reported occurrences of radiolarians from the Upper Miocene to Quaternary sequence drilled by DSDP Leg 19 but proposed no zonation. Foreman (1975) established a radiolarian zonation for the Upper Miocene to Quaternary from DSDP Leg 32. She distinguished eight zones including four upper zones similar to those of Hays (1970), a new Pliocene zone and three Upper Miocene zones of the lowlatitude zonation of Riedel and Sanfilippo (1970). Subsequently, Foreman's zonation has been applied to the Upper Miocene to Quaternary sedimentary sequences of Legs 55, 56, 57 and 86 in the mid-latitude Northwest Pacific (Ling 1980; Sakai 1980; Reynolds 1980; Morley 1985). In the Japan Sea, Ling (1975, 1992) and Alexandrovich (1992) studied radiolarians but established no zonation for Upper Miocene to Quaternary sediments of Legs 31, 27 and 128. The most important radiolarian biostratigraphic work from onshore Japan is that of Nakaseko and Sugano (1973). They established four assemblage zones in the upper Lower Miocene to Pliocene of onshore Japan based on radiolarian assemblages reported previously by Nakaseko and co-workers (e.g. Nakaseko 1954, 1955, 1959, 1960, 1969; Nakaseko et al. 1965, 1972; Sugano and Nakaseko 1968, 1971). Funayama (1988) defined four new zones and two new sub-zones from the upper Lower Miocene to lower Upper Miocene of the Suzu area in the Noto Peninsula, central Japan. In spite of these efforts, a Neogene radiolarian zonation for the high-latitude Northwest Pacific and Japan Sea was not established. Few thorough stratigraphic studies of radiolarians from the Upper Miocene to Pliocene of onshore Japan were conducted after Nakaseko and Sugano (1973). The objectives of this paper are to record the stratigraphic distribution of radiolarians in the DSDP Sites 192, 302 and 438A and a section in the Tsugaru Peninsula, northern Japan, in order to establish a radiolarian zonation for the upper Neogene of the subarctic Northwest Pacific region, including northern Japan and Japan Sea. It is also demonstrated that the mid-latitude radiolarian zonations proposed by Hays (1970) and Foreman (1975) cannot be applied in this region. Site 192 lies in the present-day source area of the cool-water Oyashio Current. It was chosen for this study to elucidate subpolar Neogene faunas. Site 438A provides one of the most continuous Neogene sedimentary sequences near northern Japan. micropaleontology, vol. 42, no. 3, pp. 221-262, text-figures 1-11, plates 1-7, tables 1-5, 1996 221 This content downloaded from 207.46.13.57 on Mon, 08 Aug 2016 05:53:13 UTC All use subject to http://about.jstor.org/terms Isao Motoyama: Late Neogene radiolarian biostratigraphy in the subarctic Northwest Pacific TEXT-FIGURE 2 Location of the six outcrops in the Tsugaru section: JQ, JR, JS, JT JX and USI. TEXT-FIGURE 1 Location of DSDP Sites 192, 302 and 438 and Tsugaru section. Site 302 and the Tsugaru section provide a composite sedimentary record from upper Miocene to Quaternary of the Japan Sea. These three sedimentary records approximately cover the same chronological range from Late Miocene to Quaternary. This makes it possible to test the stratigraphic usefulness (synchroneity) of various radiolarian events based on parallelism of sequential order of evolutionary events in these sections across a broad latitudinal distance (40?-53?N) or between the marginal sea and the open ocean. MATERIALS AND METHODS
Neogene
Subarctic climate
Radiolaria
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