LITHOSTRATIGRAPHY

Site 1136 is located in 1948.4 m of water on the eastern margin of the SKP, 8 km west of the Labuan Basin. This site is located 32 km east of Site 1135 and was chosen because the sediments overlying basement appeared on seismic profiles to be only ~150 m thick. Hole 1136A was rotary cored continuously to a depth of 161.4 mbsf. Sediments were recovered from approximately the upper 130 m, and basalts were recovered from approximately the lower 30 m of the hole (Fig. F4). The sedimentary section consists of ~66 m of pelagic ooze that overlies about 48 m of zeolitic calcareous silty clay and clayey sand (Table T3). We recognize five sedimentary units in Hole 1136A. The basalts at the base of the hole are designated as Unit VI and are described in "Physical Volcanology," "Igneous Petrology," "Alteration and Weathering," and "Structural Geology". Cores 183-1136A-1R (0-4.70 mbsf) and 8R (60.90-70.30 mbsf) had no recovery; hence, they are not included in any of the sedimentary units (Fig. F4). Core recovery was poor throughout the sedimentary section of the hole. Even in the upper 75 m of the hole, where recovery appears to be relatively high (Fig. F4), portions of many of the recovered intervals are very disturbed by drilling and appear to be flow-in.

Unit I is highly disturbed mixed clastic sediment (intervals 183-1136A-2R-1, 0-44 cm, and 95-118 cm) and ooze (interval 183-1136A- 2R-1, 44-118 cm). Pebble lithologies are varied, including granite and gneiss. The sand, granules, and pebbles are ice-rafted debris that was concentrated during drilling, probably by washing out of the fine-grained pelagic sediment. The boundary with Unit II is arbitrarily placed at Section 183-1136A-2R-1, 118 cm, which is the lowest occurrence of concentrated sands and pebbles. We recognize that all the sediment recovered from Unit I has been highly disturbed by drilling, and, thus, the true composition of the sediment in this unit is uncertain.

Unit II consists of white foraminifer-bearing nannofossil ooze that is similar to the ooze recovered in Subunit IIA of Hole 1135A. All of the cores in Unit II of Hole 1136A contain soupy intervals, suggesting that much of the recovered material flowed into the cores. Scattered pebbles and chert fragments are found at several locations. Within undisturbed intervals, the ooze is homogeneous and bioturbation is absent to slight. The carbonate content of this unit is 89-95 wt% CaCO₃(Fig. F4). The upper and lower boundaries of Unit II are uncertain because of the absence of recovery in Cores 183-1135A-1R and 8R.

This unit consists predominantly of very light brown foraminifer-bearing calcareous ooze. Some undisturbed intervals contain sediment that is partially cemented and lithified. The carbonate is predominantly micrite, and a sample of ooze from the top of the unit yielded a carbonate content of 92 wt% CaCO₃ (Fig. F4). X-ray diffraction (XRD) analyses indicate the presence of only minor amounts of clay minerals and possibly quartz (Table T4). The light brown color is caused by disseminated (<5%), silt-sized palagonitized basaltic glass shards. These yellowish grains are as much as 0.3 mm in diameter, generally oblate or tabular, and subangular to subrounded. They are nonvesicular and do not contain crystallites; however, they display internal polygonal ("jigsaw") quench textures. Inoceramid fragments are present in Sections 183-1136A-9R-6 and 9R-CC. A pebble at the top of Core 183-1136A-9R was most likely emplaced during drilling. Several intervals of the core are disturbed or soupy.

The dominant lithology in Unit IV is brown zeolitic calcareous volcanic clayey sand, which constitutes a 1-m-thick bed in Core 183-1136A-10R. This bed is reversely graded, with medium sand in interval 183-1136A-10R-1, 0-25 cm, and fine sand below. The sand is well sorted and individual grains are subangular to subrounded and moderately spherical. Several rounded, centimeter-sized pebbles in the middle of the bed are of a similar dark brown indurated lithology. The composition of the sand is predominantly white grains of carbonate and aggregates of zeolite. Minor amounts of orange to yellow palagonitized basaltic glass shards, minor green glauconitic foraminifer casts, and trace amounts of opaque silt also are found in the sand. The palagonitized shards range up to 1 mm in diameter, are nonvesicular, do not contain crystallites, and display granulated textures suggestive of quenching. These shards resemble those in the overlying Unit III. Calcareous nannofossils are rare.

In smear slides, abundant zeolites are observed to be tabular prisms as much as 20 µm long. XRD indicates that these zeolites are dominantly clinoptilolite with minor heulandite. XRD also indicates that minor amounts of nontronite (a dioctahedral Fe smectite) and plagioclase are present. X-ray fluorescence (XRF) analysis of two samples indicates a total Fe content of 13%-14%, which is consistent with the estimated abundance of the Fe-bearing components (palagonite, glauconitic foraminifer casts, and nontronite).

A thin interval of breccia at the base of Unit IV (interval 183-1136A-1R-1, 87-92 cm) consists of brown angular fragments in a brown matrix. The material is noncalcareous and possibly silicified. The fragments are internally brecciated, indicating repeated brecciation. The contact between this breccia and the overlying sand appears to be sharp; however, the relationship between this breccia and the overlying sand is not clear.

Unit V is dark greenish gray carbonate-bearing zeolitic silty clay. This unit is massive, except for a thin sand bed in interval 183-1136A-14R-1, 30-40 cm, and a thin, normally graded sand bed with a sharp base in interval 183-1136A-14R-1, 139-140 cm. Fossil debris is common, including bryozoans, bivalves, and sponge spicules. The fossils do not appear to be in growth positions; however, the presence of an articulated bivalve suggests a quiet depositional environment. Foraminifers, including both planktonic and benthic species and calcareous nannofossils, are present (<5%). Palagonitized basaltic glass shards also are present (<5%). Burrows are common. Dark green clay is found in streaks and as halos around burrows.

The carbonate content of Unit V sediment is relatively low (12-18 wt% CaCO₃) (Fig. F4). The XRD analyses indicate calcite, clinoptilolite, heulandite, glauconite, and minor nontronite are present in the sediment. An analysis of the dark green clay from Core 183-1136A-15R indicates that it is a mixture of celadonite, saponite, and nontronite. XRF analyses indicate 5-8 wt% total Fe₂O₃, which probably is in celadonite, palagonite, glauconite, and/or nontronite.

Unit V appears to directly overlie the basalt basement; however, the nature of this lower contact is uncertain. Drilling records indicate that basalt was encountered at ~133 mbsf (see "Operations"), slightly deeper than the base of Unit V, which was placed at the contact between Unit V sediments and basalt (128.08 m) in Core 183-1136A-15R. Three pieces from Core 183-1136A-15R, presumably from the base of Unit V, are particularly rich in fossil fragments, including bryozoans, bivalve, and ostracode fragments.

Lower Cretaceous basalt flows (Unit VI) are mantled by fine-grained volcanic sediments of Unit V, which were deposited in a neritic environment beginning in early Albian time. This sediment was deposited below wave base and the high diversity of shelly fossils (bivalves, bryozoans, ostracodes, and benthic foraminifers) suggests a shallow shelf environment, whereas the presence of calcareous nannofossils indicates communication with the open ocean. The oldest sediments were altered, along with the uppermost basalts (which host identical celadonite veins, see "Alteration and Weathering") in this shelf environment. The overlying brown volcanic sand (Unit IV) was probably also deposited in a shelf environment, and its grain size and size-sorting, and the presence of rounded pebbles indicate a somewhat higher energy environment. The intervening breccia (at the base of Unit IV) is probably a result of hydrothermal alteration rather than subaerial exposure.

The calcareous ooze of overlying Unit III records deepening to pelagic conditions by Turonian/Santonian time. Sparse palagonitized basaltic glass shards, with internal quench textures, indicate either minor submarine volcanism through this interval or reworking of older material. The oozes of Unit II document pelagic sedimentation during Eocene time, with major hiatuses before (Late Cretaceous to Paleocene) and after (Oligocene to Pleistocene) deposition of this unit.