Shallow-Water Skeletal Debris and Larger Foraminifers from Deep Sea Drilling Project Site 462, Nauru Basin, Western Equatorial Pacific

Shallow-water larger foraminifers and associated forms adapted to a reef environment were recovered from Deep Sea Drilling Project Hole 462, drilled in 5181 meters water depth in the Nauru Basin, south of the Marshall Islands. Shallow-water material, mainly reworked, (1) is frequently associated with volcaniclastic breccias to coarse sands, (2) occurs episodically interbedded within a turbiditic sequence whose clasts are pelagic components (planktonic foraminifers, radiolarians, and calcareous nannoplankton). More than 60 species of larger foraminifers were identified, and most of them are age-diagnostic. They range in age from Campanian to late Oligocene; only a few forms are attributable to the mid-Cretaceous. A comparison was made with similar shallow-water assemblages recovered from deep sites previously drilled in the central western Pacific (Line Islands, Tuamotu Ridge). On the basis of the occurrence of larger foraminifers (1) the existence of volcanic edifices capped by reefs is demonstrated for the mid-Cretaceous; late Campanian through late Maestrichtian; early, middle, and late Eocene; early Oligocene; and late Oligocene; (2) five main discrete erosional events are identified since the late Cretaceous; they are dated at middle(? )-late Maestrichtian, late(? ) Eocene, early late Oligocene, and middle Miocene; (3) larger-foraminifer assemblages display great affinities with the Caribbean bioprovince during the Cretaceous, whereas they contain mainly Tethyan and/or Indo-Pacific elements during the Tertiary; and (4) the main direction of faunal migration from westward (Caribbean) during the Late Cretaceous was reversed during the Tertiary. INTRODUCTION The present chapter treats larger foraminifers and other shallow-water debris, primarily recovered from DSDP Hole 462, drilled at 5181 meters water depth, in the Nauru Basin, western equatorial Pacific. Shallow-water faunas and floras with forms adapted to the reef environment, anomalously occurring at such oceanic depth, are frequently associated with volcaniclastic material and are contained in generally graded layers, including volcaniclastic breccias, fine conglomerates, and coarse foraminifer sands. Such coarse layers are episodically interbedded in a turbiditic sequence, where the dominating grain-size ranges from a foraminifer and/or radiolarian sand to a nannofossil silt (see Premoli Silva and Violanti, this volume). The age of planktonic faunas and floras associated and/or containing the coarse layers ranges from late Campanian through Quaternary. Such layers are unevenly distributed throughout the stratigraphic column: they are more common during intervals of major erosion (late Campanian, middle-late Maestrichtian, late Eocene, early late Oligocene, late Oligocene, and middle Miocene. Autochthonous sediments are red clay, yielding mainly fish debris and abyssal, non-calcareous, benthic foraminifers, and occasionally possible autochthonous radiolarians (see Premoli Silva and Violanti, and Premoli Silva and Sliter, this volume). The oldest shallow-water fossils are Late Cretaceous (CampanianMaestrichtian); they are partly reworked into the Tertiary. Shallow-water fossils are also diagnostic of early Initial Reports of the Deep Sea Drilling Project, Volume 61. Eocene, middle and late Eocene, early late Oligocene and late Oligocene ages. A few forms, mainly in the upper levels, are probably Neogene or younger. The most commonly represented taxonomic groups are larger foraminifers, including Discocyclinidae, Miogypsinidae, Nummulitidae, Orbitoididae, Pseudorbitoididae, bryozoans, and more rarely coralline algae. Mollusks, corals, and echinoids are minor components and commonly very fragmented. COMPOSITION OF THE COARSER LAYERS On the basis of their composition, both organogenic and inorganic, the coarser layers can be grouped as follows: 1) Layers from the upper (younger) part of the sequence (mainly from Core 5, Section 7, through Core 6, Section 2), in which shallow-water material is scarce, poorly diversified, never exceeding the 3 to 5% of the > 63-μm fraction. Volcanic material mainly consists of fresh glass and ash, which sometimes cements planktonic foraminifers and radiolarians. Volcanic material may represent 50% of the >63-μm fraction. 2) Layers corresponding to the typical foraminifer sands, in which shallow-water material is scarce, poorly diversified, never exceeding 15% of the >63-μm fraction, dominated by planktonic foraminifers. The larger foraminifers cover a long time interval, but are few and mainly reduced to small fragments, often with very chalky, poorly preserved inner parts. Volcanic material is present as a minor component of the > 63-μm fraction. Those layers are common in Cores 14, 23, 25, and 27. 3) Layers, recorded from Core 21, Section 1; Core 22, Section 1; Core 32, Sections 1 and 2; and Core 34, Sections 1 and core catcher in which the shallow-water