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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.

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