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Our study in lithologically diverse lower Paleogene deposits of the former southern USSR and West Siberia resolves three sedimentary provinces: (i) Southern (Crimea-Caucasus and Central Asia), dominated by biogenic calcareous sediments of the deepest basinal portions; (ii) Transitional (southern Russian craton and Turan plate), displaying calcareous and siliceous clayey deposits; and (iii) Northern (central Russian craton and West Siberian plate), showing biogenic siliceous and terrigenoussiliceous sediments of basinal margin.We used standard and regional scales based on seven microfossil groups (planktonic and benthic foraminifers, nannoplankton, radiolaria, diatoms, silicoflagellates, and dinocysts) to correlate various sediment facies among ∼ ∼80 reference sections across the study area. Correlation was performed for the Transitional province, where the presence of both calcareous and siliceous facies affords the use of calcareous and siliceous planktonic and dinocyst scales with varying resolutions. Besides, in the Transitional province, we established regional sedimentary cycles corresponding to the late Thanetian, Ypresian, and late LutetianBartonian. These cycles are traceable into the Northern province, enabling us to determine or refine ages of the sequences, and into the Southern province.Each sedimentary supercycle falls into three units, often with hiatus at the base, correlatable to 3rd order eustatic cycles and featured by distinctive facies. The facies succession appears to reiterate through the upper Thanetian, Ypresian, and upper Lutetian-Bartonian cycles.The lower units of each supercycle are composed of calcareous sediments rich in nannofossils and foraminifers and are traceable from the Southern province into the Transitional province, the late Lutetian unit extending as far as the Northern province. 239Radionova, E.P.,

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On DSDP Leg 67, drilling was done at seven sites (494-500) during the months of May and June, 1979 (Fig. 1). These sites form a transect across the Middle America Trench off Guatemala. Nannoplankton were recovered at each site; biostratigraphic results are summarized in Table 1. The nannoplankton zonation is based on that of Bukry (1973; 1975). Stratigraphic distribution of taxa are shown in Figures 2 through 9. SITE RESULTS Site 494 (Holes 494 and 494A) Located at the foot of the landward slope, this site was planned to examine the validity of the accretionaryprism-trench-slope hypothesis. Three defects of the section concern this study: (1) the low percentage of core recovery, (2) the high degree of disturbance, and (3) many intervals are without nannoplankton or contain very poor nannoflora. Nevertheless, this site is the most interesting one of the Leg, owing to its unexpected stratigraphy (Fig. 2). Quaternary. Emiliania huxleyi/Gephyrocapsa oceanica Zones, Crenalithus doronicoides Zone (Samples 494-1-1, 30-32 cm to 494-4.CC; Samples 494A-1-1, 40-42 cm to 494A-16,CC). Nannoflora are relatively diverse and numerous in diatomaceous mud and mudstone of Hole 494 and in Cores 1 to 8 of Hole 494A. Beneath Core 494A-9 there are very rare samples with poor nannoflora only. The assemblage of the Emiliania huxleyi/Gephyrocapsa oceanica Zones consists of G. oceanica (rare to common), small Gephyrocapsa and Emiliania (rare to common), Helicosphaera carteri, Calcidiscus leptoporus, Emiliania ovata, and E. annula (Samples 494-1-1, 30-32 cm to 494-4,CC; Samples 494A-1-1, 40-42 cm to 494A-2-5, 30-32 cm). The lower Pleistocene Crenalithus doronicoides Zone (Samples 494A-4,CC to 494A-16,CC) contains a similar assemblage but without G. oceanica. Upper Pliocene. Discoaster brouweri Zone (Samples 494A-17-1, 44-46 cm and 494A-17-2, 44-46 cm). The scanty assemblage of this interval includes D. brouweri (few), small Gephyrocapsa and Emiliania, and Helicosphaera carteri. Core 18 to Section 494A-20-4 are barren. Middle Miocene. ?Discoaster exilis Zone (Samples 494A-20-4, 125 cm to 494A-22-3, 20-22 cm). The upper

Summary The Middle America Trench off Guatemala was transected by 24-channel seismic reflection surveys, seismic-refraction surveys, and drilling with the Glomar Challenger . The drilling was done at three sites on the oceanic Cocos plate and four sites on the Caribbean plate. These plates converge at about 10 cm yr −1 . At all drill sites sediment of upper Miocene to Quaternary age is almost entirely hemipelagic mud with interbedded thin volcanic ash, except in the trench where mud and fine sand turbidites less than 400 000 yr old are ponded. However, the underlying rocks are very different. On the oceanic Cocos plate a basal chalk sequence of lower and middle Miocene age is overlain by a thin section of abyssal clay. At a site only 3 km landward of the trench axis where drilling penetrated the slope deposits we recovered a Cretaceous to lower Miocene claystone sequence resting on a section containing igneous rock of continental affinity. A large net subduction of sediment along with ocean crust has occurred during the present (Miocene-Quaternary) episode of subduction and perhaps parts of the continental framework have been subducted as well. However, no current model satisfactorily explains the surprising occurrence of Cretaceous-Miocene claystone at the foot of the trench slope.