This report deals with results of the first stage of a complex study of the predominantly Lower Cretaceous sediments cored at Site 398 at depths from 945 to 1740 meters. The section studied is overlain by 35 meters of Cenomanian sediments, which are similar lithologically to the underlying sequence. We studied 370 samples using the same methods as described in our chapter in Volume 47, Part 1. The main goal is to define more precisely the stratigraphy of Lower Cretaceous sediments and provide some tentative conclusions on evolution and environmental conditions of the depositional basin. Lithology and mineralogy were compiled by V.Ya. Kaban'kov, B.G. Lopatin, and D.K. Patrunov; biostratigraphy by V.A. Basov; clay minerals by V.M. Lazurkin; geochemistry of clays by I.S. Gramberg; and organic matter by A.I. Danyushevskaya. The conclusions were written jointly and edited by B.G. Lopatin. LITHOLOGY AND MINERALOGY Lithologic units: We recognized three major lithologic units within the Lower Cretaceous (plus Cenomanian) sequence. Upper Unit 3 is divided into Subunits a, b, c, and d, each being gradational to one another. The lower unit (1) corresponds to Unit 5 of the Shipboard Summary and Unit 2 corresponds to the lower part of Unit 4c. The contact between our Subunits 3a and 3b is close to the contact between Sub-units 4c and 4b. Finally, the boundary between our Sub-units 3c and 3d is close to that of Sub-units 4a and 4b of this summary. In summary, our lithostratigraphic column does not differ much from that of the shipboard scientists. As usual, the lithostratigraphic and biostratigraphic boundaries are not coincident (Figure 1). Unitl Unit 1 (Cores 138 and 130) is of Hauterivian(?)-Barremian age and is composed mainly of massive or laminated pelitomorphic limestone with varying amounts of clay and silt. The limestone is interbedded with thinly laminated calcareous-clayey siltstones. Brecciated structure in the limestone is common as a result of slumping, deformation or partial scour and redeposition of calcareous clays. Silt-sized fragments of quartz, feldspar, and muscovite are common, along with carbonate clasts.

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