The lithostratigraphy of Site 1192 comprises five main units (Table T3; Fig. F2) defined by variations in carbonate content, texture, and the presence or absence of quartz, glauconite, and phosphate grains. Unit definition depended upon changes in sediment characteristics as described below. The entire succession ranges from unlithified at the top to lithified at the base, indicating continued induration and late cementation after burial. These sediments are generally heavily bioturbated and reveal distinctive ichnofossils such as Chondrites, Planolites, Skolithos, Palaeophycus, and Teichichnus (Fig. F3). Color variations are generally subtle with most of the succession being light to dark green-gray or olive-green in color. The greenish color indicates the presence of terrigenous clay, which is reflected in decreased lightness and increased calcium carbonate content (Fig. F4). Because of the presence of clay, sediments and rocks have been classified as foraminifer grainstone, packstone, or wackestone with clay (see "Lithostratigraphy and Sedimentology" in the "Explanatory Notes" chapter for principal lithology definitions). Planktonic foraminifers dominate the sand-sized fraction of sediments in Units I-III, whereas benthic foraminifers were more dominant in Units IV-V. Collectively, the succession of these units indicates a range of physical energy conditions representing a deepening-upward trend.
Unit I is a 2.4-m-thick light yellow, tan to brown, poorly sorted, planktonic foraminifer grainstone containing gravel-sized cemented intraclasts (Fig. F2). These intraclasts are dominantly cemented planktonic foraminifers. Other allochems include echinoderm fragments and spines, as well as gastropod and bivalve shells. Bedding is indicated by gradual color changes. Little to no terrigenous material is present. Bioturbation is rare. Thin section analysis indicates mixing of grains with different types of intraparticle infilling (Fig. F5). The lower boundary between Units I and II is sharp.
Unit II is characterized by a heavily bioturbated, planktonic foraminifer mudstone to packstone with clay (Fig. F2). The sediments are mostly light greenish gray with variations in color resulting from variations in clay content. This unit also has centimeter-sized, black framboidal pyritic stains scattered throughout. Commonly, these stains are associated with burrows. Core catcher descriptions from the >63-µm fraction of Unit II indicate that benthic foraminifers, bryozoans, pteropods, ostracodes, arthropod spines, and mollusk fragments are rare relative to the planktonic foraminifers.
Unit II is divided into two subunits based upon color reflectance data (Fig. F4) and textural changes occurring at 103 mbsf. Significantly, geochemical and core physical properties data record changes at this depth as well (see "Geochemistry" and "Core Physical Properties").
Subunit IIA is essentially a skeletal packstone to wackestone with clay and is light olive-gray to light gray (Fig. F6). Planktonic foraminifers are the dominant skeletal component. The upper portion of Subunit IIA is a packstone, which grades downward to a wackestone at the base of this subunit. Several meter-scale to submeter-scale mudstone units appear near the base of Subunit IIA. Significant bioturbation with distinct ichnofossils (e.g., Scolicia, Thalassinoides, and Taenidium) begins at ~39 mbsf extending downcore. Color reflectance values indicate greater clay content as compared to Subunit IIB. These data also clearly indicate significant lithologic variations at a scale of meters to tens of meters.
Subunit IIB consists of mudstone that begins to coarsen downcore at 203 mbsf with small units of mudstone, wackestone, and packstone extending to the base of the subunit. Light greenish gray is the most persistent color. Below 135 mbsf, Chondrites are distinctly recognizable. Portions of Subunit IIB are characterized by meter-scale lithologic variations showing variations in texture and degree of bioturbation (Fig. F3). A typical lithologic variation begins with a sharp base (sometimes topographically irregular) interpreted to be a firmground or distal turbidites. Above this contact is a moderately sorted, unburrowed, foraminifer packstone several tens of centimeters thick. This grades upward into a burrowed wackestone and mudstone forming a complete fining-upward sequence, which is capped by another firmground. The mudstone beneath the firmground contains distinct, large (1-2 cm diameter) burrows, which have been infilled by the overlying packstone. These burrows crosscut earlier, smaller diameter (3-5 mm) mud-filled burrows of Chondrites. The cyclic lithologic variations described above can be found from 116 to 135 and from 203 to 246 mbsf in Subunit IIB, but not between 184 and 203 mbsf, which is a section of monolithic, heavily bioturbated mudstone with clay.
The top of Unit III (Fig. F2) is defined by the appearance of more abundant quartz, glauconite, and phosphate grains set within the foraminifer packstone with clay that dominates the unit (Fig. F7). These distinctive greenish black and black grains become more common downsection. Additionally, Amphistegina sp. and Lepidocyclina howchini, both shallow-water benthic foraminifers, appear near the top of this unit (Fig. F7). Drilling disturbance resulting from XCB coring and expressed as biscuiting is common and is partially the result of increased lithification. Unit III is bioturbated with Skolithos(?) ichnofossils.
At ~307-313 mbsf, another dark, greenish gray banded glauconite-rich packstone to grainstone unit, which contains shark teeth, occurs. This interval also contains dolomitic rhombs and coccoliths as seen in smear slides.
The base of Unit III contains a thin (<1 m) bed of coarse, glauconitic and bioclastic grains. The contact between Units III and IV is sharp.
Unit IV (Fig. F2) consists of alternating units of planktonic foraminifer mudstone and packstone with clay (Fig. F8). The dark olive-green to dark greenish gray color of this unit is darker than the color of Unit III. Glauconite and quartz grains are present. Together, Units III and IV show an overall upward increase in grain size, with Unit III having less mud. Dolomitic rhombs appear at the base of this unit, as well as possibly quartz silt.
Unit V consists of a silt-sized dolomitic grainstone with clay that becomes coarser toward the bottom of the hole (Fig. F2). Smear slide and thin section analyses show that silt-sized dolomitic rhombs and angular quartz are common. Based on its well-sorted character and its low matrix content, this sediment is classified as a grainstone because particles can be individually recognized in macroscopic examination and they are generally in grain contact. At the base of this unit, a coarse grainstone was recovered with planktonic foraminifers being the dominant particles. Even though only 9 m of Unit V was recovered, its distinctive lithology dictates its recognition as a separate lithologic unit.
The periplatform sediments in the lower portion of this site (Units V-III) have been reworked and transported from a neritic environment as suggested by the presence of shallow-water benthic foraminifers. Seismic data show that this portion of the sedimentary section occurs within the distal slope facies of the NMP (see "Seismic Stratigraphy"). Thus, these sediments probably were transported down a very low gradient slope within density underflows (hyperpycnal flows) and turbidity currents. The well-sorted nature of these sediments, as well as the extensive bioturbation, precludes any recognition of primary sedimentary structures and thus more detailed interpretation of depositional processes.
The upper portion of Unit III may correlate with the drowning and/or exposure phase of the NMP. The shutdown of this carbonate factory resulted in sediment starvation on the upper slopes of the platform, thus favoring glauconite formation and the development of a phosphate hardground, which may be the source of the distinct, dark grains seen at the top of this unit. The quartz grains admixed with the platform carbonates probably originate from the northeast Australian margin.
Sediments in the upper portion of this site (Units II-I) reflect deposition in an open-platform, hemipelagic environment with variations in energy level at and near the seafloor. The firmground and fining-upward beds between 116-135 and 203-246 mbsf in Unit II indicate discrete sedimentation events that could be either turbidity currents or distinct periods of contourite deposition. The latter interpretation is supported by the fact that Unit II lies within the seismic facies that is interpreted to be a sediment drift complex (see "Seismic Stratigraphy"). However, the extensive bioturbation observed has removed most, if not all, of the primary sedimentary structures that might confirm either interpretation.
Magnetic susceptibility and grain density data show greater variations above ~100 mbsf than below (see "Core Physical Properties"). This is consistent with the abrupt onset of terrigenous clay deposition from the northeast Australian margin, which is transported out to the Marion Plateau through hyperpycnal flows. Physical properties data also show small-scale variations supporting this interpretation, as do the color reflectance data (Fig. F4).
Sedimentation rates based on the age model (see "Age Model") indicate a maximum rate (~75 m/m.y.) occurring within the late Miocene Subunit IIB. This rate decreased to a minimum (~10 m/m.y.) near the Pliocene/late Miocene boundary or at ~80 mbsf, in the lower part of Subunit IIA. These data suggest that in addition to the increase in clay content expressed by geochemical and physical properties data, there must have been a corresponding decrease in carbonate productivity as well. However, the overall pattern in sedimentation within Unit II probably reflects lateral shifts in the positions of the drift deposit centers.
Unit I includes the modern seafloor, which has undergone disturbances allowing cementation and reworking, as indicated by the intraclasts with brownish iron oxide stains on these sediments. Site survey data indicate that portions of the modern seafloor are characterized by current-swept hardgrounds representing various stages of lithification. The depth at which modern physical processes rework Unit I sediments is unknown.