Scanning electron microscope (SEM)-based analyses of the laminated diatom oozes encountered during Leg 138 reveal three major laminae types. The first lamina type is composed of multiple layers of approximately 20 mu m thick diatom mats, which form laminae dominated by assemblages of the pennate diatom, . More than one variety/subspecies of T. longissima occurs within these laminae (referred to as the T. longissima Group). The second lamina type is composed of a mixed-assemblage of several species of diatoms (centric and pennate varieties), calcareous nannofossils, and subordinate quantities of radiolarians, silicoflagellates and foraminifera. The third lamina type is dominated by an assemblage of nannofossils and minor amounts of those fossil components mentioned above. This last form of lamination is compositionally similar to the background sediment type, foraminifer-nannofossil ooze (F-NO). Two lamina associations occur within the laminated intervals; the first comprises of alternations of T. longissima Group and mixed-assemblage laminae (average thickness is approximately 6 mm) and the second is composed of T. longissima and nannofossil-rich laminae (average thickness is approximately 3.5 mm). The arrangement of laminae probably originates from the deposition of multiple layers of 20 mu m thick mats from one mat-flux episode. The much thinner nannofossil-rich laminae are interpreted to represent periods of more 'normal' deposition between mat-flux episodes. The occurrence of several varieties/subspecies of T. longissima within individual mat layers is consistent with observations of Rhizosolenia diatom mats in the modern world ocean.