At Site 1205 one hole was RCB drilled, penetrating sediment before reaching basement at 42.7 mbsf. Five sediment cores with very low recovery (2%-16%) were obtained. The character of the sediment changes considerably between cores, and we identified five sedimentary units (I-V) in the sequence, which are described below (Fig. F2). Chemical analyses show a rapid downhole decrease in carbonate (C and CaCO3) abundance (Fig. F3; Table T2). Several types of calcite-cemented silty sandstone were recovered, containing well-rounded to subrounded basalt clasts, bioclasts, and glassy ash fragments, as well as minor clay (Fig. F4A; also see "Site 1205 Smear Slides"). A notable component of the recovered sediment is silt-sized grains of hematite and other Fe oxides (Fig. F4B; see also "Site 1205 Smear Slides").
Bioclasts include several recognizable fossil species, indicating varying but relatively shallow-water depths for the depositional environment, as documented by previous results of DSDP Leg 55, which penetrated the same sedimentary section at DSDP Site 432 (Jackson, Koizumi, et al., 1980).
This interval consists of olive to dark gray silty sandstone cemented by calcite and containing well-rounded to subrounded basalt fragments, bioclasts, volcanic glass fragments, and minor clay. The bioclasts are benthic foraminifers, bryozoan fragments, and coralline red algae (Fig. F5). The inferred bryozoan fragments and the rounded pebbles of basalt and basaltic minerals (olivine, clinopyroxene, and plagioclase) are abundant.
Dark yellowish brown (10YR 4/4) fine-grained vitric sandstone contains palagonitic volcanic glass and hematite in a calcite cement. Bioclasts are benthic foraminifers, bryozoan fragments, and coralline red algae. Volcanic glass fragments in different stages of palagonitization dominate. Opaque grains of hematite are common. Bryozoan "worm fragments" increase downhole, giving the sediment a whitish yellow-brown appearance.
Unit III is a dark olive-gray (5Y 3/2) to light gray sandy siltstone with prominent "wriggling" wisps of white bryozoans. Another distinguishing feature is indistinct "mud clasts," which are mainly present in interval 197-1205A-2R-1, 22-60 cm, and are composed of opaque minerals (15%), brown clay (60%), and calcite (25%). Rounded rock fragments (basalt and tuff) are present together with bioclasts (benthic foraminifers, red algae, and bryozoans).
Although separated from Section 197-1205A-3R-1 by a nonrecovered interval as thick as 17 m, we infer that Unit III continues through Core 197-1205A-4R, where it is a dark grayish brown (10YR 4/1) sandy siltstone with prominent white bryozoans throughout. In this lower part, the core is highly fractured by drilling. Carbonate debris of bryozoans and foraminifers is present with volcanic clasts and opaque minerals cemented by calcite. In interval 197-1205A-4R-1, 0-112 cm (23.7-24.82 mbsf), the sediment is a dark olive-gray (5Y 3/2) calcareous siltstone with abundant secondary phillipsite and opaque minerals (probably hematite). Bryozoans stand out clearly from the dark silty background.
Unit III has a fragile porous appearance in Sections 197-1205A-2R-1 and 3R-1. Farther downcore it contains a more loosely packed siltstone disturbed by drilling. At 24.3 mbsf the recovered sediment has been completely disturbed and reworked into a drilling slurry. The core catcher contains a 3-cm-long calcite-filled vug and shell fragments, which are indicative of a nearshore depositional environment.
Unit IV contains a very dark grayish brown (2.5Y 3/2) to very dark gray (5Y 3/1) weakly graded silty sandstone. Beds of brownish yellow (10YR 6/8) shell fragments are present in intervals 197-1205A-5R-1, 9-12 cm, 39-42 cm, and 59-61 cm. The bioclastic fragments have a size of ~5 mm. Charcoal and wood fragments are present in Section 197-1205A-5R-1 at 26 and 41 cm.
This unit is a calcareous conglomerate (Fig. F5B) with 5- to 90-mm rounded to subrounded clasts of fine-grained to aphanitic dark green basalt as well as secondary hematite fragments. The matrix is calcite cement with bioclastic and volcaniclastic components ranging in size from 2 to 12 mm. In thin section the matrix displays a plethora of bioclastic forms. The observed species are bivalves (pelecypods [45%]), red algae (40%), foraminifers (5%), algal borings (8%), and micrite envelopes (2%).
The color varies in shades of red, green, yellow, and gray, depending on the proximity of particular clasts. The volcanic components are highly altered, and it is difficult to recognize the protolith.
The sandstone consists primarily of resedimented volcanic material, where the chief constituent is slightly rounded altered glass fragments, indicating minimal abrasion during transport. Bryozoans and red algae live in water depths of 20-30 m or less. The basaltic clasts are well rounded, suggesting transport and deposition in a relatively high-energy erosional environment. These varying indicators all suggest deposition in a beach or near-beach environment.
Farther downhole, horizons of red soil indicate subaerial weathering between lava flows (see "Physical Volcanology and Igneous Petrology"). The geologic history revealed at Site 1205 documents a subsiding volcanic island, resulting in subaerial units becoming capped by marine sediment deposited in a shallow-water, high-energy environment (see also Jackson, Koizumi, et al., 1980). The siltstone interval 197-1205A-4R-1, 0-112 cm, contains secondary phillipsite, indicating substantial burial of the sequence, which is typical of subsiding volcanic islands.