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Deep Sea Drilling Project Initial Reports Volume 16

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Deep Sea Drilling Project Initial Reports Volume 16 36. BIOSTRATIGRAPHY OF THE EQUATORIAL EAST PACIFIC RISE1 David Bukry, United States Geological Survey, La Jolla, California, Menno G. Dinkelman, Oregon State University, Corvallis, Oregon, and Ansis Kaneps, Scripps Institution of Oceanography, La Jolla, California INTRODUCTION ZONE AND SERIES BOUNDARIES Late Cretaceous microfossil assemblages are present at Leg 16 completed the Deep Sea Drilling Project coring Site DSDP 163 at the western margin of the East Pacific program in the eastern equatorial Pacific, which also Rise. This is the first known occurrence of Mesozoic includes coring done during Legs 5, 8, and 9. Data and assemblages from the ocean floor east of the Line Islands materials obtained permit evaluation of regional biostrati- and the Hawaiian Islands. A 114-meter section of Cam- graphic correlation, fossil preservation, and paleontologic panian and early Maestrichtian calcareous ooze is zoned and evidence for large-scale tectonic movement. Many of the correlated primarily by coccoliths, the major sediment most significant sites are continuously cored and reach component. Small numbers of benthonic foraminifers, basalt (Figure 1). Comparison of the relative position of planktonic foraminifers, and radiolarians occurring through zonal boundaries for the three major microfossil groups — the section also indicate the Late Cretaceous. The coccolith foraminifers, coccoliths, and radiolarians — reveals some zonal sequence is similar to that previously observed at differences from hole to hole. Such offset relations are other DSDP sites, but no late Maestrichtian assemblages are usually within the limits of a single zone for a given DSDP present, and an abbreviated Lithraphidites quadratus Zone leg. Some of the variation results from a contrasting is overlain by barren zeolitic clay. The Campanian- preservation state of fossil groups related to their particular Maestrichtian boundary occurs within the Tetralithus depth of deposition on the west flank of the East Pacific trifidus Zone, as indicated by more abundant planktonic Rise. Leg 16 sites, cored on a west-trending transect at foraminifers in other areas, as at Atlantic Sites DSDP 10 10°N, are arrayed down the flank at water depths ranging and DSDP 21. Owing to a lack of well-preserved radiolarian from 4484 meters at DSDP 159 to 5320 meters at DSDP and foraminiferal assemblages and because the upper 163. Maestrichtian is nonfossiliferous or uncored, no compari- Although Late Cretaceous and Eocene to Pleistocene sons of the microfossils at the top of the stage, and thus at sediments are identified at the sites of Leg 16, no the top of the Upper Cretaceous Series, are possible. calcareous microfossils younger than middle Miocene were Paleocene microfossils are not present in samples from found, owing to calcite dissolution. Older calcareous Leg 16. The earliest Cenozoic assemblages recovered (DSDP assemblages also show the effects of solution, as foramini- 162, Core 17) are late early Eocene. Zonal relations for this fers are only sporadically present. Etched coccolith and other Cenozoic assemblages are summarized in Figure assemblages are common, and only radiolarian assemblages 2. Where feasible, planktonic foraminifers were used to are generally well preserved and present throughout almost define series and subseries boundaries. Although all three all of the cored section. Previously recognized low-latitude fossil groups — foraminifers, coccoliths, and radiolarians — zonations are applicable at all the sites, although modifica- permit zonation of late early Eocene sediment, the subse- tion of some parts of the Paleogene radiolarian and quent early middle Eocene is represented in a strongly coccolith zonations is indicated (Dinkelman, this volume; dissolved facies, where only radiolarian assemblages can be Bukry, this volume). zoned with assurance. The lower Eocene-middle Eocene The age of fossil assemblages in Oligocene and older subseries boundary is based partly on the Discoasteroides sediment directly above basalt is considerably younger than kuepperi Zone of coccoliths, which has previously been the prescribed age estimated from the associated magnetic correlated with the lower Hantkenina aragonensis Zone of anomalies. This difference in indicated age is greater at foraminifers in DSDP 94, Sections 26-3 to 28-3 (Worzel, older sites. But the age of the basal sediment and the depth Bryant et al., in preparation). At DSDP 162 the foram- to basement below the sea floor do show a general increase iniferal assemblage is considered to be transitional between away from the rise crest. The axes of thickest pelagic the Globorotalia palmerae Zone and the H. aragonensis sediment for each geologic series indicate a northward Zone. post-depositional offset of older equatorial centers of The middle Eocene-upper Eocene boundary, as deter- deposition. The apparent northward translation was only mined by radiolarian assemblages, lies near the upper about 0.1°/m.y. during the Neogene; it was about 0.5°/m.y. boundary of the Podocyrtis chalara Zone and the lower during the Eocene-Oligocene. boundary of the Podocyrtis goetheana Zone. At DSDP 161A and DSDP 162 this boundary can be clearly identified. But because of intense vertical mixing within the cores at DSDP 163, the presence of this boundary can only 1 Publication authorized by the Director, U.S. Geological Survey. be inferred from the relative abundance of a sphaeroid 915 D. BUKRY, M. G. DINKELMAN, A. KANEPS 160° 150° 140° 130c 120' © PACIFIC OCEAN ISLANDS 41,2^40 . W •42 ®159 ©163 @160 .69 • 71 ©79 o72 80 73 © 10c ®75 1000 kilometers at the equator 20c Figure 1. Location of DSDP coring sites and the Experimental Mohole (EM) on the west flank of the East Pacific Rise. Holes continuously cored shown as solid circles; those that terminate in basalt as double circles. Sites from DSDP 159 to DSDP 163 were cored during Leg 16. radiolarian, probably Periphaena decora, in samples from Dinkelman (this volume), and to consider other upper both zones, as this species is dominant in samples from the Eocene species, like Lophocyrtis{1) jacchia (Ehrenberg) and same interval at DSDP 161A and DSDP 163 (Plates 1 Cyclampterium(l) milowi Riedel and Sanfilippo, to estab- and 2). lish a new upper Eocene zonation. Riedel and Sanfilippo (1970) divided the upper Eocene The Eocene-Oligocene series boundary is identified in into two zones based on radiolarians, the Thyrsocyrtis DSDP 161A and DSDP 162 at a distinct lithologic change tetracantha Zone and the Thyrsocyrtis bromia Zone, plus from radiolarian-rich brown clay below to grayish orange an unzoned interval at the base of the upper Eocene. This coccolith-radiolarian ooze above. Among radiolarians, a interval was later placed by Moore (1971) into the discontinuity occurs in the morphologic lineage of Theo- Podocyrtis goetheana Zone. The Thyrsocyrtis tetracantha cyrtis tuberosa at both sites. The basal Oligocene coccolith Zone was not observed at any of the three Leg 16 sites subzone is missing, and the late Eocene is so decimated by which cored Eocene sediments. At every site Thyrsocyrtis solution of calcareous material that part of that interval bromia appeared before the first occurrence of Thyrsocyrtis also may be missing. At DSDP 161A the Eocene-Oligocene tetracantha. Examination of the data presented in the boundary is drawn at the same level, between the Dis- radiolarian reports of Legs 4, 7, and 8 (Riedel and coaster barbadiensis Zone and Helicopontosphaera reticu- Sanfilippo, 1970, 1971; Moore, 1971) does not substantiate lata Zone of coccoliths and between the Thyrsocyrtis the presence of the T. tetracantha Zone in either the bromia Zone and Theocyrtis tuberosa Zone of radiolarians; Caribbean or the Pacific. Thus it seems appropriate to foraminifers are absent or nondiagnostic. This coincidence eliminate the use of the T tetracantha Zone in future of boundaries is at a lithologic discontinuity between Cores radiolarian work. Hence, the upper Eocene is divided into 9 and 10. At DSDP 162 the top of the D. barbadiensis Zone two zones, the P. goetheana Zone and the T. bromia Zone. and T. bromia Zone are offset so that the upper part of the It may even be advisable to omit the Thyrsocyrtis bromia T. bromia Zone of radiolarians is associated with the H. Zone as well, considering the wide morphological variety of reticulata Zone of coccoliths. Again, samples are barren or the nominate species, as discussed and illustrated in nondiagnostic on the basis of foraminifers. 916 BIOSTRATIGRAPHY OF THE EQUATORIAL EAST PACIFIC RISE Zones and Subzones Approx. Age, Epoch (m.y.) Foraminifer Coccolith Radiolarian Sphenolithus heteromorphus Calocycletta costata Helicopon tosphaera ampliaperta/ 15 — Globigerinatella insueta Sphenolithus belemnos Calocycletta Miocene virgims Catapsydrax stainforthi/ 20 — Catapsydrax dissimilis Triquetrorhabdulus Discoaster druggii carinatus Globorotalia kugleri Discoaster deflandrei Lychnocanoma Dictyococcites abisectus bipes Globigerina ciperoensis 25 — Dorcadospyris Sphenolithus ciperoensis papilio Globorotalia opima opima Theocyrtis Sphenolithus distentus annosa Oligocene 30 — Globigerina ampliapertura Sphenolithus predistentus Theocyrtis ____ 9 _ _ _ _ 35 — tuberosa Helicopon tosphaera reticulata Thyrsocyrtis bromia 40 — Discoaster barbadiensis Podocyrtis goetheana Reticulofenestra Discoaster saipanensis Podocyrtis chalara umbilica Discoaster bifax Podocyrtis mitra 45 Podocyrtis ampla Coccolithus staurion Nannotetrina Chiasmolithus gigas Thyrsocyrtis triancantha quadrata
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