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Carbonate turbidites deposited on the floor of the Palau Trench
Marine Geology, 82 (1988) 217 233
217
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
CARBONATE TURBIDITES DEPOSITED ON THE FLOOR OF THE PALAU TRENCH SATOSHI YAMAMOTO 1, HIDEKAZU TOKUYAMA 2, KANTARO FUJIOKA 2, AKIRA TAKEUCHI 3 and HIROSHI UJIIl~ 1 IDepartment of Marine Sciences, University of the Ryukyus, Nishihara, Okinawa 903-01 (Japan) 2Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164 (Japan) 3Department of Geology, College of Liberal Arts, Toyama University, Toyama 930 (Japan) (Received December 18, 1987; revised and accepted February 17, 1988)
Abstract Yamamoto, S., Tokuyama, H., Fujioka, K., Takeuchi, A. and Ujii6, H., 1988. Carbonate turbidites deposited on the floor of the Palau Trench. Mar. Geol., 82: 217-233. Three major turbidite sequences of carbonate sediments are interbedded with a background of brown clay and monospecific diatom ooze in an 11 m long piston-core sequence recovered from the 8053 m deep flat floor of the Palau Trench. The carbonate turbidites are composed of graded sand and silt laminae derived from coral-reef lagoons, and of homogeneous marly chalk thicker than 7 m. These turbidites do not show evidence of strong dissolution, except for erosion resuspension on bed surfaces and leaching of carbonate-bound Sr from some sand laminae. This suggests that these carbonate turbidites were transported very quickly or as massive beds into a trench basin far below the calciumcarbonate compensation depth.
Introduction Because carbonate sediment dissolves on the seafloor at deeper levels t h a n the calciumcarbonate compensation depth (CCD), it can be used as a paleo-depth indicator. However, on some rare occasions, carbonate sediments can be deposited as turbidites on the seafloor deeper than the CCD. There are two welldocumented examples of carbonate turbidites below the CCD; one in the Puerto Rico Trench (Ericson et al., 1952; Ewing and Heezen, 1955; Ericson et al., 1961), and one in the Nauru Basin off the Ontong J a v a Plateau (Shipboard Scientific Party, 1981). Additionally, we report here another example of a carbonate turbidite on the fiat floor of the Palau Trench far below the CCD, although a short preliminary report has previously been made (Fujioka et al., 1986).
The carbonate turbidite sequences recovered from the flat floor of the Palau Trench include both carbonate-sand turbidites (Ericson et al., 1952) and chalky carbonate ooze of turbiditic origin (Shipboard Scientific Party, 1981) in an 11 m long sequence sampled by piston corer. This report describes in detail the stratigraphic and lithologic characteristics of the carbonate turbidite sequences and discusses their depositional processes on the deep Palau Trench floor below the CCD.
Bathymetry o f the sampling site The Palau Trench is located at the southwestern end of the South Mariana Trench system ( M a r i a n a - Y a p - P a l a u trenches) and is surrounded by three major abyssal basins; the West Philippine Basin in the west, the West
218 Mariana (Parece Vela) Basin in the northeast, and the West Caroline Basin in the southeast (Fig.l). The northern end of the Palau Trench is open owing to the fact t h a t the flat trench floor developed there is one of the widest trench basins in the Western Pacific. In 1985, the R.V. Takuyo of the Hydrographic Department of Japan carried out a detailed bathymetric survey on the northern Palau Trench using multibeam sonar (Kato et al., 1985). The 130 °
132 °
134 °
(N)
broad flat floor, "tear-dropped" in shape, is about 20 km long and 10 km wide, and at the deepest point is as deep as 8040 m at Teardrop Flat in the northern Palau Trench (Kato et al., 1985). The seismic survey across the Palau Trench was carried out during cruise KH84-1 of the R.V. Hakuho Maru (Tokuyama et al., 1985), and a part of the results is shown in Fig.2. Steep landward and seaward slopes adjoin the 136 °
138 °
14Q°E
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12 °
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10 °
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Fig.1. M a j o r b a t h y m e t r i c c o n f i g u r a t i o n s a n d s a m p l i n g site on the P a l a u T r e n c h floor. B a t h y m e t r i c c o n t o u r s are s h o w n (km).
Topographic map of the Palau Trench modifiedafter Kato et al. (1985).
[35°E
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134°50 '
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--10
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220 flat trench floor which is underlain by an alternation of acoustically transparent and opaque layers. The ridge to the west is interpreted as an accretionary terrace (Kato et al., 1985). Both the broad and flat floor and the ridge adjacent to the flat floor are similarly recognized in topographic features of the Puerto Rico Trench (Ewing and Heezen, 1955). Because of the similar topographic and tectonic environment, the similar depositional condition of being surrounded by carbonate platforms, and based on the previous coring results from the Puerto Rico Trench floor, there was an expectation t h a t a carbonate turbidite might be recovered from the Palau Trench floor, and in fact, we have recovered carbonate turbidites from site KH86-1 P-2 in 8053 m corrected water depth.
Stratigraphic characteristics of carbonate turbidite Analytical methods on recovered core The concentrations of combustible organic matter at 550°C (COM) and CO 2 evolved from the carbonates were analyzed according to the method described by Dean (1974) for all of the important lithologic horizons and at horizons systematically selected at 10cm intervals (Fig.3). The core horizons for chemical leaching were again selected on the basis of the fluctuations of COM and CO2. Two-hundred milligrams of the powdered and homogenized samples were used for the chemical leaching by 10 ml of 60% ethanol dissolved with 0.75N LiC1 and 0.25N CsC1 (abbreviated as Li-Cs-ethanol) and the pH 5.2 acetic acid buffered with 1.0N ammonium acetate (abbreviated as AA), according to the procedures described by Yamamoto (in press). The subtracted concentrations of the AA-leachable fractions by the L i - C s - e t h a n o l leachable fractions were considered as the lattice-bound concentrations of carbonate minerals. The analytical results are shown in Table 1. The bulk mineralogy of the samples was determined by the X-ray powder diffraction
method (XRD). Sedimentary structures of the core were investigated in detail using soft-Xray apparatus. Assignments of biostratigraphic age were based on observations of Foraminifera. Sand-grain compositions of the sand laminae were determined by point counting more than 200 grains in the 250-500 ~m size fraction. Stratigraphic characteristics As suggested from the stratigraphic fluctuations of CO2 (Fig.3), there is a gradual increase of carbonates (0.42% CO2/m) in the marly chalk downwards between 390 and 1088.5 cm in the core. The marly chalk is in gradational contact with an overlying 25 cm thick diatomaceous bed, and the decrease of the CO2 concentrations through the boundary may indicate some reworking processes of the dissolution-erosion-resuspension types on the marly chalk bed before or when the diatom ooze was deposited. The very gradual downward increases of carbonate-bound Ca and Sr concentrations in the homogeneous marly chalk are shown in Fig.4, whereas the Li-Cs-ethanol-leachable concentrations of Na, K and Ca decrease gradually down this lithologic column (Fig.5). The weak increase of carbonates with depth and the decrease of the Li-Cs-ethanol-leachable Na, K and Ca in the marly chalk can be interpreted as a result of authigenic calcite being precipitated inorganically at the deeper core horizons from interstitial pore water after deposition. The increase of carbonates with depth in the core does not demonstrate any evidence of the flow-in structure for the recovered core. Figure 6 illustrates the stratigraphic fluctuations of chemical elements for two flow-in cores of homogeneous lithology. These flow-in cores come from the hemipelagic gray clay sampled by piston corer in the trench slope off Ishigaki Island, and do not indicate any systematic trend along the core column (Fig.6). On the basis of megascopic observation the marly chalk unit between 390 and 1088.5 cm
'~u.m[t pa~aadn qa!~ 8u.~ppoq pop.aS =6 '4a~uoa [~npsa8 = 8 '.aasauoa [~uoD~p~aB =2 '4amuoa da~qs =9 :spu~q :Iasp pu~ u~oaq ~o suo!asmtus[=9 '.ozoo snooa~tuo~!p *;~aS-qs.~oi[o¢;= ~ !pu~s snooasal~a /;~a~-o^t.[o aq~![=g '.~IIsqa ¢;[a~tu pu~ pnva snooa~a[~a u~oaq-o^!io aqS!I o:~ ¢;~a8 qs!~oiio.( = g './~i a u~.~io!p~a u~oaq = ? :puo8o'-I 'spu~s ou~ ~la!qa tuua 8 pu~ 0I jo spoqao~ut, m.q:t o ~ =~ !pu~s oug ~[a!q:t mm 0I =9 '.pu~s mnt.potu ~la.~q~urm 0Z = 9 '.pu~s [~aoj~.u!ureaoj oug ~la!q~ mtu 0I =/7 ~pu~s osa~oa o~ aunt po~aos ~iaood ~la!q~ tutu 0I = ~ '.pu~s I~aOj!U!tU~aO;toug ~Ia!q~ tutu ~ = g '.pu~s oug ~ta!q~ uatu ~I = I "(sosoq:tuoa~d u! saoquanu) o~u!ua~ I pu~ S "po~a!pu~. oa~ O pu~ H 'V s~!un o~uoqa~a 30 suoD.tsod a.~qd~a~D~a~S "tua ~'~901 pu~ 06g uoo~:toq ~no!^qo ~. ~uo!~a~uoauoa rOD jo o~oaau! ~t~OA~ 'o~u!m~ I p u ~ uo pos~q oa~ ~ o l d oiSu~!a~ ~[a~lq pu~ soldures *;ppnm uo ~onl~^ [~at.~/;i~u~ moa~ pos~q oa~ S~old oIaa!a uod O "Z'd I'9~HYI oaoa Jo £z~tutun~ a!qdmt~.~a~soq~!rI 'g'8!eI
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9.1 8.0 5.4 7.6 5.6 6.0 7.0
9.3 9.9 9.0 12.1 13.1 10.9 13.0 9.7 9.2 8.6 10.8 9.4 11.6 10.8 10.9 9.7 8.0 10.3 17.8 12.5 11.2 8.6 9.4 6.8 6.8 7.0 6.1 7.0 5.7 6.9
C O M * (%)
20.8 20.7 28.2 15.9 26.8 23.3 21.4 --
6.5 5.6 5.9 8.3 9.3 13.2 10.1 14.5 6.2 6.5 5.4 7.6 7.4 8.9 14.0 18.8 21.7 20.0 5.5 11.8 15.9 20.6 21.0 24.8 26.5 27.1 27.0 27.2 28.3 26.9
CO2"* (%)
14.15 13.49 22.55 10.29 21.90 17.38 15.45 20.47
0.0653 ND 0.0170 3.143 4.074 6.839 5.536 8.288 ND 0.0688 0.0227 0.1740 1.634 2.381 6.828 12.68 15.25 14.13 5.713 6.827 9.320 14.14 14.89 18.58 19.64 19.61 20.39 20.40 21.17 21.18
Ca (%)
3840 3360 6340 5950 36{}0 5850 2860 7190
156 93.0 149 1600 1620 2200 1940 1870 1150 780 116 185 1780 2070 1620 1830 1890 2040 769 892 1370 1590 1860 2110 2110 2240 2100 2010 2080 2200
Mg
Carbonate-bound fraction
1420 1270 2830 833 2190 1380 1100 966
20.1 10.9 13.4 513 826 1160 947 1210 7.9 35.7 ND 21.5 234 210 627 833 959 1110 451 513 689 925 971 1160 1160 1170 1230 1210 1240 1230
Sr
1890 1800 2390 2000 3040 1770 1460 494
239 209 170 197 378 1090 lllO 1880 2230 1660 248 13(}0 988 4110 2210 2080 2840 1660 705 682 780 806 807 940 807 808 772 760 762 785
Mn
2.1 0,5 5.9 ND 2.7 2.7 2.1 7.4
3.6 ND 2.8 3.5 4.9 4.9 2.8 2.8 ND ND 0.7 ND 0.7 ND 3.5 5.6 5.6 3.5 2.8 4.2 5.6 6,2 8.3 9.7 8.3 8.3 9.0 8.3 8.3 9.0
Fe
2.13 2.26 1.12 1.87 1.14 1.49 1.62 0.106
2.68 2.66 2.66 2.58 2.62 2.56 2.62 2.43 2.64 2.62 2.64 2.64 2.58 2.56 2.54 2.25 1.94 2.19 2.56 2.39 2.29 2.11 2.07 1.51 1.41 1.47 1.27 1,25 1.13 1.16
N a (%)
1350 1380 969 1500 986 1210 1350 915
2630 2290 2260 1950 1990 1950 2090 1920 2270 2290 2830 2560 2270 2020 1870 1620 1180 1560 2020 1870 1700 1450 1490 1290 1220 1180 1160 1180 1140 1160
K
833 1320 741 1130 778 889 954 343
3810 3760 3560 2220 2210 2100 2090 1930 3040 3090 3360 3240 2560 2210 2140 1540 1310 1410 1930 1670 1360 1220 1280 893 918 905 874 905 757 796
Mg
911 1020 689 884 717 800 772 2720
1520 1330 1200 1830 1880 1680 1880 1750 1420 1660 823 1200 1540 1860 1790 1570 1400 1570 1990 1790 2360 1500 1280 830 880 1160 1040 869 813 757
Ca
Li C s - e t h a n o l - l e a c h a b l e f r a c t i o n
Concentrations are expressed as dry weight and unless stated otherwise, as parts per million. ND (not detected): < 10 ppm for Ca; < 0.5 ppm for other elements. AA-leachable Cr concentrations are all ND ( < 0.5 ppm). *Combustible organic matter at 550°C. **CO2 evolved from carbonates including crystalline water. ***Dark band in alternating lamination. ****Sieved sand fraction.
96 100-102 108 234 265 297 318 940-942****
Sand beds
KH86-1 P-2 0 2 10 12 20-22 40 42 48-50 60-62 70 72 90-92 107 109"** 120 122 140-142 200 202 230-232 250-252 270-272 290-292 310 312 330 332 340-342 350-352 360-362 390-392 400-402 410-412 500-502 600 602 700 702 800-802 900 902 1000 1002
(cm)
Intervals
Chemical leaching results for KH86-1 P-2 sediments from the Palau Trench floor
TABLE 1
13.8 17.5 12.5 15.0 15.0 13.8 16.3 28.1
14.3 12.9 11.6 11.6 14.3 15.7 17.1 14.3 17.1 14.3 18.5 28.5 23.5 2L0 28.5 23.5 16.6 23.5 26.0 23.5 16.6 23.5 13.5 15.2 11.8 15.2 15.2 11.8 15.2 13.5
Sr
23.9 29.2 19.3 23.9 22.7 24.4 18.2 43.4
5.5 9.3 8.1 ND ND 4.2 11.9 20,3 29.6 20,3 39.7 48.0 17.0 10.0 14.9 21.1 28.4 11.9 11.3 9.3 8.7 6.8 6.1 6.1 5.5 4.2 4.0 4.0 2.6 4.0
Mn
0.5 2.7 0.5 2.1 0.5 1.1 0.5 ND
2.0 2.7 1.4 0,7 0.7 0~7 1.4 1A 2.0 1.4 0.7 1.4 0.7 1.4 0.7 ND ND 0.7 ND ND ND 0.7 ND ND ND Nil 0.7 Nll ND 0.7
Fe
6.9 6.9 5.6 25.7 6.9 5.6 16.0 44.4
38.0 32,6 33.7 23.3 24.3 12.5 10.2 7.0 546 146 2.3 3.9 3.2 133 12.5 6.3 8.6 15.1 10.0 8.3 10.8 19.8 13.3 19.0 9.2 10.8 9.7 9.7 10.5 8.1
Cu
5.0 5.8 6.7 5.0 5.8 5.0 3.3 5.0
5.2 4.3 4.3 4.3 5.2 3.4 1.7 3.4 215 55.1 2.6 0.9 1.7 21.6 3.6 3.6 3.6 5.4 2.7 2.7 3.6 3.6 5.4 5.4 5.4 5.4 5.4 5.4 5.4 5.4
Ni
ND 1.3 1.9 1.3 3.6 0.8 0.9 7.4
3.5 2.3 2.1 2.9 1.5 1.3 1.3 0.6 5.1 2.8 ND 0.3 ND 2.8 1.7 2.8 1.5 1.1 ND ND 1.1 1.5 2.6 2.6 1.4 1.6 2.4 3.7 3.1 1.2
Zn
A A leachable fraction
4.2 4.2 7.3 2.1 5.2 4.2 4.2 4.2
ND ND ND 1.8 1.8 2.7 1.8 2.7 ND Nil ND ND ND ND 3.6 4.5 5.4 2.7 ND ND 3.6 3.6 5.4 2.7 5.4 5.4 4.5 3.6 5.4 5.4
Co
3.8 1.9 5.8 3.8 5.8 5.8 5.8 19.2
ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND 5.8 5.8 3.8 3.8
Pb
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does not indicate any apparent sedimentary structure. The X-radiographs of this unit show possible weak bedding planes which are also conspicuously inclined (Figs.7-9). There remains a possibility that the unit between 390 and 1088.5 cm is part of a slump fold, although the identification of folded structure is difficult in the case of homogeneous lithology, such as the marly chalk in this unit. There are two other carbonate units (the units are termed A, B, and C as shown in Fig.3) which are interbedded in hemipelagic-pelagic
brown clay above the massive marly chalk of unit C. Unit A is 47 cm thick (between 64 and 111 cm), and unit B is 90 cm thick (between 250 and 340 cm). Several 5-20 mm thick laminae of fine to medium calcareous sand, accompanied by graded beds, form each unit (Fig.3). The "background" hemipelagic-pelagic brown clay also shows turbiditic fabrics. The parallel laminations of dark and brown alternating bands just below carbonate unit A indicate redox cycles, and anomalous enrichments of heavy metals such as Cu and Ni accompany
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Fig.5. Stratigraphic fluctuations of Li-Cs-ethanol-leachable elements in bulk dry-weight sediments. Note decrease with depth in horizons between 390 and 1088.5 cm. this l a m i n a t i o n (Table 1). An e n r i c h m e n t zone of c a r b o n a t e - b o u n d M n is o b s e r v e d b e t w e e n 60 and 3 3 2 c m (Table 1; Fig.4). This zone m a y r e p r e s e n t a r e d u c i n g e n v i r o n m e n t in the t r e n c h basin w h e r e the b r o w n clay is deposited, and t h e p o s t d e p o s i t i o n a l diffusion of M n from t h e b r o w n clay into the c a r b o n a t e u n i t s A and B in an u p w a r d and d o w n w a r d d i r e c t i o n respectively. T h e g r a d e d c a l c a r e o u s sands of units A and B commonly occur above sharp contacts, w h i c h m a y imply some e r o s i o n a l processes. S e v e r a l m i l l i m e t e r - t h i c k l a m i n a t i o n s of fine to m e d i u m sand are e q u i v a l e n t to B o u m a divi-
sions B or D (Bouma, 1962). N o r m a l l y , several millimeters to a few c e n t i m e t e r s of pelite divisions of the B o u m a E division are deposited above the l a m i n a t e d sand of the B o u m a B or D division. T h e pelite s e q u e n c e s show an u p w a r d d e c r e a s e of c a r b o n a t e c o n t e n t s (Figs.3 and 4), in a d d i t i o n to the u p w a r d fining of g r a i n size. The f o r a m i n i f e r a l sand l a m i n a in the b r o w n clay (lamina 4 in the lithologic c o l u m n of Figo3) m a y n o t be i n t e r p r e t e d as p a r t of the B o u m a sequence, b u t it can be e x p l a i n e d as a deposit from suspension. T h e age of t h e b o t t o m of the r e c o v e r e d core s e q u e n c e is P l e i s t o c e n e and possibly y o u n g e r
225 Li-Cs-ethanol leacIlable
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P-4
Fig.6. Stratigraphic fluctuations of carbonates and Li-Cs-ethanol-leachable elements for two flow-in sections of homogeneous sediments from the Ryukyu Trench slope off Ishigaki Island. Water depths and locations for these two sites are as follows: RN87 P-3: 24°07.5'N, 124°24.1'E; water depth (corrected)= 1665 m. RN87 P-4: 23°50.2'N, 124°24.1'E; water depth (corrected) = 2513 m.
than 0.69 Ma B.P., as judged from the abundances of right-coiling P u l l e n i a t i n a spp. (Saito, 1976) in the core column (Table 2). Some index planktonic Foraminifera of the Middle Pliocene are contained in some horizons; however, they are judged to be reworked material. Benthic foraminifera species derived from shallow coral-reef lagoons are contained in various parts of the turbidite sequences (Table 2). The diatomaceous bed just above the marly chalk bed is composed of abundant broken frustules of the huge diatom, E t h m o d i s -
cus rex (Rattray) Hendey. They often occur as
monospecific diatomaceous deposits of a few tens of centimeters in thickness deposited during the last 400,000 yrs (Tanimura, 1981). M i n e r a l o g i c a l characteristics
The carbonate-bound C a - M g and Ca-Sr relationships (Fig.10) indicate compositional differences of carbonate mineralogy among the three units of the carbonate turbidite sequence. Carbonate minerals of unit C are
X
X
X
X
X
X
4.8 5.7
X
X
X
X
X
12.9 7.0
X
X
X
0.0 10.0
340 342
X
X
X
X
12.2 9.4
X
1000/o
x?
410412
X
X
X
X
X
18.6 8.8
X
0.0 2.8
100%
100%
X
X
1010 1012
X
480 482
42.9 3.6
x?
1040 1042
All samples contain the large diatom Ethmodiscus rex which ranges from a b u n d a n t to present, radiolarians occur frequently and sponge spicules occasionally. x - - present.
X
X
X X
Alcyonarian sclerites Gastropod shells Branching bryozoan fragments Echinoid spines
X
17.6 13.8
Halimeda
Biogenic fragments derived from reefal shallow waters
11.4 10.5
23.5 6.2
Ratio of shallow-water species to total benthic species Ratio of benthic species to total foraminiferal species
X X
X
X
X
X
Others
X
X
100°/'o
100%
100%
100%
X
X
X
330 332
x
X
320 322
X
100%
310 312
X
x
X
290 292
x?
X
x
240 242
Core interval (cm)
Amphistegina lessonii Amphistegina radiata Amphisorus hemprichii Baculogypsina spengleri Calcarina hispida Cymbaloporetta squammosa Elphidium jenseni
Reefal shallow water benthic foraminiferal species
Index planktonic foraminiferal taxa Reddish Globigerina rubescens (above N23 base) Hastigerinella adamsii (above N23 base) Sphaei'oidinella dehiscens excavata (above N23 base) Pulleniatina finalis (above N22 base; 1.9 Ma) Globorotalia truncatulinoides (above N22 base) Globorotalia tosaensis (above N21 base; 3.0 Ma) Globoquadrina spp. (below PL5 base ?) Globigerina nepenthes (below PL2 base) Right-coiling Pulleniatina spp.
Species/occurrence
Occurrence of datable planktonic Foraminifera and reefal benthic Foraminifera
TABLE 2
227 6.5
cm
6.5 I
441
cm
641
\ \
,!
f
[
a
J
f t ] f
!
f
/
f
J
f
J f /
I
J
/
f
/
f !
)
[
/
\ f !
459 cm
f
659 cm
Fig.7. X-radiograph of homogeneous section of the marly chalk bed. Some inclined bedding planes are recognizable. Possible layering is shown with dashed lines.
Fig.8. X-radiograph showingreversal of the bedding plane inclination compared to Fig.7.
composed mainly of low-Mg calcite with slight enrichments of Mg and very small amounts of aragonite, as indicated by XRD in Fig.ll. Carbonate minerals of units A and B are essentially composed of high-Mg calcite, highSr and low-Sr aragonites, and low-Mg calcite (Figs.10 and 11). The carbonate-bound Ca-Sr relationship suggests t h a t unit A contains slightly higher concentrations of high-Sr aragonite t h a n unit B. This tendency is indicated further by the point-count data on the sand compositions (Table 3); the sand grains in unit A are slightly higher regarding the concentra-
tion of alcyonarian sclerites, bryozoans and Halimeda, but lower in the concentration of planktonic Foraminifera than unit B. No heavy leaching of Mg or Sr from sands composed of high-Mg calcite and high-Sr aragonite is evident from the carbonate-bound C a - M g and Ca-Sr relationships. In contrast, Gomberg and Bonatti (1970) have observed dissolution evidence and subsequent leaching of Mg and Sr in a 143 cm thick turbidite of coarse calcareous sand on the 4230 m deep slope off the Tuamotsu Archipelago in the South Pacific. The only leaching evidence from
228 6 cm
900
I
1
and in the pelite sequences of units A and B at contacts with the background brown clay (Fig.10). Generally, the turbiditic sands deposited at the Palau Trench well below the CCD are thought to be free of dissolution because the sands were transported and buried very quickly by turbidity currents and the turbiditic sands were not exposed in the deep sea for a very long period. Episodic sedimentation of carbonate turbidite
920 cm
Fig.9. X-radiograph showing obvious horizontal contact (arrow) between beds.
the Palau Trench carbonates is observed for the carbonate-bound Sr in unit A (Fig.10). The leaching of Sr occurs in the Botuna D sand laminae of the unit A turbidite sequence. This kind of leaching of Sr without the association of Mg can be interpreted as a stability relationship of carbonate minerals where high-Sr aragonite is more unstable t h a n high-Mg calcite (Friedman, 1965). The carbonate-bound Mg is enriched in the planktonic Foraminifera
The carbonate sequences of units A, B and C are all considered to be turbidite deposits derived from the adjacent carbonate platforms and slopes. The sandy carbonate turbidites of units A and B could be transported by turbidity currents from the shallow coral-reef lagoons or aprons developed in the off-bank carbonate slope (Davies, 1968; Crevello and Schlager, 1980; Ujii~ et al., 1983; Yamamoto and Ujii~, 1983; Mullins et al., 1984; Ujii~ and Nagano, 1984; Hoskins et al., 1986; Midorikawa and Ujii6, 1987). The marly chalk turbidite of unit C could have been derived from the periplatform ooze (Mullins et al., 1984), which may be distributed widely on the carbonate slope off Palau Island in an approximate depth range of 2-4km. The periplatform calcareous ooze could be supplied from the ridges and slopes shallower than 4 km which are developed on both the seaward and landward slopes facing Teardrop Flat in the Palau Trench (Kato et al., 1985). Carbonate turbidites have been reported from various types of modern carbonate slopes (Davies, 1968; Gomberg and Bonatti, 1970; Bornhold and Pilkey, 1971; Huang and Pierce, 1971; Bennetts and Pilkey, 1976; Crevello and Schlager, 1980; Mullins et al., 1984) as well as in ancient depositional environments (Carozzi and Frost, 1966; Tucker, 1969; Scholle, 1971a, b; Hobson et al., 1985). A limited number of reports have described carbonate turbidites deposited on the abyssal seafloor well below the CCD and have discussed their significance. Huang and Pierce (1971) have reported several
7
10.0 2.5 ND ND ND 0.8 3.3 9.2 ND 27.5 13.3 0.8 14.2 ND ND 3.3 13.3 1.7 99.9
96
Interval (cm)
All values are expressed as volume percent. ND (not determined) : <0.1%. *Sieved sand fraction.
Numbers indicated in parentheses in the stratigraphic column of Fig.3
Quartz and feldspars Opaque heavy m i n e r a l s Lithic fragments Total
E t h m o d i s c u s rex
Coralline algae Bryozoans Echinoid (spines) Mollusca Decapoda Planktonic Foraminifera Benthic Foraminifera Microcrystalline clump Sponge spicules Radiolarians
Halimeda
Corals Alcyonarian sclerites Calcareous ascidian spicules
Sand grains
6
15.1 5.0 2.2 0.7 0.7 5.0 3.6 3.6 ND 23.0 6.5 7.2 15.8 ND ND 3.6 6.5 1.4 99.9
100 102
Petrographic point-count data for sand beds, KH86-1 P-2
TABLE 3
4
18.2 ND 1.6 ND ND 2.1 2.1 9.0 ND 31.9 6.9 1.1 8.0 0.5 ND 5.3 11.7 1.1 99.5
23.3 2.2 1.6 0.3 3.5 2.8 0.6 15.4 ND 11.7 4~1 0.3 3.5 1.0 ND 7.9 15.8 6.0 100.0
5
234
108
3
25.2 ND 0.7 ND 2.0 1.0 0.7 19.6 ND 18.0 3.3 ND 5.2 2.0 ND 2.9 7.2 12.4 100.2
265
2
14.3 1.2 1.2 ND 1.7 0.3 ND 4.0 0.9 32.1 6.6 1.2 0.6 ND ND 7.5 16.3 12.3 1~.2
297
1
17.0 1.4 0.4 ND 3.1 1.4 1.7 2.8 0.7 24.7 3.8 1.0 2.8 ND ND 12.2 23.6 3.5 100.1
318
ND ND ND ND ND ND ND ND ND 39.4 5.6 ND 17.4 14.6 20.2 0.9 ND 1.9 1~.0
930 932*
230 Legend
% 0.8-
a
Unit A
mud
•
Unit A sand
A
Unit B mud
•
Unit B sand
0
Unit C
•
Background clay unit
II~
Sieved sand fraction
Mg Sr
940-942
% • 234
0.6
A 297
/
0.3-
j~ ,'
~08
/
•
/
/2 ,
/
.S~//
o.,
0.2-
c~ching ~?>
, / ' a318
//
60-62
/~ /_/
~
-
/
/
C
40,-5o'- ~
~o/~
/
/9~ . ' / /
./~°//~,:<
,,_,(o,o-,Y,~,/ ,~j~-""% ,,og,,
.~ / / A ~y)90-392 / 270-272w----tg°-29~ 40;4~/ ~-Y-360-362
.o,/
u
o<,I0ta0-202 "°'" 0
/ / I_
~o ~33%23'.,,~" <;'o (oo, O.1-
'
y/
,,/////B
~]00-102
"k/~o350 C 2
0-22 1107_202 ] "0
210
Ca
I
3o
%
0
I
~o
I
Ca
20
I
30
%
Fig.10. Carbonate-boundCa-Mg and Ca Sr relationships. Territories are encircled accordingto samples in carbonate units A, B and C. All core intervals (cm) are given.
graded sandy carbonate turbidites interbedded in the pelagic clay on the 4770 m deep abyssal floor off the Bahama Islands. Carbonate turbidites deposited on the abyssal trench floor far below the CCD (Berger and Winterer, 1974) are probably restricted only to the Puerto Rico Trench (Ericson et al., 1952) and to the Palau Trench (Fujioka et al., 1986). Some episodic cycles of graded sand beds are comparable between the Puerto Rico and Palau Trench floor cores, and the cycles are assumed to have occurred episodically in the pelagic-hemipelagic brown clay deposited in the abyssal basin. Estimating the sedimentation rate of the background brown clay to be about 1 cm/1000 yrs, the sandy turbiditic sequences of units A and B are assumed to have been deposited once every 60,000 yrs or so
on the Puerto Rico and Palau Trench floors. The thick homogeneous chalky bed, although the base of which was not recovered by coring, is a unique stratigraphic assemblage in the Palau Trench carbonate turbidites. However, similar massive and homogeneous chalk beds of turbiditic origin have been reported from the Nauru Basin in 5191 m of water. The Shipboard Scientific Party (1981) concluded that the episodic cycles of turbidity currents had transported and deposited the massive beds of chalky ooze from the carbonate platform of the Ontong J a v a Plateau 400 km away. Although more comparative stratigraphic analyses are required to clarify the mechanism for initiations of episodes of mass transport, several combined mechanisms may be appropriate. One is sea-level fluctuation due to
231 35 KV 10 mA Cu-Ka Radiation Slits: Div. I- Scat. 2" Rec.
Ni filter Scan speed I~(2~))/min. 0.15 i11m Time const, i sec.
KHB~-]
P-2
Lc 50-62 cm (i;lu d )
O I
C
H
Pl A
A
O
I
ii mc
j~ Ii
108 cm
I
I
( sand )
I
i~ I
I
:1
j
250-252
or/
I
(n/ud/
II II li
318
il
{sand)
I
cm
ii I
[
Jl
I
Ji iI rl
I
I
I
|
40
J
f
I
J
|
I
I
J
I [ I
I
J
f
I
l
700-702 cm (mud)
r
J
l
F
30
I
I
J
I
I
20 DEGREES
I
P
J
I
I
[
r
J
I
I
I
I
I
i
i
10
[2el
F i g . l l . XRD diffractograms of r e p r e s e n t a t i v e c a r b o n a t e turbidites. L c - - low-Mg calcite; H c - - h i g h - M g calcite; C - calcite; A - - aragonite; Q - - quartz; P l - - plagioclase; H - halite; O l - - olivine(?); R h - - rhodochrosite; C h l - - chlorite; K a o l - kaolinite; M - smectite; ? - - unidentified.
either eustatic or tectonic movements. The fluctuation of sea level in the carbonate platforms alters the sedimentary environment by causing progradation of coarse sediments seawards (Schlager, 1981; Read, 1985). Sealevel changes during the Pleistocene-Holocene periods have been known for the carbonate-reef islands of the Western Pacific region (Konishi et al., 1974). Other possible mechan-
isms include catastrophic events, including meteorologic (mainly typhoons) and tectonic (earthquake) catastrophes. Summary The l l m long core recovered from the 8053 m deep floor of the Palau Trench includes three major carbonate turbidite sequences
232 interbedded with a background brown clay. Carbonate sands of turbiditic origin do not show evidence of strong dissolution, except for a few l a m i n a e o f t h e B o u m a D d i v i s i o n , although they were transported and deposited to the abyssal basin far below the CCD. The homogeneous bed of marly chalk, thicker than 7 m, is a l s o i n t e r p r e t e d a s a t u r b i d i t e , a n d i t may have been transported to the abyssal basin as a massive bed without experiencing dissolut i o n . T h i s t y p e o f d e p o s i t i o n is c o m p a r a b l e t o the extraordinarily thick micritic limestone of turbiditic origin of the Monte Antola Flysch in the Northern Apennines, as interpreted by S c h o l l e (1971a). L a c k o f i n t e n s e d i s s o l u t i o n i n both the carbonate-sand and carbonate-mud turbidites suggests fast transport by mass m o v e m e n t s o n t o t h e a b y s s a l t r e n c h floor, w e l l below the CCD. Not all carbonate turbidites deposited in the abyssal basin below the CCD are devoid of dissolution effects, the latter depending on the amount of ~ponded" bed and t h e m e c h a n i s m c a u s i n g t h e t u r b i d i t e flow.
Acknowledgements W e a r e g r a t e f u l t o C a p t a i n I. T a d a m a , C h i e f E n g i n e e r Y. M a t s u m o t o a n d c r e w m e m b e r s o f t h e R.V. Hakuho Maru, a n d to P r o f . Y. T o m o d a and scientific members of cruise KH86-1 for their cooperation during sampling operations. The two flow-in cores from the Ryukyu Trench slope were sampled during the joint educational cruise of the University of the Ryukyus a n d N a g a s a k i U n i v e r s i t y o n t h e R.V. Nagasaki Maru. M e s s r s . K. K o g a a n d S. K u r a m o t o a r e acknowledged for their assistance during photographing production of the X-radiographs of the piston core. The manuscript was critically reviewed by Profs. G.M. Friedman, D.F. Merr i a m , J . W . H a r b a u g h , J . D . M i l l i m a n a n d H. Kagami.
References Bennetts, K.R.W. and Pilkey, O.H., 1976. Characteristics of three individual turbidites from the Hispaniola Caicos Basin. Geol. Soc. Am. Bull., 87: 1291-1300. Berger, W.H. and Winterer, E.L., 1974. Plate stratigraphy
and the fluctuating carbonate line. In: K.J. Hsfi and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Int. Assoc. Sedimentol. Spec. Publ., 1: 11-48. Bornhold, B.D. and Pilkey, O.H., 1971. Bioclastic turbidite sedimentation in Columbus Basin, Bahamas. Geol. Soc. Am. Bull., 82:1341 1354. Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits. Elsevier, New York, 168 pp. Carozzi, A.V. and Frost, S.H., 1966. Turbidites in dolomitized flank beds of Niagaran (Silurian) Reef, Lapel, Indiana. J. Sediment. Petrol., 36:563 573. Crevello, P.D. and Schlager, W., 1980. Carbonate debris sheets and turbidites, Exuma Sound, Bahamas. J. Sediment. Petrol., 50:1121 1148. Davies, D.K., 1968. Carbonate turbidites, Gulf of Mexico. J. Sediment. Petrol., 38: 1100- 1109. Dean Jr., W.E., 1974. Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. J. Sediment. Petrol., 44:242 248. Ericson, D.B., Ewing, M. and Heezen, B.C., 1952. Turbidity currents and sediments in the North Atlantic. Bull. Am. Assoc. Pet. Geol., 36:489 511. Ericson, D.B., Ewing, M., Wollin, G. and Heezen, B.C., 1961. Atlantic deep-sea sediment cores. Geol. Soc. Am. Bull., 72: 193-286. Ewing, M. and Heezen, B.C., 1955. Puerto Rico Trench topographic and geophysical data. Geol. Soc. Am. Spec. Pap., 62:255 268. Friedman, G.M., 1965. Occurrence and stability relationships of aragonite, high-magnesian calcite, and lowmagnesian calcite under deep-sea conditions. Geol. Soc. Am. Bull., 76:1191 1196. Fujioka, K., Furuta, T., Kimura, G., Kodama, K., Koga, K., Kuramoto, S., Matsugi, H., Seno, T., Takeuchi, A., Watanabe, M. and Yamamoto, S., 1986. Sediments and rocks in and around the Palau and Yap trenches. In: Y. Tomoda (Editor), Preliminary Report of the Hakuho Maru Cruise KH86-1. Ocean Res. Inst., Univ. Tokyo, Tokyo, pp.38 148. Gomberg, D.N. and Bonatti, E., 1970. High-magnesian calcite: leaching of magnesium in the deep sea. Science, 168:1451 1453. Hobson, J.P., Caldwell, C.D. and Toomey, D.T., 1985. Early Permian deep-water allochthonous limestone facies and reservoir, West Texas. Bull. Am. Assoc. Pet. Geol., 69: 2130 2147. Hoskins, C.M., Reed, J.K. and Mook, D.H., 1986. Production and off-bank transport of carbonate sediment, Black Rock, Southwest Little Bahama Bank. Mar. Geol., 73: 125 144. Huang, T.C. and Pierce, J.W., 1971. The carbonate minerals of deep-sea bioclastic turbidites, southern Blake Basin. J. Sediment. Petrol., 41: 251-260. Kato, S., Kato, G. and Asada, A., 1985. Detailed bathymerry of the northern Palau Trench by multi-beam sonar. Proc. Joint Meet. U.S. Jpn. Coop. Program Nat. Resourc., Sea-Bottom Surv. Panel, 14th (Tokyo), pp.179 193.
233 Konishi, K., Omura, A. and Nakamichi, O., 1974. Radiometric coral ages and sea level records from the Late Quaternary reef complexes of the Ryukyu Islands. Proc. Int. Coral Reef Symp., 2nd. Great Barrier Reef Comm., Brisbane, pp.595-613. Midorikawa, Y. and Ujii~, H., 1987. Bottom sediments of the Sekisei Lagoon, the Yaeyama Islands. The Earth Mon., 9:152 158 (in Japanese). Mullins, H.T., Heath, K.C., Van Buren, H.M. and Newton, C.R., 1984. Anatomy of a modern open-ocean carbonate slope: northern Little Bahama Bank. Sedimentology, 31: 141 168. Read, J.F., 1985. Carbonate platform facies and models. Bull. Am. Assoc. Pet. Geol., 69: 1-21. Saito, T., 1976. Geologic significance of coiling direction in the planktonic foraminifera P u l l e n i a t i n a . Geology, 4: 305 309. Schlager, W., 1981. The paradox of drowned reefs and carbonate platforms. Geol. Soc. Am. Bull., 92: 197-211. Scholle, P.A., 1971a. Sedimentology of fine-grained deep water carbonate turbidites, Monte Antola flysch (Upper Cretaceous), northern Apennines, Italy. Geol. Soc. Am. Bull., 82:629 658. Scholle, P.A., 1971b. Diagenesis of deep-water carbonate turbidites, Upper Cretaceous Monte Antola flysch, northern Apennines, Italy. J. Sediment. Petrol., 41: 233 250.
Shipboard Scientific Party, 1981. Site 462: Nauru Basin, western Pacific Ocean. In: R.L. Larson, S. Schlanger et al., Initial Reports of the Deep Sea Drilling Project, Leg 61, Vol. 61. U.S. Gov. Print. Off., Washington, D.C., pp.19-177. Tanimura, Y., 1981. Late Quaternary marine diatom E t h m o d i s c u s rex from the northwestern Pacific Ocean. Bull. Natl. Sci. Mus., Tokyo, Ser. C, 7:91 96. Tokuyama, H., Asanuma, T., Nishiyama, E., Hatori, H., Chiba, H., Ueno, S. and Tomita, N., 1985. Multichannel seismic reflection survey. In: K. Kobayashi (Editor), Preliminary Report of the Hakuho Maru Cruise KH84-1. Ocean Res. Inst., Univ. Tokyo, Tokyo, pp.282 291. Tucker, M.E., 1969. Crinoidal turbidites from the Devonian of Cornwall and their palaeogeographic significance. Sedimentology, 13:263 280. Ujii~, H. and Nagano, K., 1984. Sediments of Haneji Inlet and its environs, Okinawa, subtropical Japan. Galaxea, 3: 81-95. Ujii~, H., Yamamoto, S., Okitsu, M. and Nagano, K., 1983. Sedimentological aspects of Nakagusuku Bay, Okinawa, subtropical Japan. Galaxea, 2: 95-117. Yamamoto, S., in press. Fractionation for carbonate-bound metals in deep-sea sediments from the Japan Trench basin. Sedimentology. Yamamoto, S. and Ujii~, H., 1983. Calcareous sediment around coast of the Okinawa-jima Island. News Osaka Micropaleontol., 11:48 62.
217
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
CARBONATE TURBIDITES DEPOSITED ON THE FLOOR OF THE PALAU TRENCH SATOSHI YAMAMOTO 1, HIDEKAZU TOKUYAMA 2, KANTARO FUJIOKA 2, AKIRA TAKEUCHI 3 and HIROSHI UJIIl~ 1 IDepartment of Marine Sciences, University of the Ryukyus, Nishihara, Okinawa 903-01 (Japan) 2Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164 (Japan) 3Department of Geology, College of Liberal Arts, Toyama University, Toyama 930 (Japan) (Received December 18, 1987; revised and accepted February 17, 1988)
Abstract Yamamoto, S., Tokuyama, H., Fujioka, K., Takeuchi, A. and Ujii6, H., 1988. Carbonate turbidites deposited on the floor of the Palau Trench. Mar. Geol., 82: 217-233. Three major turbidite sequences of carbonate sediments are interbedded with a background of brown clay and monospecific diatom ooze in an 11 m long piston-core sequence recovered from the 8053 m deep flat floor of the Palau Trench. The carbonate turbidites are composed of graded sand and silt laminae derived from coral-reef lagoons, and of homogeneous marly chalk thicker than 7 m. These turbidites do not show evidence of strong dissolution, except for erosion resuspension on bed surfaces and leaching of carbonate-bound Sr from some sand laminae. This suggests that these carbonate turbidites were transported very quickly or as massive beds into a trench basin far below the calciumcarbonate compensation depth.
Introduction Because carbonate sediment dissolves on the seafloor at deeper levels t h a n the calciumcarbonate compensation depth (CCD), it can be used as a paleo-depth indicator. However, on some rare occasions, carbonate sediments can be deposited as turbidites on the seafloor deeper than the CCD. There are two welldocumented examples of carbonate turbidites below the CCD; one in the Puerto Rico Trench (Ericson et al., 1952; Ewing and Heezen, 1955; Ericson et al., 1961), and one in the Nauru Basin off the Ontong J a v a Plateau (Shipboard Scientific Party, 1981). Additionally, we report here another example of a carbonate turbidite on the fiat floor of the Palau Trench far below the CCD, although a short preliminary report has previously been made (Fujioka et al., 1986).
The carbonate turbidite sequences recovered from the flat floor of the Palau Trench include both carbonate-sand turbidites (Ericson et al., 1952) and chalky carbonate ooze of turbiditic origin (Shipboard Scientific Party, 1981) in an 11 m long sequence sampled by piston corer. This report describes in detail the stratigraphic and lithologic characteristics of the carbonate turbidite sequences and discusses their depositional processes on the deep Palau Trench floor below the CCD.
Bathymetry o f the sampling site The Palau Trench is located at the southwestern end of the South Mariana Trench system ( M a r i a n a - Y a p - P a l a u trenches) and is surrounded by three major abyssal basins; the West Philippine Basin in the west, the West
218 Mariana (Parece Vela) Basin in the northeast, and the West Caroline Basin in the southeast (Fig.l). The northern end of the Palau Trench is open owing to the fact t h a t the flat trench floor developed there is one of the widest trench basins in the Western Pacific. In 1985, the R.V. Takuyo of the Hydrographic Department of Japan carried out a detailed bathymetric survey on the northern Palau Trench using multibeam sonar (Kato et al., 1985). The 130 °
132 °
134 °
(N)
broad flat floor, "tear-dropped" in shape, is about 20 km long and 10 km wide, and at the deepest point is as deep as 8040 m at Teardrop Flat in the northern Palau Trench (Kato et al., 1985). The seismic survey across the Palau Trench was carried out during cruise KH84-1 of the R.V. Hakuho Maru (Tokuyama et al., 1985), and a part of the results is shown in Fig.2. Steep landward and seaward slopes adjoin the 136 °
138 °
14Q°E
WEST
MARIANA BASIN
12 °
pH~IL Ipp INEW'SEA BASIN
10 °
-
YAP
ISLANDS
8o
134°50 ! --
% '?, c9
4°
i
~l
/
i
'
135 ° i
i
135°10 ' I 8°
I
WEST CAROLIBASI NE N f~"~~87! 7!!f] t 7~40 ' 7°30'
0 o
Fig.1. M a j o r b a t h y m e t r i c c o n f i g u r a t i o n s a n d s a m p l i n g site on the P a l a u T r e n c h floor. B a t h y m e t r i c c o n t o u r s are s h o w n (km).
Topographic map of the Palau Trench modifiedafter Kato et al. (1985).
[35°E
135°I0 '
L
7
6"
,
W i r ~
I
Fig.2. Multichannel seismic profile across the Palau Trenvh. Vertical scale: two-way travel time (s).
134°50 '
,E
CONDS
--10
--9
--8
--7
--6
--5
~D
bD
220 flat trench floor which is underlain by an alternation of acoustically transparent and opaque layers. The ridge to the west is interpreted as an accretionary terrace (Kato et al., 1985). Both the broad and flat floor and the ridge adjacent to the flat floor are similarly recognized in topographic features of the Puerto Rico Trench (Ewing and Heezen, 1955). Because of the similar topographic and tectonic environment, the similar depositional condition of being surrounded by carbonate platforms, and based on the previous coring results from the Puerto Rico Trench floor, there was an expectation t h a t a carbonate turbidite might be recovered from the Palau Trench floor, and in fact, we have recovered carbonate turbidites from site KH86-1 P-2 in 8053 m corrected water depth.
Stratigraphic characteristics of carbonate turbidite Analytical methods on recovered core The concentrations of combustible organic matter at 550°C (COM) and CO 2 evolved from the carbonates were analyzed according to the method described by Dean (1974) for all of the important lithologic horizons and at horizons systematically selected at 10cm intervals (Fig.3). The core horizons for chemical leaching were again selected on the basis of the fluctuations of COM and CO2. Two-hundred milligrams of the powdered and homogenized samples were used for the chemical leaching by 10 ml of 60% ethanol dissolved with 0.75N LiC1 and 0.25N CsC1 (abbreviated as Li-Cs-ethanol) and the pH 5.2 acetic acid buffered with 1.0N ammonium acetate (abbreviated as AA), according to the procedures described by Yamamoto (in press). The subtracted concentrations of the AA-leachable fractions by the L i - C s - e t h a n o l leachable fractions were considered as the lattice-bound concentrations of carbonate minerals. The analytical results are shown in Table 1. The bulk mineralogy of the samples was determined by the X-ray powder diffraction
method (XRD). Sedimentary structures of the core were investigated in detail using soft-Xray apparatus. Assignments of biostratigraphic age were based on observations of Foraminifera. Sand-grain compositions of the sand laminae were determined by point counting more than 200 grains in the 250-500 ~m size fraction. Stratigraphic characteristics As suggested from the stratigraphic fluctuations of CO2 (Fig.3), there is a gradual increase of carbonates (0.42% CO2/m) in the marly chalk downwards between 390 and 1088.5 cm in the core. The marly chalk is in gradational contact with an overlying 25 cm thick diatomaceous bed, and the decrease of the CO2 concentrations through the boundary may indicate some reworking processes of the dissolution-erosion-resuspension types on the marly chalk bed before or when the diatom ooze was deposited. The very gradual downward increases of carbonate-bound Ca and Sr concentrations in the homogeneous marly chalk are shown in Fig.4, whereas the Li-Cs-ethanol-leachable concentrations of Na, K and Ca decrease gradually down this lithologic column (Fig.5). The weak increase of carbonates with depth and the decrease of the Li-Cs-ethanol-leachable Na, K and Ca in the marly chalk can be interpreted as a result of authigenic calcite being precipitated inorganically at the deeper core horizons from interstitial pore water after deposition. The increase of carbonates with depth in the core does not demonstrate any evidence of the flow-in structure for the recovered core. Figure 6 illustrates the stratigraphic fluctuations of chemical elements for two flow-in cores of homogeneous lithology. These flow-in cores come from the hemipelagic gray clay sampled by piston corer in the trench slope off Ishigaki Island, and do not indicate any systematic trend along the core column (Fig.6). On the basis of megascopic observation the marly chalk unit between 390 and 1088.5 cm
'~u.m[t pa~aadn qa!~ 8u.~ppoq pop.aS =6 '4a~uoa [~npsa8 = 8 '.aasauoa [~uoD~p~aB =2 '4amuoa da~qs =9 :spu~q :Iasp pu~ u~oaq ~o suo!asmtus[=9 '.ozoo snooa~tuo~!p *;~aS-qs.~oi[o¢;= ~ !pu~s snooasal~a /;~a~-o^t.[o aq~![=g '.~IIsqa ¢;[a~tu pu~ pnva snooa~a[~a u~oaq-o^!io aqS!I o:~ ¢;~a8 qs!~oiio.( = g './~i a u~.~io!p~a u~oaq = ? :puo8o'-I 'spu~s ou~ ~la!qa tuua 8 pu~ 0I jo spoqao~ut, m.q:t o ~ =~ !pu~s oug ~[a!q:t mm 0I =9 '.pu~s mnt.potu ~la.~q~urm 0Z = 9 '.pu~s [~aoj~.u!ureaoj oug ~la!q~ mtu 0I =/7 ~pu~s osa~oa o~ aunt po~aos ~iaood ~la!q~ tutu 0I = ~ '.pu~s I~aOj!U!tU~aO;toug ~Ia!q~ tutu ~ = g '.pu~s oug ~ta!q~ uatu ~I = I "(sosoq:tuoa~d u! saoquanu) o~u!ua~ I pu~ S "po~a!pu~. oa~ O pu~ H 'V s~!un o~uoqa~a 30 suoD.tsod a.~qd~a~D~a~S "tua ~'~901 pu~ 06g uoo~:toq ~no!^qo ~. ~uo!~a~uoauoa rOD jo o~oaau! ~t~OA~ 'o~u!m~ I p u ~ uo pos~q oa~ ~ o l d oiSu~!a~ ~[a~lq pu~ soldures *;ppnm uo ~onl~^ [~at.~/;i~u~ moa~ pos~q oa~ S~old oIaa!a uod O "Z'd I'9~HYI oaoa Jo £z~tutun~ a!qdmt~.~a~soq~!rI 'g'8!eI
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9.1 8.0 5.4 7.6 5.6 6.0 7.0
9.3 9.9 9.0 12.1 13.1 10.9 13.0 9.7 9.2 8.6 10.8 9.4 11.6 10.8 10.9 9.7 8.0 10.3 17.8 12.5 11.2 8.6 9.4 6.8 6.8 7.0 6.1 7.0 5.7 6.9
C O M * (%)
20.8 20.7 28.2 15.9 26.8 23.3 21.4 --
6.5 5.6 5.9 8.3 9.3 13.2 10.1 14.5 6.2 6.5 5.4 7.6 7.4 8.9 14.0 18.8 21.7 20.0 5.5 11.8 15.9 20.6 21.0 24.8 26.5 27.1 27.0 27.2 28.3 26.9
CO2"* (%)
14.15 13.49 22.55 10.29 21.90 17.38 15.45 20.47
0.0653 ND 0.0170 3.143 4.074 6.839 5.536 8.288 ND 0.0688 0.0227 0.1740 1.634 2.381 6.828 12.68 15.25 14.13 5.713 6.827 9.320 14.14 14.89 18.58 19.64 19.61 20.39 20.40 21.17 21.18
Ca (%)
3840 3360 6340 5950 36{}0 5850 2860 7190
156 93.0 149 1600 1620 2200 1940 1870 1150 780 116 185 1780 2070 1620 1830 1890 2040 769 892 1370 1590 1860 2110 2110 2240 2100 2010 2080 2200
Mg
Carbonate-bound fraction
1420 1270 2830 833 2190 1380 1100 966
20.1 10.9 13.4 513 826 1160 947 1210 7.9 35.7 ND 21.5 234 210 627 833 959 1110 451 513 689 925 971 1160 1160 1170 1230 1210 1240 1230
Sr
1890 1800 2390 2000 3040 1770 1460 494
239 209 170 197 378 1090 lllO 1880 2230 1660 248 13(}0 988 4110 2210 2080 2840 1660 705 682 780 806 807 940 807 808 772 760 762 785
Mn
2.1 0,5 5.9 ND 2.7 2.7 2.1 7.4
3.6 ND 2.8 3.5 4.9 4.9 2.8 2.8 ND ND 0.7 ND 0.7 ND 3.5 5.6 5.6 3.5 2.8 4.2 5.6 6,2 8.3 9.7 8.3 8.3 9.0 8.3 8.3 9.0
Fe
2.13 2.26 1.12 1.87 1.14 1.49 1.62 0.106
2.68 2.66 2.66 2.58 2.62 2.56 2.62 2.43 2.64 2.62 2.64 2.64 2.58 2.56 2.54 2.25 1.94 2.19 2.56 2.39 2.29 2.11 2.07 1.51 1.41 1.47 1.27 1,25 1.13 1.16
N a (%)
1350 1380 969 1500 986 1210 1350 915
2630 2290 2260 1950 1990 1950 2090 1920 2270 2290 2830 2560 2270 2020 1870 1620 1180 1560 2020 1870 1700 1450 1490 1290 1220 1180 1160 1180 1140 1160
K
833 1320 741 1130 778 889 954 343
3810 3760 3560 2220 2210 2100 2090 1930 3040 3090 3360 3240 2560 2210 2140 1540 1310 1410 1930 1670 1360 1220 1280 893 918 905 874 905 757 796
Mg
911 1020 689 884 717 800 772 2720
1520 1330 1200 1830 1880 1680 1880 1750 1420 1660 823 1200 1540 1860 1790 1570 1400 1570 1990 1790 2360 1500 1280 830 880 1160 1040 869 813 757
Ca
Li C s - e t h a n o l - l e a c h a b l e f r a c t i o n
Concentrations are expressed as dry weight and unless stated otherwise, as parts per million. ND (not detected): < 10 ppm for Ca; < 0.5 ppm for other elements. AA-leachable Cr concentrations are all ND ( < 0.5 ppm). *Combustible organic matter at 550°C. **CO2 evolved from carbonates including crystalline water. ***Dark band in alternating lamination. ****Sieved sand fraction.
96 100-102 108 234 265 297 318 940-942****
Sand beds
KH86-1 P-2 0 2 10 12 20-22 40 42 48-50 60-62 70 72 90-92 107 109"** 120 122 140-142 200 202 230-232 250-252 270-272 290-292 310 312 330 332 340-342 350-352 360-362 390-392 400-402 410-412 500-502 600 602 700 702 800-802 900 902 1000 1002
(cm)
Intervals
Chemical leaching results for KH86-1 P-2 sediments from the Palau Trench floor
TABLE 1
13.8 17.5 12.5 15.0 15.0 13.8 16.3 28.1
14.3 12.9 11.6 11.6 14.3 15.7 17.1 14.3 17.1 14.3 18.5 28.5 23.5 2L0 28.5 23.5 16.6 23.5 26.0 23.5 16.6 23.5 13.5 15.2 11.8 15.2 15.2 11.8 15.2 13.5
Sr
23.9 29.2 19.3 23.9 22.7 24.4 18.2 43.4
5.5 9.3 8.1 ND ND 4.2 11.9 20,3 29.6 20,3 39.7 48.0 17.0 10.0 14.9 21.1 28.4 11.9 11.3 9.3 8.7 6.8 6.1 6.1 5.5 4.2 4.0 4.0 2.6 4.0
Mn
0.5 2.7 0.5 2.1 0.5 1.1 0.5 ND
2.0 2.7 1.4 0,7 0.7 0~7 1.4 1A 2.0 1.4 0.7 1.4 0.7 1.4 0.7 ND ND 0.7 ND ND ND 0.7 ND ND ND Nil 0.7 Nll ND 0.7
Fe
6.9 6.9 5.6 25.7 6.9 5.6 16.0 44.4
38.0 32,6 33.7 23.3 24.3 12.5 10.2 7.0 546 146 2.3 3.9 3.2 133 12.5 6.3 8.6 15.1 10.0 8.3 10.8 19.8 13.3 19.0 9.2 10.8 9.7 9.7 10.5 8.1
Cu
5.0 5.8 6.7 5.0 5.8 5.0 3.3 5.0
5.2 4.3 4.3 4.3 5.2 3.4 1.7 3.4 215 55.1 2.6 0.9 1.7 21.6 3.6 3.6 3.6 5.4 2.7 2.7 3.6 3.6 5.4 5.4 5.4 5.4 5.4 5.4 5.4 5.4
Ni
ND 1.3 1.9 1.3 3.6 0.8 0.9 7.4
3.5 2.3 2.1 2.9 1.5 1.3 1.3 0.6 5.1 2.8 ND 0.3 ND 2.8 1.7 2.8 1.5 1.1 ND ND 1.1 1.5 2.6 2.6 1.4 1.6 2.4 3.7 3.1 1.2
Zn
A A leachable fraction
4.2 4.2 7.3 2.1 5.2 4.2 4.2 4.2
ND ND ND 1.8 1.8 2.7 1.8 2.7 ND Nil ND ND ND ND 3.6 4.5 5.4 2.7 ND ND 3.6 3.6 5.4 2.7 5.4 5.4 4.5 3.6 5.4 5.4
Co
3.8 1.9 5.8 3.8 5.8 5.8 5.8 19.2
ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND 5.8 5.8 3.8 3.8
Pb
223 CO 2 (%) i0
20
30
Ca (%) 40
0
i0
20
Mg (PPm) 30
0
2000
Mn (ppm)
SF (ppm) 0
0
i000 I
2000
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o
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Fig.4. Stratigraphic fluctuations of carbonates and carbonate-bound elements in bulk dry-weight sediments. Note the weak increase with depth and the reduced rate of increase at horizons between 390 and 1088.5 cm.
does not indicate any apparent sedimentary structure. The X-radiographs of this unit show possible weak bedding planes which are also conspicuously inclined (Figs.7-9). There remains a possibility that the unit between 390 and 1088.5 cm is part of a slump fold, although the identification of folded structure is difficult in the case of homogeneous lithology, such as the marly chalk in this unit. There are two other carbonate units (the units are termed A, B, and C as shown in Fig.3) which are interbedded in hemipelagic-pelagic
brown clay above the massive marly chalk of unit C. Unit A is 47 cm thick (between 64 and 111 cm), and unit B is 90 cm thick (between 250 and 340 cm). Several 5-20 mm thick laminae of fine to medium calcareous sand, accompanied by graded beds, form each unit (Fig.3). The "background" hemipelagic-pelagic brown clay also shows turbiditic fabrics. The parallel laminations of dark and brown alternating bands just below carbonate unit A indicate redox cycles, and anomalous enrichments of heavy metals such as Cu and Ni accompany
224 Na (%) I
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Fig.5. Stratigraphic fluctuations of Li-Cs-ethanol-leachable elements in bulk dry-weight sediments. Note decrease with depth in horizons between 390 and 1088.5 cm. this l a m i n a t i o n (Table 1). An e n r i c h m e n t zone of c a r b o n a t e - b o u n d M n is o b s e r v e d b e t w e e n 60 and 3 3 2 c m (Table 1; Fig.4). This zone m a y r e p r e s e n t a r e d u c i n g e n v i r o n m e n t in the t r e n c h basin w h e r e the b r o w n clay is deposited, and t h e p o s t d e p o s i t i o n a l diffusion of M n from t h e b r o w n clay into the c a r b o n a t e u n i t s A and B in an u p w a r d and d o w n w a r d d i r e c t i o n respectively. T h e g r a d e d c a l c a r e o u s sands of units A and B commonly occur above sharp contacts, w h i c h m a y imply some e r o s i o n a l processes. S e v e r a l m i l l i m e t e r - t h i c k l a m i n a t i o n s of fine to m e d i u m sand are e q u i v a l e n t to B o u m a divi-
sions B or D (Bouma, 1962). N o r m a l l y , several millimeters to a few c e n t i m e t e r s of pelite divisions of the B o u m a E division are deposited above the l a m i n a t e d sand of the B o u m a B or D division. T h e pelite s e q u e n c e s show an u p w a r d d e c r e a s e of c a r b o n a t e c o n t e n t s (Figs.3 and 4), in a d d i t i o n to the u p w a r d fining of g r a i n size. The f o r a m i n i f e r a l sand l a m i n a in the b r o w n clay (lamina 4 in the lithologic c o l u m n of Figo3) m a y n o t be i n t e r p r e t e d as p a r t of the B o u m a sequence, b u t it can be e x p l a i n e d as a deposit from suspension. T h e age of t h e b o t t o m of the r e c o v e r e d core s e q u e n c e is P l e i s t o c e n e and possibly y o u n g e r
225 Li-Cs-ethanol leacIlable
5
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COM (%) 4 5 6
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Na (%)
25
0,8
1.0
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Mg (PPm)
I000 500
600
Ca {PlDm) 150
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I L J
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Fig.6. Stratigraphic fluctuations of carbonates and Li-Cs-ethanol-leachable elements for two flow-in sections of homogeneous sediments from the Ryukyu Trench slope off Ishigaki Island. Water depths and locations for these two sites are as follows: RN87 P-3: 24°07.5'N, 124°24.1'E; water depth (corrected)= 1665 m. RN87 P-4: 23°50.2'N, 124°24.1'E; water depth (corrected) = 2513 m.
than 0.69 Ma B.P., as judged from the abundances of right-coiling P u l l e n i a t i n a spp. (Saito, 1976) in the core column (Table 2). Some index planktonic Foraminifera of the Middle Pliocene are contained in some horizons; however, they are judged to be reworked material. Benthic foraminifera species derived from shallow coral-reef lagoons are contained in various parts of the turbidite sequences (Table 2). The diatomaceous bed just above the marly chalk bed is composed of abundant broken frustules of the huge diatom, E t h m o d i s -
cus rex (Rattray) Hendey. They often occur as
monospecific diatomaceous deposits of a few tens of centimeters in thickness deposited during the last 400,000 yrs (Tanimura, 1981). M i n e r a l o g i c a l characteristics
The carbonate-bound C a - M g and Ca-Sr relationships (Fig.10) indicate compositional differences of carbonate mineralogy among the three units of the carbonate turbidite sequence. Carbonate minerals of unit C are
X
X
X
X
X
X
4.8 5.7
X
X
X
X
X
12.9 7.0
X
X
X
0.0 10.0
340 342
X
X
X
X
12.2 9.4
X
1000/o
x?
410412
X
X
X
X
X
18.6 8.8
X
0.0 2.8
100%
100%
X
X
1010 1012
X
480 482
42.9 3.6
x?
1040 1042
All samples contain the large diatom Ethmodiscus rex which ranges from a b u n d a n t to present, radiolarians occur frequently and sponge spicules occasionally. x - - present.
X
X
X X
Alcyonarian sclerites Gastropod shells Branching bryozoan fragments Echinoid spines
X
17.6 13.8
Halimeda
Biogenic fragments derived from reefal shallow waters
11.4 10.5
23.5 6.2
Ratio of shallow-water species to total benthic species Ratio of benthic species to total foraminiferal species
X X
X
X
X
X
Others
X
X
100°/'o
100%
100%
100%
X
X
X
330 332
x
X
320 322
X
100%
310 312
X
x
X
290 292
x?
X
x
240 242
Core interval (cm)
Amphistegina lessonii Amphistegina radiata Amphisorus hemprichii Baculogypsina spengleri Calcarina hispida Cymbaloporetta squammosa Elphidium jenseni
Reefal shallow water benthic foraminiferal species
Index planktonic foraminiferal taxa Reddish Globigerina rubescens (above N23 base) Hastigerinella adamsii (above N23 base) Sphaei'oidinella dehiscens excavata (above N23 base) Pulleniatina finalis (above N22 base; 1.9 Ma) Globorotalia truncatulinoides (above N22 base) Globorotalia tosaensis (above N21 base; 3.0 Ma) Globoquadrina spp. (below PL5 base ?) Globigerina nepenthes (below PL2 base) Right-coiling Pulleniatina spp.
Species/occurrence
Occurrence of datable planktonic Foraminifera and reefal benthic Foraminifera
TABLE 2
227 6.5
cm
6.5 I
441
cm
641
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f
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459 cm
f
659 cm
Fig.7. X-radiograph of homogeneous section of the marly chalk bed. Some inclined bedding planes are recognizable. Possible layering is shown with dashed lines.
Fig.8. X-radiograph showingreversal of the bedding plane inclination compared to Fig.7.
composed mainly of low-Mg calcite with slight enrichments of Mg and very small amounts of aragonite, as indicated by XRD in Fig.ll. Carbonate minerals of units A and B are essentially composed of high-Mg calcite, highSr and low-Sr aragonites, and low-Mg calcite (Figs.10 and 11). The carbonate-bound Ca-Sr relationship suggests t h a t unit A contains slightly higher concentrations of high-Sr aragonite t h a n unit B. This tendency is indicated further by the point-count data on the sand compositions (Table 3); the sand grains in unit A are slightly higher regarding the concentra-
tion of alcyonarian sclerites, bryozoans and Halimeda, but lower in the concentration of planktonic Foraminifera than unit B. No heavy leaching of Mg or Sr from sands composed of high-Mg calcite and high-Sr aragonite is evident from the carbonate-bound C a - M g and Ca-Sr relationships. In contrast, Gomberg and Bonatti (1970) have observed dissolution evidence and subsequent leaching of Mg and Sr in a 143 cm thick turbidite of coarse calcareous sand on the 4230 m deep slope off the Tuamotsu Archipelago in the South Pacific. The only leaching evidence from
228 6 cm
900
I
1
and in the pelite sequences of units A and B at contacts with the background brown clay (Fig.10). Generally, the turbiditic sands deposited at the Palau Trench well below the CCD are thought to be free of dissolution because the sands were transported and buried very quickly by turbidity currents and the turbiditic sands were not exposed in the deep sea for a very long period. Episodic sedimentation of carbonate turbidite
920 cm
Fig.9. X-radiograph showing obvious horizontal contact (arrow) between beds.
the Palau Trench carbonates is observed for the carbonate-bound Sr in unit A (Fig.10). The leaching of Sr occurs in the Botuna D sand laminae of the unit A turbidite sequence. This kind of leaching of Sr without the association of Mg can be interpreted as a stability relationship of carbonate minerals where high-Sr aragonite is more unstable t h a n high-Mg calcite (Friedman, 1965). The carbonate-bound Mg is enriched in the planktonic Foraminifera
The carbonate sequences of units A, B and C are all considered to be turbidite deposits derived from the adjacent carbonate platforms and slopes. The sandy carbonate turbidites of units A and B could be transported by turbidity currents from the shallow coral-reef lagoons or aprons developed in the off-bank carbonate slope (Davies, 1968; Crevello and Schlager, 1980; Ujii~ et al., 1983; Yamamoto and Ujii~, 1983; Mullins et al., 1984; Ujii~ and Nagano, 1984; Hoskins et al., 1986; Midorikawa and Ujii6, 1987). The marly chalk turbidite of unit C could have been derived from the periplatform ooze (Mullins et al., 1984), which may be distributed widely on the carbonate slope off Palau Island in an approximate depth range of 2-4km. The periplatform calcareous ooze could be supplied from the ridges and slopes shallower than 4 km which are developed on both the seaward and landward slopes facing Teardrop Flat in the Palau Trench (Kato et al., 1985). Carbonate turbidites have been reported from various types of modern carbonate slopes (Davies, 1968; Gomberg and Bonatti, 1970; Bornhold and Pilkey, 1971; Huang and Pierce, 1971; Bennetts and Pilkey, 1976; Crevello and Schlager, 1980; Mullins et al., 1984) as well as in ancient depositional environments (Carozzi and Frost, 1966; Tucker, 1969; Scholle, 1971a, b; Hobson et al., 1985). A limited number of reports have described carbonate turbidites deposited on the abyssal seafloor well below the CCD and have discussed their significance. Huang and Pierce (1971) have reported several
7
10.0 2.5 ND ND ND 0.8 3.3 9.2 ND 27.5 13.3 0.8 14.2 ND ND 3.3 13.3 1.7 99.9
96
Interval (cm)
All values are expressed as volume percent. ND (not determined) : <0.1%. *Sieved sand fraction.
Numbers indicated in parentheses in the stratigraphic column of Fig.3
Quartz and feldspars Opaque heavy m i n e r a l s Lithic fragments Total
E t h m o d i s c u s rex
Coralline algae Bryozoans Echinoid (spines) Mollusca Decapoda Planktonic Foraminifera Benthic Foraminifera Microcrystalline clump Sponge spicules Radiolarians
Halimeda
Corals Alcyonarian sclerites Calcareous ascidian spicules
Sand grains
6
15.1 5.0 2.2 0.7 0.7 5.0 3.6 3.6 ND 23.0 6.5 7.2 15.8 ND ND 3.6 6.5 1.4 99.9
100 102
Petrographic point-count data for sand beds, KH86-1 P-2
TABLE 3
4
18.2 ND 1.6 ND ND 2.1 2.1 9.0 ND 31.9 6.9 1.1 8.0 0.5 ND 5.3 11.7 1.1 99.5
23.3 2.2 1.6 0.3 3.5 2.8 0.6 15.4 ND 11.7 4~1 0.3 3.5 1.0 ND 7.9 15.8 6.0 100.0
5
234
108
3
25.2 ND 0.7 ND 2.0 1.0 0.7 19.6 ND 18.0 3.3 ND 5.2 2.0 ND 2.9 7.2 12.4 100.2
265
2
14.3 1.2 1.2 ND 1.7 0.3 ND 4.0 0.9 32.1 6.6 1.2 0.6 ND ND 7.5 16.3 12.3 1~.2
297
1
17.0 1.4 0.4 ND 3.1 1.4 1.7 2.8 0.7 24.7 3.8 1.0 2.8 ND ND 12.2 23.6 3.5 100.1
318
ND ND ND ND ND ND ND ND ND 39.4 5.6 ND 17.4 14.6 20.2 0.9 ND 1.9 1~.0
930 932*
230 Legend
% 0.8-
a
Unit A
mud
•
Unit A sand
A
Unit B mud
•
Unit B sand
0
Unit C
•
Background clay unit
II~
Sieved sand fraction
Mg Sr
940-942
% • 234
0.6
A 297
/
0.3-
j~ ,'
~08
/
•
/
/2 ,
/
.S~//
o.,
0.2-
c~ching ~?>
, / ' a318
//
60-62
/~ /_/
~
-
/
/
C
40,-5o'- ~
~o/~
/
/9~ . ' / /
./~°//~,:<
,,_,(o,o-,Y,~,/ ,~j~-""% ,,og,,
.~ / / A ~y)90-392 / 270-272w----tg°-29~ 40;4~/ ~-Y-360-362
.o,/
u
o<,I0ta0-202 "°'" 0
/ / I_
~o ~33%23'.,,~" <;'o (oo, O.1-
'
y/
,,/////B
~]00-102
"k/~o350 C 2
0-22 1107_202 ] "0
210
Ca
I
3o
%
0
I
~o
I
Ca
20
I
30
%
Fig.10. Carbonate-boundCa-Mg and Ca Sr relationships. Territories are encircled accordingto samples in carbonate units A, B and C. All core intervals (cm) are given.
graded sandy carbonate turbidites interbedded in the pelagic clay on the 4770 m deep abyssal floor off the Bahama Islands. Carbonate turbidites deposited on the abyssal trench floor far below the CCD (Berger and Winterer, 1974) are probably restricted only to the Puerto Rico Trench (Ericson et al., 1952) and to the Palau Trench (Fujioka et al., 1986). Some episodic cycles of graded sand beds are comparable between the Puerto Rico and Palau Trench floor cores, and the cycles are assumed to have occurred episodically in the pelagic-hemipelagic brown clay deposited in the abyssal basin. Estimating the sedimentation rate of the background brown clay to be about 1 cm/1000 yrs, the sandy turbiditic sequences of units A and B are assumed to have been deposited once every 60,000 yrs or so
on the Puerto Rico and Palau Trench floors. The thick homogeneous chalky bed, although the base of which was not recovered by coring, is a unique stratigraphic assemblage in the Palau Trench carbonate turbidites. However, similar massive and homogeneous chalk beds of turbiditic origin have been reported from the Nauru Basin in 5191 m of water. The Shipboard Scientific Party (1981) concluded that the episodic cycles of turbidity currents had transported and deposited the massive beds of chalky ooze from the carbonate platform of the Ontong J a v a Plateau 400 km away. Although more comparative stratigraphic analyses are required to clarify the mechanism for initiations of episodes of mass transport, several combined mechanisms may be appropriate. One is sea-level fluctuation due to
231 35 KV 10 mA Cu-Ka Radiation Slits: Div. I- Scat. 2" Rec.
Ni filter Scan speed I~(2~))/min. 0.15 i11m Time const, i sec.
KHB~-]
P-2
Lc 50-62 cm (i;lu d )
O I
C
H
Pl A
A
O
I
ii mc
j~ Ii
108 cm
I
I
( sand )
I
i~ I
I
:1
j
250-252
or/
I
(n/ud/
II II li
318
il
{sand)
I
cm
ii I
[
Jl
I
Ji iI rl
I
I
I
|
40
J
f
I
J
|
I
I
J
I [ I
I
J
f
I
l
700-702 cm (mud)
r
J
l
F
30
I
I
J
I
I
20 DEGREES
I
P
J
I
I
[
r
J
I
I
I
I
I
i
i
10
[2el
F i g . l l . XRD diffractograms of r e p r e s e n t a t i v e c a r b o n a t e turbidites. L c - - low-Mg calcite; H c - - h i g h - M g calcite; C - calcite; A - - aragonite; Q - - quartz; P l - - plagioclase; H - halite; O l - - olivine(?); R h - - rhodochrosite; C h l - - chlorite; K a o l - kaolinite; M - smectite; ? - - unidentified.
either eustatic or tectonic movements. The fluctuation of sea level in the carbonate platforms alters the sedimentary environment by causing progradation of coarse sediments seawards (Schlager, 1981; Read, 1985). Sealevel changes during the Pleistocene-Holocene periods have been known for the carbonate-reef islands of the Western Pacific region (Konishi et al., 1974). Other possible mechan-
isms include catastrophic events, including meteorologic (mainly typhoons) and tectonic (earthquake) catastrophes. Summary The l l m long core recovered from the 8053 m deep floor of the Palau Trench includes three major carbonate turbidite sequences
232 interbedded with a background brown clay. Carbonate sands of turbiditic origin do not show evidence of strong dissolution, except for a few l a m i n a e o f t h e B o u m a D d i v i s i o n , although they were transported and deposited to the abyssal basin far below the CCD. The homogeneous bed of marly chalk, thicker than 7 m, is a l s o i n t e r p r e t e d a s a t u r b i d i t e , a n d i t may have been transported to the abyssal basin as a massive bed without experiencing dissolut i o n . T h i s t y p e o f d e p o s i t i o n is c o m p a r a b l e t o the extraordinarily thick micritic limestone of turbiditic origin of the Monte Antola Flysch in the Northern Apennines, as interpreted by S c h o l l e (1971a). L a c k o f i n t e n s e d i s s o l u t i o n i n both the carbonate-sand and carbonate-mud turbidites suggests fast transport by mass m o v e m e n t s o n t o t h e a b y s s a l t r e n c h floor, w e l l below the CCD. Not all carbonate turbidites deposited in the abyssal basin below the CCD are devoid of dissolution effects, the latter depending on the amount of ~ponded" bed and t h e m e c h a n i s m c a u s i n g t h e t u r b i d i t e flow.
Acknowledgements W e a r e g r a t e f u l t o C a p t a i n I. T a d a m a , C h i e f E n g i n e e r Y. M a t s u m o t o a n d c r e w m e m b e r s o f t h e R.V. Hakuho Maru, a n d to P r o f . Y. T o m o d a and scientific members of cruise KH86-1 for their cooperation during sampling operations. The two flow-in cores from the Ryukyu Trench slope were sampled during the joint educational cruise of the University of the Ryukyus a n d N a g a s a k i U n i v e r s i t y o n t h e R.V. Nagasaki Maru. M e s s r s . K. K o g a a n d S. K u r a m o t o a r e acknowledged for their assistance during photographing production of the X-radiographs of the piston core. The manuscript was critically reviewed by Profs. G.M. Friedman, D.F. Merr i a m , J . W . H a r b a u g h , J . D . M i l l i m a n a n d H. Kagami.
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233 Konishi, K., Omura, A. and Nakamichi, O., 1974. Radiometric coral ages and sea level records from the Late Quaternary reef complexes of the Ryukyu Islands. Proc. Int. Coral Reef Symp., 2nd. Great Barrier Reef Comm., Brisbane, pp.595-613. Midorikawa, Y. and Ujii~, H., 1987. Bottom sediments of the Sekisei Lagoon, the Yaeyama Islands. The Earth Mon., 9:152 158 (in Japanese). Mullins, H.T., Heath, K.C., Van Buren, H.M. and Newton, C.R., 1984. Anatomy of a modern open-ocean carbonate slope: northern Little Bahama Bank. Sedimentology, 31: 141 168. Read, J.F., 1985. Carbonate platform facies and models. Bull. Am. Assoc. Pet. Geol., 69: 1-21. Saito, T., 1976. Geologic significance of coiling direction in the planktonic foraminifera P u l l e n i a t i n a . Geology, 4: 305 309. Schlager, W., 1981. The paradox of drowned reefs and carbonate platforms. Geol. Soc. Am. Bull., 92: 197-211. Scholle, P.A., 1971a. Sedimentology of fine-grained deep water carbonate turbidites, Monte Antola flysch (Upper Cretaceous), northern Apennines, Italy. Geol. Soc. Am. Bull., 82:629 658. Scholle, P.A., 1971b. Diagenesis of deep-water carbonate turbidites, Upper Cretaceous Monte Antola flysch, northern Apennines, Italy. J. Sediment. Petrol., 41: 233 250.
Shipboard Scientific Party, 1981. Site 462: Nauru Basin, western Pacific Ocean. In: R.L. Larson, S. Schlanger et al., Initial Reports of the Deep Sea Drilling Project, Leg 61, Vol. 61. U.S. Gov. Print. Off., Washington, D.C., pp.19-177. Tanimura, Y., 1981. Late Quaternary marine diatom E t h m o d i s c u s rex from the northwestern Pacific Ocean. Bull. Natl. Sci. Mus., Tokyo, Ser. C, 7:91 96. Tokuyama, H., Asanuma, T., Nishiyama, E., Hatori, H., Chiba, H., Ueno, S. and Tomita, N., 1985. Multichannel seismic reflection survey. In: K. Kobayashi (Editor), Preliminary Report of the Hakuho Maru Cruise KH84-1. Ocean Res. Inst., Univ. Tokyo, Tokyo, pp.282 291. Tucker, M.E., 1969. Crinoidal turbidites from the Devonian of Cornwall and their palaeogeographic significance. Sedimentology, 13:263 280. Ujii~, H. and Nagano, K., 1984. Sediments of Haneji Inlet and its environs, Okinawa, subtropical Japan. Galaxea, 3: 81-95. Ujii~, H., Yamamoto, S., Okitsu, M. and Nagano, K., 1983. Sedimentological aspects of Nakagusuku Bay, Okinawa, subtropical Japan. Galaxea, 2: 95-117. Yamamoto, S., in press. Fractionation for carbonate-bound metals in deep-sea sediments from the Japan Trench basin. Sedimentology. Yamamoto, S. and Ujii~, H., 1983. Calcareous sediment around coast of the Okinawa-jima Island. News Osaka Micropaleontol., 11:48 62.