Cretaceous Planktonic Foraminifers from the Nauru Basin, Leg 61, Site 462, Western Equatorial Pacific

Abstract

Cretaceous planktonic foraminifers recovered from Deep Sea Drilling Project Site 462 in the Nauru Basin of the western equatorial Pacific Ocean range in age from near the Coniacian/Santonian boundary to middle Maestrichtian. Species from Hole 462 show this stratigraphic range, whereas those from Hole 462A are Campanian and Maestrichtian in age. Assemblages are characterized by a small-sized, poorly preserved, low-diversity planktonic fauna, dominated by species of Hedbergella, Globigerinelloides, Archaeoglobigerina, Heterohelix, and Schackoina. Species of Globotruncana, Marginotruncana, Dicarinella, Praeglobotruncana, and other larger-sized genera are rare. Reworked species indicate the presence of upper Aptian, Cenomanian, and possible Turonian source areas that were not identified at Site 462. Benthic foraminifers and characteristics of the associated biogenic and lithologic constituents indicate that abyssal depositional environments greater than 4000 meters and well below the calcite compensation depth existed throughout the Mesozoic interval represented by sediments from just above the volcanic complex of probable mid-Cretaceous age to those of the modern basin, with a water depth of 5,189 meters. INTRODUCTION Cretaceous foraminifers of Coniacian/Santonian to Maestrichtian age were recovered from Site 462 in the Nauru Basin, western equatorial Pacific (Fig. 1). The site was selected to sample Late Jurassic oceanic sediments, and to penetrate the basement at magnetic anomaly M-26, approximately 150 m.y. old. Hole 462 was drilled at a water depth of 5189 meters, and 376.8 meters of core were taken at a recovery rate of 61 per cent. The hole was terminated at Core 69, in basalt sills, as the rate of penetration was very slow. Hole 462A was offset from Hole 462 by 473 meters, and drilled to Core 92, at 1068.5 meters below sea floor. Operations were terminated in a thick volcanic complex of probable midCretaceous age. Cretaceous planktonic foraminifers reported here are from Core 46, at a depth of 428 meters below sea floor, to Core 60, at 559 meters below sea floor (Hole 462), and from Core 7, at a depth of 439.5 meters below sea floor, to Core 2, at 564.2 meters below sea floor (Hole 462A). Hole 462 was continuously cored throughout, whereas Hole 462A was continuously cored only from Core 8, at a depth of 487.0 meters below sea floor, to Core 24; above Core 8, the section was washed and spot cored. Despite the different methods of recovery, the two holes correlate very well, and most of the biostratigraphic events are detectable in both holes. Cretaceous sediments recovered from Site 462 represent a succession of fine-grained resedimented-carbonate layers that alternate with sediments devoid of carbonate. The carbonate content is very unevenly distributed throughout the Cretaceous sequence and is related preInitial Reports of the Deep Sea Drilling Project, Volume 61. dominantly to the presence of calcareous nannoplankton, and to a lesser extent the amount of calcareous foraminifers, both planktonic and benthic. In comparison to the Tertiary sequence, the autochthonous zeolitic claystones become more abundant in the Cretaceous sequence, and increase down-hole to become the dominant sediment type, where they finally intercalate with the volcanic complex. Coarse to very-coarse volcaniclastic sediments are interbedded in the late Campanian to Maestrichtian portion of the Cretaceous sequence; they consist of volcanic rock fragments and shallow-water bioclasts, along with a fine fraction composed mainly of planktonic and small benthic foraminifers. These redeposited interbeds display a variety of structures, including graded bedding, cross-laminations, and parallel laminations. BIOSTRATIGRAPHY The occurrence of planktonic foraminifers at Site 462 is associated with layers of redeposited volcaniclastic material. The more-diverse assemblages are associated with the coarse-grained layers that contain shallowwater debris. Planktonic foraminifers from the finegrained layers are rare. Preservation of specimens in all sediment types from this deep basin, well below the Cretaceous CCD, is characteristically poor, and specimens are always strongly recrystallized. All assemblages exhibit varying degrees of size sorting, and most are composed of small-sized individuals. Despite the obvious evidence of displacement, the distribution of foraminifers follows the normal succession of biostratigraphic events. Obviously, the transport mechanisms and timing were largely penecontemporaneous. Nevertheless, several points need to be considered: (1) there is no continuity from one assemblage to the next; (2) the first appearance of a taxon must be