Site 1198 was drilled through a sequence of hemipelagic drift deposits into underlying talus and slope sediments of the Southern Marion Platform (SMP). Below, Site 1198 penetrated hemipelagic sediments, a thin veneer of deep euphotic carbonates, and basaltic basement. The ~517-m-thick sediment sequence was divided into five lithologic units according to variations in sedimentary structures and texture, grain size, terrigenous vs. carbonate content, degree of lithification, and biotic assemblages (Fig. F3; Table T3).
Unit I is a 200.6-m-thick upper Pliocene to Pleistocene sequence of alternating mudstone to skeletal grainstone intervals deposited in a hemipelagic environment with varying input of terrigenous constituents. At 200.6 mbsf, a 10- to 25-cm-thick layer of brown phosphate nodules corresponds to a hardground that marks the top of Unit II (200.6-397.6 mbsf). Unit II comprises two intervals of fine-grained skeletal packstone to grainstone rich in neritic constituents, deposited in a periplatform environment (Subunits IIA and IIC). Between these subunits, the 96-m-thick Subunit IIB consists of coarse-grained skeletal grainstone to rudstone with abundant larger benthic foraminifers deposited at the base of the SMP escarpment, 5 km to the southeast. The boundary between Units II and III at 397.6 mbsf corresponds to a sharp contact between silt-sized packstone with neritic debris, underlying variously colored mudstone to packstone deposits without neritic components. Unit IV consists of skeletal floatstone to rudstone with larger benthic foraminifers and rhodoliths, interpreted to be deposited in a deep euphotic environment. Acoustic basement (Unit V; below 513.2 mbsf) is characterized by an olivine basalt.
Sediments recovered at Site 1198 are unconsolidated in the upper 60 mbsf, become more consolidated to 200.6 mbsf, and are semilithified for the remainder of the drilled interval.
Core recovery at this site is good above 200 mbsf, very poor within the semilithified, friable sediments of Subunits IIA and IIB, moderate in Subunit IIC, and good below 397 mbsf (Units III-V) (Fig. F3). The poorly recovered interval corresponds to the seismic Megasequence C (see "Seismic Stratigraphy").
In this chapter, noncarbonate content, as determined by X-ray diffraction (XRD) analysis, is inferred to reflect clay content. This assumption is based on the fact that little coarse-grained siliciclastic material was observed in low-calcium carbonate intervals.
Unit I is a light gray to olive-gray, moderately to well-sorted succession characterized by alternations of wackestone to grainstone. Pyrite is present to common throughout this unit. This large package of sediments is divided into two subunits on the basis of sedimentary structures, texture, grain size, calcium carbonate content, degree of lithification, and biotic assemblages.
Subunit IA shows a large variability in sediment texture, gradually alternating from unconsolidated dark greenish gray skeletal wackestone to light gray grainstone, both with minor amounts of clay. These changes are well reflected in calcium carbonate data (Fig. F3, column C) (also see "Geochemistry"). The biota found within this unit comprises a broad range of species, dominated by planktonic foraminifers with minor amounts of small benthic foraminifers, pteropods, echinoderm fragments, and scaphopods. Smear slide analysis showed the presence of common silt-sized skeletal components dominated by mollusk fragments. Synsedimentary slump structures are located in the upper 60 m of Hole 1198A (see Fig. F4). Within the upper ~30 m, glauconite is rare to present, mostly infilling foraminiferal tests. At the base of this subunit, a thin layer of light greenish mudstone marks the boundary with Subunit IB. Additionally, XRD analyses show the appearance of aragonite above 117 mbsf, with a continuously increasing trend upcore, showing values of ~30 wt% at the seafloor.
In Subunit IB, the lithology changes to a succession of well-sorted, light greenish gray, firmly consolidated skeletal packstone dominated by planktonic foraminifers (62-138 mbsf). Slight variations in grain size and clay content result in changes from lighter to darker color. Clay content varies between ~10% and ~20%. Below 138 mbsf the clay content increases steadily to the base of Unit I, with a maximum content of ~30% (Fig. F3), resulting in an overall downcore color change from olive-gray to pale olive. In the lower part of Subunit IB, the texture of the sediment is more variable, ranging from skeletal wackestone to packstone. Within Subunit IB, a few layers containing small carbonate concretions (Cores 194-1198A-9H, 12H, 18H, and 21H) indicate early cementation. Furthermore, the abundance of dark brownish, fine to medium sand-sized phosphatic grains (up to 1-5 mm) increases within the lowermost 3.5 m of Unit I.
The boundary between Units I and II coincides with the Megasequence C/D boundary (see "Seismic Stratigraphy") and is marked by a 10- to 25-cm-thick layer consisting mostly of olive to dark olive phosphatic nodules (see Fig. F5) up to 6 cm in diameter. The texture within each nodule is skeletal packstone dominated by planktonic foraminifers and minor amounts of small benthic foraminifers.
Unit II is a poorly recovered sequence of periplatform sediments that was divided into three subunits: an upper (Subunit IIA) and lower (Subunit IIC) succession of mostly fine to very fine grained skeletal neritic detritus and an interbedded coarser unit (Subunit IIB) rich in larger benthic foraminifers.
Subunit IIA consists of silt- to fine sand-sized, well- to moderately sorted, white to pale brownish skeletal packstone to grainstone. Planktonic foraminifers still predominate, but larger benthic foraminifers and other neritic skeletal components, such as mollusks, bryozoans, echinoid spines, and shell fragments also occur. At the base of Subunit IIA, a small 1.5-cm-thick lamination consisting of imbricated larger benthic foraminifers is observed. Mineralogical analyses indicate up to 20 wt% dolomite within this section (see Fig. F3 and "Geochemistry").
Subunit IIB is characterized by white to pale yellowish, medium to coarse sand-sized, moderately to well-sorted skeletal grainstone/floatstone to rudstone. Within the sediment, larger benthic foraminifers (e.g., Lepidocyclina sp., Operculina sp., and Amphistegina sp.) are abundant. In addition, fragmented neritic skeletal grains are common, such as rhodoliths, red algae, sponges, echinoderms, and bryozoans (see Fig. F6). Small benthic and planktonic foraminifers are also present. These sediments are weakly cemented and friable. Within Cores 194-1198B-7R through 12R, repetitive changes from skeletal floatstone/grainstone to grainstone and wackestone were observed. The base of this subunit coincides roughly with the seismic Megasequence B/C boundary at 330.3 mbsf (see "Seismic Stratigraphy").
The top 30 m of Subunit IIC consists of brownish white fine-grained skeletal packstone to grainstone, which gradually changes downcore into light gray to light olive-gray silt-sized packstone to grainstone with minor clay. The most striking characteristic defining this subunit is the presence of fine skeletal grains dominating the sediment texture. Planktonic and small benthic foraminifers are abundant to dominant, making up the coarse fraction with fine sand- to silt-sized neritic skeletal grains present, mainly as abraded larger benthic foraminifers, bryozoans, red algae, and mollusks (Fig. F7).
The transition between Units II and III is marked by the appearance of firm, nonlithified layers of mudstone to wackestone with clay in Cores 194-1198B-22R to 23R (397-417 mbsf; see Table T3). This interval is also marked by a sudden downcore decrease in calcium carbonate content and the disappearance of fine neritic grains. In addition, distinct changes in seismic velocity and grain density occur (see "Core Physical Properties").
Unit III is a light gray to dark olive, moderately to well-sorted interval with alternations of silt to sand-sized wackestone to grainstone. Within this unit, clay shows an overall increase from 5% to 10% at the bottom to 50% in the top (Fig. F3). Pyrite is rare. Discernible small-scale, rhythmic changes of texture and color also occur. Unit II is divided into two subunits.
Subunit IIIA consists of silt- to fine sand-sized, well to moderately sorted, lithified skeletal wackestone to packstone with clay. Color ranges from light to dark greenish gray. Planktonic and small benthic foraminifers are the most abundant skeletal components, but mollusks and larger benthic foraminifer fragments also occur. Rare faint, wavy laminations and crude bedding structures were observed in the upper 30 m. Small-scale (up to 20 cm) fining-upward sequences occur at the base of wackestone intervals in Cores 194-1198B-23R through 27R (407-455 mbsf), which distinguish this subunit from Subunit IIIB. High-amplitude changes in the ratio of calcium carbonate to noncarbonate and magnetic susceptibility are another characteristic feature of this subunit (see "Geochemistry" and "Paleomagnetism").
Subunit IIIB is characterized by alternations of silt-sized, moderately to well-sorted, greenish gray to olive-gray colored packstone and grainstone with clay. Facies changes appear either gradational or bioturbated. Clay content is generally lower than in Subunit IIIA. In the lowermost 7.5 m of Subunit IIIB, small (up to 5-cm-thick) laminated intervals occur. The base of this unit exhibits a sharp, discontinuous contact to underlying Unit IV (Fig. F8).
The top of this interval is encrusted with alternations of submillimeter-scaled layers of phosphate and pyrite indicating a hardground surface (Fig. F9). Below this hardground, the uppermost 50 cm of Unit IV consists of a light yellowish brown to white lithified skeletal floatstone with large rhodoliths and larger benthic foraminifers. Below this floatstone occurs a lithified skeletal floatstone to rudstone dominated by larger benthic foraminifers (mostly Lepidocyclina spp. and Cycloclypeus sp. common) and large platy-shaped rhodoliths (Fig. F10). The upper meter of this lithology is stained brownish red. Rhodoliths and larger benthic foraminifers are oriented horizontally within a light greenish gray clay- to silt-sized matrix with a laminated texture. Minor skeletal components include bryozoans, planktonic foraminifers, and coralline algae. The lamination disappears toward the base of this unit, when smaller, more robust foraminifers (Amphistegina sp.) and more spheroidal rhodoliths appear. Toward the bottom of Unit IV, glauconite grains are present.
Olivine basalt was recovered below a sharp contact with the overlying carbonate-rich Unit IV. This lithology represents acoustic basement as seen on seismic data (see "Seismic Stratigraphy"). The groundmass of the rock consists of a darkish green matrix of plagioclase phenocrysts. Some mineral-infilled veins are present. The rock is heavily altered as indicated by the presence of zeolites. Quartz crystals found in this rock do not show undulatory extinction, indicating that they have not been deformed.
Based on benthic foraminifer faunal assemblages, the sediments recovered at Site 1198 were deposited on the talus or proximal slope adjacent to the SMP in a water depth of >200 m (upper bathyal) ("Biostratigraphy and Paleoenvironments"). The only exception is the few meters of sediment above basement (see below), which was deposited in a deep euphotic environment (~50-150 m).
The age and origin of the olivine basalt acoustic basement is uncertain, but is interpreted as synrift basaltic flows.
Abrupt flooding of the basement is indicated by a short interval containing spheroidal rhodoliths and robust foraminifers of the genus Amphistegina, followed by a few-meters-thick series of skeletal floatstone to rudstone, mainly composed of larger benthic foraminifers (Lepidocyclina sp.) and rhodoliths. This skeletal assemblage is indicative of deep euphotic water depths of ~50-150 m (Tsuji, 1993) (see "Biostratigraphy and Paleoenvironments"), and it may have been deposited on the flanks of a carbonate buildup, located northwest of Site 1198, as seen in the seismic data (see "Seismic Stratigraphy"). Reddish to brownish color toward the top of this unit might show some influence of subaerial weathering, indicating probable shallowing of the depositional environment. The top of Unit IV is marked by a phosphatic hardground, suggesting a hiatus between Units IV and III.
The base of Unit III is characterized by hemipelagic limestones with clays and a high calcium carbonate content with silt- to fine sand-sized (skeletal?) grains. Upsection, the clay content increases, and small fining-upward sequences occur more frequently. In the same interval, the amplitude of the carbonate vs. noncarbonate increases upsection. A similar trend of increasing clay content was observed for the interval spanning the early to middle Miocene at Site 1194 (Subunits VB to IVB) (see Fig. F3). This trend is interpreted as a deepening-upward trend in a hemipelagic setting. The fining-upward trend further suggests a reduction of carbonate and deposition in a more distal platform environment. In addition, the observed high-amplitudinal, high-frequency changes in the ratio of calcium carbonate vs. noncarbonate deposition, as observed in the upper part of Subunit IIIA, may have been controlled by sea level changes.
Above 400 mbsf (equivalent to <15 Ma [Zones N9-N15]; see "Biostratigraphy and Paleoenvironments"), a prominent increase to higher calcium carbonate content distinguishes Unit III from Unit II. The boundary between Subunits IIB and IIC, defined lithologically and geochemically, coincides with the Megasequence B/C boundary at ~320 mbsf (see "Seismic Stratigraphy"). The carbonate-rich sedimentary package of Subunit IIC, which may have been deposited during the major late middle Miocene sea level drop that seismically and biostratigraphically correlates with the carbonate package built on the slope of the Northern Marion Platform (NMP) (Unit III at Site 1194) (see "Seismic Stratigraphy"and "Lithostratigraphy and Sedimentology" in the "Site 1193" chapter, and "Seismic Stratigraphy" and "Lithostratigraphy and Sedimentology" in the "Site 1194" chapter.) At Site 1198, the source for the neritic material deposited in Unit II was likely to be the SMP.
Late Miocene platform growth at Site 1198 is represented by Subunits IIB and IIA, which consist of a talus to proximal slope facies deposited adjacent to the SMP. Subunit IIB sediments (Cores 194-1198B-7R through 12R) were probably shed across the nondepositional escarpment 5 km east of Site 1198 and then mixed with pelagic constituents. Eventually, during the late Miocene, the SMP platform drowned as documented at Site 1198 by the end of neritic-derived deposition at the top of Subunit IIA. Once neritic carbonate shedding ceased, a phosphatic hardground with rounded phosphatic nodules formed, representing a nondepositional period of ~4 Ma (see "Age Model"). During the onset of hemipelagic deposition of Unit I, this hardground became reworked as shown by fine to medium sand-sized phosphatic grains (up to 1-5 mm in size) in the lowermost 3.5 m of Unit I.
During the Pliocene and Pleistocene, a 200-m-thick hemipelagic mudstone to grainstone unit was deposited in an upper bathyal environment. Bedding geometries seen on seismic data (see "Seismic Stratigraphy") indicate that Unit I sediments are drift deposits. The appearance of aragonite at about 117 mbsf, with a continuously increasing trend upcore, can be either a primary input or a diagenetic signal. Within the upper 70 m of Site 1198, soft sediment deformation indicates the presence of slumps, which also can be traced on the seismic section (see "Seismic Stratigraphy").