Initial Reports of the Deep Sea Drilling Project, vol. 85

Covering Leg 85 of the cruises of the Drilling Vessel Glomar Challenger Los Angeles, California, to Honolulu, Hawaii March-April 1982. Includes six chapters: 1. INTRODUCTION: BACKGROUND AND EXPLANATORY NOTES, DEEP SEA DRILLING PROJECT LEG 85, CENTRAL EQUATORIAL PACIFIC 2. SITE 571 3. SITE 572 4. SITE 573 5. SITE 574 6. SITE 57

Mid-Quaternary Paleoceanographic Trend in Near-shore Waters of the Northwest Pacific : A Case Study Based on an Offshore Well

An exploratory oil and gas well was drilled on the continental shelf in water 38 m deep at lat. 36°9′33.8″N., long. 140°43′49.3″E., 13 km off the coast of Kashima, Ibaraki Prefecture, Kanto District, Japan. This well sank to the total subbottom depth of 2100 m penetrated sediments ranging back to Late Cretaceous in age. The upper 500 m of well sequence which comprises largely Quaternary sediments was subjected to paleoenvironmental analyses by jointly analyzing microfossils (calcareous nannoplankton and benthic and planktonic foraminifera), mineral content, and oxygen and carbon isotope stratigraphies. Three dated horizons, 0.128, 0.27 and 0.46 Ma, were established by recognizing one marked excursion to negative values in the oxygen isotope record and two calcareous nannoplankton datum levels in the well sequence. A possible hiatus at about 465 m depth terminates this mid-Quaternary sequence which is underlain by late Pliocene strata of about 1.9 Ma and older. Mid-Quaternary paleobathymetric trends were reconstructed for the well site area mainly on the basis of Q-mode cluster analysis of benthic foraminiferal faunas. Deposition of the mid-Quaternary sequence began in the upper bathyal zone and a steady shallowing followed through outer shelf environments to the present-day inner neritic zone. Other paleo-depth indicators such as abundance variations of nannoplankton Florisphaera profuuda and planktonic versus total foraminifera ratios, and variations of ^C isotope record are all supportive of this benthic foraminifera-based depth-trend. Paleotemperature fluctuations were estimated by analyzing oxygen isotope compositions of benthic and planktonic foraminifera and by calculating with the aid of paleoecological transfer functions quantitative estimates of past winter temperatures. The paleotemperature fluctuations estimated for the well site area by these two methods agree well for the lower part of the well sequence, but begin to diverge from the mid-sequence and upwards, probably reflecting increasing effects of local water masses to the oxygen isotope variation. In the Quaternary isotope record of Kashima SK-1 well, no definite interval correlatable with the even-numbered isotope stages was observed. In view of the fact that distinct hiatuses occur in the onshore Quaternary record of Kanto District located to the west of the well site, the lack of even-numbered isotope stages is interpreted to signify periods of greatly reduced sedimentation or possibly non-deposition in a broad coastal region encompassing also the continental shelf around the well site. Mid-Quaternary paleoceanography around the well site prior to about 0.35 Ma was under the general influence of cold Oyashio Current with its strength varying greatly from time to time, whereas the latter time period responding to the diminishing Oyashio Current saw a seesaw game of two warm currents, Kuroshio and Tsugaru, expanding and retreating alternatively over a broad region of the sea off Northeast Honshu, Japan

Biostratigraphy, paleomagnetism and sedimentology of Late Cenozoic sediments in northwestern Hokkaido, Japan

Four Neogene sedimentary basins in northwestern Hokkaido (latitudes 44°-45° N.), the northernmost island of Japan, were studied to establish a combined microfossil-paleomagnetic sedimentologic stratigraphy. The measured section near Atsuta encompasses a shoaling depositional sequence from upper bathyal depths in the lower part to stagnant, nearshore conditions near the top of the sequence. Magnetostratigraphic evidence and age-diagnostic diatoms indicate the section to be Late Miocene age covering the interval from early Epoch 5 through Epoch 6, and possibly to late Epoch 7. The second measured section located north of Shosanbetsu exposes sediments derived from a volcano-clastic source and deposited on a continental slope with oversteepened slopes. Rich diatom floras from the sequence are assignable to the Denticula seminae var. fossilis-D. kamtschatica and D. kamtschatica Zones. A magnetic signature containing one reversal was observed in the upper part of the section, although the data seem to indicate the lower section to be dominantly normally magnetized. In the northwest Pacific deep-sea sequences, the zonal boundary of these two diatom zones lies between the Nunivak ( = "b") and "c" Events of the Gilbert Epoch, thus the distinct magnetic reversal observed in the upper part of the sequence is correlated with the top of the Nunivak Event. Neogene strata developed near Wakkanai City, the northwestern tip of Hokkaido, appear to have been deposited during a time interval of predominantly reversed geomagnetic polarity. Diatom floras suggest a correlation of these strata with the Denticula kamtschatica Zone which in turn corresponds, in the northwest Pacific deep-sea sequences, to the lower Gilbert Series below a horizon midway between the Nunivak and "c" Events. Diatom rich sediments of the Shimo-Ebekorobetsu area belong to the Yuchi Formation and comprise two assemblages assignable to the D. kamtschatica Zone and the superjacent Denticula seminae var.fossilis-D. kamtschatica Zone. From northern Honshu through Hokkaido to Sakhalin, in the western Pacific coastal region, beds containing the large pecten Fortipecten takahashii (YOKOYAMA) constitute a marker horizon useful for inter-regional correlation. In its southern range of distribution, F. takahashii is a diagnostic species in the lower part of the Tatsunokuchi Formation from which diatom floras assignable to the D. seminae var. fossilis-D. kamtschatica Zone are described. The F. takahashii bed occurs in the Shosanbetsu section, lying within the same diatom zone as the Tastunokuchi Formation and is in a reversely magnetized interval above the Nunivak Event (3.9 m.y. B.P.). The Neogene marine sediments of northwestern Hokkaido were deposited in the back-arc basin and reveal evidence of active arc magmatism at the time of deposition. In this tectonically active back-arc region, sediment-collecting basins shifted from area to area, accumulating sediments only for a relatively short period of time when spurts of active subsidence occurred in a given sedimentary basin

Occurrence and estimated abundance of planktonic foraminifers at DSDP Leg 85 holes

Tropical planktonic foraminifers occur throughout the sequences at all sites of Leg 85, and the standard planktonic foraminiferal zonation of Blow (1969) is applicable to most of the recovered sequences. However, the abundance and state of preservation of foraminifers decline markedly in certain intervals because of the effects of dissolution. Although siliceous microfossils studied on this leg indicate recovery of nearly complete records for the Pleistocene to Oligocene interval, the planktonic foraminiferal biostratigraphy is interrupted by strongly dissolved sections at all sites. Particularly, faunas assignable to Zone N7 (early Miocene) and Zone N15-16 (early late Miocene) are almost totally unrecognizable throughout the eastern equatorial Pacific. Well-preserved and diverse planktonic foraminifers occur in the lower middle Miocene, where the evolutionary developments of Orbulina universa d'Orbigny and Globorotalia fohsi Cushman and Ellisor are well represented. The Orbulina first appearance datum is observed to be nearly coincident with the last occurrence level of the diatom Annellus californicus Tempère, thus .establishing an age of 15 Ma for this datum by using the paleomagnetic calibration of the diatom datum. Moderately well-preserved late Eocene planktonic foraminifers occur in the carbonate sediments immediately overlying the basalt basement at Sites 573 and 574. The Eocene-Oligocene faunal transition, however, is masked at both sites by an intercalation of metalliferous layers containing no planktonic foraminifers

Miocene Planktonic Foraminifera from Honshu, Japan

The planktonic foraminiferal assemblages from the important Miocene sedimentary basins in Honshu, Japan are described giving full account of their systematics, stratigraphic distribution, and paleoecology. Eight zonal subdivisions of the Japanese Miocene are proposed on the basis of local planktonic foraminiferal sequences ascertained in the individual areas studied. The correlation of the Japanese Miocene with the Caribbean as well as the mutual relationships of the Miocene formations within Japan is attempted in terms of the established eight planktonic foraminiferal zones. The Miocene boreal planktonic faunas are discussed, and their significance and characteristics described. In the systematics, among the determined total of 11 genera, 58 species and 14 subspecies, two species are proposed as a new to science

(Table 6) Occurrence and estimated abundance of planktonic foraminifers at DSDP Site 85-575

Tropical planktonic foraminifers occur throughout the sequences at all sites of Leg 85, and the standard planktonic foraminiferal zonation of Blow (1969) is applicable to most of the recovered sequences. However, the abundance and state of preservation of foraminifers decline markedly in certain intervals because of the effects of dissolution. Although siliceous microfossils studied on this leg indicate recovery of nearly complete records for the Pleistocene to Oligocene interval, the planktonic foraminiferal biostratigraphy is interrupted by strongly dissolved sections at all sites. Particularly, faunas assignable to Zone N7 (early Miocene) and Zone N15-16 (early late Miocene) are almost totally unrecognizable throughout the eastern equatorial Pacific. Well-preserved and diverse planktonic foraminifers occur in the lower middle Miocene, where the evolutionary developments of Orbulina universa d'Orbigny and Globorotalia fohsi Cushman and Ellisor are well represented. The Orbulina first appearance datum is observed to be nearly coincident with the last occurrence level of the diatom Annellus californicus Tempère, thus .establishing an age of 15 Ma for this datum by using the paleomagnetic calibration of the diatom datum. Moderately well-preserved late Eocene planktonic foraminifers occur in the carbonate sediments immediately overlying the basalt basement at Sites 573 and 574. The Eocene-Oligocene faunal transition, however, is masked at both sites by an intercalation of metalliferous layers containing no planktonic foraminifers

(Table 5) Occurrence and estimated abundance of planktonic foraminifers at DSDP Site 85-574

Tropical planktonic foraminifers occur throughout the sequences at all sites of Leg 85, and the standard planktonic foraminiferal zonation of Blow (1969) is applicable to most of the recovered sequences. However, the abundance and state of preservation of foraminifers decline markedly in certain intervals because of the effects of dissolution. Although siliceous microfossils studied on this leg indicate recovery of nearly complete records for the Pleistocene to Oligocene interval, the planktonic foraminiferal biostratigraphy is interrupted by strongly dissolved sections at all sites. Particularly, faunas assignable to Zone N7 (early Miocene) and Zone N15-16 (early late Miocene) are almost totally unrecognizable throughout the eastern equatorial Pacific. Well-preserved and diverse planktonic foraminifers occur in the lower middle Miocene, where the evolutionary developments of Orbulina universa d'Orbigny and Globorotalia fohsi Cushman and Ellisor are well represented. The Orbulina first appearance datum is observed to be nearly coincident with the last occurrence level of the diatom Annellus californicus Tempère, thus .establishing an age of 15 Ma for this datum by using the paleomagnetic calibration of the diatom datum. Moderately well-preserved late Eocene planktonic foraminifers occur in the carbonate sediments immediately overlying the basalt basement at Sites 573 and 574. The Eocene-Oligocene faunal transition, however, is masked at both sites by an intercalation of metalliferous layers containing no planktonic foraminifers