Seymour O. Schlanger, Department of Geological Sciences, Northwestern University and Ralph Moberly, Hawaii Institute of Geophysics2


The sedimentary section recovered during Leg 89 at Deep Drilling Project Site 525 provided a record of volcan- ism, plate motion, and other events of the Cretaceous and Cenozoic history of the East Mariana Basin. This part of the Pacific lithosphere had formed in the Jurassic; by late Aptian time it was at about 22°S and had subsided to a depth of about 5000 m. In the Aptian and Albian, eruptions built major volcanic edifices nearby that were the sources of mas- sive volcaniclastic debris flows and turbidites found at Site 525. The now deeply altered sediment originally was of -normative tholeiitic composition, with some later alkalic basalt contributions. Reefal debris, ooids, and displaced benthic foraminifers within the hyaloclastic and other volcanogenic components show that some of the volca- noes reached shallow water. They are now the seamounts and guyots north and south of the eastern end of the East Mariana Basin. Recovered cores show evidence of the Cenomanian-Turonian oceanic anoxic event in debris rich in or- ganic carbon redeposited in turbidites, of passage under the equator in Campanian-Maestrichtian time from core Pa- leomagnetism, and of the Oligocene (P21-P22) fall of sea level that exposed seamounts and thereby caused renewed tur- bidite activity. The Nauru Basin igneous complex discovered during Leg 61 yielded cores of 12 additional volcanic units as a result of the deepening of Hole 462A during Leg 89. The mineralogical and geochemical homogeneity within these (and over- lying) sheet flows, as well as their little-altered nature and small amount of interlayered sediment, indicate that the great volume of midplate magma was generated and erupted rapidly. The lowest flows drilled during Leg 89 probably are 130 Ma (Valanginian). They lie on M26-aged ridge-crest crust (about 145 Ma, Oxfordian), and underlie volcanogenic sediment and sills representing at least two other episodes of midplate igneous activity, namely in the Aptian and Albian (about 100 Ma) and in the Campanian (about 75 Ma).

INTRODUCTION flow complex found there. A third objective was to oc- cupy Site 289, drilled on Leg 30, and by use of the rela- The major drilling target for Leg 89 was the section tively new hydraulic piston core obtain undisturbed sam- of Cretaceous and Late Jurassic pelagic sediments pre- ples of the pure carbonate pelagic sequence there to sumed to overlie the Jurassic-age lithospheric basalt plate depths of approximately 300 m. This drilling effort was in the western Pacific. The JOIDES Paleoenvi- assigned to Leg 89 in order to shorten the transit time ronment Panel had as one of its concerns during the for Leg 90 whose primary objective was to drill a north- decade of the 1970s recovery of cores of Mesozoic pe- south transect from the Ontong-Java Plateau (Site 289) lagic sediments in order to decipher the paleoceanogra- to the latitude of New Zealand as part of the JOIDES phy of the old Pacific superocean. Previous drilling in planning for Neogene paleoceanographic objectives. Re- the area had revealed the presence of widespread and in- sults of this drilling are reported in both this volume and tensive Cretaceous volcanism; products of this volcanic in the Leg 90 volume. activity in the form of widespread basalt flows, hyalo- To these ends Glomar Challenger left Yokohama, Ja- clastite sheets, and large edifices had either covered or pan, shortly after midnight on 11 October 1982 and ar- destroyed the Early Cretaceous and Late Jurassic open- rived at Noumea, New Caledonia, on the evening of 29 ocean pelagic record over wide areas of the western Pa- November. cific. Following the discovery, on Leg 61, of thick basalt Despite the fact that the Jurassic sedimentary objec- flows of Early Cretaceous age in the Nauru Basin that tives were not reached, Leg 89 did add to our knowledge thwarted efforts to penetrate the pelagic sediments thought of the timing and areal extent of the mid-Cretaceous vol- to overlie the deeply buried lithospheric plate, new site canic events that have left their imprint over much of surveys were commissioned to explore for drill sites in the western Pacific. the Mariana Basin. Thus the basic plan for Leg 89 was This chapter summarizes the sedimentary and volcanic to drill a multiple reentry hole in the East Mariana Ba- history revealed by drilling in the East Mariana Basin at sin to Jurassic sediments and, it was hoped, into the Site 585 (MZP-6 of the Leg prospectus); new data from ~ 145-Ma plate. A secondary objective was to deepen the reoccupation of Site 462 (MZP-2 of the Leg prospec- Site 462 drilled on Leg 61 in the hope of penetrating the tus) is included as it is relevant to that history. For addi- tional details on results from the drilling on the Ontong- Java Plateau at Site 586 (SWP-9 of the Leg prospectus), 1 Moberly, R., Schlanger, S. O., et al., Init. Repts. DSDP, 89: Washington (U.S. Govt. Printing Office). readers are referred to the Leg 90 Initial Reports (Ken- ^ Addresses: (Schlanger) Dept, of Geological Sciences, Locy Hall, Northwestern Univer- nett, von der Borch et al., in press). Sites occupied on sity, Evanston, Illinois 60201; (Moberly) Hawaii Institute of Geophysics, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI. Leg 89 are shown in Figure 1.





Site MZP-6 Guam *

10c Site MZP-2 (DSDP 462)

10c 140° 150° 160° 170° 180° Figure 1. Magnetic lineations in the western Pacific in relation to proposed Leg 89 drilling Sites MZP-6 (Site 585), MZP-2 (a reentry at DSDP Site 462, Leg 61), and SWP-9 (a reoccupation and HPC drilling at DSDP Site 289, Leg 30).

EAST MARIANA BASIN 199 and 200, 201, and 202 were drilled (Fig. 2); 200 and 201 are in close proximity to 202. At Site 199 566 m Site 585 (MZP-6) is located near the eastern end of of section was penetrated and spot-cored to a depth of the Mariana Basin, an elongate deep, surrounded in the 457 m. The upper 209 m consisted of early Pliocene to east by submarine volcanic edifices of Cretaceous age middle Miocene turbidites made up of nannofossil ooze, (Fig. 2). The water depth in the Basin of -6100 m is in brown silty zeolitic clay, and radiolarian ooze; this sec- accord with the presumed plate age of Jurassic accord- tion was interpreted (Heezen, MacGregor, et al., 1973) ing to accepted subsidence curves for cooling lithosphere. as indicating that the Caroline Abyssal Plain existed as The penetration of a thick sequence of Early Cretaceous early as the middle Miocene. Below this section, to a volcanic flows in the Nauru Basin at Site 462 on Leg 89 depth of 447 m, late Paleocene to early Maestrichtian appeared to confirm the argument that the anomalously nannofossil chalk, chert, and siliceous limestone were shallow depth of the Nauru Basin could be ascribed to a encountered; in the early Maestrichtian strata tuffs were Cretaceous thermal regeneration and uplift of the Nau- common. The deepest core, at 447 to 457 m, consisted ru Basin that resulted in a resetting of the subsidence of brown zeolitic made up almost exclusively of glass path (Larson and Schlanger, 1981). It was thought that shards. Heezen, MacGregor, et al. (1973) interpreted this because the Mariana Basin was at its predicted depth tuff as a record of the final eruption of the volcanoes the influence of Cretaceous thermal resetting would be that formed the guyots that now lie along the eastern lacking there. margin of the Caroline Abyssal Plain. Since the drilling Previous drilling in the Mariana Basin took place on of Leg 20, this Late Cretaceous volcanism has been re- Leg 20 (see Heezen, MacGregor, et al., 1973) when Sites corded over large areas of the western Pacific (Schlanger









154° 155° 156° 157° 158° 159° 160° Figure 2. Bathymetric chart of the East Mariana Basin and surrounding seamounts showing the location of MZP-6 (Site 585, Leg 89). Drilling on Leg 20 (Heezen, MacGregor, et al., 1973) revealed Late Creta- ceous pelagic sediments at Site 199 and shallow-water carbonate sediments atop Ita Maitai Guyot at Site 282, which was presumed to be a Cretaceous volcanic edifice (bathymetry in fathoms after Chase et al., 1970). and Premoli Silva, 1981). Sites 200, 201, and 202 of Leg (Larson and Schlanger, 1981) and Late Cretaceous vol- 20 were drilled atop Ita Maitai Guyot (Fig. 4). At Sites canism was recorded in the East Mariana Basin on Leg 200 and 201 the drilling penetrated the acoustically de- 20 (Heezen, MacGregor, et al., 1973), the depth of the termined pelagic cap of early Eocene to Quaternary Glo- Mariana Basin suggested that intensive Early Cretaceous bigerina ooze; a few basalt fragments were found in the volcanism had not occurred there. Therefore the East ooze. At Site 202, however, tan oolitic limestone was en- Mariana Basin was chosen for a site survey as shown in countered at 100 m sub-bottom depth. Below the oolitic Figure 3 (see Shipley et al., 1983; Petersen et al., this limestone were lime muds containing bryozoans and gas- volume). tropods. The oolitic limestones and lime mud were in- Mapped magnetic lineations of the Mesozoic M-series terpreted as pre-early Eocene shallow-water materials de- (Fig. 3) indicated that MZP-6 was on old Pacific litho- posited atop a Late Cretaceous volcanic edifice. sphere that formed ~ 145 Ma. The survey track crossed Site 199 on a course parallel to the east-west axis of the Site Survey and Selection Mariana Basin (Fig. 4). Seismic profiles (Figs. 5 and 6) Although volcanism of both Early and Late Creta- showed a thick sedimentary section along the axis of the ceous age was revealed in the Nauru Basin on Leg 61 basin, and Site MZP-6 was chosen in the thickest part of







Palau Truk ' Ponape

10c 130° 140° 150° 160° 170° 180° Figure 3. Site survey track of Rana Keoki KK810626 Leg 4 cruise. Large arrows point to the location of the presumably oldest crust in the Pacific basin (see Peter sen et al., this volume). the section. Interpretation of water gun profiles by the final decision by the operations people to drill the site as site survey team (Shipley et al., 1983; Petersen et al., this a single bit hole. Hole 585 was terminated at 763.7 m volume) are shown in Figure 6. From the seafloor to 590 T.D. due to a stuck misfit core barrel sub; a second at- m at 8.74 s two-way traveltime, the section at MZP-6 tempt ended at 892.8 m upon total bit failure. The sec- was taken to be equivalent to the section drilled at Site tion drilled is summarized on Figure 7. As predicted by 199. Between 590 and 850 m, that is, 8.74 to 9.0 s two- the site survey, the section down to approximately 490 m way traveltime, the section at MZP-6 was predicted to be duplicated the section at Site 199. Between 490 and 590 m made up of biogenic pelagic sediments. From 850 to a section of middle Cenomanian through Santonian stra- 1000 m, down to the 9.1-s reflection, limestones depos- ta was drilled. Below 590 m to the total depth of 892.8 ited above the calcite compensation depth (CCD) were m the drilled section is markedly different from the sec- predicted to be lying directly above the Jurassic plate. tion predicted by the site surveyors. Instead of Creta- The section from 850 to 1000 m sub-bottom depth was ceous and Jurassic pelagic carbonates, a thick section of therefore considered the prime target for Leg 89. late Aptian to middle Albian volcanogenic turbidites rich in ooids and shallow-water skeletal debris was found. The postdrilling correlation of seismic stratigraphy and Stratigraphic Section lithology is shown on Figure 8. Two holes were drilled at Site 585. Although the JOIDES Planning Committee had approved the site as a Sedimentology reentry attempt, we were informed by DSDP engineers The sedimentary section recovered at Site 585 was di- the day before departure that reentry would not be at- vided into six lithologic units based on composition and tempted because of the ~6100-m water depth at the site. degree of diagenesis and lithification (Fig. 7). At the site, weather conditions were good but calcula- The highest unit is 6.8 m of lower Pleistocene to Re- tions of dynamic stress on the drill string resulted in the cent brown clay and yellowish brown to brown clay-bear-


ing nannofossil ooze. No cores were taken between 6.8 and 256 m. The next unit recovered is middle Eocene to Maes- trichtian light gray, nannofossil chalk between 256 and 399 m sub-bottom depth. Varying amounts of limestone, silicified limestone, chert, and zeolitic clay indicate in- creased diagenesis with greater depth. The Appendix of the Site 585 report shows the composition changes graph- 14°N ically; they are described by Baltuck (this volume). From 399 to 426 m sub-bottom depth the section is Maestrichtian to upper Campanian dark brown zeolit- ic claystone of variable carbonate content. These beds, many of which show subtle grading, lie on Campanian chert interbedded with brown zeolite-bearing claystone (426-485 m). The silicification, which commonly is re- placement of pore filling of limestone, is complex (Bal- tuck, this volume). From 485 to 590 m sub-bottom depth the section is Campanian to middle Albian claystone with variable amount of zeolite, carbonate, and radiolarians. Special attention was given to description and interpretation of the turbidite sequences (Whitman et al., this volume), secondary silicate (Chamley et al., this volume), and organic geochemistry (Schaefer and Leythaeuser, this 13°N volume). A thick section of volcaniclastic sediments lies below \ DSDP Sites \ \2OO,2O1 590 m, extending at least to the total depth of 893 m \ \ \ \l\ sub-bottom. These middle Albian to upper Aptian sand- 156°E 157°E stones, siltstones, claystones, and breccias were explained Figure 4. Glomαr Challenger Leg 89 track across the East Mariana Ba- as turbidites and debris flows (Whitman et al., this vol- sin. Bathymetry contoured in seconds of two-way traveltime from ume), from volcanic edifices (Floyd, this volume; Vier- Petersen et al. (this volume), Chase et al., (1970), and Heezen, eck et al., this volume), some of which were sufficiently MacGregor, et al. (1973). One second is about 750 m depth; the shallow to provide the mainly hyaloclastic debris with abyssal plain of the Mariana Basin is slightly deeper than 6000 m. ooid and reefal admixtures (Haggerty and Premoli Sil- va, this volume).

MZP-6 West East

Figure 5. Seismic profile along Rana Keoki track shown in Figure 4.

657 oo

GMT 28 September 1981 East West Interpretation Age Velocity Seismic character Lithology 2315 2300 2200 (m/s) (Ma) 8.10- Close but variably spaced, Zeolitic clays, accumulating moderate amplitude, parallel, below the CCD and turbidites high-continuity reflections, composed of allochthonous

_, traceable for 50 to 100 km, f nannofossils and radiolarians tj>asin leveling sequence j derived from the surrounding Closely spaced, moderate and seamounts ^- BP - &•&.'Φ&$m MZP-6 '/ low amplitude, hummocky (?), Upper section: more distal low-continuity reflections, turbidites containing less 8.33- -210- i jonlap on highs, basin leveling/ biogenic material ^sequence Middle section: more proximal turbidites and debris flows Variably spaced, moderate with more biogenic, shallow- and high amplitude, parallel, water material - -330- moderate continuity Some ash layers reflections, onlap on highs, Sediments transitional \ basin leveling sequence between overlying Low to moderate amplitude, turbidites and underly- 8.60- -450- parallel, moderate continuity ing pelagics j— reflections, some onlapping of I basin margin in upper portion,' Biogenic pelagic sedi- meπts, composed of -500 „ <£ 8.67-|-520• more sheet draping in lower section interlayered nannofossil o o! E Uniformly spaced, low to chalk and chert, becom- ' 590- moderate amplitude, parallel, ing more clay-rich in moderate continuity lower two-thirds of sec- reflections, some onlapping tion, occasional ash or near base, some thickening tuff layer and thinning in subunits, mod- Equivalent depth of hole est onlapping on basin margin bottom of Site 199 at but more of a sheet drape unit, 8.67 s some discrete, high-amplitude reflections occur, becoming very high amplitude to the 155- 9.00- -850 iwest / "160" Closely spaced, high ampli- Limestones, perhaps tude, parallel to hummocky, interbedded with minor 3000? moderate continuity volcanics, carbonate reflections, internal geometry accumulated on young indistinct but sequence crust above the CCD 9.10- 1000 appears to be infilling underly- 170- -1000 ing surface irregularities Basalt

16 August 1982 Figure 6. Seismic reflection section in the East Mariana Basin across proposed Site MZP-6 (Site 585). The interpretations of the section in terms of depth to reflector, interval velocities, seismic character, and Hthology were made prior to drilling at Site 585 (from Shipley et al., 1983). See also Whitman (this volume). SEDIMENTARY AND VOLCANIC HISTORY

Cores Lithology »el o •* ö S Description Age Hole Hole Hole Hole 585 585A 585 585A 80 Brown clay, nannofossil ooze Recent— I I I I e. PleistJ 50- 1.50 (1) 100- i Miocene- middle 150- Eocene


250- • ••• Nannofossil chalk, limestone, 1.89 chert, zeolitic claystone m. Eocene (2) Thin ash beds 300- 8 A . A. 9 10 11 e. Eocene 12 350- 13 14 15 16 , I. Paleocene, 17 -§ 400- 18 Maestrichtian 19 Zeolitic claystone, nannofossil 20 claystone 21 I. Campanian 22 Chert, zeolitic claystone E 450- 23 o 24 IV Campanian 25 2.01 26 Zeolitic claystone 500 27 Santonian (3) 28 Radiolarian-rich graded silt- 29 ^Indeterminate. 30 sandstone. Organic carbon-rich 31 Turonian 32 beds at Cenomanian Turonian 33 boundary ΛI. Cenomanian/ 550- 34 35 Claystone, radiolarian-rich m. Cenomanian 36 \ I. Albian 37 silt-sandstone 38 600- 39 debris forming 40 numerous graded sequences of 41 m. Albian 42 volcanogenic breccias, sand- 650- 43 stone, siltstone. Fining upward 44 sequences up to several meters 45 thick. Shallow-water skeletal 46 debris from echinoids, molluscs. 47 e. Albian 2.18 700- 48 In lower part of section algal and (4) 49 50 rudist fragments are common - 51 VI as are ooids. 52 750- 53 Orbitolinid foraminifers are com- 54 mon. Fragments of calcite- I. Aptian 55 hi cemented, sorted, ooid and T.D.= 12 Orb/to//πa-bearing grainstone 800- 763.7 m 13 present in turbidites 14 15 16 17 850- 18 3.2 19 20 (5) 21 I 22 900- IT.D. = l892.8m I I I I

t TI Clay, claystone zzz j Zeolitic Foraminifer ooze | -~- [Radiolarian ooze [v v v v| Basalt ,_—_—jSilt, siltstone Nannofossil ooze Foraminifer chalk [^—^| Radiolarite CT βJ Reef debris ...... j Volcanogenic sand, F>

659 s

Depth (m) Lithology Age East GMT 28 September 1981

l> — 2330 2315 2300 2245 Recent 7.5-i. e. Pleistocene — 50 —

0.20 1.5 — 100 —

— 150 —

0.10 — 200 —

— 250 — A A • m.Eocene 1.9 [2.5] — 300 — e. Eocene 0.21 — 350 — • 4- A-Sr Paleocene - 400 -7,-zrn— — Maestrichtian I. Camp. 450 — Campanian

— 500 — 2.0 •±r=r=J r. ^Con-SantV 0.14 [3.5] Turonian — 550 — -^ ^~ ~^~ Cenomanian

I. Albian — 600 — vsssssjπmrsssj m. Albian — 650 —

2.2 0.19 — 700 — e. Albian [4.5]

80 in. water gun - 750 — I. Aptian

800 —

3.2 — 850 — [6] — 900 —

Figure 8. Seismic profile at Site 585 interpreted in terms of drilling results from Holes 585 and 585A. See Figure 6 in order to compare predrilling and postdrilling interpretations. SEDIMENTARY AND VOLCANIC HISTORY

History of Plate Motions reinforces the reliability of the computed paleolatitude curve shown on Figure 9. Paleolatitude from Paleomagnetism Three clusters of Cretaceous paleolatitude determi- Paleolatitude data derived from Holes 585 and 585A nations can be differentiated (Fig. 9): (1) a Paleocene- can be interpreted as indicating a steady northward mo- Maestrichtian group centered at about 1°S latitude, (2) tion of the Pacific plate underlying the East Mariana a Santonian-Turonian (plus some Campanian and Ce- Basin at a rate of 32.5 km/yr. from late Aptian through nomanian) cluster centered at about 7°S latitude, and Paleocene time (Ogg, this volume). The paleolatitude da- (3) an Albian-late Aptian cluster centered at 15°S lati- ta sets from the two holes coincide very well and display tude. There was inadequate recovery of sediments and no systematic offset from each other. This coincidence paleomagnetic sampling at Site 585 to derive reliable pa-

Ma (time scale of Harland et al., 1982) 100 80 60 40

Reconstruction model (Whitman, 1981) 10

A 15


Hot-spot model 25 £ (Lancelot, 1978)


Gordon and Cox, 1980

35 Gordon, 1983

Figure 9. The amount of northward drift of the western Pacific, as computed from the paleolatitudes of Sites 585 (solid dots), 462 (open dots), and 289 (open triangle), relative to the present latitude of those sites. For comparison, the predicted amounts of northward drift for the location of Site 585 from the Pacific poles (boxes), computed by Gordon (1982), Gordon and Cox (1980), and Gordon (1983), from the hot-spot model of Lancelot (1978), and from the reconstruction model of Whitman (1981) are also plotted (from Ogg, this volume).


leolatitudes for the Campanian or Cenomanian or for Basin and the surrounding seamounts (see Sliter, this vol- the Tertiary. Therefore, whether the site had a constant ume). Age-diagnostic species that are contained in the northward drift during the Cretaceous or underwent ep- turbidite fill of the Mariana Basin range from late Apti- isodes of rapid movement between relative standstills can- an to Maestrichtian in age. Displaced species in sedi- not be determined. An average northward drift rate of ments derived from the tops and flanks of nearby sea- 32.5 km/m.y. (3.25 cm/yr.) is shown as an approximate mounts were deposited sporadically on the Basin floor fit to the three clusters. This average Cretaceous value is well below the CCD at abyssal depths of 5000 to 6000 m. less than the 50 km/m.y. predicted by the Pacific hot- These abyssal depths, characterized by an indigenous as- spot model of Lancelot (1978) and greater than the rates semblage of benthic foraminifers, recrystallized radio- predicted for the latest Cretaceous and early Tertiary by larians, fish debris, and sponge spicules, existed in the Whitman's model (1981), which uses global reconstruc- Mariana Basin from late Aptian to the present. Early tions on magnetic anomalies. Albian and older edifice-building volcanism had reached Several paleomagnetic poles have been computed for the photic zone and shallow-water bank or reef environ- the Pacific plate by combining data sets from DSDP sites, ments existed atop seamounts. The ooids and shallow- from modeling of seamount magnetism, and from skew- water foraminifers deposited from Ita Maitai Guyot can ness of marine magnetic anomalies. Gordon (1982) com- be now considered as Albian-Aptian in age. By middle puted a late Maestrichtian (~69 Ma) paleomagnetic pole Albian, the dominant source areas subsided to outer- for the Pacific plate as 71 °N, 9°E, which would predict neritic to upper-bathyal depths. Major volcanic activity a paleolatitude for Site 585 at 2.7°S (95% confidence ceased and fine-grained sediments were deposited by dis- level of 2°). This paleolatitude compares excellently with tal turbidites, although intermittent volcanism and the the line-fit value for Site 585 of 3° S. influx of rare neritic material continued until the late Gordon and Cox (1980) computed an average Campa- Albian. By the Cenomanian to Turonian, upper- to mid- nian (81 Ma; chron 33R) paleomagnetic pole for the Pa- dle-bathyal depths were reached by the dominant source cific plate at 56.6°N, 353.5°F, which would predict a areas, and the sediments recovered from this interval in- paleolatitude of 18.6°S (95% confidence level = 4°) for clude organic, carbon-rich layers. Rare benthic foramin- Site 585. The actual paleolatitude is 6.5°S using the ifers from the Coniacian-Santonian interval indicate a 81-Ma intersection of the line fit to the data, or up to continuation of dominantly middle-bathyal source areas. 8.5°S ±2.2° if the average Campanian-Santonian value A change in sedimentation during the Campanian-Maes- of Hole 585 is used. Therefore Site 585 was 12° farther trichtian from older zeolitic claystone in the Maestrich- north than predicted by the pole of Gordon and Cox tian resulted from migration of the site beneath the equa- (1980). A Pacific paleomagnetic pole for 90 Ma was torial productive zone due to northwestward plate mo- computed by Gordon (1983) to be at 54°N, 334°E, which tion. The appearance of rare middle-neritic and upper would predict a paleolatitude for Site 585 of 22.5° (95% bathyal species in the Maestrichtian interval associated confidence level = 6°). The actual value is tightly con- with volcanogenic debris gives evidence of the remobili- strained to be 9°S (± 2.5°). Therefore the site was about zation and downslope transport of pelagic deposits as a 13.5° farther north than predicted by the pole of Gor- result of renewed Late Cretaceous volcanism in the area. don. It is beyond the scope of this chapter to attempt to re- Subsidence History of the East Mariana Basin solve the differences between the predictions of the Late Based on the assumption that the Pacific plate litho- Cretaceous Pacific poles by Gordon (1983) and Gordon sphere below the East Mariana Basin could be as old as and Cox (1980) and the actual paleolatitudes of Site 585. ~ 170 Ma (Whitman et al., this volume), the subsidence It is interesting that the amount of change in paleolati- history can be modelled by taking into account thermal tudes agrees well with the Late Cretaceous results from subsidence and isostatic effect of loading by sediments. Site 462 in the nearby Nauru Basin (Steiner, 1981a, b). Figure 10 shows a subsidence curve for the basement in Perhaps the western Central Pacific Basin had differen- this of the Basin with the subsidence history of tial movement from the North Pacific Basin and eastern the sedimentary section superimposed. The backtrack- Central Pacific Basin (where Gordon's poles are derived) ing of horizons is based on sedimentation rates, the thick- during the Late Cretaceous. The predictions agree with ness and density of the rocks in the lithologic units, and the actual paleolatitude of Site 585 in the Maestrichtian, the age versus depth equation of Parsons and Sclater therefore such a theoretical displacement would have been (1977). No corrections were made for compaction and completed by that stage. The spurt of rapid northward density changes through time. This technique yields an motion of the Pacific plate during the mid-Late Creta- estimate for the Mesozoic ridge-crest depth of 3110 m. ceous proposed by Gordon (1983) is not apparent in the Our ridge-crest depth predicted by Whitman et al. (this data from Site 585 or Site 462. Instead, a fairly constant volume) is significantly deeper than present ridge-crest northward drift component of about 23.5 km/m.y. would depths. Although this prediction supports 170 Ma as a fit the late Aptian through early Tertiary results. minimum age estimate for crust in this region, it raises questions about the origin of the crust and its history Paleobathymetry from Paleontology since formation. If the sedimentary sections extend to Cretaceous benthic foraminifers from Site 585 in the 9.5 s, an initial ridge-crest depth of almost 3400 m is in- Mariana Basin, western , provide an envi- dicated, further emphasizing that the subsidence history ronmental, Paleobathymetric, and tectonic history of the of this Basin has been very different from other areas of


c to .2 c to. cu.™ II I p to c c to Q. σ ";= P 3 to to P to to to 38 .8 ü WO I > o

50 100 150 170 3000 Stratigraphic age (Ma) /-


T.D. for Site 585 Passed beneath equator - 5000 S

05 00 T-IO UK IVN _ - 6000 ^Subsidence curve for unloaded crust

7000 ^– Crust —-

i i i i i i i i i i i i i i 170 150 100 50 Age of crust (Ma)

Figure 10. Backtracking curve of the Mariana Basin near Sites 199 and 585. Lithologic units recov- ered at Site 585 (I-VI), total depth (T.D.) for Site 585, and subsidence curve for 170-m.y.-old un- loaded crusts are indicated. Thermal subsidence (Parsons and Sclater, 1977) and isostatic correc- tion for sediment loading have been applied. Time scale is based on Hardenbohl and Berggren (1978), Berggren and van Couvering (1974), and van Hinte (1976a, b). the , beginning with creation at an anomalously sidence curve to flatten out and approach an asymptotic deep spreading center. value for older crust. To resolve this problem, they pro- Anomalously shallow depths in the Pacific have been posed an exponential relationship between age and depth attributed to many causes (see Schlanger, in press, for for crust older than 70 m.y. Our subsidence curve (Fig. review), including the thermal effects of widespread mid- 10) has been produced using the two separate equations plate volcanism during the Late Cretaceous (155-70 Ma) they proposed. in the Nauru Basin (Schlanger et al., 1981), the emplace- Calculations by Whitman et al. (this volume), how- ment of sills within the sedimentary section of the Nau- ever, for the depth to crust at 170 m.y., isostatically cor- ru Basin during the Late Cretaceous (Larson and Schlang- rected for sediments, falls very close to a curve of t1/2. If er, 1981), the formation of "asthenospheric bumps" the crust at this site has followed only the linear t1/2 rela- caused by the patterns of convection cells (Menard, 1973), tionship, it implies that the lithosphere beneath this Ba- arches formed in crustal deformation due to the load of sin has not received additional heating to maintain a plate oceanic volcanoes (Menard, 1964; McNutt and Menard, of constant thickness and that the plate has continued 1978), and crustal thinning produced by heating from an to thicken beyond the maximum value of 125 km sug- underlying hot spot (Detrick and Crough, 1978; Crough, gested by Parsons and Sclater (1977). Pollack et al. (1981) 1978). have contoured the thickness of the lithosphere, and the Anomalies of excessive depth have received very little western Pacific is a region of greater thickness than most treatment by previous authors. Because the Mariana Ba- of the ocean basins, but their resolution is not good sin is anomalously deep, it cannot have undergone the enough to prove or disprove the theory at this point. reheating that affected other areas of the western Pacif- Whitman et al. (this volume) offer the hypothesis that ic, but instead it raises the question of whether the con- the crust of the Mariana Basin is above a "cold spot," verse, excessive cooling, occurred in the area of this Ba- receiving less heating by the mantle, producing a thick- sin. Parsons and Sclater (1977) found that the age versus ened lithosphere and excess subsidence. depth relationship deviates from a linear square root of age (t1/2) model for oceanic crust older than 70 m.y. They Summary of Plate History proposed that heating at the bottom of the lithosphere Taking the paleolatitude analysis of Ogg (this volume), maintains a constant plate thickness and causes the sub- the Paleobathymetric interpretations of Sliter (this vol-


ume), and the subsidence history of Whitman et al. (this boundary argue for the interpretation that these Corg- volume) as discussed above into account, the following rich layers are manifestations of the Cenomanian-Turo- history of the evolution of the East Mariana Basin can nian OAE. However, as it can be clearly seen in figure 15 be proposed. After formation of the plate at a ridge crest of the Site 585 report (this volume), these bands are in in Jurassic time, this basement reached a paleolatitude turbidite sequences. Further, benthic foraminifers from of 22°S in late Aptian time. At this point the Basin was above and below the Corg-rich layer in Section 585-32-3 at a depth of from 5000 to 6000 m. In Aptian-Albian consist of species most representative of upper to middle time major volcanic edifices grew up to sea level and bathyal environments mixed with rare elements of the provided the source of much of the volcanogenic and bi- indigenous deep abyssal assemblage (Sliter, this volume). oclastic turbidite debris. These edifices then subsided as Also, extensive bioturbation in the late Cenomanian- the plate moved northward and crossed the equator in early Turonian section shows that the deep bottom wa- Campanian-Maestrichtian time. Further volcanic activ- ters were oxygenated. It seems clear then that these Corg- ity took place in the region, but subsidence continued as rich bands were redeposited from the flank of the sur- the Basin moved northward to its present position at rounding seamounts, the slopes of which were within an 13.5°N. O2-minimum zone during Cenomanian/Turonian bound- ary time (Whitman et al., this volume). The stratigraph- The Cenomanian-Turonian Oceanic Anoxic Event in ically restricted occurrence of the Corg layer in the East the Mariana Basin Mariana Basin argues for the interpretation put forward One of the objectives of Leg 89 was to seek evidence, by Schlanger and Jenkyns (1976) that a short-lived global in the Cretaceous strata to be drilled, for the occurrence expansion of the O2-minimum zone took place at the of the Cenomanian-Turonian oceanic anoxic event in the Cenomanian/Turonian boundary and that this new East western Pacific. Based on earlier DSDP results and a Mariana Basin data fit the global picture of the ubiqui- study of outcrops, Schlanger and Jenkyns (1976) had ty of Corg-rich black bands at this stratigraphic horizon postulated the occurrence of oceanic anoxic events (OAE) (Schlanger et al., in press; Arthur et al., in press). during stratigraphically restricted intervals. During these events strata containing significant amounts of organic Oligocene Sea Level Falls Recorded by Turbidite Events carbon were deposited within an expanded O2 minimum zone in the global ocean (see also Arthur and Schlanger, DSDP sites in the Line Islands and Marshall Islands 1979; Jenkyns, 1980). One of these OAE's took place atoll provinces were selected, among other reasons, to at, or very close to, the Cenomanian/Turonian bound- study the stratigraphy of the deep-water archipelagic ary. Stratigraphic sequences of late Cenomanian and ear- aprons around the atolls. The assumption was that tur- ly Turonian age that contain organic carbon-rich "black bidite beds made up of fossil skeletal debris eroded from shales" have been described throughout the Tethys, the the nearby atolls and associated shallow banks would Atlantic Basin, in European shelf settings, in African record the history of atoll development including peri- marginal basins, or plateaus in the Pacific and in the ods of emergence (Winterer, Ewing, et al., 1973; Schlang- U.S. Western Interior Basin (Schlanger et al., in press). er, Jackson, et al., 1976; Larson, Schlanger, et al., 1981). In both Holes 585 and 585A thin beds rich in kerogen At DSDP Site 462 drilled on Leg 61 in the Nauru Basin of marine algal origin were cored. In Section 585-32-3, a and at Sites 165 and 315 in the Line Islands drilled on 2-cm-thick band of black, pyritic, silty clay stone was re- Legs 17 and 33, distinct coarse-grained, graded turbidite covered (see fig. 15, Site 585 report; and Frontispiece, units made up of tests of age-diagnostic large benthic this volume). Shipboard analysis of Corg in samples from foraminifers, and fragments of calcareous algae, bryo- this band showed values of 5.6% and 5.1%; reanalysis zoans, mollusks, and echinoids of reefal and shallow- ashore (see Site 585 report; and Schaefer, this volume) bank origin were cored and were found to be restricted to Late Cretaceous, Oligocene, middle Miocene, and Pli- showed a Corg value of 9.9%. A high hydrogen index of ocene-Pleistocene stratigraphic intervals (Premoli Silva 383 mg/g Corg reflects the high content of both dinofla- gellates and unicellular marine algae. The Cenomanian/ and Brusa, 1981; Schlanger and Premoli Silva, 1981). Turonian boundary is placed within Core 585-32 with Premoli Silva (this volume) has revised the stratigraphic the latest Cenomanian faunal assemblage occurring in ages assigned to these turbidite horizons. In the Leg 61 report Premoli Silva and Brusa reported that the ero- Core 585-34. Thus this Corg-rich black band is within a meter or two of the Cenomanian/Turonian boundary. sional events that produced the turbidites took place at In Core 585A-8,CC a thin lamina of C and radiolari- 37 Ma, 32 Ma, 26 Ma, and 13 Ma (1981). The events are org now dated (Premoli Silva, this volume) at 30 Ma, 28 an-rich sediment contains 1.45% Corg according to ship- board analysis and 2.6% according to shore lab analy- Ma, 26 Ma or younger, and 12 Ma, respectively. The fol- lowing section is summarized from Schlanger and Pre- ses. The Corg is made up of immature, marine algal-de- rived carbon. The Cenomanian/Turonian boundary in moli Silva (in press). Hole 585A is placed at the level of Core 585A-9 ~ 50 cm The data and interpretations presented here are from below the black lamina. a midplate setting in the central Pacific, a region where The presence of these Corg-rich layers at, or very close subsidence rates are well-constrained and atolls and as- to, the Cenomanian/Turonian boundary, their content sociated sediments contain records of relative sea level of marine-derived Corg, and the lack of any other Corg- changes. The interpretations of seismic stratigraphic sec- rich layers above or below the Cenomanian/Turonian tions of passive margins argue for the occurrence of a

664 SEDIMENTARY AND VOLCANIC HISTORY major coastal of flap event and an inferred global sea ord revealed by deep drilling operations on Enewetak level fall at 30 Ma (Vail et al., 1977) or 29 Ma (Vail and Atoll (Ladd et al., 1953; Cole, 1957; Schlanger, 1963). Hardenbohl, 1979), correlative with the planktonic fora- Guyots and atolls are excellent recorders of sea level falls miniferal Zone P21 {Globorotalia opima opimá), with and subsidence of the surrounding seafloor. Although lesser falls at 22 and 24 Ma (Vail and Hardenbohl, 1979). in most cases the carbonate-producing capacity of reef Stratigraphic studies of exploratory wells on the New Jer- ecosystems allows the atoll to maintain itself at sea level sey margin and DSDP sites on the Goban Spur of the despite basement subsidence and sea level rises, sea level Irish continental margin have defined a series of Oligo- lowerings cause the atoll to emerge as a high limestone cene of flap events that produced stratigraphic hiatuses island. The limestone above sea level and within the Ghy- in the shelf sequences (Poag, 1980; Olsson et al. 1980; ber-Herzberg freshwater lens that develops within the Graciansky et al., 1984; Miller et al., 1984; Miller et al., emergent atoll is subjected to massive diagenesis involv- 18 1985). Miller et al. (1985) correlated increases in δ θ ing solution of much original aragonite skeletal materi- values from ~ 1.5‰ to ~2.0‰ (Miller et al,. 1985; Keig- al and the deposition of calcite cement and dolomitiza- win and Keller, 1984; Savin et al., 1975; Boersma and tion. Karstic surfaces develop atop the emerged island. Shackleton, 1977), taken as indicative of glacial build- Upon resubmergence the new capping unaltered sediment ups, with these hiatuses and inferred three Oligocene gla- may remain atop the diagenetically altered older atoll. cio-eustatic sea level falls: a minor fall at approximately The contact between the altered and unaltered limestones 36 Ma, and major falls with significant offlaps at 31 Ma is a solution unconformity (Schlanger, 1963). in the P21 Zone (G. opima opima) and 25 Ma in the P22 The stratigraphic record at Enewetak Atoll is charac- Zone {Globigerina ciperoensis). Because all of this work terized by several solution unconformities of Oligocene, is based on stratigraphic sections deposited in passive middle Miocene and Pliocene-Pleistocene age. Dating margin tectonic settings and the exact relationship be- of these atoll limestones is determined by large benthic tween global glacio-eustatic sea level swings, offlap events, foraminiferal assemblages using the Letter Code and margin subsidence history are not completely deter- and correlated with planktonic foraminiferal zones (Cole, mined, it is important that evidence for Oligocene sea 1957; Haak and Postuma, 1975). The basalt edifice be- level falls be sought in other tectonic settings such as the low the present atoll formed in Paleocene-early Eocene mid-Pacific. time (Kulp, 1963). Subsidence took place throughout Eo- The turbidites drilled in the Marshall and Line islands cene time with the deposition of at least 420 m of reef provinces contain several distinct foraminiferal assem- and shallow-bank limestone atop the volcanic edifice; blages of Cretaceous through late Oligocene age and lie the limestones directly below the solution unconformi- enclosed in archipelagic carbonates of mid-Oligocene age ty, now 850 m below sea level, contain late Eocene (Tb in the P21 {Globorotalia opima opima) Zone of late Oli- zone) large benthic foraminifers (Cole, 1957). The sec- gocene age in the P22 {Globigerina ciperoensis) Zone. tion is completely lacking in foraminifers characteristic We propose here that these Oligocene turbidites were de- of the early Oligocene (Tc and Td) state, and the entire posited during sea level falls and that they are correla- Tb section is characterized by petrographic features in- tive with the two major passive margin offlap events de- dicative of subaerial exposure or diagenesis in the fresh- scribed by Miller et al. (1985). Miller et al. (1985) corre- water Ghyben-Herzberg lens. The missing Tc-Td sec- 18 late three δ θ events, taken to indicate the development tion and the presence of late Tb fossils indicate that the of significant glaciation, with offlap events and glacio- atoll was emergent in Oligocene time and that some Tc- eustatic sea level falls that caused the development of hi- Td sediments were deposited and later removed during atuses in the stratigraphic sections drilled on the New the lowering of the atoll surface by solution. Deposition Jersey and Irish (Goban Spur) passive margins. These of reefal carbonate at Enewetak resumed in late Oligo- workers recognized that in passive margin settings ero- cene (Te!_4) time, but the exact time of resumption with- sion and offlap should occur during times of maximum in that interval is unknown. The P21 and P22 turbidite rate of eustatic sea level fall that they date as about 36 events at DSDP Sites 462 and 315 took place in Tei_4 Ma (latest Eocene to Eocene/Oligocene boundary), about time. Thus at Enewetak the span of the hiatus repre- 31 Ma in the P21 Zone, and about 25 Ma (late Oligo- sented by the missing Tc-d section and the turbidite cene). The turbidites at both DSDP Sites 462 and 315 in events would be correlative with the major offlap events the P21 Zone correlate with the most marked Oligocene on the New Jersey and Irish passive margins. hiatus in both the New Jersey and Irish continental mar- 18 The magnitude of the Oligocene sea level fall relative gins and the related δ θ, offlap, and maximum rate of to present sea level in P21 Zone time can be determined sea level lowering events. The turbidites at DSDP Sites by combining data on the subsidence path of Enewetak 462, 315, and 165 that lie in the P22 Zone correlate with Atoll (Cole, 1957; Kulp, 1963; Crough, 1978) and the the late Oligocene hiatus and the related δ18θ, offlap, present depth to the solution unconformity at 850 m be- and maximum rate of sea level lowering events described 18 low sea level in light of late Paleocene and early Eocene by Miller et al. (1985). The approximately 36 Ma δ θ sea level estimates (Vail and Hardenbohl, 1979). At the and sea level lowering event seen on the passive margins, time of the formation of the volcanic edifice, sea level however, is not clearly recorded in the mid-Pacific sec- was, using Vail and Hardenbohl's (1979) estimate, ~ 150 tions. to 200 m above present sea level. The top of the volcanic Further evidence for Oligocene sea level falls in the peak must have been at or above sea level in order that mid-Pacific region is derived from the stratigraphic rec- the late Eocene reef limestone could form. The subsi-

665 S. O. SCHLANGER, R. MOBERLY dence path of Enewetak Atoll can be determined from north of the site. Assuming no significant change in the the drilling data on present depths to dated horizons and velocity-depth function, and that the records do show a predicted path based on the argument of Crough (1978) basement rather than a layer over it, Whitman et al., that Enewetak is subsiding as if it was now on Mesozoic (this volume) predict basement to be 1200 m (9.2 s) be- oceanic crust that had been uplifted by thermal rejuve- low the seafloor, or 300 m below the total depth of Hole nations to an elevation characteristic of seafloor 25 m.y. 585A. It may, however, be at 1700 m (9.5 s). They sug- old. Thus the top of the volcanic edifice has subsided gest that Jurassic carbonates may be present below the from -200 m above present sea level to 1270 m below volcanogenic section and above basement. present sea level. During P21 Zone time at 30 Ma the Thus drilling at Site 585 provided no information about volcanic basement was therefore 450 m below present the nature of the crust. Its specific age also remains un- sea level and at least 410 m of Eocene limestone and known, but its Jurassic age prediction has been strength- some unpreserved thickness of Oligocene limestone had ened by its record of subsidence. An alternative to the accumulated prior to the formation of the solution un- Cande et al. (1978) Jurassic interpretation was given by conformity. In order for the solution unconformity to Kroenke (in Hilde et al., 1977) as Cretaceous. The subsi- have developed at 30 Ma sea level must have been at dence curve, however, shows that a 1700-m (9.5 s) base- least 100 m below the present sea level. ment for the East Mariana Basin, isostatically corrected In summary, the presence of turbidites, composed of for sediment loading, would be too deep for Cretaceous- reef and shallow-bank skeletal debris, and the Oligocene age lithosphere. Indeed, Whitman et al., (this volume) hiatus below Enewetak Atoll point to major emergence interpret the subsidence as being too deep for the Par- events on mid-Pacific atolls during P21- and P22-Zone sons and Sclater (1977) age versus depth relationship be- 1/2 times at the same time that hiatuses were forming on cause it fits almost exactly a simple t curve rather than passive margins. The analysis of Schlanger and Premoli the Parsons and Sclater (1977) empirical relationship that 1/2 Silva (in press) argues for the truly global extent and combines t younger than 70 Ma with a flatter curve synchroneity of the previously postulated Oligocene sea older than 70 Ma. To account for the depth, Whitman level falls. Further, their estimate of mid-Oligocene sea et al., (this volume) suggest that a "cold spot" in the level relative to present sea level is in accord with the es- mantle may underlie the East Mariana Basin. Rather than timate of Vail and Hardenbohl (1979), who proposed a that explanation, we prefer an explanation that the Scla- 1/2 mid-Oligocene sea level at ~ 150 m below present based ter curve should be t both before and after 70 Ma, on seismic stratigraphic studies of passive margins. These with the East Mariana Basin as a valid point on it. We estimates are, however, significantly greater than the 35-m believe that depth points selected by Parsons and Sclater relative sea level fall during mid-Oligocene time proposed in the Central and Western Pacific basins are in loca- by Miller et al. (1985), which is based on correlation of tions of Cretaceous midplate volcanism, and therefore high δ18θ values with ice volume changes. had their thermal histories interrupted. Midplate volcan- ism in the East Mariana Basin was not at Site 585 but Oceanic Crust rather was at the seamounts around it. The main purpose for drilling at Site 565 was to re- cover deposits of the Jurassic-age megaocean and its base- Cretaceous Volcanism in the East Mariana Basin ment. Extrapolation 1000 km northwest from mapped magnetic anomaly M-25 of the Phoenix lineations (Can- Lowest Record de et al., 1978) suggested a basement age for the site of Volcanic and carbonate sediments, emplaced by grav- about 150 to 170 Ma (Callovian to Bajocian). ity-flow processes, are evidence of the mid-Cretaceous The site survey (Shipley et al., 1983) showed a promi- growth of volcanic seamounts and islands around the nent reflection at 9.0 s, which was predicted to be the East Mariana Basin, and also record the history of the top of Jurassic limestones found above the CCD on the carbonate caps and later subsidence of the volcanic edi- flanks of the ridge crest. Some irregular deeper reflec- fices. Although anticipated by members of the scientific tors at 9.1 s were thought to be the basement. Depths party of the leg, extensive volcanic sedimentary rocks to the 9.0- and 9.1-s reflectors were estimated to be at were not predicted in the site survey report (Shipley et about 850 and 1000 m. al., 1983). Drilling at Site 585 encountered a thick mid-Creta- The volcanogenic sandstones, siltstones, clay stones, ceous section of volcaniclastic sedimentary rocks. Drilled and breccias from 590 to 893 m (total depth) at Site 585 thickness and seismic-velocity properties of cores allowed (shown on Fig. 7) have been assigned to Unit VI (Site correlation of the 9.0-s reflector with the marked de- 585 report, this volume). Most of the grains are hyalo- crease in drilling rate at 800 m in Hole 585A, within the clastite, volcanic glass broken by internal stresses from mid-Cretaceous section. Subsequent interpretation of digi- sudden quenching as erupted in contact with wa- tally recorded seismic profiles obtained on Leg 89 al- ter. The granulated glass, now pervasively altered, lowed correlation of the 8.96-s reflector, and a 45% ve- was transported from its original location to the basin locity increase, with the 800-m level (Whitman, this vol- floor and deposited there. Tracing along seismic reflec- ume). Glomar Challenger records show reflections deeper tion profiles confirms the evidence from included shal- than observed during the site survey and deeper than we low-water carbonate debris that seamounts around the drilled. Acoustic basement is at least 9.2 s deep at Site rim of the eastern East Mariana Basin were sources for 585, and appears to be at 9.5 s where profiled a few km Unit VI. Its age is late Aptian to early Albian.


The lower part of Unit VI is mainly a breccia charac- Or terized by abundant angular to subrounded tabular clasts of hyaloclastic sandstone and siltstone, as well as basalt. The basalt clasts are pebbles, some cobbles, and a few boulders, supported by a matrix of sand- and silt-sized hyaloclastite glass. That fabric, along with a general lack of bedding, indicates that the rocks below about 850 m were deposited from debris flows. Petrographic and geo- chemical analysis of several of the larger pebbles shows them to be - and Plagioclase phyric hypocrystalline as well as composed of cemented glass fragments. All are quartz-normative tholeiites that had undergone slight submarine weather- ing before they slumped and were incorporated into de- bris flows (Floyd, this volume). The grains are weakly to nonvesicular and altered , with ves- icle sizes suggesting they had erupted at submarine depths of perhaps 300 to 400 m. The pebbles and sand formed nonexplosively as particles fairly high on volcanic slopes that became unstable and slumped. The slumped masses broke into debris flows that were deposited on the plain of the East Mariana Basin. The bottom two cores of Hole 585A lack carbonate debris, but the upper part of the debris-flow sections has ooids, oncoids, and fragments of mdist pelecypods. By that time in the late Aptian one or more of the volca- noes had reached shallow water. Their tholeiitic compo- sition was slightly more enriched than MORB (Viereck et al., and Floyd, this volume). Main Turbidite Record The section between 590 and about 850 m, which is the main part of Unit VI, is a sequence of graded turbi- dites of dominantly sand-sized hyaloclastite. There is, however, a textural range from fine pebbles at the base of some turbidites to silt throughout the beds, and com- 15 ponents other than hyaloclastite characterize certain beds. Nearly 400 turbidites were identified and their thick- nesses measured (Whitman et al., this volume). More than two dozen are thicker than 1 m and a few are near- ly 4 m thick, but most are thinner than 0.5 m. The thick- est turbidites are in the middle and lower part of the sec- tion. The turbidites have a variety of sedimentary struc- tures, including scoured and load-cast bases, grading, ripple cross-bedding, and bioturbation of pelagic layers, which show the Bouma divisions. Many contain the com- plete A through E sequence (Fig. 11). Characteristical- 20L ly, the base of division A may contain fine pebbles and sand of shallow-water carbonate debris (Haggerty and Figure 11. Carbonate and volcaniclastic turbidite, from the core catcher Premoli Silva; and Whitman et al., this volume) and of of Core 89-585-39, showing complete Bouma sequence. vitric, lithic, and crystal volcanic debris (Floyd; Floyd and Rowbotham; and Viereck et al., this volume). icle size and abundance that indicates its hydroclastic compositions show two volcanic sources, origin. Most apparently formed by nonexplosive spall- tholeiitic basalt as well as (Floyd and Row- ing below the volatile compensation depth (Fisher and botham, this volume). Some hawaiitic and mugearitic Schmincke, 1984). Throughout the section, and especial- basalt clays are tentatively identified (Viereck et al., this ly in one 11-m interval (Core 47 and the upper part of volume). According to the geochemistry, mineralogy, and Core 48, near 680 m) the abundant highly vesicular ba- petrology of the volcanic debris, the seamounts and is- salt glass suggests and relatively shallow-water subma- land sources for the mid-Cretaceous turbidites at Site rine eruption. 585 resembled island chains like Hawaii (Fig. 12). The provenance of the turbidites was mainly the up- The turbidite sand has angular, equant, blocky to cusp- per submarine slopes and shallow submarine caps of one ate shapes, altered sideromelane composition, and ves- or more of the volcanic seamounts and islands around


501 Floyd, Floyd and Rowbotham, Chamley et al., and Vier- eck et al. is ubiquitous. The phyllosilicates are smec- tites, interlayered smectite-chlorites, and celadonite. Cli- noptilolite and sodium-rich chabazite or phillipsite are more common zeolites than are natrolite and heuland- ite. Analcime is especially common. Calcite and small amounts of barite, hematite, palygorskite, a calcium sul- fate, apophyllite, leucoxene, albite, and opal are other secondary components. Textural sites of secondary min- 210 erals are the glassy grains and coarser clasts, the matrix of these clasts, in clasts, vesicles, pores and vugs, and late veins. Palagonitized glass is most commonly yellow to brown 8 5 DC and isotropic to weakly birefringent, typically has shrink- age cracks, and may be banded in shades of green or brown. Its composition is mainly cryptocrystalline smec- tite with large amounts of absorbed water. Spherical leu- // Hawaiian Islands and Loihi Seamount tholeiites coxene aggregates characterize the darker bands. Other areas may be enriched in Fe or Mg. Granular to fibrous Young tholeiitic seamounts Pacific Rise or platy microcrystalline green or yellow smectite replac- es palagonite, beginning at grain and vesicle surfaces. Or, in many instances, zeolite replaces the palagonite. I I I I I I I I I I I I I I The paragenesis may differ from sample to sample and La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu can be complex (Viereck et al.; and Floyd, this volume). Figure 12. Chondrite-normalized REE pattern of Mariana Basin Ap- tian volcanoes compared to patterns for midplate volcanoes and Smectites are the phyllosilicate minerals of greatest ridge-crest volcanoes. Samples 6 and 8 are, respectively, the most abundance in the Cretaceous volcaniclastic section. In primitive members of Floyd's (this volume) groups B (Plagioclase the lowest part cored a highly crystallized and sodium- and olivine-phyric tholeiitic basalt) and C (plagioclase-phyric tho- rich magnesium smectite, probably a saponite, is associ- leiitic basalt). ated with celadonite, amorphous aluminum oxides, and lesser chlorite and mixed-layer clays. The smectite re- the basin. Some of the ooids have nuclei of sand-sized places glass fragments and matrix, and fills parts of zoned basalt, altered glass, or clinopyroxene. In addition, Vier- vesicles. Smectite or chlorite or both replaces olivine phe- eck et al., (this volume) interpret some vesicular clasts nocrysts and microlites and fills vesicles of these Aptian and all crystalline basaltic clasts to have formed and erod- turbidites and debris flows. ed above sea level. Holocrystalline basalt grains may Above Core 48 (686 m) are reworked and graded hya- make as much as 25% of samples in a debris flow (Core loclastites and the highest of the coarser turbidite beds. 54, near 740 m) within the mainly turbidite section. It Their smectites are less well crystallized, having fleecy to should be noted that this same level also yielded the lathed morphology. Chlorite is more abundant but shows clast of fine-grained epidote amphibolite (Floyd, this vol- poorer crystallinity, and traces of illite are present as a ume) that perhaps came from metamorphosed ocean crust micaceous phyllosilicate, but celadonite has disappeared. or deep within a volcanic edifice, and was subsequently Mg-beidellite and Mg-chlorite occur in Core 46. ejected or eroded from a fault scarp. From the uppermost Albian to upper Campanian The upper one-third of the turbidite section of Unit (590-476 m) the beds are fine grained but still contain a VI generally has thinner turbidites than below. Lithic large volcanogenic component. Smectites are poorly crys- grains decrease upward in size and abundance. Crystal tallized, typically having a fleecy morphology. The clay- fragments are small, but include rare apatite and titan- like palygorskite occurs in Cores 34 and 35. Clay , also suggesting that some of the volcanism was mineralogy, chemistry, and morphology of the Cretaceous alkaline (Floyd, this volume). sediments are described in detail by Chamley et al. (this volume). Floyd (this volume) and Viereck et al. (this vol- Alteration of Mid-Cretaceous Volcanogenic Sediments ume) describe the petrographic relationships of replace- The volcaniclastic section of Site 585 is pervasively ment and vesicle filling as well as provide additional min- altered to palagonite, phyllosilicates, zeolites, analcime, eralogical and geochemical information. and other secondary minerals. Preserved sideromelane Clinoptilolite is the zeolite of the claystones in the is rare, whereas fresh phenocrysts and microlites of py- Cretaceous section above 590 m. Its habit is chiefly as roxene and Plagioclase are moderately common. Clino- granular to radiate-fibrous zeolite spheroids. A contro- ptilolite and phillipsite in the mineral assemblages are versy exists about the zeolite in the coarse-grained sec- characteristic of the lower end of the zeolite facies, rep- tion below that depth. Floyd (this volume) and Viereck resenting alteration temperatures certainly less than 60° C. et al. (this volume) each give several chemical analyses Chapters in this volume describing the alteration are by that overlap one another and that overlap the composi-

668 SEDIMENTARY AND VOLCANIC HISTORY tional ranges both of phillipsite and of chabazite. Phil- donite and saponite smectite are the two characteristic lipsite, however, differs structurally from chabazite, and clays that form in the near-surface alteration of ocean- both Floyd and Viereck et al. used X-ray diffraction floor basalts (Cann, 1981). The analcime below about methods of identification. Floyd reports abundant phil- 670 m, well-crystallized smectite below about 686 m, and lipsite containing a wide range of alkali content; Viereck the zeolite veining (including heulandite) and zeolite- et al. report abundant sodium-rich chabazite, recogniz- rimmed vugs below about 820 m, along with the down- ing phillipsite only as relict structures now replaced with hole decrease in clinoptilolite, are suggestions of increased later analcime. diagenesis in the deepest sediments, but well within the In the deepest sections, natrolite fibers and plates are zeolite facies. present, replacing crystals or filling vesicles. Natrolite Overall in the Cretaceous, Chamley et al. (this vol- and heulandite also may fill veins cutting the sandstones ume) characterize the late Aptian to Maestrichtian non- and breccias. Analcime, considered to be a zeolite by biogenic sedimentary history as being first dominated some mineralogists, is common in the reworked hyalo- by closely adjacent volcanogenic sources, then by more clastites and other turbidites and debris flows in the mid- distal volcanic sources, and finally by the development Cretaceous section. Clean analcime replaces glass (or pa- of distant detrital sources. lagonitic glass) and, less commonly, Plagioclase. Inclu- sion-rich analcime has filled pores, where apparently it THE NAURU BASIN IGNEOUS COMPLEX has replaced earlier prismatic phillipsite cement, and may also have replaced margins of glassy grains. Sheet-Flow Basalts Recovered on Leg 89 The occurrences of small amounts of such minerals During the Leg 89 reoccupation of Site 462, we reen- as calcite, barite, leucoxene, albite, and opal in the vol- tered and cored Hole 462A an additional 140.5 m to a caniclastic sediments are noted in several chapters in this total sub-bottom depth of 1204 m. The newly recovered volume (Chamley et al., Viereck et al., and Floyd et al.). basalts of Cores 93 through 109 were described and ana- These same writers also describe the alteration histo- lyzed, and the new results were compared with the earli- ry of the volcaniclastic sediments at Site 585. Alteration er Leg 61 observations and conclusions. took place in two stages. The earlier alteration occurred An additional 12 volcanic units, numbered 45 through on the flanks and near the tops of the seamounts that 56 and shown in Figure 13 were recognized below the were the sources of the sediments. Some evidence cited volcanic units cored and described during Leg 61 (Bati- by Floyd (this volume) includes ooids nucleated on smec- za et al., 1981). These deeper units are interlayered aphyr- tite-replaced vesicular glass, and sideromelane enclosing ic and sparsely clinopyroxene-plagioclase-olivine phyric, a palagonitized fragment with smectite-infilled vesicles. nonvesicular basalts. The textures and other characteris- This early stage is represented by development of palag- tic features indicate that the 12 units are all sheet flows. onite, smectite, minor hematite, and possibly of cela- Thin, reworked hyaloclastite layers overlie units 45, 46, donite. Subsequently, the partly altered to fresh hyalo- and 47, and resemble the thin sedimentary intercalations clastite slumped from the flanks of the seamounts and described within the higher basalt layers (Moberly and was transported in debris flows and turbidity currents to Jenkyns, 1981). The lavas are olivine-normative and the location of Site 585. quartz-normative tholeiite basalts with low abundances Floyd (this volume) sees no evidence that there was of the incompatible trace elements. They are a deeper an epiclastic or pyroclastic component of the earliest vol- continuation of the type 2 or type B basalts (Larson, canogenic sediment, and Viereck et al. (this volume) con- Schlanger, et al., 1981) of Cores 80 to 92. Although in clude that the earliest sediment in sideromelane and general little systematic mineralogical or chemical chan- tachylite hyaloclastite formed mainly by quenching and ges occur between samples, there is enough to allow spalling of submarine pillow lavas and sheet-flow lavas. three groupings based on content of siderophile elements Chamley et al. (this volume) believe, however, that the and in compatible trace elements, and on slight differ- abundance of fixed or absorbed sodium in most of the ences in TiO2 content (Floyd, this volume). The three- secondary minerals is a marker of subaerial volcanism. way division of the whole complex by Batiza et al. (1980) The textural evidence seems far more important than the was also based on TiO2 content, as well as on other cri- chemical evidence. teria, but those TiO2 differences are larger than those of Burial of the volcanogenic sediment was rapid, as the the three so-called megagroups (Fig. 14) of Floyd within secondary minerals gained much less K (in phillipsite and the type 2 basalts. According to Floyd (this volume), the celadonite), Rb, and Ba than expected in comparison to chemical variation displayed can be related to fractiona- many other sites where the effects of seafloor alteration tion of the phases. Floyd's megagroup A, in have been studied (Viereck et al., this volume). The mod- units 45 to 47, apparently resulted from fractionation of est increase in K above Core 585A-16 may relate to cela- clinopyroxene, and probably also of olivine, but altera- donite abundance, with the celadonite having replaced tion obscures the trends. Megagroup B is in units 48 trachytic rocks, after a change from the initially tholeiit- through 52 (upper part), and probably resulted mainly ic volcanism, according to Chamley et al. (this volume). from fractionation of olivine and Plagioclase with rela- Clinoptilolite and phillipsite, along with the abundant tively little contribution from fractionation of clinopy- smectite and celadonite, characterize the lower end of roxene. Plagioclase fractionation is the important factor the zeolite facies, and represent alteration temperatures leading to megagroup C, found in the lower part of unit at the seafloor or under modest depths of burial. Cela- 52 into unit 56, the deepest unit cored. Further division


Lithology and main features II oI 2 I

93 Aphyric to sparsely clinopyroxene—Plagioclase phyric basalt 1080- • Glomerophyric glassy basalt near top of unit 94 45 • Olivine becomes phenocryst phase toward base in subvariolitic glassy basalt • Numerous closely spaced horizontal fractures at base


Moderately olivine—clinopyroxene phyric basalt 1100- 96 • At top of unit below hyaloclastite layer is quench-textured moderately phyric basalt * \ • Patchy development of glass-rich zones (glass altered to smectite) 97 -– 46 throughout suggest unit may be composed of a number of smaller flow units • Plagioclase phyric quench-textured subunit at base with chilled top 98 1120-

Olivine—clinopyroxene phyric basalt µ 99 47 • Oxidized reworked volcaniclastic sediments at top and bottom of unit • Top of flow represented by spalled glass in smectite—zeolite matrix 100 |5 Aphyric basalt 101 o. 1140- 48 • Often glassy, now represented by smectite patches throughout • Grades downward into sparsely olivine—clinopyroxene phyric basalt • Fine-grained thin subunit near base 102 Clinopyroxene—Plagioclase phyric basalt 16 49 • Glomerophyric texture; glassy quench-textured subunits present (f) Plagioclase—clinopyroxene—olivine phyric basalt 103 50 • Glassy throughout and quench-textured 1160- • Vesicular or vuggy base on irregular surface of unit below Moderately clinopyroxene—olivine ± Plagioclase phyric basalt 104 51 • Number of small cooling units • Quench-textured throughout

Aphyric glassy basalt • Quench-textured top and bottom to unit 1180 ΛΛΛ IΛΛΛ 52 • Various sparsely phyric variants in central portion 107 • Grain-size increases downward from basalt to fine-grained dolerite

Clinopyroxene—Plagioclase phyric basalt 108 Clinopyroxene—Plagioclase phy ic basalt • Number of thin quench-textured units present 1200- 55 Moderately clinopyroxene—Plagioclase phyric basalt 109 Moderately clinopyroxene—Plagioclase phyric basalt 56 • Quench-textured; grain-size variation suggests thin subunits

/ × Basalt x × Clinopyroxene-phyric Quench-textured

ΛΛΛ Dolerite o o Plagioclase-phyric

Reworked Hyaloclastite • Olivine-phyric

Figure 13. Section of sheet-flow basalts drilled in Hole 462A during Leg 89, showing lithology of the 12 volcanic units recovered.


FeOVMgO TiO, Cr Zr Ce Zr/Y La/Nb o £ Ni

1.6 1.8 2.0 1.2 1.3 1.4 120 160 80 90 60 70 10 20 2.0 2.6 0.5 1.5 I I I I I I i r I I I I

> < f 1080H 1 i 95 J à i / 1100H


1120H *>.



N E 1140H

49 i. \

50 1160 H V 51 105 i

1180—I ,


1200H 55


Mainly aphyric units Mainly phyric units

Figure 14. Chemical stratigraphy of basalts drilled in Hole 462A during Leg 89, showing the distribution of Floyd's (this volume) groups A, B, and C, and their subgroups.

671 S. O. SCHLANGER, R. MOBERLY into 22 small geochemical subgroups may have resulted those of N-type MORB, their 87Sr/86Sr ratios resemble from eruption of separate magma pulses undergoing fur- those of ocean-island basalts. ther minor fractionation or flow differentiation. Com- plexities in the rapid cooling of the flows is reflected in Origin of the Igneous Complex their anisotropic physical properties (Fujii and Hama- no, this volume). Magma Generation and Evolution Clinopyroxene glomerocrysts are mainly Ti-poor au- Although the rocks of the Nauru Basin and flow gites, whereas the quenched clinopyroxenes are relatively complex generally resemble one another in gross chemi- richer in Ti and show considerable varieties in Ca abun- cal and mineralogical aspects, different levels or degrees dance. are mainly labradorites, although of difference do exist and can be used to indicate the or- some zoned Plagioclase glomerocrysts range from An73 igin and evolution of their magmas. (bytownite) to An2o (oligoclase). The more calcic plagio- At the highest level the basalt types differ in their clases generally have higher Mg and Fe contents. These gross TiO2 and La contents (Fig. 17) and also in their basalts underwent low-temperature submarine anoxic al- stratigraphic grouping and paleomagnetic inclinations teration to golden brown palagonite and brownish smec- (Castillo et al., this volume). Type 1 basalt has the low- tite that replaced interstitial glass and celadonite that re- est TiO2 values, and occupies the highest 250 m of the placed Plagioclase (Floyd and Rowbotham, this volume). sheet-flow section (Figs. 17 and 18). Below it is type 2 Alteration, as indicated by H2O and Fe2O3/FeO analy- basalt, with intermediate TiO2 values; the new samples ses (Fig. 15), is relatively slight, suggesting that these ba- from Leg 89 coring of deeper sheet flows are all of this salts erupted too rapidly for extensive submarine weath- type. Also, type 2 basalt occurs in the lower sills (Cores ering. 36-41) of the interbedded sills and sediments that com- Nauru Basin basalts have ratios of La/Sm, La/Yb, prise the upper part of the complex. Type 1 basalt, of Nb/Zv, La/Ta, and Th/Hf that are chondritic. La/Ta highest TiO2 values, is found in the highest sills. Saun- for these and other basalts is shown on Figure 16. The ders (this volume) also recognizes the division of the sheet ratios of La/Yb, Nb/Zr, and Ta/Zr are higher than flows (his II equals type 1 of Castillo et al., and III N-type MORB, more closely resembling T-type MORB equals lower 2), but Saunders put all sill rocks into one from the Reykjanes Ridge, transitional between ridge- group (his I), perhaps because he doubts that they are crest and ocean-island basalts for these trace elements sills and had only a few analyses. (Saunders, this volume). Castillo et al. (this volume) show Isotopic evidence bears heavily on the problem of the that whereas Nauru Basin 143Nd/144Nd ratios resemble origin of these magmas. According to Castillo et al. (this volume), all basalts of the igneous complex came from a common source, because the 87Sr/86Sr ratios vary only slightly. Differences among the three types are ascribed to different degrees of partial melting in the source re- gion of the mantle (Castillo et al.; and Notsu et al., this volume); examination of simple models suggests that the onset of volcanism resulted from a batch of magma that Hydrothermal underwent a modest amount of partial melting (type 2), alteration trend followed by the peak of volcanism with more extensive partial melting (type 1). Considerably later (see age dis- cussion below), the sills came from magma batches that formed from modest (type 2) and slight (type 3) amounts of melting. Submarine As several of the contributors to this volume stated +- trend (and as Floyd shows in detail), the next level of magma o differentiation has resulted from fractional crystalliza- tion of the phenocryst phases. The type 2 flows have at least three such groups. The finest scale or level of dif- ferentiation seems to have occurred as separate magma pulses underwent additional fractionation or flow dif- ferentiation. Fresh MORB Tectonic Setting A major question arose from the Leg 61 discoveries. How can a laterally extensive, tabular mass of MORB- 0.5 1.0 like basalt (Fig. 19) more than 600 m in thickness have Fe2O3/FeO been emplaced without disturbing the underlying crust

+ enough to destroy the M-series magnetic anomalies in Figure 15. Hydration (H2O ) and oxidation (FejCyFeO) of Hole 462A basalts relative to typical trends of alteration of basalt (See Floyd, the wide, flat-floored Nauru Basin? This problem was this volume.) addressed by Larson and Schlanger (1981). Structurally,


33 I -

















16 Chondrite 15 + 14






8 • • I 0.01 10.0 Ta (ppm) Figure 16. Plot of La/Ta versus Ta, showing Nauru Basin analyses relative to other oceanic basalts. (See dis- cussion in Saunders, this volume.) the MORB-like basalt must have erupted much like con- of the Nauru Basin igneous complex, whose basalts fit tinental floor basalts (Saunders, this volume), that is to best with island-arc tholeiites (Fig. 20) or mid-ocean ridge say, rapidly and from fissures and not from edifice-build- tholeiites (Figs. 16 and 19). ing central vents. Regionally, there is no evidence what- soever of a hot-spot plume trace. Perhaps there were nu- Episodes of Igneous Activity in the Nauru Basin merous short-lived propagating , giving numerous Site 462 is on Anomaly M-26. There has been no new tiny, ephemeral microplates (Castillo et al., this volume), information to alter our opinion that the lithosphere at as had been suggested for a Cretaceous western Pacific Site 462 formed at M-26-time, about 145 Ma. ridgecrest setting by Kroenke (in Hilde et al., 1977), ex- The sheet-flow basalt lavas provided radiometric ages cept the preserved M-series anomalies show there was of 131 Ma (Ozima et al., 1981, age spectrum ragged) no significant spreading. from Core 462A-50, 127 Ma (S. Hart quoted by Castillo It is interesting that various tectonic discrimination et al., this volume) from an undesignated part of the diagrams give little help in indicating the tectonic setting sheet flows, and 129 Ma (Takigami et al., this volume)


£ TiC "A \vTl• /O/ La (ppm) TiO La (ppm) o c u 1.0 1.5 2.0 2 4 6 C o 1.0 1.5 2.0 2 4 565- 1 1 1 Φ ' T5 900- 20 * t • s 70 600- S2 S

25 35 12 % 75 30 • • 80 s 1000- I. i * # x: 700-_—_ α 40 ft x> 21 o •X s I botto m dep t

CO 22 S u

45 95 J 1100-

— 25 800- = 55

ti 50 — — 10b

— 30 • 1200- 1 09 65 < 56

Figure 17. Variation in TiO2 and La in the Nauru Basin igneous complex. Dashed vertical lines separate three basalt types: (1) low Tiθ2and La values in flows between Cores 43 and 79; (2) intermediate values in sills between Cores 36 and 43 and in flows between Cores 80 and 109, and (3) high values in sills between Cores 14 and 32. (See discussion in Castillo et al., this volume.) from Core 462A-108 (T.D.). Thus the likely radiometric reworking from an older sediment source exposed in the age of these rapidly erupted flows is about 127 to 131 Basin. Ma, or Valanginian. The sample with rare, poorly pre- Above the flows is a sedimentary section extending served radiolarians in Core 99 yielded only one species from the Aptian to the Recent, with sills intruded into that could be positively identified; it ranges from latest sediments as high as upper Albian or Cenomanian (i.e., Jurassic to earliest Aptian (Schaaf, this volume) and so the highest sills should be no older than late Albian, is compatible with an age in the 127 to 131 Ma range. about 100 Ma). The Aptian and Albian hyaloclastic sed- As pointed out by Schaaf, the presence of a late Titho- iments intruded by the sill complex are by themselves ev- nian-early Berriasian fauna in Core 462A-46 indicates idence that the Aptian-Albian Pacific igneous episode


Core Lithology Basalt type Descriptions 1 2 3

Sedimentary section


400 -

Igneous complex of sills, flows, and sediments 14 600 H Upper unit (Cores 14—42) consists of basalt 32 sills with intercalated volcanogenic sediments. 36 41 43 Lower unit (Cores 44—109) consists of basalt sheet flows of variable thickness. 800-

Basalt sheet flows.

Red brown silt with radiolarians, fish debris, 1000- 79 80 and agglutinated foraminifers of bathyal facies.

Basalt sheet flows.

Zeolitic hyaloclastic siltstone with radiolarians.

1200-LJ09 Basalt sheet flows. Figure 18. Generalized stratigraphic relations in the igneous complex in Hole 462A, Nauru Basin. affected the Nauru Basin or its borders. Moreover, the activity in the Pacific. There is great difficulty, however, carbonized terrestrial plant remains (Jenkyns and Schlang- in reconciling these specific ages with the paleontologi- er, 1981) in the upper Aptian section indicate that some cal ages, namely of intruding 120-Ma (early Barremian Aptian volcanoes were built above sea level. age), or even 110-Ma (later Aptian age), sills into sedi- Of the two sets of sills distinguished from each oth- ments deposited in late Albian or Cenomanian time (Lar- er on stratigraphic, geochemical, and paleomagnetic son, Schlanger, et al., 1981; about 100 Ma). grounds, the upper set (above Core 462A-32) presum- There is no doubt of igneous activity in or near the ably is the older, because its average paleomagnetic in- Nauru basin in the late Campanian (about 75-70 Ma), a clinations dip south at steeper angles than those of the time of volcanism for so much of the Pacific. There are lower set, and the Pacific plate was moving northward. major volcanic components in the Campanian sedimen- The ^Ar/^Ar ages reported by Ozima et al. (1981) are tary section at Site 462. At the present time we authors 110 Ma for a sample from the lower sills (Core 462A-32; remain of different opinions about the time of sill injec- age relatively reliable) and 120 Ma from the upper sills tion. One of us (RM) suggests there may be moderate (Core 60; approximate age only). Thus the radiometric errors in the radiometric dating of the sills and that their ages seem to support the sequence of the upper sill as time of intrusion may correspond to the Campanian vol- being older than the lower sills, and they are mid-Creta- canism. The other (SOS) suggests there are smaller er- ceous ages, now recognized as a time of major igneous rors in the radiometric dating, that the sills are of Apti-


0.5 1.0 1.5 2.0 1.0 1.5 2.0

2.0 MORB 20

1.5 15

1.0 10






4 0 1.0 1.5 2.0 TiO, TiO,

Figure 19. Major-element variation of Nauru Basin basalts relative to fields of analyses from part of the Mid-Atlantic Ridge (FAMOUS), mid-ocean ridge basalts (MORB), ocean-island basalts (OIB), and ocean-island tholeiites (OIT). (See discussion and references in Castillo et al., this volume.) an and Albian age, and that the Campanian volcanism Arthur, M. A., Schlanger, S. O., and Jenkyns, H. C, in press. The is a separate event. Cenomanian-Turonian oceanic anoxic event, II, paleoceanograph- ic controls on organic matter production and preservation. In Thus the igneous history of the Nauru Basin can be Brooks, J., Fleet, S. (Eds.), Marine Petroleum Source Rocks. Geol. summarized as follows: (1) formation of lithosphere in Soc. London Spec. Pub. the Oxfordian, about 145 Ma, along the Pacific-Phoe- Batiza, R., 1981. Trace-element characteristics of Leg 61 basalts. In nix spreading center; (2) eruption of an extensive and Larson, R. L., Schlanger, S. O., et al., Init. Repts. DSDP, 61: Washington (U.S. Govt. Printing Office), 689-695. thick mass of flows from numerous small rifts during Batiza, R., Larson, R. L., Schlanger, S. O., Shcheka, S. A., and To- the Early Cretaceous, probably Valanginian, about 130 kuyama, H., 1980. Trace element abundances in basalts of the Na- Ma; (3) nearby volcanism and perhaps intrusion of sills uru Basin (DSDP Leg 61): Late Cretaceous off-ridge volcanism in in two episodes in mid-Cretaceous time, in the Aptian the western Pacific. Nature, 186:476-478. or Albian or both, about 100 Ma; and (4) nearby vol- Batiza, R., Shcheka, S., Tokuyama, H., Muehlenbachs, K., Valuer, T. L., Lee-Wong, F., Seifert, K. E., Windom, K. E., Book, P., and canism, and perhaps intrusion of the sills in two epi- Fujii, N., 1981. Summary and index to petrologic and geochemical sodes, in the Campanian, about 75 Ma. studies of Leg 61 basalts. In Larson, R. L., Schlanger, S. O., et al., Init. Repts. DSDP, 61: Washington (U.S. Govt. Printing Of- ACKNOWLEDGMENTS fice), 829-839. We thank the scientists on Legs 61, 62 and 89 for their interest in Berggren, W. A., and van Couvering, J. A., 1974. The late Neogene: the sedimentary and volcanic history of the western Pacific. This is biostratigraphy, geochronology, and paleoclimatology of the last HIG Contribution Number 1653. 15 million years in marine and continental sequences. Paleogeogr. Paleoclimatol. Paleoecol., 16:1-216. REFERENCES Boersma, A., and Shackleton, N., 1977. Oxygen and carbon isotope Arthur, M. A., and Schlanger, S. O., 1979. Cretaceous "oceanic anox- record through the Oligocene, DSDP Site 366, equatorial Atlantic. ic events" as causal factors in development of reef-reservoired gi- In Lancelot, Y., Seibold, E., et al., Init. Repts. DSDP, 41: Wash- ant oil fields. Bull. Am. Assoc. Pet. Geol., 63:870-885. ington (U.S. Govt. Printing Office), 957-961.


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677 S. O. SCHLANGER, R. MOBERLY van Hinte, J. E., 1976a. A Cretaceous time scale. Bull. Am. Assoc. Winterer, E. L., Ewing, J., et al., 1973. Init. Repts. DSDP, 17: Wash- Pet. Geol., 60:498-516. ington (U.S. Govt. Printing Office). van Hinte, J. E., 1976b. A Jurassic time scale. Bull. Am. Assoc. Pet. Geol., 60:489-497. Whitman, J. M., 1981. Tectonic and bathymetric evolution of the Pa- cific Ocean since 74 Ma [Master's dissert.]. University of Miami, Date of Initial Receipt: 15 October 1985 Florida. Date of Acceptance: 15 October 1985

1 0.1 0.3 0.5 Ocean-floor basalts -1.3

~. 5000

-1.5 Island-arc tholeiites

MORB 1000 -1.7 50 100 500 1000 Cr (ppm)

Ti 10 TiO Nauru basalts (Leg 89) .•"'•; Continental •../' basalts

Y× 3 MnO × 10 P2O5x10

Figure 20. Distribution of Hole 462A basalt analyses on various tectonic discrimination diagrams. There is no geo- morphic or geophysical evidence, however, to suggest that an occupied the Nauru basin in the Creta- ceous. (Diagrams referenced in Floyd, this volume.)