(Table 1) Light minerals in sand at DSDP Leg 84 holes

The Middle America active continental margin is the best-sampled active plate margin to date, having been drilled during Legs 84, 67, and 66. With nine sites drilled on the continental slope of Guatemala and an additional site drilled on the Costa Rican slope, a summary of slope sediments and sedimentary processes can be made. Sediments are easily subdivided into a thick apron of Neogene and Quaternary volcanically derived hemipelagic and turbidite mud and mudstone and a thinner, more varied assemblage of mostly Paleogene mudstone, radiolarian mudstone, and limestone. This latter assemblage may contain hiatuses or be completely lacking between slope deposits and basement. Cores from the foot of the continental slope (Core 567A-19) consist of Campanian micrite. The pre-Neogene section is much thicker and of more terrigenous provenance beneath the forearc basin landward of the forearc structural high than on the continental slope. Sedimentary processes of the Neogene and Quaternary slope sediments include reworking of hemipelagic and turbidite deposits. Redeposition by slumping, plastic flow, and turbidity current-documentable through benthic foraminiferal analysis-occurs in intracanyon and canyon settings. Erosion by slumping and by turbidity current and deposition of mud or sand in canyons and in local depressions on the continental slope and different rates of sediment accumulation result in dramatic thickness variations of lithologic units over small distances in localized pockets of sand in small filled canyons on the slope or in sediment ponds, and in high-relief basement topography. The age of sediment overlying igneous basement ranges from Cretaceous to Quaternary. Gas hydrate was visible or inferred present at every site drilled during Leg 84. Nevertheless, except for a small amount in the last core, it was not recovered in sufficient quantities to be visible at Site 568, a site specifically chosen for the study of hydrate and located near Site 496, which was abandoned during Leg 67 because of the dangerous abundance of hydrates. The association of hydrate with porous, coarser sediment results in a distribution as localized and unpredictable as the slope sands off Guatemala, which do not occur in beds coherent enough to produce acoustic reflection. Although the normal lithologic section at Sites 567 and 496 limits the volume of sediment that could be part of an accretionary prism offshore Guatemala and the volume of sediment in the Trench axis is not sufficient to argue for significant accumulation of Cocos Plate sediments, the varied lithology and attenuated thickness of pre-Neogene sediment seaward of the forearc structural high do not exclude earlier accretion from the history of the Guatemalan continental margin.

Calcareous nannofossil stratigraphy of ODP Holes 173-1065A and 173-1069A

Upper Jurassic calcareous nannofossil assemblages have been studied from strata cored over basement blocks now buried under the Iberia Abyssal Plain (Ocean Drilling Program Leg 173 Sites 1065 and 1069). The youngest Jurassic assemblages at each site are Tithonian in age, the same as those at nearby Leg 149 Site 901, an age that predates the breakup of the Iberia continental margin. The paucity of the assemblages, the prevalence of coccospheres, and the relatively high organic contents of the fine clastic sediments in which they occur are characteristic of a restricted interior basin that had little communication with the open ocean. During the major rifting episode (a Berriasian event), the Jurassic sequences were dispersed along with their underlying blocks of presumed continental crust across the ocean-continent transition of the Iberia Abyssal Plain, probably as a result of detachment faulting.

Abundance of benthic foraminifera in late Paleocene to eraly Pliocene sediments of DSDP Hole 74-525A on the Walvis Ridge, South Atlantic (Table 1)

Benthic forammifers in the size-fraction greater than 0.073 mm were studied in 88 Paleocene to Pleistocene samples from Deep Sea Drilling Project Site 525 (Hole 525A, Walvis Ridge, eastern south Atlantic). Clustering of the samples on the basis of the 86 most abundant foramimfers (in total, 331 taxa were identified) allowed separating two major assemblage zones: the Paleocene to Eocene interval, and the Oligocene to Pleistocene interval. Each of these, in turn, were subdivided into three minor subzones as follows: lower upper Paleocene (approx. 62.4 to 57 8 Ma); upper upper Paleocene (56.6 to 56 2 Ma), lower and middle Eocene (55.3 to 46 8 Ma); upper Oligocene to middle Miocene (25.3 to 16 Ma), middle Miocene to Pliocene (15.7 to 4.2 Ma), and lower Pleistocene (0.4 to 0.02 Ma), with only minor differences with the previous zone. Some very abundant taxa span most of the column studies (Bolivina huneri, Cassidulina subglobosa, Eponides bradyi, E. weddellensis, Gavelinella micra, Oridorsalis umbonatus, etc.). Several of the faunal breaks recorded coincide with conspicuous minima in the specific diversity curve, thus suggesting that the corresponding turnovers signal the final stages of periods of faunal impoverishment. At least one major bottomwater temperature drop (as derived from delta18O data) is synchronous with a decrease in the forammiferal specific diversity. On the other hand, a specific diversity maximum in the middle Miocene might be associated with a delta13C increase at approx 16 to 12 Ma. Highest foraminiferal abundances (up to 600-800 individuals per gram of dry sediment) occurred in the late Paleocene and in the early Pleistocene, in coincidence with the lowest diversity figures calculated. The magnitude of the most important faunal turnover recorded, between the middle Eocene and the late Oligocene, is magnified in our data set by the large hiatus which separates the middle Eocene from the upper Oligocene sediments. Considerably smaller overturns occurred within the late Paleocene (in coincidence with changes in the specific diversity, absolute abundance of forammiferal tests, and delta13C), and in the middle Miocene (in coincidence with a specific diversity maximum and a delta13C excursion). New reformation on the morphology and the stratigraphic ranges of several species is furnished. For all the taxa recorded the number of occurrences, total number of individuals identified and first and last appearances are listed.