39. PLANKTONIC MICROFOSSIL BIOSTRATIGRAPHY OF THE NORTHWESTERN PACIFIC OCEAN David Bukry1, U.S. Geological Survey, La Jolla, California, Robert G. Douglas, Case Western Reserve University, Cleveland, Ohio, Stanley A. Kling, Cities Service Oil Company, Tulsa, Oklahoma, and Valeri Krasheninnikov, Academy of Sciences of the U.S.S.R., Moscow CONTENTS Page Page Introduction 1253 Regional Correlation 1281 Zonal Comparison 1254 Calcareous Nannoplankton 1281 Planktonic Foraminifera, Mesozoic 1281 Upper Cretaceous-Paleocene Boundary 1255 California 1281 Paleocene-Eocene Boundary 1259 Japan 1285 Eocene-Oligocene Boundary 1259 West Pacific 1286 Oligocene-Miocene Boundary 1259 Australia 1287 Miocene-Pliocene Boundary 1260 Planktonic Foraminifera, Cenozoic 1288 Pliocene-Pleistocene Boundary 1261 Solomon Islands 1288 Zonal Summary 1261 Mariana Islands 1288 The Philippines 1288 Paleoecology 1261 Taiwan 1289 Calcareous Nannoplankton 1261 Japan 1289 Radiolaria 1267 Kamchatka Penisula 1290 California 1290 Preservation 1267 Radiolaria 1291 Calcareous Nannoplankton 1267 Sedimentation Rates 1291 Foraminifera 1269 Relationship to Acoustostratigraphy 1294 Radiolaria 1279 References 1296 INTRODUCTION A comparison of zonal units of calcareous nannoplank- ton, foraminifera, and radiolarians in the same strata Biostratigraphic evidence obtained from the north- shows only few cases of exact coincidence of zonal western Pacific Ocean as a result of coring carried out limits, especially if coincidences at the top or bottom by the Glomar Challenger during Leg 6 of the Deep of the standard 9-meter coring runs are dismissed as Sea Drilling Project from Hawaii to Guam is considered artificially induced owing to gaps in sediment recovery. here mainly from the standpoint of three dominant Exact coincidence of zonal limits within coring runs marine planktonic microfossil groups—calcareous nan- are most notable for the Upper Paleocene sediment of noplankton, foraminifers, and radiolarians. The co- the Shatsky Rise, where the Heliolithus kleinpelli Zone occurrence of these microfossils in various types of (nannoplankton) is equivalent to Globorotalia pseudo- sediment in 684 meters of core recovered along a menardii Subzone (foraminifera), the Heliolithus riedeli 9000-kilometer traverse of the Pacific (Figure 1) pro- Zone (nannoplankton) to Globorotalia velascoensis vides the opportunity to compare zonal units, paleo- Zone (foraminifera), and the Discoaster multiradiatus ecology, preservation, correlation to land sections, and Zone (nannoplankton) to Globorotalia subbotinae Zone variation in sedimentation rates as determined by these (foraminifera). A significant example of noncoincidence fossil groups. occurs at the Cretaceous-Tertiary boundary of the Shatsky Rise. Although both calcareous nannoplankton and planktonic foraminifera underwent profound ex- Publication authorized by the Director, U. S. Geological Sur- tinctions and rejuvenation near this boundary world- vey. wide, evidence from our coring indicates that the great 1253 The state of preservation of radiolarian assemblages is mainly indicated by the number of taxa present that are least resistant to solution. Following complete dis- solution of less-resistant taxa, specimens of more- resistant taxa are not noticeably altered. PACIFIC Paleoecologic biogeographic patterns are best repre- 40° sented by radiolarians, which exhibit a much greater $/ 51 49&50 distinction in species composition between assemblages * •• 47 & "* from our northern and southern sites than do the cal- „ SHATSKY 52 careous nannoplankton and planktonic foraminifera. RISE Nannoplankton assemblages from the open ocean areas PHILIPPINE investigated are distinctly smaller in number of species 20° BASIN ^53 than nearshore and basin assemblages of the same age. * 54 60 OCFΔN HAWAII W 1/ L. M I V • GUAM HQR |Z0N 59 A comparison of the biostratigraphic data accumulated during Leg 6 with studies of outcrop sections in the Circum-Pacific region and Pacific islands demonstrates ->v that few studies have attempted to correlate through the Pacific region. The micropaleontologic content of 1000 KILOMETERS AT EQUATOR sediment sections cored by the Deep Sea Drilling Project provides a basis for comparison of taxonomic, paleo- ecologic, and preservational relations that can initiate a 140" 160" 180" 160° synthesis of the geologic history of the vast Pacific realm. Figure 1. Location of drilling sites in the northwestern Pacific Ocean, Deep Sea Drilling Project, Leg 6. ZONAL COMPARISON Series boundaries are based on certain stage boundaries change in assemblages of planktonic foraminifera oc- where major changes in megafossil assemblages occur. curred before that of the calcareous nannoplankton. These major boundaries were established originally in The uppermost Maestrichtian Tetralithus mums Zone sections of shelf and nearshore marine sediment crop- (nannoplankton) is recognized in samples that contain ping out on land. The correlation of zones of planktonic foraminiferal assemblages dominated by species repre- open-ocean microfossils to these type stages often is senting the lowermost Danian Globigerina taurica Zone presumptive because the lithologic facies of these stages and Globorotalia trinidadensis Zone of foraminifera. in type areas do not necessarily include such micro- fossils. Therefore, using diverse lines of evidence and Interpretation of zonal and paleoecologic data inti- reasoning, workers studying planktonic microfossils mately involves consideration of the effects of preser- have developed various concepts of series boundaries that are based on particularly significant zonal changes vation of microfossils. In general, sediment containing for their groups. Leg 6 cores provide a basis for a com- all three microfossil groups has been little effected by parison of series boundaries determined from a variety dissolution. For the two main types of dissolution of fossil groups. resulting from a great water depth during deposition, planktonic foraminifera are completely removed, where- as the calcareous nannoplankton and radiolarians are Comparison of zonal scales based on nannoplankton, either represented only by resistant species or only the Radiolaria, and planktonic foraminifera shows generally most resistant radiolarians remain. In calcareous oozes close coincidence of boundaries of zonal subdivisions. deposited at moderate depths, calcareous nannoplank- Some discrepancies are accounted for mainly by a ton, especially the star-shaped discoasters, are over- gradual change of faunas and floras at boundaries of grown with secondary calcite. The siliceous radiolarians stratigraphic units. Uninterrupted and high sedimenta- are absent from several of our shallow carbonate ooze tion rates often serve to magnify zonal boundaries and samples, suggesting strong dissolution control of radio- lead to the recognition of transitional sequences not larian assemblages in relatively shallow ooze. Whereas represented in zones known from type stages. Partly, planktonic foraminifera and calcareous nannoplankton the discrepancies are the result of different understand- show many degrees of preservation, owing to a range ing of the essence of zones (chronostratigraphic zones of overgrowths and dissolution of individual specimens, of intercontinental extent or biostratigraphic provincial radiolarians do not show evident signs of such alteration. and local zones). 1254 Comparatively more significant discrepancies are ob- Tetralithus mums Zone, whereas the upper part of the served with respect to understanding stratigraphic units core contains species representing both the T. mums of higher rank (stage, sub series, series). These may result and C. tenuis Zones and is considered transitional in part from general shortcomings of the international Cretaceous-Tertiary. stratigraphic scale, owing to such factors as unconform- ities and reinterpretations of paleontologic evidence. According to foraminiferal faunas, Core 11 contains the lowermost Paleocene Globorotalia trinidadensis In sediment cores from several drilling sites, the co- and Globigerirta taurica Zones, with small numbers of occurrence of abundant assemblages of foraminifera species of the Upper Maestrichtian Abathomphalus and nannoplankton, commonly also with radiolarians, mayaroensis Zone throughout, though more common allows a comparison within the same strata of zonal in the bottom part of the core. Below this core, charac- boundaries and series boundaries based on these teristic assemblages of the A. mayaroensis Zone occur groups (Figure 2). This comparison allows assessment with the uppermost Maestrichtian Tetralithus mums of the potential for combination zones based on changes Zone of nannoplankton. in assemblages of two or more groups and assessment of the relative control of environmental factors on the In detail, the identification of only late Maestrichtian microfossil groups. nannoplankton in samples from the lower part of Core 11 (Sections 3,4,5 and 6), which contains foraminifera The method of nannoplankton zonation, like that used representative of early Danian and late Maestrichtian, for foraminifera, does not rely solely on first or last indicates that the great evolutionary changes that occurrences of single species, but instead takes into affected these microfossils did not have a precisely account the overall aspect of the changes occurring synchronous effect. Tertiary planktonic foraminiferal within the assemblages. Radiolarian zonation is keyed species appeared before the calcareous