Dark greenish gray, dark gray, and greenish gray-colored sediments are pervasive, although minor gray to black volcanic ash layers occur intermittently. Terrigenous components such as quartz and feldspar are relatively invariant across the boundary of Units I and II, in contrast to the downsection disappearance of biogenic material. The primary lithologies of Unit III are dark greenish gray to very dark greenish gray silty clay, clay with silt, clay, and clay with diatoms. No biogenic sediment is found in the unit except for trace amounts of siliceous microfossils within the upper portion. The coarse fraction of Unit IV is similar to Unit III, except that it contains slightly smaller amounts of quartz and larger amounts of sulfides.
Biogenic material occurs sporadically and intermittently throughout the sequence.
In Unit I it includes both calcareous and siliceous material, but in Unit III it consists solely of siliceous material. The alternating biogenic-bearing and nonbiogenic sediments may reflect climatically driven changes over a long time scale. The boundary between Units II and III probably reflects the onset of increased glaciation during the Pliocene. At this level, quartz and feldspar increase markedly upsection, the first dropstones occur, and changes in the suite of clay minerals occur as indicated by changes in natural gamma-ray emissions. The geochemistry of the sedimentary sequence is characterized by processes typical of sulfate reduction, and processes reflecting alteration of volcanic ash within the sediments and basement basalts below.