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Leg 54 of the Deep Sea Drilling Project was designed to study young oceanic igneous rocks and associated geochemical phenomena in the tropical eastern Pacific Ocean. Several upper Pliocene to Quaternary biogenous sediment sections were cored incidentally to the primary igneous coring program. Two of the sites (Sites 424 and 425) are in the southern part of the Panama Basin on the flanks of the Galapagos spreading center. The others are clustered on the western flank of the East Pacific Rise at the junction of the Siqueiros fracture zone (Figure 1). The sediment sections of the Siqueiros area, Sites 419 through 423 and 427 through 429, contain sufficient silicoflagellates to recognize several previously described zones (Figure 2). Dissolution thinning of specimens is common and the uppermost Quaternary Dictyocha aculeata aculeata Zone is missing in all the Siqueiros sites but Site 427. One new silicoflagellate, Dictyocha aculeata subaculeata Bukry, is described from Leg 54. The mid-Quaternary Mesocena quadrangula Zone is well represented in the Siqueiros and Galapagos areas. At Site 422, the regional acme of Mesocena quadrangula is found above and below an intercalated basaltic rock unit. The negative magnetization reported for the unit and the M. quadrangula abundance signify it is not part of the Jaramillo polarity event, but is probably only slightly older (Hays et al., 1969; Saito and Burckle, 1977) — about 1 m.y. At Site 425 near the Galapagos Islands, silicoflagellates are especially abundant, diverse, and well preserved. This site was clearly affected by the high fertility of the Peru Current, since the assemblages are similar to those cored at DSDP Sites 157 and 321 to the south. The basal Quaternary samples contain sparse Dictyocha perlaevis ornata, a typical upper Pliocene and lowest Quaternary taxon in the Atlantic. This species may prove useful in biogeographic studies between the Pacific and Atlantic oceans, but was not recognized at the Siqueiros sites because of dissolution. The occurrence of silicoflagellates at Sites 419, 420, 422, 425, 427, and 428 is shown in six separate figures which are interspersed through the text. These figures also record the number of opal phytoliths and Thalassiosira leptopus elliptica (diatom) encountered during the silico flagellate counts. Opal phytoliths are skeletal elements from terrestrial grasses that are an aeolian addition to the sediment (Bukry, this volume). Thalassiosira leptopus elliptica is a large distinctive form which was reported to occur only in the Quaternary of the tropical eastern Pacific (Bukry and Foster, 1973). That relation is supported by its near absence from the Pliocene at the four sites of Leg 54 where Pliocene was recovered, dissolution withstanding. See Barron (this volume) for taxonomic and stratigraphic discussion of this subspecies. Presence, absence, and blooms of the giant diatom Ethmodiscus rex are also indicated on the figures.

Silicoflagellate Biostratigraphy and Paleoecology in the Eastern Equatorial Pacific, Deep Sea Drilling Project Leg 54