The diatom-rich unit occurs from the lower part of Sections 199-1219A-20H-4 through 22H-2. The interval of maximum diatom content is clearly revealed by a color change from reddish brown sediments dominated by radiolarians to grayish green diatomaceous sections. The color change is gradual in the lower part of the unit and more abrupt (but still gradual) in the upper part of the unit. The sediments are intensely bioturbated.
Preservation of diatom flora is moderate to poor because of intense dissolution of the tests (J. Fenner, pers. comm.). Dissolution of diatoms is species-selective (see discussion in Berger, 1976). Robust species of the Centrales group are predominant, whereas other taxa are almost impossible to identify, mostly because of their fragmented occurrence. In contrast to this, radiolarians are mostly well preserved, although dissolution features are present in delicate shells. Some of the robust radiolarians show rounded protuberances (e.g., the spines of the actinommids or periphaenas), and the ragged outlines of many spongodiscids is also evidence of dissolution. Artostrobids are typically dissolved at their collarlike base.
The radiolarian shells settled gently to the bottom, in water that may evidently have been moving slowly; afterward came bioturbation and compaction of the sediment. The reason for suggesting that there may have been slow-moving currents is that there is evidence in some places of both original layering and size-sorting of shells. This layering has been modified by differential compaction, forming small lenses in which grains are concentrated. The lenticular spaces of low grain density are filled with long-spined radiolarians and ornamented forms. Between the larger radiolarians, the rock matrix is composed of small radiolarians, diatoms, calcareous nannofossils, and clay.
The entire diatom-rich unit is composed of three parts. The basal part, above the color change from the reddish radiolarites to grayish to greenish diatom-radiolarian mixed lithologies, is characterized by thin-layered banding. The central part of the unit begins at a relatively abrupt color change, with thick color bands retaining the color of the basal part. The unit terminates with a gradational color change from greenish gray to reddish gray and reddish, returning to the normal Eocene-type radiolarite. The upper part of the unit still contains considerable numbers of diatoms in Core 199-1219A-20H.
Smear slides of older samples show that there is a precursor diatom event within Section 199-1219A-23H-4 (middle Eocene; radiolarian Zone RP-14; Podocyrtis mitra Zone) (cf. Sanfilippo and Riedel, 1985). Above Zone RP-15, the diatom content of late Eocene radiolarian sediments is 0%–5%, which does not represent a distinct diatom interval. The Zone RP-15 diatom episode is important because high numbers of calcareous nannoplankton are present. The sedimentologic overview (Fig. F3) demonstrates that calcareous nannoplankton occurs earlier than the diatoms that start just at the beginning of the Podocyrtis chalara Zone in Section 199-1219A-24H-2. The alternating layers of diatom and radiolarian deposits are characterized by a distinct color banding (Fig. F4).