Site 1132 is located at a water depth of 218.3 m immediately seaward of the shelf-slope break of the eastern Eyre Terrace in the western Great Australian Bight. A Quaternary-Eocene succession of carbonate sediments, 603.2 m thick, was penetrated. It records the temporal evolution from shallow neritic Eocene, through bathyal early Oligocene and middle Miocene, to outer neritic-upper bathyal late Pleistocene and Holo-cene deposition. The lower Oligocene and middle Miocene intervals are thickly developed, the lower Miocene is represented by a major hiatus, the upper Miocene-lower Pliocene is condensed, and the Pleistocene interval is relatively thick (see "Biostratigraphy"). The succession shows a wide variety of lithologies, and a total of six litho-stratigraphic units are recognized (Fig. F3). Core recovery is good for the upper 250 mbsf, and lithostratigraphic boundaries are well defined. Recovery is poor below this level, and the positions of boundaries are accordingly more tentative. The most remarkable lithology encountered at the site is a thickly developed bryozoan floatstone-dominated package characterizing the upper 100 mbsf. Seismic evidence shows that deposition of this interval took place in a complex of overlapping and laterally migrating low biogenic mounds (see "Seismic Stratigraphy").
Unit I consists of bryozoan floatstone and rudstone alternating with bryozoan packstone; the lower part also includes bioclastic wackestone with bryozoans. The sediment is predominantly unlithified, although thin, widely spaced, partially lithified beds are present below ~100 mbsf. The abundant bryozoan fauna is highly diverse and includes a variety of growth forms (e.g., Bone and James, 1993). A unique benthic foraminifer assemblage was found in association with the bryozoan mounds of Subunits IA and IB (see "Benthic Foraminifers"). The sediments are unlithified and burrow mottled, and the color is dominantly light gray with pale olive and white intervals. The unit represents a major bryozoan mound complex composed of well-defined lithostratigraphic packages that form the basis for division into five subunits. The boundaries between subunits are defined at sharp textural and compositional changes between lithologic types.
Subunit IA consists of unlithified, thick-bedded bryozoan floatstone, together with a few beds of bryozoan rudstone and packstone, mainly <0.5 m thick. The color is light gray, light olive gray, and pale yellow. The floatstone contains granule- to cobble-sized flat robust branching, encrusting, and delicate branching bryozoan growth forms floating in a matrix consisting of fine sand-sized bioclasts, debris of delicate branching and articulated zooidal bryozoans, and soft fecal pellets. Serpulids are common in the lower part, together with coralline algal rods. In addition, the matrix fine fraction contains coccoliths, bioclasts, benthic and planktonic foraminifers, echinoid spines, ostracodes, and sponge and tunicate spicules (see "Site 1132 Smear Slides"). The rudstone and packstone beds contain the same components, but the packstone is dominated at some levels by bryozoan debris and other bioclasts. The sediment is strongly bioturbated, and distinct burrows can be recognized at some levels.
Subunit IB consists of packstone with less than 15% "floating" granule-sized bryozoan fragments. The upper boundary is placed where the packstone is overlain by the thick bryozoan floatstone-dominated Subunit IA. The color is light gray, light olive gray, and pale olive. The most common larger bryozoan growth forms are delicate branching and flat robust branching. Serpulids occur scattered throughout. The matrix contains articulated zooidal bryozoans, bioclasts, benthic foraminifers, sponge spicules, planktonic foraminifers, echinoid spines, gastropods, and blackened grains. The sediment is strongly bioturbated, although well-defined burrows are rare to absent.
Subunit IC consists mainly of bryozoan floatstone, with intercalated bryozoan rudstone and packstone beds in the upper part, mainly <1 m thick, and with thicker bryozoan packstone beds in the lower part. A 35-cm-thick wackestone occurs between 51 and 52 mbsf. The upper boundary of Subunit IC is placed where a thick floatstone is overlain by the uniform packstone of Subunit IB. The color is light gray and light olive gray, and white intervals occur in the lower part. The floatstone contains granule- to pebble-sized nodular, arborescent, delicate branching, and flat robust branching bryozoan growth forms, and serpulids are common at certain levels. The matrix between the large bryozoans consists of fine to medium sand-sized bioclastic packstone that contains dominant bioclasts and abundant benthic foraminifers and bryozoans. Bivalves, sponge spicules, ostracodes, and planktonic foraminifers are also present. The rudstone, packstone, and floatstone are of similar composition, but the packstone also contains numerous (<25%) blackened grains.
Subunit ID is dominated by bioclastic packstone packages, ~2.5-5 m thick, intercalated with two floatstone-rudstone packages, 0.8-1.5 m thick. The upper boundary is placed where the upper packstone is abruptly overlain by the bryozoan floatstone of Subunit IC. The color is light gray, pale yellow, and white. The bioclastic packstone is poorly sorted very fine to fine sand, and the content of dark grains increases upward within each package. The lower packstone contains scattered flat robust branching, fenestrate, and nodular bryozoans. The sand fraction of the packstone contains dominant bioclasts, present benthic and planktonic foraminifers, and traces of echinoid spines. The fine fraction includes abundant bioclasts and coccoliths, common tunicate spicules, present sponge spicules, rare dolomite rhombs, and traces of benthic foraminifers and pyrite (see "Site 1132 Smear Slides"). The sediment is strongly bioturbated throughout, as reflected by muddy and grainy patches with diffuse outlines.
Subunit IE consists of alternating packages of bioclastic grainstone, packstone, and wackestone, together with two beds of bryozoan floatstone, 10 and 40 cm thick, at ~109 mbsf. The upper boundary is placed at the contact where the bryozoan-rich Subunit ID overlies lithologies with few or no bryozoans. Packstone is the dominant lithology of Subunit IE, and the scarcity or near absence of bryozoans, except for the thin floatstone beds, is a characteristic feature. The color is gray, light gray, light greenish gray, and white. The grainstone and packstone sand fraction consists of dominant bioclasts, common benthic foraminifers, present to rare planktonic foraminifers, echinoid spines, sponge spicules, tunicate spicules, quartz grains, and traces of bryozoans and glauconite. Blackened grains are common at many levels. The two floatstone beds contain medium sand to granule fragments of bryozoans, dominated by flat robust branching, delicate branching, articulated zooidal, and encrusting growth forms. The fine fraction includes abundant bioclasts and benthic foraminifers, rare sponge spicules, and traces of planktonic foraminifers. The wackestone contains bioclasts, minor benthic and planktonic foraminifers, and tunicate spicules. The subunit is intensely bioturbated, and some burrow fills are grainy and slightly carbonate cemented.
Unit II consists of bioclastic packstone with bryozoans. The sediments are mainly unlithified, but thin partially lithified horizons are present below 130 mbsf. The color is dominantly light gray, with light olive gray, olive, and white intervals. The lithology alternates between packstone with well-sorted bioclastic debris and packstone with a diverse fauna of centimeter-sized bryozoans floating in the matrix. The sand-sized matrix includes articulated zooidal and other bryozoan fragments, benthic and planktonic foraminifers, sponge spicules, small spines of infaunal echinoids, glauconite, and blackened grains. The larger grains comprise a diverse, well-preserved fauna of bryozoans, including delicate branching, flat robust branching, nodular, and arborescent forms that commonly float in the matrix with apparently random orientations. Serpulids constitute a significant faunal component, and partially lithified gray intraclasts are present at some levels. The sediment is completely bioturbated and scattered discrete burrows are recognized, mostly at minor lithologic changes.
Unit III consists of unlithified bioclastic packstone and minor wackestone and grainstone, with a thin package of foraminiferal ooze and chalk at the base. The upper boundary is placed at a prominent firmground where the light olive gray, partially lithified packstone with bryozoans of Unit II overlies gray unlithified packstone. The color is dominantly light olive gray with thinner olive, pale olive, and white intervals. The sediments are strongly burrowed. Unit III is dominantly unlithified; however, there are a few thin partially lithified intervals down to 168 mbsf, and the unit is partially lithified below that level. The presence of several prominent firmgrounds associated with minor but distinct lithologic and textural changes allows division into five subunits. Core recovery is good in the upper part but decreases below ~240 mbsf.
Subunit IIIA consists of unlithified bioclastic packstone. The color is gray, light gray, and light olive gray. The upper boundary of the subunit coincides with the top of Unit III, and the base is placed at a prominent firmground where unlithified bioclastic packstone overlies the burrowed bioclastic wackestone of Subunit IIIB. The sand fraction is well sorted, very fine to fine grained, and contains dominant bioclasts, common benthic and planktonic foraminifers, and present articulated zooidal bryozoans and echinoid spines. The sediment is strongly bioturbated, and abundant Chondrites occur below the top firmground.
Subunit IIIB consists of bioclastic packstone. The color is light olive gray, pale olive, and white. The sediment is unlithified in the upper 4 m and partially lithified below that level. The lower subunit boundary is placed at a distinct firmground where a partially lithified, light gray bioclastic packstone overlies a light olive gray bioclastic wackestone (Fig. F4). The sand fraction is well-sorted very fine and fine sand and contains dominant bioclasts and present benthic and planktonic foraminifers, echinoid spines, articulated zooidal bryozoans, and glauconite. The sediment is strongly bioturbated and distinct burrows occur, most commonly in the lower part.
Subunit IIIC consists of bioclastic wackestone in the top 50 cm, overlying bioclastic packstone. The color is light gray and light olive gray, and the sediment is partially lithified. The sand fraction is poorly sorted fine sand and contains dominant bioclasts; common to present benthic foraminifers, tunicate spicules, and quartz; and rare planktonic foraminifers and sponge spicules. The sediment is strongly burrowed.
Subunit IIID consists of bioclastic packstone and two minor intervals of bioclastic wackestone, one forming the top 2.5 m, and the other at 207.2-208.2 mbsf. The color is light gray, light olive gray, pale olive, olive, and pale yellow, and the sediment is partially lithified. The lower boundary is placed where light olive gray bioclastic packstone of Subunit IIID overlies white foraminiferal ooze of Subunit IIIE. Although the boundary occurs within an uncored interval between Cores 182-1132B-27X and 28X, the lithologic contrast is marked. The sand fraction is poorly sorted fine sand and contains dominant bioclasts, common sponge spicules, common to present benthic foraminifers, and present to rare bryozoans, serpulids, ostracodes, and glauconite. The grains are commonly partially covered by small carbonate crystals. The basal 50 cm of the subunit contains granule-sized intraclasts and macrofossils, including bryozoans and serpulids. The sediment is strongly bioturbated, with common subhorizontal burrows including Planolites with a coarse, commonly greenish fill.
Subunit IIIE consists of foraminiferal ooze and foraminiferal chalk, together with two thin beds of partially lithified bioclastic packstone. The color is white, light gray, and gray. Core recovery is poor above and below the subunit, and, as a result, the upper boundary cannot be precisely placed. The dominant ooze and chalk lithologies are, however, markedly different from both the packstone lithology of Subunit IIID and the chert-bearing grainstone and packstone lithologies of the underlying Unit IV. The chalk and the packstone are partially lithified, and it is possible that the unconsolidated nature of the ooze is due to drilling disturbance. The sand fraction of the ooze and chalk includes dominant planktonic foraminifers, common benthic foraminifers and bioclasts, present glauconite, rare ostracodes, and traces of echinoid spines and bryozoans. The fine fraction includes dominant nannofossils, common benthic foraminifers, and present planktonic foraminifers. The packstone sand fraction contains dominant bioclasts, common planktonic foraminifers, common to present glauconite, rare benthic foraminifers, and traces of echinoid spines.
The interval containing Unit IV is characterized by very poor core recovery. Recognition of a distinct lithostratigraphic unit that differs markedly from the overlying and underlying deposits is, however, possible. Unit IV consists of partially lithified grainstone with light gray, dark gray, and almost black chert. It is significant that the chert is restricted to this single unit and has not been observed in overlying or underlying units. The chert is thus considered an important primary constituent of Unit IV, and it cannot have been emplaced in the cores by drilling contamination. The upper boundary is placed at the first downhole occurrence of chert; the lower boundary, by the last occurrence (LO) of chert. The first appearance is at roughly the same depths in Holes 1132B and 1132C. The color of the grainstone is light gray, olive gray, and white in one case, and the sediments are partially to strongly lithified. The sand fraction contains abundant bioclasts and planktonic foraminifers; common benthic foraminifers; and present to rare echinoid spines, sponge spicules, quartz, and glauconite. A thin section (Sample 182-1132C-13R-CC, 10-12 cm) shows a gray, very fine grained, partially dolomitized microbioclastic packstone with ostracode and zooidal? bryozoan fragments and planktonic foraminifers. The matrix consists of silt-sized microbioclasts. Silt- and sand-sized dolomite rhombs are common.
The chert is light gray or, more commonly, dark gray to almost black. Recovered fragments have mostly been crushed or brecciated by drilling, although centimeter-sized nodules with their original shapes preserved are common. The nodules have a white rim, 1-2 mm thick, consisting of poorly silicified carbonate. Abundant ghosts after small, partially silicified calcareous fossils occur within the nodules. Porous, partly silicified, white to light gray carbonate infills of commonly cylindrical burrows penetrate many nodules. The chert nodules were formed by partial to complete silicification of grainstone, by replacement of carbonate.
The interval containing Unit V is characterized by very poor core recovery. The available material shows, however, that it is lithologically distinct from overlying and underlying units. The unit consists of bioclastic and foraminiferal packstone and grainstone, without chert. The upper boundary is placed at the top of bioclastic grainstone lacking significant amounts of chert, and the lower boundary is placed at a prominent mineralized and bored hardground where white bioclastic grainstone overlies pale yellow echinoid wackestone (Fig. F5). The color of the subunit is pale yellow, very pale brown, and pale brown, and the sediment is strongly lithified. The sand fraction is very fine to fine sand and contains bioclasts, benthic and planktonic foraminifers, and glauconite. The lowest grainstone (516.8-517.7 mbsf) contains delicate branching bryozoans and serpulids. Dolomite and abundant glauconite are present at 507.1-507.3 mbsf. A thin section of Sample 182-1132C-26R-CC, 7-9 cm (468.7 mbsf), consists of gray, very fine to fine-grained foraminiferal packstone to local wackestone. Planktonic and benthic foraminifers dominate, and brachiopod, echinoid, ostracode, and bryozoan fragments are common. The matrix is a microsparite. The echinoderm grains are dolomitized and overgrown with dolomite rhombs. A thin section of Sample 182-1132C-31R-1, 56-59 cm (517.3 mbsf), from the base of Unit V, consists of white to gray, very fine to fine-grained, partially dolomitized planktonic foraminiferal wackestone to local packstone (see "Site 1132 Thin Sections"). Particles are dominated by planktonic and benthic foraminifers, ostracodes, and echinoid fragments; clear, sand-sized dolomite rhombs are common.
Unit VI is complex and may well represent several discrete packages that could have been defined as subunits, if recovery had been better. The unit is represented by five cores containing lithologies that are different from core to core and with recovery gaps of 5-9 m between cores. The individual packages are described in turn from top to bottom to allow a possible future assignment to separate lithostratigraphic subunits or even units. Thin sections yield important information on the rather fragmentary succession and are, therefore, also described (see "Site 1132 Thin Sections").
The top interval of Unit VI, between 517.7-520.6 mbsf (Section 31R-1, 100 cm, through 31R-4), contains several firmgrounds and hardgrounds (Figs. F6, F7, F8). The upper boundary of the unit is a mineralized hardground showing brecciation, borings, and possible neptunian dikes (Fig. F6). It forms the top surface of a lithified, pale yellow echinoid wackestone, 55 cm thick, that contains brachiopods and bryozoans—in addition to echinoid fragments in the very coarse sand fraction—and a dolomite content of ~20%. A thin section of Sample 182-1132C-31R-2, 55-58 cm (518.8 mbsf), consists of gray to white, fine to medium sand-sized, partially dolomitized planktonic foraminiferal wackestone to packstone. The sediment is dominated by planktonic foraminifers with common bryozoan fragments (mostly delicate branching forms), glauconite grains, clear dolomite rhombs, and medium to coarse sand-sized echinoid and brachiopod fragments.
The echinoid wackestone overlies a pale yellow lithified bioclastic packstone that is rich in glauconite and contains bryozoans, solitary corals, echinoid fragments, and gastropods. The sediments are strongly bioturbated and include Thalassinoides burrows. A thin section of Sample 182-1132C-31R-3, 36-38 cm (520.1 mbsf), consists of white to gray, fine-grained, glauconitic, partially dolomitized chalk with a bioclastic wackestone to local packstone texture. The dominant particles are planktonic foraminifers, common benthic foraminifers, glauconite, and fine sand-sized dolomite rhombs. Echinoid and bryozoan particles are also present.
The packstone rests on a burrowed firmground at 520.3 mbsf (Section 31R-3) that forms the top surface of a bioclastic wackestone with floating bryozoans, at least 15 cm thick, which in turn overlies a mineralized hardground at 545.5 mbsf (Section 31R-CC) with sponge borings and debris of bryozoans, serpulids, echinoids, and bivalves (Figs. F7, F8).
The interval from 526.3 to 529.7 mbsf (Core 32R) consists of lithified bioclastic packstone with abundant, green, very fine grained material that is possibly a precursor clay to glauconite. The packstone is light gray to pale yellow, and the green material occurs in densely spaced, gently curved, wispy solution seams, which give the sediment package a characteristic finely striped appearance. Scattered centimeter-sized mollusk and bryozoan fragments are common, and the sediment is strongly bioturbated. A thin section of Sample 182-1132C-32R-1, 40-42 cm (526.7 mbsf), consists of gray to white, fine to coarse sand-sized bryozoan wackestone to local packstone. Bryozoan particles dominate and represent a diverse fauna with many different growth forms. Planktonic foraminifers and echinoid particles with replacing limonite, glauconite, and angular quartz silt are also present. A thin section of Sample 182-1132C-32R-2, 124-126 cm (528.9 mbsf), consists of gray to white, fine-grained, partially dolomitized planktonic foraminifer wackestone. Planktonic foraminifers dominate, and bryozoan, echinoid and ostracode fragments, benthic foraminifers, quartz silt, and clear dolomite rhombs are common.
A package at 535.9-536.4 mbsf (Core 33R) consists of lithified yellow to reddish yellow bioclastic grainstone. The grains are medium sand-sized in the upper part and fine to medium sand-sized in the lower part. Bryozoan fragments are common. The sediment is strongly bioturbated. Burrows filled with reddish, fine sand-sized bioclastic packstone are common in the lower part.
The interval from 545.5 to 547.5 mbsf (Core 34R) consists of yellow to red bioclastic grainstone and packstone. The grainstone contains granule-sized bryozoan fragments and whitish burrow fills. The grains are mainly medium to coarse sand sized. The packstone is composed of fine to medium sand-sized bioclasts; scattered granule-sized bryozoan fragments are also present.
The lowest cored material at 555.1-555.9 mbsf (Core 35R) consists of pale to dark red bioclastic grainstone and packstone. The grains are fine to medium sand sized, and scattered granule-sized bryozoan and shell fragments are present. Two pebbles of coarse sand- to granule-sized calcareous sandstone are composed of subrounded to subangular quartz grains, green lithic fragments, and sedimentary rock fragments. A thin section of Sample 182-1132C-35R-1, 45-47 cm (551.6 mbsf), consists of orange, medium to coarse sand-sized bryozoan wackestone and packstone. Most grains are somewhat rounded and abraded, with conspicuous microborings filled by limonite. The dominant grains are bryozoans, with mostly flat robust branching, articulated branching, and nodular growth forms. Benthic and planktonic foraminifers are common, and dolomite rhombs are rare. Presence of the planktonic foraminifer Acarinina collactea gives a middle-late Eocene age for the deepest cored part of the succession (see "Biostratigraphy").
The succession encountered in Holes 1132B and 1132C is 556 m thick and spans the Eocene-Pleistocene time interval. It shows an overall punctuated deepening to shallowing trend, from an Eocene shallow-marine shelf, to mid-Oligocene and middle Miocene deep-slope conditions, and then back to upper bathyal to outer neritic water depths in the late Pliocene-Holocene.
The oldest cored sediments, Unit VI of middle-late Eocene age, were deposited in a high-energy shallow-marine environment as reflected by the body and trace fossil fauna and rapidly varying lithology. Succeeding deposition took place in progressively deeper water, probably in an upper slope environment with relative sea-level falls reflected by repeated occurrences of firmgrounds and mature, mineralized hardgrounds with sponge borings. There is no direct evidence for latest Eocene-earliest Oligocene deposition, and the Eocene Unit VI is apparently overlain by mid-Oligocene chert-free slope carbonates of Unit V. The early Oligocene hiatus and the marked landward facies shift probably reflect sea-level rise, backstepping, and marked condensation in the middle and lower slope regions. A second major hiatus appears to coincide with the boundary between the chert-free mid-Oligocene Unit V and chert-bearing middle Miocene slope carbonates of Unit IV. The cause of the hiatus is unclear, but a direct sea-level control seems unlikely because the carbonate facies of the two units appear similar. The hiatus is more likely to reflect scouring, winnowing, or nondeposition related to a change in deeper water-current regimes, which may have some combination of climatic, plate tectonic-paleogeographic, or sea-level causes. However, the poor core recovery prevents firm conclusions on this aspect at present. Late Miocene-early Pleistocene deposition of Unit III reflects gradual shallowing, represented by stacking of probably middle to upper slope carbonate packages topped by firmgrounds. Upper slope depths were reached at the Pliocene-Pleistocene transition, recorded by the incoming of delicate bryozoans floating in the fine-grained carbonates of Unit II. This can be considered the first phase of widespread bryozoan colonization of the seafloor, which eventually led to establishment in the Pleistocene of the bryozoan mound complex of Unit I. This type of ecological facies succession is well known from the upper Maastrichtian-lower Danian carbonates of Denmark (Surlyk, 1997). The mound complex shows a cyclic internal upbuilding characterized by packages, which coarsen upward from packstone and grainstone to floatstone and rudstone with rich and diverse bryozoan faunas. Several orders of cycles are recognized, corresponding to Subunits IA to IE. The larger cycles illustrate the establishment, aggradation, and lateral migration of major mound systems. The smaller cycles may reflect possible Milankovitch-driven rhythmicity in either productivity or current velocity. The main subunit boundaries within the bryozoan mound complex appear to correlate well to seismic reflectors seen in dip sections across the shelf-slope break (Feary and James, 1998, reprinted as Chap. 2). Combined analysis of facies successions and trends in the mound complex and seismic reflection patterns will yield important information on the dynamic architecture and temporal evolution of the impressive >100-m-thick bryozoan mound complex. There are no signs of deposition within the photic zone. During the Holocene, the long-term slope progradation recorded at Holes 1132B and 1132C reached water depths corresponding to the present-day position of the site, immediately beyond the shelf-slope break in slightly more than 200 m of water.