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The Upper Cretaceous and Cenozoic facies along the distal continental margin to the west of Portugal at Site 398 includes deposits of mass-gravity flows and the suspended-sediment regime. The deposits record the interplay of fine-grained sediment deposition, bottom currents, mass-gravity flows, physical oceanography, and tectonic activity. This continental margin is a passive type where deposition is characterized by a predominance of sediments which originated from differential pelagic settling processes. Deposits from suspensates grade from carbonate-rich pelagic deposits, to calcareous poor hemipelagic deposits, to carbonate-free claystones. The distribution of these three main sediment types is controlled primarily by terrigenous dilution, dissolution of calcareous components, and bottom currents. Turbiditic deposits are a ubiquitous lithofacies component of the stratigraphic section, but are more important in the lower Cenozoic than in the other stratigraphic intervals. Deposits of slumps, debris, and mud flows are well represented in some core sections, but are volumetrically less important compared to the other lithofacies. Two facies associations, based on petrologic and structural characteristics, represent distinct natural lithologic affinities. A rhythmic association consists of alternating turbiditic and pelagic sediments. Each rhythm is composed of turbiditic and pelagic divisions, and represents deposition by a turbidity flow of local importance. Deposits of slumps, debris, and mud flows are also present in this facies association. A cyclic association consists of alternating beds of carbonate-rich pelagic deposits and carbonate-poor hemipelagic deposits. Base cut-out turbiditic sequences are irregularly distributed throughout this association. At Site 398, the lower carbonate-poor division of the cycle results from increased terrigenous dilution, associated with a minor loss of calcite through dissolution. The carbonate cycles reflect major changes of the environmental conditions associated with deposition. In contrast to the deposits of the rhythmic association, the carbonate cycles might be correlated along extensive areas of the continental margin. The vertical stratigraphic succession at Site 398 is characterized by five facies units: (a) Unit 1 (Upper Cretaceous-Paleocene) is a pelagic facies which developed near or below the carbonate compensation depth; (b) Unit 2 (Paleocene-early Miocene) represents the rhythmic facies association and reflects a major influx of clastic sediments off the continental margin; (c) and (d) Units 3 and 4 (Miocene) are transitional facies between the rhythmic and the cyclic facies associations; and (e) Unit 5 (Pliocene-Pleistocene) is a carbonate cycle which records climatic and eustatic changes during the Quaternary. INTRODUCTION gal on the western flank of a deep trough. The trough, trending northeast-southwest across the slope and conSite 398 is located on the lower continental rise in a tinental rise between the Iberian Peninsula and Vigo small bathyal platform, at a water depth of 3910 meSeamount, at present precludes major access of sediters on the southern margin of Vigo Seamount (Figure ment input from the continent to Site 398. This region 1). The site was drilled 200 km off the coast of Portubelongs to the passive continental margin province of

Upper Cretaceous and Cenozoic Depositional Processes and Facies in the Distal North Atlantic Continental Margin off Portugual, DSDP Site 398