Thickness Distribution of Reddish Brown Clay in the Western North Pacific*

Home , Hemipelagic sediment, Pelagic red clay

Journal of the Oceanographical Society of Japan Vol.43, pp.139 to 148, 1987

Satoshi Yamamotot

Abstract: Thickness and stratigraphy of reddish brown clay were compiled from lithologic descriptions of various types of cored surface sediments in the western North Pacific. Thin brown clays usually overlie gray clay in hemipelagic deposi- tional environments and the thickness of the brown clay increases offshore, as results of the decreasing rate of sedimentation of terrigenous organic matter. The hemipelagic gray clay disappears in pelagic environments and several tens of meters of pelagic red clay overlies the basement sedimentary sequences of bedded chert and chalk in the pelagic seafloor. In the boundary region of the hemipelagic and pelagic sequences at the outer open floor of the trench basin, the pelagic facies of the red clay-bedded chert-chalk assemblage underlie the hemipelagic gray clay. This stratigraphy might indicate subduction of pelagic facies below the hemipelagic sequences.

1. Introduction pelagic reddish clay; what underlies the hemi- Since the scientific expedition of H. M. S. pelagic grayish clay or the pelagic reddish clay; Challenger, it has been recognized that two types etc. All these questions are concerned with the of clays of reddish brown and greenish gray genesis and sedimentary processes of these grayish (bluish) are distributed very widely in the world and brownish clays in the ocean. Although an ocean (Murray and Hjort, 1912). The reddish isopach map of the brown clay in hemipelagic brown clays are commonly observed in the open environments was published for the marginal sea, whereas the greenish gray clays are found areas of the eastern Pacific Ocean (Lyle, 1983), in the continental margin area. However, the the same kind of map for the continental margin stratigraphic features of these two types of clays in the western North Pacific has not yet been in the ocean floor still require study. Numerous constructed. Masuzawa (1983) has investigated coring operations on the ocean floor have re- the relatidnship between water depth and the vealed that a few centimeters of brown clay thickness of the oxidized brown clay in the Japan usually overlies gray mud in hemipelagic environ- Sea by compiling lithologic data on sediment- ments (Lynn and Ponatti, 1965; Lyle, 1983). cores from many cruise reports of R/V Haku- The color difference between the reddish brown ho Maru, Hakurei Maru, Vema and Conrad; and greenish gray sediments can be explained however, the geographical distribution of thick- as results of oxidation and reduction of iron (Fe) ness of the oxidized brown clay was not shown and manganese (Mn) products in the sediment, by his study. so that the brown-gray transition boundary in The present report is intended to construct the hemipelagic sediment can be interpreted as an isopach map of the oxidized brown clay on a redox transitional zone (Lyle, 1983). the grayish clay in the marginal area' of the The question arises, however; whether or not Japanese Islands, by compiling lithologic descrip- the brown-gray redox boundary can be observed tions on sediment-cores from several cruise re- in the pelagic deposits of reddish clay; how ports of R/V Hakuho Maru, Hakurei Maru, thick is the hemipelagic grayish clay or the and D/V Glomar Challenger, and other scientific * Received 9 January 1987; in revised form 14 reports available to the author. Stratigraphic May 1987; accepted 23 May 1987. features of the hemipelagic sediments and pelagic

† Department of Oceanography, Ryukyu University, red clay are summarized here from the lithologic Nishihara, Okinawa 903-01. data to investigate the genesis of these sedi- 140 Yamamoto

Fig. 1. Bathymetry and sampling sites for surface cores in the western North Pacific. Sampling sites are labeled and shown by various symbols in legend: 1; Hakurei Maru sites (labeling number indicates site designation). 2; Hakuho Maru site (cruise number). 3; Tansei Maru site (cruise number). 4; Nagasaki Maru site (cruise number). Major bathymetric contours are in km.

ments in the ocean. The compiled region is cruise reports) mainly restricted to the North Pacific region. (2) Cruise reports by the Geological Survey of Japan (G. S. J.); 2. Information source and data analysis No.4, No.5, No.6, No.7, No.8, No.9, The lithology and thickness of sediments were No.10, No.11, No.12, No.13, No.14, judged from the lithologic descriptions of core- No.15, No.17, No.18, No.19, No.20. samples recovered during scientific cruises. The (3) Initial reports of the Deep Sea Drilling lithologic information includes color and grain Project (DSDP); size (e. g. clay, silt, mud, sand, gravel, etc.) of Vol.5, Vol.6, Vol.7, Vol.9, Vol.16, sediments. The compiled scientific reports are: Vol.17, Vol.18, Vol.19, Vol.20, Vol.31, (1) Preliminary reports of Hakuho Maru Vol.32, Vol,33, Vol.55, Vol.56, Vol.57, cruises; Vol.58, Vol.59, Vol.61, Vol.62, Vol.63, KH68-3, KH68-4, KH69-2, KH71-1, KH71-5, Vol.85, Vol.86, Vol.87, and Newsletter on KH72-2, KH75-3, KH77-1, KH77-3, KH78-3, Leg 89. KH79-3*, KH80-1*, KH80-3, KH81-3*, For the oceans surrounding the Ryukyu Islands, KH82-4, KH84-1, KH86-1.(* unpublished the data were also compiled from the unpublished Thickness Distribution of Reddish Brown Clay in the Western North Pacific 141

Fig. 2. Isopach map of oxidized brown clay in the western North Pacific. Isopach contours are in cm. Areas with thicknesses larger than 500cm are hatched by lines. Trench axis is shown by dashed line. reports of RN cruises (Ryukyu University and hues with very low chroma and intermediate Nagasaki University joint educational cruises) lightness, or hues 5GY and 10GY)-including by R/V Nagasaki Maru (Yamamoto et al., 1984) greenish gray, dark gray, and brownish gray. and KT84-14 cruise by R/V Tansei Maru (Ono Black mud was not a dominant lithology for et al., 1987). The compiled data include 247 oceanic sediments, as far as the present data piston cores, 62 box cores, 153 grab sampler's compilation was concerned. Whitish mud was cores, and 10 Smith-McIntyre sampler, gravity, often considered as calcareous ooze, in which and other similar kinds of cores covering the CaCO3 content is more than about 80%, for entire Pacific region. oceanic sediments. There might exist a brown- Colors in lithologic descriptions of clayey sedi- gray redox boundary in the calcareous ooze as ments, which were followed descriptions in the a very obvious color contact. However, the published reports, are grouped into two essential analysis for lithologic colors was restricted to types; (1) reddish brown (hues 5R, 10R, 5YR, clayey sediments in which CaCO3 may be less 10YR, 5Y and 10Y, excluding too low lightness than about 50%. The colors of sands can be and too low values of chroma: Rock-Color Chart, also grouped into gray and reddish brown types; Geological Society of America) - including however, the sand-dominated sediments as mas- such colors as brown, yellowish brown, dark sive beds are restricted to the continental shelf brown, and pinkish brown; and (2) gray (any areas (Boggs, 1984). 142 Yamamoto

Thickness (cm) of brown mud overlying on gray mud

Leg end

(Total 141 points)

Fig. 3. Relationship between thickness of brown mud overlying gray mud and water depth.

The color change from reddish brown to the ever, the type of contact was not compiled in greenish gray muds can be considered as the this study, because many descriptions did not change of oxidized to reduced states. Although describe the contacts precisely. There are a few the exact match of the oxidation-reduction con- cases in which the brownish and grayish muds ditions to colors was not clearly demonstrated were deposited cyclically with approximate thick- by chemical measurements (Pantin, 1969; Lyle, ness of 100 cm for each bed. In this case, how- 1983), the essential mechanism to cause the ever, the lithology changed cyclically to the color change can be considered as oxidation- underlying unit's lithology of grayish mud. The reduction of various Fe products contained in boundary in this kind of transitional zone was the sediment (Heller-Kallai and Rozenson, 1978; determined arbitrarily judging from the lithologic Rozenson and Heller-Kallai, 1978). descriptions. Data for thickness of the reddish brown mud on the gray mud were compiled from the litho- 3. Thickness distribution of oxidized brown logic descriptions. The contact between the clay on gray clay reddish brown and grayish muds was of various Figure 1 shows the site locations of the cores types; sharp, gradual, and gradational. How- used to construct the isopach map of the brown Thickness Distribution of Reddish Brown Clay in the Western North Pacific 143

Fig. 4. Bathymetry and sites for surface cores and thickness (cm) of brown clay in the Pacific. Area A is illustrated in Fgs. 1 and 2. Area GH is represented by numerous (more than 200) surface cores of brown clays which did not reach to the base within sampler's depth (<5 m), based on Hakurei Maru sampling sites. Other sites were sampled by Hakuho Maru, and their labels indicate recovered thickness (cm) of reddish brown clay. Bathymetric contours are in km. clay overlying gray sediments in the adjacent of the regional differences of basins. The thick- seafloors of the Japanese Islands. The contoured ness of the brown clay in the trench basins of map for the thickness of brown clay is shown greatest water depths is not as much as that in in Fig. 2, which indicates that the marginal the open seafloors, since the distance to the area of the islands is covered by gray sand and trench floors from the islands is shorter than mud without brown clay (thickness is 0 cm) and that to the open seafloors. Possibly, the thick- that the thickness increases from nearshore to- ness increases with decreased sedimentation rates wards offshore. The relationship between the of terrigenous matter (Lyle, 1983). The trench thickness of the brown clay and the water depth basins are places where terrigenous matter (Fig. 3) demonstrates, in general trends, that accumulates. Excluding the the seamount areas the maximum thickness of the brown clay in- where deep-sea carbonates are thickly deposited, creases with increasing water depth, regardless the open seafloor is covered by brown mud 144 Yamamoto

Legend

Fig. 5. The depth (m) to the base of pelagic red clay below seafloor in the North Pacific. All sites are from the DSDP. The sites which represent the presence of pelagic red clay are only indicated together with thickness (the depth to the base below seafloor) of red clay in meters. The thickness is contoured and in the hatched area thickness is less than 50m. The area which is covered by terrigenous/hemipelagic sediments is symbolized by zigzag lines. Lithology contacted at the base of red clay is classified by patterns in legend. 1; bedded cherts. 2; chalk and chalky limestone. 3; basalt. 4; volcanics other than basalt. 5; base is unknown because the drilling did not reach. Area A is shown in Fig. 6.

thicker than about 10m. overlying the gray clay is restricted to the hemi- Figure 4 shows the site locations and thick- pelagic marginal seafloors. Therefore, the brown ness of surface brown clays in other parts of clay in the open seafloor should be called pelagic the Pacific Ocean, based on data of the Hakuho red clay for differentiation from the hemipelagic Maru and G. S. J. cruise reports. This brown brown clay. clay overlies gray clay in the marginal areas, such as in the Bering Sea, Palau Trench floor, 4. Thickness distribution of pelagic red clay and near the islands of New Zealand. The cores The thickness of the pelagic red clay varies in other parts of the open seafloor do not reach little, in the range of 50 to 100 m, in wide areas to the lowest contact of the brown clay within of the central part of the North Pacific (Fg. 5). the range of core-depths of the piston corer. The pelagic red clay thins out near the East The thickness of brown clay in the open sea- Pacific Rise (EPR), as seen in Fg. 5. The floors of the Pacific, except for shallow sea- lithology with which the pelagic red clay contacts mounts, was investigated using the lithologic is chalk in the peripheral areas of the EPR and data of the Initial Reports of the DSDP (Fg. changes to bedded cherts and chalk in some 5). As recognized from Fig. 5, a few tens of places in the central North Pacific. The thick- meters of reddish-brown clay overlies chert, ness of the pelagic red clay increases gradually chalk, basalt, and many types of volcanics. Tt northwest in the North Pacific. As shown in is understood that the grayish clay disappears Fig. 6, the northwestern portion of the North in the open seafloors and that the brown clay Pacific is covered by about 100 to 200 m of Thickness Distribution of Reddish Brown Clay in the Western North Pacific 145

Fig. 6. The depth (m) to the base of pelagic red clay below seafloor in the western North Pacific. All sites are from DSDP. Explanations for patterns of sites and labeled numeri- cal values are similar to Fig. 5. Sites in which pelagic red clay is overlain by hemipelagic gray clay are marked by labels G and hatched by lines. pelagic red clay, which overlies bedded cherts. ing sites, no speculation can be made on the The Philippine Sea basin is also covered by about changes of thickness of the hemipelagic gray 50 to 100 m of pelagic red clay, although the clay and of the pelagic red clay which underlies lithology that contacts the red clay is rather the gray clay. complicated. Pelagic red clay can be differentiated from The depth to the base of the pelagic red clay terrigenous marine sediments and hemipelagic below the seafloor increases in a northwest direc- clays in terms of mineralogical composition, be- tion in the North Pacific, and the depth may cause the pelagic red clay is characterized by also increase towards the same direction in the authigenic clay minerals, such as Fe-rich smectites Philippine Sea basin. As illustrated in Fig. 7, and zeolites (Velde, 1985; Walter and Stoffers, the pelagic red clay un derlies hemipelagic gray 1985; Yamamoto, 1986, 1987b). In addition to clay which also underlies the thin hemipelagic the mineralogical composition, lithostratigraphic brown clay. The boundary where the hemi- features can discriminate the terrigenous pelagic gray clays thin out and disappear was marine, hemipelagic, and pelagic sediments (Fig. not able to be determined from the lithologic 8). Terrigenous marine sediments are usually descriptions of piston cores, but was roughly composed of gray sand and mud without cover determined from deep-sea drilling cores. Be- of brown mud. Hemipelagic deposits are charac- cause of the very sporadic distribution of sampl- terized by thin cover of brown mud on the gray 146 Yamamoto

Fig. 7. Lithologic columns of reddish brown clay accompanied with hemipelagic gray clay in the western North Pacific. DSDP site numbers are shown. Explanations for legend: 1; thin cover of oxide brown clay may be expected, because of drilling's disturbance of surface structure. 2; hemipelagic gray clay. 3; reddish brown clays (hemipelagic and pelagic). 4; bedded cherts with interbeds of pelagic red clay. 5; chalk or chalky limestone, with occasional occurrence of nodular flints. 6; basalt. 7; sandstone. 8; gray mudstone. 9; basalt gravels.

mud which may be interbedded with thin gray- facies of the red clay-bedded chert-chalk assem- ish sand on some occasions. Pelagic red clay blage have moved towards an approximately is a thick and reddish colored deposit of authi- northwestern direction from the EPR passing the genic clay minerals with some enrichment of equator and subducted below the outer portion radiolarian ooze. Pelagic red clay is also under- of the trench floor, by showing the bending lain by bedded chert, chalk, basalt and other structure of the bedded chert. However, more types of volcanics. observations would be required to clarify the As illustrated in Fig. 8, the hemipelagic gray processes involved in the subduction of pelagic clay is underlain by pelagic red clay and bedded red clay beneath the hemipelagic gray clay; e. g., chert in the outer portion of the trench floor in by making more drillholes in these areas. the hemipelagic zone. The pelagic red clay and It can be suspected that the pelagic red clay bedded chert underlying the gray clay may be was deposited in the open seafloors at the depths pelagic facies that have been subducted beneath below the calcite-compensation depth and later the hemipelagic deposits (Yamamoto, 1987a). the red-clay beds have moved to subduct below Yamamoto (1987a) has shown that the pelagic the hemipelagic sediments. According to the Thickness Distribution of Reddish Brown Clay in the Western North Pacific 147

Fig. 8. Schematic sketch of stratigraphic profile across the Northwest Pacific Ocean. Cross section presents profile between the Japanese Islands and the EPR passing the equator. Vertical scales indicate both water depth and sediment thickness. Explanations of legend: 1; oxidized brown clay both as hemipelagic brown clay and pelagic red clay. 2; hemipelagic gray clay. 3; pelagic red clay. 4; bedded cherts with interbeds of red clay. 5; chalk or chalky limestone with occasional occurrence of nodular flints. 6; basalt. 7; probable presence of sedimentary sequences older than Jurassic. Distribution of hemipelagic brown clay is limited within the same area as hemipelagic gray clay. assumptions of plate tectonics, the pelagic red reducing matter. clay is able to underlie the hemipelagic gray clay, if it can move laterally towards the sites 5. Conclusions where the high deposition rate of grayish clay The analysis of lithologic descriptions of piston of terrigenous and hemipelagic origin occurs. cores, box cores, and other similar types of sur- The reason why the thickly deposited pelagic face cores, and deep-sea drilling cores demon- red-clay does not include any reducing gray clay strates that two types of reddish brown clays, in its stratigraphy may be explained by too low hemipelagic brown clay and pelagic red clay, sedimentation rate of organic matter to reduce can be recognized in the modern ocean floor. the red clay and by thickening of the red clay The hemipelagic brown clay overlies gray mud due to the lateral movement of plate motion. and the thickness of the hemipelagic brown clay The lower sedimentation rates of organic matter increases generally towards offshore, depending result in thicker oxide brown clays towards off- on the sedimentation rate of terrigenous organic shore, if the oxygen contents in seawater and matter. The pelagic red clay overlies bedded pore-water are assumed to be of the same order cherts, chalk, basalt, and other volcanics, and between the nearshore and offshore areas (Lyle, generally the thickness of the pelagic red clay 1983). While the hemipelagic brown clay can increases from the EPR in a northwestern direc- be reduced after burial, the pelagic red clay may tion in the North Pacific. In the boundary not be reduced because the red clay is mainly region of the hemipelagic and pelagic sequences, composed of authigenic clay minerals without the stratigraphy indicates some subduction of 148 Yamamoto pelagic red clay below the hemipelagic gray clay. ations of carbonate and interstitial-exchangeable The reason why the pelagic red clay does not elements in sediments from the ocean surround- include any reduced gray clay may be explained ing the Ryukyu Islands. The Compass (in press). by the fact that the pelagic red clay is deposited Pantin, H. M. (1969) The appearance and origin of at a place where no or too small supply of colours in muddy sediments around New Zealand. N. Z. J. Geol. Geophys., 12, 51-66. terrigenous reducing matter occurs. The thicken- Rock-Color Chart (1979): The Geological Society of ing of pelagic red clay towards the northwestern America, Boulder, Colorado. Pacific can be explained by the lateral motion Rozenson, I. and L. Heller-Kallai (1978): Reduction of the Pacific plate. and oxidation of Fe-3+ in dioctahedral smectite -3 .Oxidation of octahedral iron in montmoril- Acknowledgments lonite. Clays Clay Miner., 26, 88-92. This study was supported in part by funds Velde, B.(1985): Clay Minerals. Elsevier, Am- from Cooperative program (No. 85110) provided sterdam, 427 pp. by the Ocean Research Institute, University of Walter, P. and P. Stoffers (1985): Chemical charac- Tokyo. I thank Dr. H. Kagami for arranging teristics of metalliferous sediments from eight the program and his advice on the study and areas on the Galapagos Rift and East Pacific Rise between 2•‹N and 42•‹S. Mar. Geol., 65, Prof. H. Ujiie for reviewing the manuscript. 271-287.

Yamamoto, S. (1986): Correlation between iron and

References magnesium and its significance on the distribution Boggs, S., Jr. (1984): Quaternary sedimentation in of heavy metals in deep-sea cherts. Sediment. the Japan arc-trench system. Geol. Soc. Am. Bull., Geol., 49, 261-280.

95, 669-685. Yamamoto, S. (1987a): Thickness distribution of

Heller-Kallai, L. and I. Rozenson (1978): Removal pelagic red clay and bedded chert in the North of magnesium from interestitial waters in reducing Pacific. Abstr. with Programs, 94th Annu. environments-the problem reconsidered. Geo- Meeting Geol. Soc. Japan, p.328 (in Japanese). chim. Cosmochim. Acta, 42, 1907-1909. Yamamoto, S. (1987b): Ferromagnesian and metal-

Lyle, M. (1983): The brown-green color transition liferous pelagic clay minerals in oceanic sedi- in marine sediments: A marker of the Fe(III)- ments. In: Diagenesis (Developments in Sedi- Fe(II) redox boundary. Limnol. Oceanogr., 28, mentology), ed. by G. V. Chilingar and K. H. 1026-1033. Wolf, Elsevier, Amsterdam, (in press). Lynn, D. C. and E. Bonatti (1965): Mobility of man- Yamamoto, S., S. Yada, M. Kimura and Y. Kato ganese in diagenesis of deep-sea sediments. Mar. (1984): Bottom samples collected from the East Geol., 3, 457-474. China Sea during oceanographic cruises of 1979 Masuzawa, T. (1983): The changes of redox con- through 1983. Bull. Coll. Sci. Univ. Ryukyus,

ditions in the bottom waters of the Japan Sea. 38, 117-129 (in Japanese with English abstract). Mar. Sci. Monthly, 15, 68-77 (in Japanese). Note: Lists on the referred Preliminary Reports Murray, J. and J. Hjort (1912): The Depths of the of the Hakuho Maru Cruise, Cruise Reports Ocean. Macmillan and Co., London, 821 pp. by the G. S. J., and Initial Reports of the DSDP Ono, T., S. Yamamoto and H. Ujiie (1987): Fluctu- are omitted in this paper.

北西太平洋における赤褐色粘土の層厚分布山本聡*

要旨:北西太平洋域において採取されたさまざまのコア洋性環境には堆積せず,遠洋域の海底では,数十メート状の表層堆積物についての岩相記載記録から,赤褐色粘ルの厚さを持つ遠洋赤粘土が層状チャートやチョークの土層の厚さ及びその層序の特徴をまとめた.半遠洋性の基底上に堆積している.海溝外側域の大洋底における半堆積環境では一般に薄く褐色粘土が灰緑色粘土の上に載遠洋性と遠洋性堆積層の境界域では,赤粘土一層状チャーつている.この褐色粘土層は,陸源有機物の堆積速度のトーチョークという遠洋堆積層が,半遠洋性の灰緑色粘減少する外洋域へと厚くなっている.灰緑色の粘土は遠土の下位に見られる.このような層序は,多分,遠洋性 *琉球大学理学部海洋学教室堆積物が半遠洋性堆積物の下位に潜り込んでいることを〒903-01沖縄県西原町字千原1番地示すと考えられる.