LITHOSTRATIGRAPHY

Hole 1184A was washed to 134.4 mbsf, then continuously cored to 538.8 mbsf. Recovery was excellent. We divided the recovered sequence into two units: calcareous ooze above 201.1 mbsf and generally coarse-grained volcaniclastic rocks below. An ~1-cm-thick ferromanganese crust separates the two units. Seismic reflection data show that the boundary between these two units dips 4° to the north-northeast (18°). Reflectors within Unit II dip ~9° in about the same direction (0°), whereas reflectors within Unit I are horizontal and onlap the surface between the units (see "Background and Objectives"). Paleontological results suggest that most of the volcaniclastic rocks were deposited during the middle Eocene and that the ~65 m of ooze recovered was deposited during the early Miocene. The sedimentary record of the ~20 m.y. between the middle Eocene volcaniclastic phase and the early Miocene pelagic phase of deposition is missing or represented by the ferromanganese crust.

Sedimentary intervals recovered from Hole 1184A are composed of an upper interval of calcareous ooze (Unit I) overlying a lower interval of volcaniclastic rocks (Unit II). The volcaniclastic rocks are divided into five subunits (Fig. F10).

Unit I

Interval: 192-1184A-2R-1, 0 cm, to 8R-CC, 10 cm

Depth: 134.4-201.1 mbsf

Age: early Miocene

Lithology: nannofossil foraminifer ooze

Unit I (Fig. F11) is composed predominantly of white, homogeneous, nannofossil foraminifer ooze to foraminifer nannofossil ooze (95 wt% CaCO₃; Table T3) and contains 10% siliceous microfossils. Coring began in this unit at 134.4 mbsf (Section 192-1184A-2R-1, 0 cm). The calcareous ooze probably extends up to the seafloor. The recovered interval contains early Miocene microfossils (see "Biostratigraphy"). In Core 192-1184A-8R, the ooze is broken into 10- to 20-cm-long pieces (biscuits) set in a slurry of material disturbed by drilling. This type of recovery is common in ooze near the chalk-ooze transition, but no chalk was recovered from Hole 1184A. Thus, Unit I in Hole 1184A corresponds to Subunit IA (Neogene ooze) at those other sites drilled on the Ontong Java Plateau in which the sedimentary sequence was divided into units (e.g., Andrews, Packham, et al., 1975; Kroenke, Berger, Janecek, et al., 1991; see "Lithostratigraphy" in the "Site 1183" chapter). The base of Unit I is placed at 201.1 mbsf between the ooze and a ferromanganese crust on top of volcaniclastic Unit II. The unit boundary occurs between Cores 192-1184A-8R and 9R. The unit boundary shows up as a step in color and magnetic susceptibility traces (Fig. F10), but, because the unit boundary is between cores, the sharpness of the contact is unknown.

Variations in color and texture within Unit I are generally very subtle. The white ooze has a slight greenish tinge in Cores 192-1184A-2R and 4R, and the greenish intervals seem to contain a slightly higher abundance of siliceous microfossils and foraminifers than the whiter intervals. Faint burrow mottling and discrete burrows are present where color varies. We observed no other trace fossils or physical sedimentary structures.

Volcanic ash is a minor component of the sediment in Cores 192-1184A-4R through 7R. Ash-rich layers are present in Cores 192-1184A-4R, 5R, and 7R and show up as small spikes in the magnetic susceptibility record (Fig. F11). In addition, isolated burrows in Cores 192-1184A-4R to 7R are filled by a light gray mixture of ash and ooze, suggesting additional eruptive events.

Very faint, 1- to 2-cm-thick, subhorizontal color bands are present in Cores 192-1184A-6R and 7R. These bands are broader and fainter than, but otherwise similar to, the Liesegang banding within the calcareous ooze and chalk of Hole 1183A (see "Lithostratigraphy" in the "Site 1183" chapter). Rare dark flecks throughout Unit I may represent authigenic pyrite or ferromanganese oxide grains.

Unit II

Interval: 192-1184A-9R-1, 0 cm, to 46R-1, 132 cm

Depth: 201.1-538.8 mbsf

Age: middle Eocene

Lithology: tuff, lapilli tuff, and lapillistone

Unit II represents a succession of >330 m of volcaniclastic rocks. The most common lithologies are tuff, lapilli tuff, and lapillistone containing lithic and vitric clasts (Fig. F12). The tuff is dominated by sand-size volcaniclastic material, whereas the lapilli tuff and lapillistones are characterized by pebble-size volcaniclastic material in a sand-size matrix (see "Lithostratigraphy" in the "Explanatory Notes" chapter). Accretionary and armored lapilli are present throughout most of Unit II (Fig. F13). We interpret most of the unit as redeposited material derived from primary deposits of explosive hydroclastic volcanic eruptions (see "Igneous Petrology"). Rare ash-fall layers, wood, and organic-rich intervals are also present.

Nannofossils are present in >75% of the tuffs examined, suggesting that most of Unit II is marine; the nannofossils suggest deposition during the middle Eocene (see "Biostratigraphy"). The top of Unit II is an ~1-cm-thick ferromanganese crust (Fig. F14) at the top of interval 192-1184A-9R-1, 0-1 cm (201.1 mbsf). We did not reach the base of Unit II; however, using the velocity data obtained from the volcaniclastic rocks (see "Physical Properties") to convert seismic travel times to depth, Unit II could be 1500-2000 m thick. Similar volcaniclastic units have not been encountered previously on the Ontong Java Plateau.

Rocks in Unit II can be grouped into eight facies types (Table T4). We used changes in the abundance and color of these lithologies to divide Unit II into five subunits. Interpretations in the following subunit descriptions are discussed further in "Interpretation of the Sedimentary Record at Site 1184" in this chapter.

Subunit IIA

Interval: 192-1184A-9R-1, 0 cm, to 16R-1, 81 cm

Depth: 201.1-245.21 mbsf

Age: middle Eocene

Lithology: vitric lithic tuff and vitric lithic lapilli tuff

Subunit IIA is 44 m thick. The top of the subunit is placed at the ferromanganese crust at 201.1 mbsf (interval 192-1184A-9R-1, 0-1 cm). The base of the subunit is placed at the base of 3.5 m of massive, fine- to medium-grained, thin- to medium-bedded vitric lithic tuff, which overlies an ~25-m-thick bed of massive vitric lithic lapilli tuff of uppermost Subunit IIB (245.21 mbsf; Section 192-1184A-16R-1, 81 cm).

Subunit IIA is characterized by a succession of thin- to medium-bedded vitric lithic tuff and vitric lithic lapilli tuff. Cores 192-1184A-9R through 11R and interval 12R-4, 10-75 cm, are very pale brown to yellowish red (Fig. F15); the rest of Subunit IIA is greenish gray. Some beds are massive, but beds with slightly inclined layers, normal grading, and reverse grading, are common. We tentatively interpret beds containing grading and parallel laminations as turbidity-current deposits. Massive beds within Subunit IIA are interpreted as debris-flow deposits. This subunit shows an overall fining-upward trend.

Subunit IIB

Interval: 192-1184A-16R-1, 81 cm, to 22R-6, 93 cm

Depth: 245.21-304.21 mbsf

Age: middle Eocene

Lithology: lithic vitric lapilli tuff and lithic vitric tuff

Subunit IIB is 59 m thick and composed of greenish gray to reddish brown, coarse-grained lithic vitric tuff and lithic vitric lapilli tuff. The top of the subunit is placed at 245.21 mbsf (Section 192-1184A-16R-1, 81 cm) at the top of an ~25-m-thick massive lapilli tuff. The base of the subunit is placed at the top of a tachylitic lapillistone at 304.21 mbsf (Section 192-1184A-22R-6, 93 cm).

Most beds in Subunit IIB are massive and very poorly sorted. They range in thickness from 5 cm to >25 m (Fig. F16). Grain size ranges from very fine sand to coarse pebble. Grading is absent to very subtle within most beds, but some beds show distinct inverse grading (Fig. F17). We interpret most of the beds of Subunit IIB as debris flows. Four relatively thin intervals of fine- to medium-grained tuff are present (192-1184A-18R-5, 110 cm, to 19R-1, 75 cm; 19R-8, 0-40 cm; 20R-2, 40-55 cm; 21R-2, 15-80 cm). There is a slight fining-upward trend through this subunit.

Subunit IIC

Interval: 192-1184A-22R-6, 93 cm, to 30R-5, 15 cm

Depth: 304.21-380.52 mbsf

Age: middle Eocene

Lithology: vitric lithic lapilli tuff and vitric lithic lapillistone

Subunit IIC is 76 m thick and composed of red, vitric lithic lapilli tuff (Fig. F18) and vitric lithic lapillistone bounded by tachylitic lapillistone (Fig. F19). The upper tachylitic lapillistone varies from reddish gray to weak red, and its top defines the top of Subunit IIC at 304.21 mbsf (192-1184A-22R-6, 93 cm). This bed spans the interval from Sections 192-1184A-22R-6, 93 cm, to 24R-1, 17 cm (304.21-316.60 mbsf). However, only ~2 m of rock was recovered in that >12-m interval, so bedding thickness is uncertain and the possibility of unrecovered interbeds cannot be ruled out. The lower tachylitic lapillistone is gray to dark gray; its base falls at 380.52 mbsf (Section 192-1184A-30R-5, 15 cm). It grades upward over 2 m into the vitric lithic lapillistone that makes up the main body of Subunit IIC.

The dominant lithologies (vitric lithic lapilli tuff and vitric lithic lapillistone) of Subunit IIC are very thick bedded and massive. They contain a high proportion of red clasts and a generally red matrix (Fig. F18). The proportion of lithic clasts is higher, and that of accretionary lapilli is lower, in Subunit IIC than in the rest of Unit II (Fig. F20). Magnetic susceptibility is higher in Subunit IIC than in the rest of Unit II and is particularly high in the tachylitic lapillistone at the top and bottom of the subunit (Fig. F10). Tachylite is indicative of subaerial cooling (see "Igneous Petrology") and the abundant red clasts in Subunit IIC are consistent with prior subaerial weathering of these clasts. A thin-bedded, fine-grained tuff layer in interval 192-1184A-27R-5, 30-35 cm, appears to be a primary ash-fall deposit.

Subunit IID

Interval: 192-1184A-30R-5, 15 cm, to 36R-4, 50 cm

Depth: 380.52-437.39 mbsf

Age: middle Eocene

Lithology: vitric lithic tuff to vitric lithic lapilli tuff

Subunit IID comprises 57 m of vitric lithic tuff and vitric lithic lapilli tuff. The top of the subunit is placed at the base of the lower tachylitic lapillistone of Subunit IIC at 380.52 mbsf (Section 192-1184A-30R-5, 15 cm). The base of Subunit IID is placed at the base of a 50-cm-thick, horizontally layered interval at 437.39 mbsf (Section 192-1184A-36R-4, 50 cm). Both contacts are gradational. Near the contacts, lithologies typical of Subunit IID are interbedded with lithologies typical of the subjacent and superjacent subunits over 5-10 m of the section.

Subunit IID is characterized by relatively well defined bedding, and inclined layers are common (Fig. F21). The top ~17 m of the subunit (interval 192-1184A-30R-5, 15 cm, to 32R-3, 150 cm) are heterogeneous. The interval includes fine to medium sand-size, medium-bedded, dark gray and greenish gray vitric lithic tuff containing accretionary lapilli and wood; inclined granule-rich layers in beds of vitric lapilli tuff; and a 1.24-m-thick bed of massive pebble-rich lapilli tuff typical of Subunit IIC (interval 192-1184A-30R-7, 10-134 cm). This heterogeneous interval is underlain by ~35 m (interval 192-1184A-32R-4, 0 cm, to 35R-8, 75 cm) of interbedded, massive, greenish gray lapillistones and lapilli tuffs with inclined layers. The lower ~5 m of the subunit (interval 192-1184A-36R-1, 0 cm, to 36R-4, 50 cm) is composed of fine- to medium-sand-size, normally and inversely bedded vitric lithic tuff with common inclined layers of coarse sand and granules. Intercalation of internally layered beds and massive beds suggests that more than one major depositional process was active. Possibilities include current or wave reworking between deposition of massive mass flows and of debris eroded from more proximal deposits.

Subunit IIE

Interval: 192-1184A-36R-4, 50 cm, to 46R-1, 132 cm

Depth: 437.39-538.8 mbsf

Age: middle Eocene

Lithology: lithic vitric tuff to lithic vitric lapilli tuff

Subunit IIE is at least 101 m thick. The top of Subunit IIE is placed below a 50-cm-thick bed of internally layered lapilli tuff typical of Subunit IID at 437.39 mbsf (Section 192-1184A-36R-4, 50 cm), but the transition is gradational. Subunit IIE extends to the base of Hole 1184A.

Subunit IIE is dominated by massive lithic vitric tuff and lithic vitric lapilli tuff. Wood-bearing intervals are present near the top of the subunit and at the base of the cored section (Fig. F22). The subunit exhibits an overall fining-upward trend and grain-size oscillations on scales from centimeters to meters. The upper ~30 m of Subunit IIE (interval 192-1184A-36R-4, 50 cm, to 39R-7, 105 cm) consists of fine to medium sand-size tuff containing accretionary lapilli and of inclined layers composed of granule and coarse sand-size grains. The underlying ~70 m (interval 192-1184A-39R-7, 105 cm, to 45R-6, 105 cm) contains interbedded lithic vitric lapilli tuff and fine to medium grained tuff with rare granule- to pebble-size clasts. Changes in grain size are gradual. Individual beds are up to 6 m thick, but bedding is generally indistinct. Finer-grained beds contain well-preserved accretionary lapilli (Fig. F13A), horizontal granular stringers, and ~10-cm-thick intervals of massive lapilli tuff. Chaotic bedding suggestive of synsedimentary slumping is present in interval 192-1184A-45R-6, 105 cm, to 45R-7, 130 cm (Fig. F23). The chaotic bed also contains wood at eight different levels (Fig. F22). One of these wood pieces displays Teredo-like mollusk borings (Fig. F24). The basal 1.3 m of Subunit IIE (Section 192-1184A-46R-1) contains three inversely graded beds, one of which is overlain by fine-grained sand and clay. Wood pieces are present, and the clay contains small black particles, which may be organic.

Inclined layers are present throughout Unit II. They are most abundant in Subunit IID and are rare in Subunit IIC (Fig. F25). We combined the dip and dip direction of 82 laminae with paleomagnetic declinations in order to orient the dip directions relative to magnetic north (see "Paleomagnetism"). Dips within Subunit IIA are generally <15°, whereas in Subunit IID dips >25° are common. Layers in the other three subunits show a range of inclinations. Throughout Unit II, the dip of the inclined layers is predominantly toward the north-northwest.

Middle Eocene: Hydroclastic Volcanism

The middle Eocene volcaniclastic beds (Unit II) at Site 1184 preserve the record of nearby hydroclastic volcanism. Characteristics of the clasts in Unit II that support a dominantly hydroclastic source are presented in "Igneous Petrology". Volcaniclastic grains make up virtually 100% of the rock, but we suggest that most beds are redeposited sediment (i.e., rocks in Unit II are not generally interpreted as primary pyroclastic fall or flow deposits). Most of the sedimentary features in Unit II are consistent with a model in which sediments were derived from primary deposits near the vent and then were redeposited on the flanks of the volcano at depths at or below wave base (Fig. F26).

Source Area(s)

Sediments preserved in Unit II were derived from a volcanically active island. Most eruptions were submarine, as demonstrated by the abundance of vitric clasts, but the eruption column often reached the atmosphere, as indicated by the presence of accretionary and armored lapilli (see "Igneous Petrology"). The presence of wood and other organic material at several intervals (Figs. F12, F22) suggests that the upper portion of the volcanic edifice became subaerial long enough to become vegetated.

One or more of the three bathymetric and free-air gravity highs within 35 km of Site 1184 may mark the location of a volcano active during the middle Eocene (Fig. F3) (see "Background and Objectives"). The preferred north to northwest dips of inclined layers in Unit II (Fig. F25) and the regional dip of ~9° to the north (see "Background and Objectives") may indicate transport from the south-southeast. However, if the regional dip is to the north, many of the low-angle inclined layers in the core could be subparallel to bedding and thus not provide independent estimates of transport direction (see "Paleomagnetism").

Depositional Setting and Paleobathymetry

Coarse, massive beds composed almost exclusively of volcaniclastic material dominate Unit II. No pelagic interbeds were observed, but nannofossils are present throughout the unit, indicating submarine deposition. The nannofossil assemblages suggest most of Unit II was deposited in ~3 m.y. or less (see "Biostratigraphy"). The short time interval represented by Unit II and the paucity of nonvolcaniclastic material argue for rapid deposition proximal to the source. The presence of relatively delicate accretionary lapilli and angular vitric clasts is consistent with short transport distances. Close to the vent, however, we would expect to find a significant number of large (>2 cm), dense clasts (e.g., blocks and spindle bombs), which we did not observe. Therefore, the rocks cored from Site 1184 were probably not the most proximal primary deposits, and we infer that Site 1184 was located on the flanks of the volcano (Fig. F26).

Gravity flows should be common on a rapidly aggrading sedimentary apron around a volcano. Features typical of volcaniclastic debris flow deposits (massive, poorly sorted, or coarse) are abundant in Unit II. Other volcanic processes can produce massive beds, but they are usually subaerial and occur in association with internal and/or interbedded, layered deposits (e.g., Fischer and Schmincke, 1984). The presence of nannofossils throughout Unit II (see "Biostratigraphy") is consistent with submarine mass-flow deposits. Similarly, the majority of internally stratified beds in Hole 1184A do not appear to represent primary volcanic layering. Fine-grained material that may be air-fall ash is present rarely, but we did not observe layering typical of coarser material that has settled through the water column. Rather, the graded beds, parallel-laminated intervals, and subhorizontal coarse sand and granule layers in Unit II are consistent with deposition from high-concentration turbidity currents. Steeply inclined layers in Subunit IID suggest sediment transport with a unidirectional component and could be crossbedding formed by currents.

As with our conclusions regarding distance to the vent, paleobathymetric inferences are based largely on negative data. Debris flow deposits provide no depth information, and high sedimentation rates (estimated to be >100 m/m.y.), coupled with possible uplift or subsidence pulses associated with the nearby active volcanism, may imply significant paleodepth changes during deposition of Unit II. With these caveats, we propose that Subunits IIA, IIB, and IIE probably accumulated below the storm-wave base (/50 m) but shallower than the present-day water depth of Unit II (~2000 m). The upper depth limit is based on the scarcity of sedimentary structures indicative of reworking or winnowing by waves or currents after each debris flow was deposited. Shallow-water bioclasts are absent, but there is also a lack of fine-grained and/or pelagic interbeds. If inclined layers (Fig. F21), which are most common in Subunit IID, reflect transport by storm-wave-generated currents, they indicate deposition near the storm-wave base.

Subunit IIC is coarse grained throughout, contains abundant red lithic clasts, and is bounded by tachylitic lapillistone (Fig. F19). The tachylitic clasts formed during subaerial eruptions (see "Igneous Petrology"). The red clasts (Fig. F18) and some aspects of alteration suggest subaerial weathering (see "Alteration"). Together, these observations suggest a significant subaerial component within the sediments of Subunit IIC, but we have little sedimentological control on the final depth of deposition.

Depositional Trends in Unit II

Debris flow deposits dominate the volcaniclastic rocks of Unit II. There is a general increase in the thickness, coarseness, and abundance of these deposits from Subunit IIE to IIC and a decrease from Subunit IIC to IIA. If the tachylite and the high proportion of red lithic clasts in Subunit IIC indicate a proximal, shallow to subaerial setting, then the trends through Unit II can be interpreted as progressive shallowing from Subunit IIE to IIC and deepening from Subunit IIC to IIA. Conversely, differences among subunits might reflect variability within a submarine facies characterized by a mosaic of depositional styles that vary as a function of sediment input (e.g., changes in debris flow frequency or clast composition caused by variations in volcanic activity).

Early Miocene: Pelagic Deposition

In the early Miocene, pelagic ooze began to accumulate at Site 1184. The ooze is typical of pelagic deposition above the calcite compensation depth (CCD) under an oxygenated water column and contains rare volcanic ash layers. Similar ash layers were found in coeval deposits from Site 1183 (see "Lithostratigraphy" in the "Site 1183" chapter). The ooze contains a bathyal to abyssal benthic foraminifer assemblage (see "Biostratigraphy"). Seismic stratigraphy shows that a thick accumulation of horizontally layered sediments in this area is currently present below the level of the oldest ooze at Site 1184 (Fig. F4). However, at Site 1184, a ferromanganese crust formed between the termination of volcaniclastic deposition during the middle Eocene and the initiation of pelagic accumulation ~20 m.y. later during the early Miocene. Ferromanganese crusts and nodules typically grow at a rate of a few millimeters per million years. The crust between Units I and II may record nondeposition during much of the middle Eocene through early Miocene hiatus. The pervasive alteration and reddish orange color of the upper 8 m of Unit II and the capping ferromanganese crust are consistent with a hiatus (Figs. F14, F15). Why the pelagic sediments onlap rather than drape the volcaniclastic surface is unresolved.