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DSDP Leg 84 recovered Tertiary and Upper Cretaceous calcareous nannofossils along the landward slope of the Middle America Trench at five sites offshore of Guatemala and one site offshore of Costa Rica. At four sites, drilling reached tectonized, ophiolitic basement that underlies Pleistocene (Hole 566), upper Miocene (Hole 566C), lower Eocene (Sites 569 and 570), and possibly lower Maestrichtian/upper Campanian (Site 567—Upper Cretaceous limestone above basement may be displaced) slope sediment. Three significant hiatuses occur within the area of drilling and encompass most of the middle to lower upper Miocene, lower Oligocene, and middle Eocene intervals. Short, localized hiatuses occur in Pleistocene and Pliocene sediment. Reworking of calcareous nannofossils caused by downslope displacement of older sediment occurs at all the sites, and is especially significant within lower Miocene (Helicosphaera ampliaperta Zone) sediment penetrated at Site 567, 3 km upslope from the trench floor. One new Miocene Discoaster species (Discoaster tuberi Filewicz, n. sp.) is described in this chapter. INTRODUCTION During Leg 84, ten holes were cored at five sites along the landward slope of the Middle America Trench (MAT) offshore of Guatemala, and one hole was cored at another site along the same slope margin offshore of Costa Rica (Figs. 1-3). Tertiary sediment containing age-diagnostic nannofossils was recovered at all six sites (along with a 1.5-m-thick Upper Cretaceous limestone in Hole 567A). Additional drilling of the trench floor, landward slope, and shelf was achieved on the first MAT transect, north of the Tehuantepec Ridge off Acapulco (DSDP Leg 66), and on the second MAT transect offshore of Guatemala (DSDP Leg 67). The primary objective of the third MAT transect drilled on Leg 84 was to determine the age, structure, and stratigraphy of the slope sediments present landward of the trench offshore of Guatemala. A similar objective was met during Legs 66 and 67 for their respective geographic positions along the MAT axis, but drilling was terminated without the desired slope penetration when gas hydrates (clathrates) with a seismically undefined base were penetrated. Leg 84 is the first cruise approved by the JOIDES advisory panels to specifically drill into gas hydrates above their seismically defined base, and to study clathrate origin and occurrence in the marine environment. A total of 700 samples from 200 cores (1043 m of section) recovered on Leg 84 was examined for nannofossils under the light microscope. Calcareous nannofossils are relatively continuous in occurrence, and provide detailed biostratigraphic correlation when present. A list of zonal and age assignments for these cores is presented in Figure 4. Assemblage preservation varies from moderate to poor, with extensive overgrowth of species morphotypes in lower Eocene (Sites 569 and 570) and lower von Huene, R., Aubouin, J., et al., Init. Repts. DSDP, 84: Washington (U.S. Govt. Printing Office). 2 Address: Union Oil Company of California, Ventura, CA 93003. Maestrichtian/upper Campanian (Site 567) limestones penetrated above basement. Assemblage numbers range from rare to abundant, with occasional dilution of the species content because of high sedimentation rates within the Holocene to Pleistocene mudstones. Thin intervals of Pliocene and upper to middle Miocene sediment are dominated by siliceous micro fossils, and are barren of calcareous nannofossils. Tertiary and Late Cretaceous nannofossil species considered in this report are listed alphabetically according to generic epithet in the Appendix. BIOSTRATIGRAPHY The Pleistocene to Holocene zonation utilized in this study is after Gartner (1977). Zonal boundaries are sometimes tentative, especially at the Emiliania huxleyi Zone/ Gephyrocapsa oceanica Zone boundary (first occurrence of E. huxleyi) and Gephyrocapsa oceanica Zone/Pseudoemiliania lacunosa Zone boundary (last occurrence of P. lacunosa), owing to (1) high sedimentation rates and clastic input, which have diluted the nannofossil, assemblage, and (2) a high abundance of siliceous microfossils, which may be due to the close proximity of these trench-slope sites to shore, where upwelling can inject surface waters with high nutrient levels favorable to diatom productivity. Intermittent downslope sediment displacement, as documented by sedimentary structures and benthic foraminifers (Baltuck et al., this volume), has sporadically reworked specimens of Helicosphaera sellii and Calcidiscus macintyrei, which are zonal guide species, thus causing further difficulty in zonal assignments for the lower Pleistocene. Stradner and Allram (1982) and Musylov (1982) experienced similar problems in their attempt to zone Pleistocene MAT-slope mudstones. The Pliocene through Eocene zonation utilized is after Bukry (1973, 1975) and Okada and Bukry (1980). The Calcidiscus macintyrei Subzone (CN12d) is relabeled Discoaster triradiatus to avoid confusion with the Pleisto-

Calcareous Nannofossil Biostratigraphy of the Middle America Trench and Slope, Deep Sea Drilling Project Leg 84