Site 1131 is located on the upper continental slope of the eastern Eyre Terrace in 332.4 m of water. It is situated immediately seaward of the modern shelf edge, where the seafloor displays an irregular mound-like topography. Underlying the mounds is a thick (>500 m) succession of offshelf-trending prograding clinoforms, which are oblique to sigmoidal and contain high-amplitude parallel reflection surfaces (Feary and James, 1995; see "Seismic Stratigraphy" in the "Site 1127" chapter). Seismic data indicate that the ridge-like topography is the surface expression of a series of mounds lying immediately below the sea-floor (Feary and James, 1998, reprinted as Chap. 2; see "Seismic Stratigraphy" in the "Site 1127" chapter). Individual mounds are stacked on top of one another, prograding upslope.
Site 1131 intersected the flank of one of the near-surface mounds and continued through the thickest part of the clinoform package, providing a record of relatively rapid and uninterrupted upper Pliocene-Pleistocene sediment production and accumulation (Fig. F3) (see "Biostratigraphy"). The base of the succession is marked by an unconformity representing a major hiatus. Recovery below this hiatus was extremely poor. Three lithostratigraphic units are recognized on the basis of differences in sediment texture, composition, and packaging.
Unit I consists of a thin succession of neritic carbonate sediments with packstone, floatstone, and rudstone textures and a few decimeter-scale wackestone layers. Unit I is divided into two subunits, based on differences in texture and bioclastic composition.
Subunit IA consists of light gray to olive gray, unlithified bioclastic packstone. It is massive and homogeneous throughout, probably the result of extensive bioturbation, although individual burrows cannot be recognized. Grain size of the >63-µm fraction is fine to very fine sand, composed mainly of bryozoans with benthic and planktonic foraminifers, ostracodes, and echinoderm spines present in subordinate amounts. The mud fraction includes nannofossils, benthic and planktonic foraminifers, and sponge and tunicate spicules (see "Site 1131 Smear Slides"). The basal contact with Subunit IB is gradational in Hole 1131A. In Hole 1131B it is burrowed, with Subunit IA material incorporated as far as 50 cm down into the top of Subunit IB.
Subunit IB is a heterogeneous 20-m-thick interval of unlithified bryozoan floatstone and rudstone, punctuated by thin (decimeter scale) layers of wackestone, packstone, and grainstone. The matrix of the floatstone and rudstone is fine grained, giving the interval an overall muddy appearance.
The gravel-sized fraction is mainly bryozoans, with accessory serpulids, whole small infaunal echinoids, infaunal bivalves, and crab claws. The bryozoan assemblage is diverse but dominated by a small number of growth forms, particularly delicate branching, flat robust branching, fenestrate, and nodular-arborescent types (Bone and James, 1993). Diversity is highest within the delicate and flat robust branching forms, with most nodular arborescent types being relatively large centimeter-sized Celleporaria sp. Articulated zooidal growth forms are not as obvious, but their remains make up a large proportion of the silt and very fine sand-sized fraction.
The sand-sized fraction is bimodal and includes medium-sized fecal pellets, fine to very fine sand-sized planktonic and benthic foraminiferal tests, serpulid worm tubes, ostracode shells, sponge spicules, infaunal echinoid spines, and bioclasts, with rare pteropod shields. The mud-sized fraction is composed of common nannofossils with accessory benthic and planktonic foraminifers, small sponge and tunicate spicules, and rare dolomite rhombs (see "Site 1131 Smear Slides").
Particle preservation is excellent in all size grades, and, although broken, fragile skeletal fragments typically retain even the most delicate surface ornamentation. The sediment appears thoroughly burrowed throughout with an overall patchy, heterogeneous texture resulting partly from the multigeneration filling of Thalassinoides-type burrows.
Unit II consists of a succession of massive, homogeneous, light gray to light olive gray bioclastic packstone to grainstone, punctuated by decimeter-scale intervals of wackestone. The contact with the overlying Unit I is gradational and placed at a strong peak in the natural gamma radiation (NGR) (principally uranium) log at 25 mbsf (see "Downhole Measurements"). Unit II is divided into three subunits, based primarily on textural changes and patterns of sediment interbedding.
Subunit IIA consists of an interval of light gray to olive gray, unlithified to partially lithified bioclastic packstone ~150 m thick. The entire interval is massive and homogeneous, and interpreted to be completely bioturbated. The grain size ranges from coarse silt to fine sand. The bioclastic fraction seems to reflect a low-diversity fauna; however, this may be a result of poor preservation, which makes individual grains difficult to identify. The identifiable components in the sand-sized (>63-µm) fraction consist of benthic and planktonic foraminifers, sponge spicules, rare ostracodes, and echinoderm spines. The fine (<63-µm) fraction contains nannofossils, planktonic foraminifers, sponge and tunicate spicules, and dolomite rhombs (see "Site 1131 Smear Slides"). The most distinguishing feature of Subunit IIA is its uniformity and the lack of any indication of variability or interruption in deposition, such as omission surfaces, firmgrounds, or hardgrounds.
Subunit IIB is a light gray to light olive gray, partially lithified bioclastic packstone and grainstone, locally interrupted by decimeter-scale wackestone intervals. The grain size is very fine to fine sand size, and the entire interval is massive to homogeneous. Subunit IIB is characterized by three 40- to 60-m-thick grainstone-dominated packages separated by two packstone intervals, each 30-70 m thick. The position of the boundaries of the subunit are defined by the first and last occurrences (FO and LO, respectively) of grainstone. Boundary locations are supported by NGR (primarily uranium) peaks, and the upper boundary shows an additional change in the FMS signal from high resistivity above to low resistivity below (see "Downhole Measurements").
The packstones are similar to those in the overlying Subunit IIA, with only subtle differences. The bioclastic assemblage is more diverse and includes the appearance of large miliolid foraminifers. Additionally, blackened grains occur immediately below the subunit boundary and maintain a significant presence, primarily in the sand-sized (>63 µm) fraction, throughout the subunit. The origin and mineralogy of blackened grains is not known, but they are commonly associated with pyrite and glauconite grains and form coarse-grained burrow fills. Additional components of the sand-sized (>63 µm) fraction include benthic and planktonic foraminifers, sponge spicules, and rare echinoderm spines, ostracodes, and bryozoans. The fine-grained fraction includes abundant nannofossils, together with benthic and planktonic foraminifers, sponge and tunicate spicules, and dolomite rhombs (see "Site 1131 Smear Slides"). Grainstone layers exhibit the same grain size and composition as the packstone intervals but lack the fine fraction.
Subunit IIB contains evidence of two possible interruptions in deposition. A possible omission surface occurs at 226 mbsf (Section 182-1131A-26X-1, 125 cm), represented lithologically by a color change from light olive to light gray in a moderately disturbed (biscuited) section of the core. It appears as a peak in the NGR log and as a change in character in the FMS log, from less to more resistant downcore (see "Downhole Measurements"). Another omission surface occurs at 250 mbsf (Section 182-1131A-28X-4, 140 cm). This omission surface is represented lithologically by a burrowed contact between packstone (above) and grainstone (below) and is reflected as a thin low-resistivity interval in the FMS log (see "Downhole Measurements").
Subunit IIC is a light gray to light olive gray, partially lithified bioclastic packstone. The grain size is very fine to fine sand size, and the entire interval is massive to homogeneous. Sediment is similar to the packstone of the overlying Subunit IIB, with only subtle differences. The bioclastic component appears less diverse and is, therefore, more similar to Subunit IIA, but blackened grains, organic (algal?) filaments, and large miliolid foraminifers (Hole 1131A, below 454 mbsf) are present. Additional components of the sand-sized (>63 µm) fraction include benthic and planktonic foraminifers, sponge spicules, and rare echinoderm spines, ostracodes, and bryozoans. The mud fraction includes abundant nannofossils with accessory benthic and planktonic foraminifers, sponge and tunicate spicules, and dolomite rhombs (see "Site 1131 Smear Slides").
A few distinctive units interrupt this apparently continuous packstone succession. A 0.5-cm-thick layer of deformed nannofossil chalk is present at 486 mbsf (Section 182-1131A-53X-3, 56 cm). The significance of this chalk is unknown, but it appears to have been squeezed between biscuits during the drilling process. Also, a thin (few centimeter) layer of celestite-filled cracks occurs at 504 mbsf (interval 182-1131A-55X-3, 1-5 cm, which may be a diagenetic consequence of the high salinities observed in pore waters (see "Inorganic Geochemistry"). Additionally, a possible omission surface occurs at 463.3 mbsf (Section 182-1131A-51X-1, 13 cm). It is sharp and irregular, but there are no obvious changes in texture or content across the surface. A strong peak on the NGR log at 465-470 mbsf (see "Downhole Measurements") may correlate with this surface and seismic surface 2c, which occurs at ~466 mbsf (see "Seismic Stratigraphy" in the "Site 1127" chapter). Consequently, the possible omission surface may represent a major boundary and should be scrutinized closely during postcruise studies.
Unit III is separated from Unit II by an unconformity representing a major hiatus (see "Biostratigraphy"). Lithologically, the boundary is represented by a sharp contact between the partially lithified packstone of Unit II above and a chert horizon below. It corresponds to the base of seismic Sequence 2, a basinwide unconformity that represents the boundary between the aggradational shelf deposits of seismic Sequence 3 and the progradational outer shelf-upper slope deposits of the overlying seismic Sequence 2 (Feary and James, 1998, reprinted as Chap. 2). Recovery is poor below the unconformity. Recovered sediments consist of chert and partially lithified bioclastic grainstone. The chert is dark to very dark gray and consists of a silicified nannofossil ooze. The grainstone is olive gray in color and of fine sand size, consisting of unidentifiable bioclasts with accessory blackened grains and glauconite. A jagged appearance in porosity and density logging data suggests that the chert and grainstone are thinly interbedded (see "Downhole Measurements").
The cores collected at Site 1131 record a late Pliocene-Quaternary history of rapid and relatively uninterrupted deposition on the outer shelf and upper slope of the eastern Eyre Terrace. An interpretation of lithostratigraphic Unit III is difficult because of poor recovery. Recovered material consists of chert and grainstone, although it is most likely an ooze sequence with horizons that have been preferentially silicified. The age is interpreted as middle Miocene (see "Biostratigraphy"). The upper boundary corresponds to the sequence boundary between seismic Sequences 2 and 3 and represents a strongly lithified surface as previously suggested by Feary and James (1998, reprinted as Chap. 2).
Lithostratigraphic Unit II corresponds to the offshelf-trending prograding clinoform package of seismic Sequence 2 (see "Seismic Stratigraphy" in the "Site 1127" chapter), interpreted to have been deposited by the offshelf transport of shallow-water material (James and von der Borch, 1991). This interpretation is corroborated by the dominantly shallow-water carbonate assemblage identified in the bioclastic fraction of these sediments. It appears that deposition was rapid and relatively uninterrupted. This is supported by the relative paucity of indicators of interrupted sedimentation, such as omission surfaces, firmgrounds, or hardgrounds. Sediment accumulation rates are calculated to be ~25 cm/k.y. (see "Biostratigraphy"), which is high for continental slopes dominated by hemipelagic deposition and higher than most carbonate slopes bordering rimmed platforms (James, 1997). Continuous deposition fed by offshelf transport may be responsible for these high accumulation rates. The timing, mechanisms, and overall controlling factors, such as climate, sea level, and oceanic currents, governing offshelf transport are not known at present. The grainstone/packstone package of Subunit IIB shows some evidence of cyclicity, which may be related to cycles of offshelf transporting activity.
Lithostratigraphic Unit I represents a change in depositional style. The morphology of the bryozoan accumulations of Subunit IB suggest in-place biogenic mounds (e.g., James and Bourque, 1990). There are no skeletons in obvious growth position, but the delicate preservation and abundant mud suggests that the skeletons have not been transported and that bottom currents were weak. Fragmentation of skeletons is probably due to biological processes. The heterogeneous texture argues for some postmortem redistribution of skeletons, probably by burrowing organisms. Following the demise of the bryozoan mounds, it appears that the pre-Holocene depositional style resumed. Pending more accurate age dating, the preferred interpretation of lithostratigraphic Unit I is as follows: the mounds grew during the uppermost sea-level lowstand (~17-22 ka) in water depths of ~200 m and were subsequently buried by the influx of relatively fine-grained sediments, probably by resumed offshelf transport, during the late Pleistocene-Holocene sea-level rise.