40. MESOZOIC MAGNETIC ANOMALIES, OCEANIC PLATEAUS, AND SEAMOUNT CHAINS IN THE NORTHWESTERN PACIFIC OCEAN1 Ralph Moberly, Hawaii Institute of Geophysics, Honolulu, Hawaii and Roger L. Larson, Lamont-Doherty Geological Observatory, Palisades, New York INTRODUCTION certainties over the nature of the contact. There have been modest revisions of the time scale originally pro- Leg 32 of the Deep Sea Drilling Project was planned posed by Heirtzler et al. (1968) for the past 80 m.y. (for with three main objectives. One was to date the example Berggren, 1972; Sclater et al., 1974); however, magnetic lineations of the oceanic crust east of Japan the present best calibration is within a few percent of the and west of the Hawaiian Ridge from the age of sedi- one originally proposed. ments lying on basement. The lineations were predicted Magnetic lineations discovered in the western Pacific to be a record of sea-floor spreading during a period of and on the border of the western North Atlantic did not late Mesozoic reversals of the magnetic field. A second fit within the Cenozoic sequence. These are northeast- interest was to establish Mesozoic and Cenozoic bio- striking lineations east of Japan (Uyeda et al., 1967); stratigraphic reference sections for the northwestern northwest-striking lineations west of the Hawaiian Pacific by continuously coring the sedimentary sections Ridge (Hayes and Pitman, 1970); east-striking lineations on Shatsky and Hess rises. Presumably, planktonic north of the Phoenix Islands (Larson and Chase, 1972); fossils that otherwise would not survive dissolution in and northeast-striking lineations off the southeastern the deep ocean might be preserved on top of these sub- United States (Vogt et al., 1971). marine plateaus. The third objective was to determine Larson and Chase (1972) correlated these Japanese, the age and paleolatitude of Köko Guyot, in the Hawaiian, and Phoenix magnetic lineations and used Emperor-Hawaiian Seamount chain. Cores of the basalt their geometry to propose a late Mesozoic history for underlying the guyot would be used to test a current the Pacific Ocean crust. Larson and Pitman (1972) cor- theory of the origin of seamount chains. related the Keathley lineations with these Pacific ones, The actual sedimentary rocks encountered on the leg, and on the basis of scanty evidence from DSDP as well as the operating capabilities and weather, were biostratigraphy proposed a geomagnetic time scale for not always as anticipated. It remains convenient, how- Mesozoic magnetic anomalies M-l (about 112 m.y.B.P.) ever, to present our sedimentary, biostratigraphic, and through M-22 (about 148 m.y.B.P.). During several tectonic history of the northwestern Pacific Ocean in million years of Middle to Late Cretaceous time, after terms of the original objectives even though not all of the M sequence and before the latest Cretaceous them were met. through Cenozoic ones, the earth's magnetic field ap- parently was of dominantly normal polarity. AGE OF MESOZOIC MAGNETIC ANOMALIES Objectives of Leg 32 Background We planned several holes on the Japanese and One of the outstanding successes of the Deep Sea Hawaiian lineation patterns of the northwestern Pacific Drilling Project has been the dating of magnetic anoma- that would test the contention of Larson and Chase ly lineations through the recovery of fossiliferous sedi- (1972) that these lineations are all expressions of the ment overlying the basement basalt. In several instances same magnetic reversal sequence recorded at two there have been some uncertainty whether the sediment different spreading centers. These sites would also test lies nonconformably on extrusive basalt generated at the and further calibrate the late Mesozoic reversal time mid-ocean ridge or whether the basalt is intrusive and scale of Larson and Pitman (1972). Initially, the time thereby younger than the true crust. Evidence is ac- scale had two points of control. Site 166 was drilled be- cumulating that the great majority of contacts either are tween M-7 and M-8 of the Phoenix set, where the oldest of sediment on extrusive basalt, or are of very shallow sediment is late Hauterivian or early Aptian in age. In intrusives penecontemporaneous with crust generation the western North Atlantic, Oxfordian and Callovian (Moberly, in preparation). Other possible sources of sediments were cored just west of the Keathley error, such as assignment of the correct biostratigraphic anomalies at Sites 100 and 105. age and uncertainty of the depth to a particular sedi- ment sample above basement, discussed by van Andel The Pacific Advisory Panel of JOIDES had a number and Bukry (1973), appear to be more important than un- of secondary reasons to drill in the western Pacific. It was hoped that basalts recovered from these sites could be examined radiometrically and paleomagnetically to gain more insight into age calibration and tectonic reconstructions, respectively. Little sediment had been 1 Hawaii Institute of Geophysics Contribution No. 676. recovered in the deep western Pacific during DSDP 945 R. MOBERLY, R. L. LARSON phases I and II, and modifications to D/V Glomar mid-Cretaceous cherty pelagic clays that grade down to Challenger had been planned so as to overcome some of cherty nannofossil oozes. This lithology is underlain by these difficulties. Fossiliferous sediment could be used weathered basalts that were cored at all three sites, to improve Mesozoic biostratigraphy and to record such although the basalt-sediment contacts were never events as the transit of the Pacific lithospheric plate un- recovered. The basalts are very fine grained, and at Sites der the equatorial zone of high productivity or under the 303 and 307 consist of several flow units, the latter site Kuroshio Extension Current. containing a large amount of hyaloclastite. The basalts The upper sedimentary section from the youngest site are moderately to highly altered, the upper portion at on the Hawaiian lineations should also yield the age of Site 307 being a "tholeiitic claystone" containing 13% the Hawaiian Ridge at that point, because the magnetic H2O. Despite this sometimes extreme alteration, it is anomalies are overlain by the Hawaiian archipelagic possible to determine, especially from the trace elements apron. This part of the Hawaiian Ridge lies between of the less-altered samples, that the rocks were originally Midway Islands and the Hawaiian-Emperor "bend," tholeiitic basalts, similar to basalts being generated at and is in an area where the age progression along the present-day ridge crests (Marshall, this volume). chain appears to be nonlinear. Although the basement contacts were not sampled at Sites 303 and 304, sediment was recovered from no more Leg 32 and Subsequent Information than three meters above this contact in both cases. These Of the four sites located on Mesozoic magnetic samples have been dated by their calcareous microfossils anomalies (303, 304, 307, and 311, Figure 1), the first at both sites, although the preservation and diversity of three successfully cored the Mesozoic sediment section, fossils at Site 304 is far superior to that at Site 303. At penetrated volcanic basement, and achieved basal age both sites the foraminifera give a Barremian or Hauteri- estimates of varying precisions. Site 311 bottomed in vian age, while the nannofossils are Hauterivian or volcanic sandstone of Oligocene age and will be dis- Valanginian. The basalt at Site 304 has a calcite vein cussed in the section on island chains. containing crustacean coproliths that are closely related The post-Mesozoic sediments at Sites 303 and 304 are to the species Favreina salevensis that is widespread in diatom-radiolarian oozes of Neogene age, whereas the Late Jurassic limestones. Since the favereinids at Site uppermost sediment at Site 307 is mainly a thin, un- 304 are not conspecific with Favreina salevensis, they are fossiliferous brown clay (Figure 2). These sediments are of limited stratigraphic value, but the possibility exists underlain, probably unconformably, at all three sites by that Site 304 is somewhat older than the Early -160° -150° MID-PACIFIC MOUNTAINS -160° -I5O€ Figure 1. Chart of the northwestern Pacific showing Leg 32 site locations relative to the magnetic lineations, rises, and seamount chains of the area. 946 Figure 2. Summary diagram showing the paleontologic ages, lithologies, and recovered intervals of the Leg 32 holes as functions of depth beneath the sea floor. R. MOBERLY, R. L. LARSON Cretaceous age given by the nannofossils and foram- be Tithonian or Kimmeridgian in age from the Larson inifera (Lehmann, this volume). For the present we will and Pitman (1972) time scale. This age range cannot be accept the Barremian to Valanginian age from the over- excluded at Site 307 because of the poor sediment lying sediments as the most likely basement age. recovery and the preliminary nature of the radiolarian Site 303 lies on magnetic anomaly M-9 and Site 304 is stratigraphy. The geology of the hole, however, sug- located on anomaly M-4 of the Japanese magnetic linea- gests an age of 5 to 10 m.y. younger than the predicted tion sequence. This information, when combined with age. This discrepancy could result for the following the early Aptian or late Hauterivian age determined at reasons. (1) Our best estimate of the basement age may Site 166 (Leg 17) between anomalies M-7 and M-8, is be too young because of the poor recovery. (2) A non- still best fit by the magnetic reversal time scale proposed depositional interval may have occurred after extrusion by Larson and Pitman (1972). Because of the multistage of the basalt at Site 307. (3) The calibration of the range of the fossils and the usual mismatch of nan- radiometric time scale of the latest Jurassic and earliest nofossil and foraminiferal ranges, the time scale repre- Cretaceous may be significantly in error.