Site 1129 is located at a water depth of 202.4 m at the present-day shelf edge of the eastern Eucla Shelf and is the shallowest site drilled during Leg 182. The site is situated at the most landward position on a transect that include Sites 1127 and 1131.
Seismic data indicate that the seafloor on the present-day shelf edge is underlain by a thick Pleistocene succession composed of a mounded facies in the upper part and a prograding clinoform package in the lower part (Feary and James, 1998, reprinted as Chap. 2; see "Seismic Stratigraphy," in the "Site 1127" chapter). The near-surface, moundlike seismic feature is inferred to represent an extensive bryozoan mound complex.
The 604.2-m-thick succession of carbonate sediments recovered at Site 1129 consists of a thick Pleistocene bryozoan floatstone-dominated package, a massive upper Pliocene-Pleistocene bioclastic packstone-dominated package, and a thin middle Miocene package composed of packstone and grainstone with chert. The boundary between the Pliocene-Pleistocene and middle Miocene strata is sharp and represents a major hiatus.
The succession is divided into three units on the basis of major sediment type and compositional changes (Fig. F3). The sediments found at Site 1129 resemble those found at Site 1131, although some differences exist. Most of the units identified at Site 1129 can be correlated with those of Site 1131.
One of the highlights at this site is the presence of a major bryozoan mound complex in the upper 150 m of the Pleistocene succession. The temporal and spatial variability within the complex provides much important sedimentological and ecological information about cool-water carbonate biogenic accumulations.
Unit I consists mainly of bryozoan floatstone to rudstone and bioclastic packstone to grainstone, with abundant bryozoan fragments. The floatstone and rudstone are pale yellow to light gray and have a very fine to medium sand-sized, poorly sorted bioclastic packstone matrix. The floatstone and rudstone contain abundant granule- to pebble-sized bryozoan fragments showing highly diverse growth forms (e.g., Bone and James, 1993). The bioclastic packstone to grainstone is pale yellow, light gray, and light olive gray, and it consists of very fine to fine sand-sized bioclasts with some coarse sand- to granule-sized bryozoan fragments.
The sediments are mainly unlithified, but some poorly consolidated lumps are present in the lower part. The unit is divided into five subunits, each forming a package grading upward from bioclastic packstone at the base to bryozoan floatstone and rudstone at the top.
Subunit IA consists of bioclastic packstone and grainstone with some thin floatstone intervals (<1 m thick) in the middle part. The boundary between different lithologies is generally gradational. The color is light gray to light olive gray. The sediment is unlithified and intensely burrowed throughout. Some of the burrows are recognized as white patches.
The packstone consists of very fine to medium sand-sized bioclasts with abundant sand- to pebble-sized bryozoan fragments floating in the packstone matrix. The granule- and pebble-sized bryozoan fragments include delicate branching, flat robust branching, arborescent/nodular, and fenestrate growth forms (Fig. F4). Granule-sized serpulid tubes are also present. The very fine to medium sand-sized fraction contains bioclasts, articulated zooidal bryozoan fragments, benthic foraminifers, sponge spicules, planktonic foraminifers, echinoid test fragments and spines, gastropods, tunicate spicules, blackened grains, and quartz grains. The bioclastic grainstone at the top of Subunit IA contains the same sand-sized components as that of the packstone, together with rare glauconite grains.
The intercalated bryozoan floatstone beds contain granule- to pebble-sized bryozoan fragments, including delicate branching, flat robust branching, nodular/arborescent, and fenestrate growth forms. The matrix between the bryozoans shows a packstone texture and contains very fine to fine sand-sized bioclasts with benthic and planktonic foraminifers, delicate branching and articulated zooidal bryozoan fragments, sponge spicules, and blackened grains.
Subunit IB consists of thick bryozoan floatstone to rudstone beds and thin bioclastic packstone beds. Floatstone and rudstone are dominant in the upper part of the subunit. The color varies between pale olive gray, light gray, and olive, and the sediment is strongly bioturbated. The upper boundary is placed where bryozoan floatstone is overlain by bioclastic packstone without the bryozoan fragments of Subunit IA.
Subunit IB has the thickest bryozoan floatstone and rudstone intervals and the most highly diverse bryozoan faunas in Unit I (Fig. F4). The bryozoan floatstone to rudstone contain abundant bryozoan fragments with a bioclastic packstone matrix. The bryozoan fauna has a high diversity and includes delicate branching, flat robust branching, foliose, fenestrate, vagrant, articulated zooidal, and arborescent/nodular growth forms. Granule-sized serpulid tubes also are present. The packstone matrix between the large bryozoan fragments is composed of fine sand-sized bioclasts, bryozoan fragments, benthic and planktonic foraminifers, pellets, sponge spicules, ostracodes, echinoid spines, and blackened grains.
The bioclastic packstone and grainstone contain granule- to pebble-sized nodular, flat robust branching, and articulated branching bryozoan fragments. The very fine to medium sand-sized fraction contains dominant bioclasts, common benthic foraminifers, articulated zooidal and articulated branching bryozoan fragments, present planktonic foraminifers, and rare sponge spicules, ostracodes, and echinoid spines. Blackened grains are present throughout. The finer matrix contains dominant nannofossils, abundant bioclasts, rare benthic foraminifers, sponge and tunicate spicules, and traces of pyrite (see "Site 1129 Smear Slides" in PDF format).
Subunit IC consists of bryozoan floatstone and rudstone and bioclastic packstone with bryozoan fragments. The color is white, pale yellow, light gray, and light olive gray. The sediment is unlithified and strongly bioturbated. The upper boundary of Subunit IC is placed where bioclastic packstone is overlain by a graded packstone bed of Subunit IB.
The bryozoan floatstone and rudstone contain dominant bryozoan fragments, including delicate branching, flat robust branching, arborescent/nodular, foliose, and vagrant growth forms. The matrix between the large bryozoans consists of bioclastic packstone. The medium to coarse sand-sized fraction contains pellets, benthic foraminifers, and serpulid tubes. The very fine to fine sand-sized fraction contains bioclasts with common benthic foraminifers, present planktonic foraminifers and sponge spicules, rare echinoid spines, and traces of ostracodes and blackened grains.
The bioclastic packstone consists of very fine to fine sand-sized bioclasts, with common benthic foraminifers, and present tunicate and sponge spicules, bryozoan fragments, ostracodes, pteropods, and blackened grains. Coarse sand- to granule-sized bryozoan and bivalve fragments are also present. The dominant forms of bryozoans are flat robust branching, articulated branching, foliose, and vagrant. Within the packstone, two subfacies are distinguished: one characterized by as much as 10% blackened grains and another characterized by abundant bryozoan fragments. The bioclastic packstone with blackened grains is dominant in the lower part.
Subunit ID consists mainly of bioclastic packstone with two bryozoan floatstone beds in the upper part. The color is pale yellow, light gray, and gray. The sediments are mainly unlithified, but partially lithified lumps are present in some intervals (i.e., Section 14H-2). The sediment is strongly bioturbated. The upper boundary is placed where bioclastic packstone with abundant delicate branching bryozoan fragments is overlain by bioclastic packstone with rare bryozoans of Subunit IC.
The two bryozoan floatstone beds at the top of Subunit ID contain dominant granule-sized, blackened, delicate branching bryozoan fragments and minor amounts of flat robust branching bryozoans. The matrix consists of fine sand-sized bioclastic wackestone to packstone composed of benthic foraminifers, bioclasts, planktonic foraminifers, sponge spicules, and echinoid spines.
The packstone is dominated by bioclasts with various amounts of sand- to granule-sized bryozoan fragments. The bryozoan fauna is dominated by delicate and robust flat branching, arborescent/nodular, and articulated zooidal growth forms. In addition to the bioclasts, the very fine to fine sand-sized fraction includes common benthic foraminifers, serpulid tubes, present planktonic foraminifers, rare ostracodes, echinoid spines, and some blackened grains. Some bioclastic grains have a coating of calcite cement.
Subunit IE consists of bioclastic packstone with five bryozoan floatstone beds, each less than 80 cm thick. The color is pale yellow to light gray. The sediment is mainly unlithified, but some partially lithified lumps are scattered throughout, and the sediment is intensely bioturbated. The upper boundary is placed where bryozoan floatstone with abundant arborescent bryozoan fragments is overlain by the fine sand-sized, well-sorted bioclastic packstone of Subunit ID.
The bioclastic packstone is poorly sorted and contains dominant very fine to fine sand-sized bioclasts with scattered granule-sized bryozoan fragments. The bryozoans are usually abraded, and arborescent, delicate branching, and flat robust branching growth forms are represented. In addition to bioclasts, major components include benthic and planktonic foraminifers, articulated zooidal and delicate branching bryozoan fragments, sponge spicules, echinoid debris, ostracodes, brown organic filaments, quartz grains, and blackened grains. The bioclasts commonly have a coating of calcite cement. The texture and composition is relatively uniform throughout. The bryozoan floatstone beds are dominated by granule and larger sized particles of arborescent, robust branching and delicate branching bryozoan growth forms.
Unit II consists of thick light gray, light olive-gray, and gray bioturbated bioclastic packstone and minor grainstone and wackestone beds, with four nannofossil chalk intervals. The sediments are unlithified in the upper part of Unit II in interval 182-1129C-17H through 25X, whereas the sediments are partially lithified and well lithified in the middle and lower part of the unit. The sediments are slightly neomorphosed and dolomitized in the lower part of Unit II, with dolomite grains constituting ~10% (see "Inorganic Geochemistry"). Body fossils commonly occur as molds. Some shells, molds, and vugs are filled with celestite. A remarkable diagenetic feature of Unit II is the presence of 3- and 6-cm oval concretions of pale yellow to yellow native sulfur in Cores 182-1129C-20H through 22H.
The sediment is bioturbated throughout and is uniform in appearance in the upper part of the unit; however, burrows are observed as diffuse green mottles in the middle to lower part. The lower part especially is dominated by abundant, well-defined burrows, including Palaeophycus heberti, Planolites, Chondrites, and Zoophycos. These burrows are filled with a variety of material, including planktonic foraminifers and unidentified green material, and commonly have a dark wall lining.
Bioclastic packstone, the dominant lithology of Unit II, is very fine to fine grained, generally well sorted, and uniform due to bioturbation. The packstone consists mainly of skeletal components with rare coarse sand- to granule-sized bryozoan fragments, represented by delicate branching and flat robust branching growth forms. The size, abundance, and diversity of bryozoan fragments are much lower than in Unit I. The other millimeter-sized fraction contains well-preserved benthic foraminifers and shell fragments. The very fine to fine sand-sized fraction contains abundant bioclasts with common benthic and planktonic foraminifers, articulated zooidal bryozoan fragments, and rare ostracodes, sponge spicules, and echinoid spines. Blackened grains are present throughout. Most of the skeletal components have calcite overgrowths.
Some grainstone beds are present in the upper and middle part of Unit II. The bioclastic grainstone beds are well sorted and have the same composition as the packstone, with very fine to medium sand-sized grains.
The lower part of Unit II is dominated by bioclastic wackestone with subordinate bioclastic mudstone and packstone. The color is light olive gray, light gray, light brownish gray, and gray, and the grain size is silt to very fine sand sized. The coarse fraction contains planktonic and benthic foraminifers, with rare sponge spicules, glauconite grains, blackened grains, and pyrite grains. Planktonic foraminifers are abundant in some levels, and the lithology is close to chalk. The sediment is intensely bioturbated, and abundant, well-defined burrows of P. heberti, Planolites, Zoophycos, and Chondrites are present. A dense assemblage of P. heberti is observed in Section 182-1129D-13R-5 (Fig. F5).
Four nannofossil chalk intervals with subordinate bioclastic mudstone, wackestone, and packstone occur in Sections 182-1129C-25X, 28X, and 47X and in Sections 182-1129D-8R through 9R and 16R. The color is light olive gray and light gray, and the grain size is silt to very fine sand sized. The chalks have mudstone and wackestone textures and consist dominantly of nannofossils with abundant bioclasts. Planktonic foraminifers are also present, as well as rare sponge spicules and traces of radiolarians and tunicate spicules. The sediment is pervasively burrowed, and burrows of Chondrites and Zoophycos are observed. Intervals with a compacted, high-density Chondrites fabric occur at several levels (Fig. F6).
The base of Unit II consists of a clean bryozoan grainstone to rudstone bed in interval 182-1129D-22R-1, 57-60 cm. It has a sharp base that forms the boundary between Units II and III (Fig. F7). The bed is slightly graded. The coarse fraction is dominated by bryozoan fragments with delicate branching, articulated branching, flat robust branching, and vagrant growth forms. The other components are blackened grains, bioclasts, benthic foraminifers, and lithoclasts of very fine grained packstone.
Five cycles are recognized in Unit II, each grading upward from a nannofossil chalk at the base to bioclastic packstone and grainstone. The upper three cycles are dominated by very fine to fine sand-sized, generally well-sorted massive bioclastic packstone, with subordinate grainstone and wackestone. The lower two cycles are characterized by muddy bioclastic mudstone, wackestone, and packstone with abundant, well-defined burrows.
Unit III is characterized by very poor core recovery. The available material shows, however, that it is lithologically distinct from the overlying deposits, and this is supported by micropaleontological study (see "Biostratigraphy"). Unit III consists of bioclastic packstone to grainstone with dark gray to black chert. The upper boundary is placed at the sharp base of the thin, graded bryozoan grainstone to rudstone bed of Unit II.
The light gray, partially dolomitized, bioclastic packstone to grainstone is well lithified. The main constituents are dolomite, bioclasts, bryozoan fragments, ostracodes, planktonic and benthic foraminifers, echinoid spines, glauconite, and quartz (see "Site 1129 Thin Sections" in PDF format). The dolomite content reaches ~40%, and the dolomite crystals are clear and silt to fine sand sized, with rhombohedral facies.
The chert is dark gray to almost black and crushed or brecciated by drilling. Chert pebbles often show ghosts after millimeter-thick burrows, probably Chondrites, and contain larger, poorly silicified, light gray to white burrow fills. Some pebbles have a 1- to 2-mm white rim of poorly silicified carbonate.
The succession recovered at Site 1129 provides a sedimentologic and paleoceanographic record of a shelf edge to upper slope setting of middle Miocene to Pleistocene age.
It is difficult to interpret Unit III because of its poor recovery. The limited material suggests that the succession consists of bioclastic packstone to grainstone with horizons that have been preferentially silicified. The age is interpreted as early-middle Miocene (see "Biostratigraphy"). The lithology is similar to that of Unit IV at Site 1132 in the same upper slope setting on the western transect (see "Lithostratigraphy" in the "Site 1132" chapter). This indicates that both sites were situated in a similar depositional environment in the lower-middle Miocene.
It is remarkable that the top part of Unit III (interval 182-1129D-22R-1, 105-109 cm) contains as much as 20% bryozoan fragments in the coarse fraction. A tentative interpretation based on thin-section observations suggests that the sediment was deposited in a deep-slope environment. This indicates that the bryozoan fragments were brought into the deep-slope setting from the shelf edge near the site by downslope transportation and that bryozoan faunas may thus have already colonized shallower water environments by early-middle Miocene time.
The sharp boundary between Units II and III corresponds to the sequence boundary between seismic Sequences 2 and 3, and it represents a major hiatus between the middle Miocene and the upper Pliocene. The unit boundary can be traced to Sites 1127 and 1131, where the late Miocene-early Pliocene age interval is also absent (see "Lithostratigraphy" in the "Site 1127" chapter and "Lithostratigraphy," in the "Site 1131" chapter). Upper Miocene sediments, however, are recovered from Sites 1130 and 1132 in the western transect (see "Lithostratigraphy," in the "Site 1130" chapter and "Lithostratigraphy" in the "Site 1132" chapter).
The seismic record indicates that the sequence boundary between seismic Sequences 2 and 3, correlated with the hiatus, truncates the top of both seismic Sequences 3 and 4, implying marine erosion controlled by current flow (Feary and James, 1998, reprinted as Chap. 2). Further, the distribution of seismic Sequence 3 extends westward, and it is the thinnest on the transect through Site 1129 (see Fig. F16, in Feary and James, Chap. 2, this volume).
Unit II corresponds to the thick prograding clinoform package of seismic Sequence 2 (see "Seismic Stratigraphy," in the "Site 1127" chapter), and Site 1129 intersects the proximal portion of the prograding wedge. The base of Unit II consists of a graded bryozoan grainstone to rudstone bed, with several growth forms. This indicates that bryozoan colonization of the upper slope and shelf edge had already occurred in the upper Pliocene. The thick bioclastic packstone-dominated package of Unit II, including neritic faunal elements, is interpreted as representing offshelf transport of shallow-water material (James and von der Borch, 1991).
The upper three cycles of Unit II are dominated by uniform bioclastic packstone, whereas the lower two cycles are characterized by muddy bioclastic sediments with abundant burrows. Sediment accumulation rates of the upper and lower part of this unit are calculated to be ~50 cm/k.y. and 20 cm/k.y., respectively (see "Biostratigraphy" and "Paleomagnetism"). The differences in lithology and sedimentation rates between the upper and lower parts of Unit II seem to indicate a different depositional environment, corresponding to the change from a distal to proximal position on the prograding slope. This interpretation is supported by the transition into the bryozoan floatstone-dominated deposits of Unit I overlying this unit, which accumulated in the upper slope to shelf edge setting. Unit II is thus interpreted as being part of this overall shallowing-upward succession. Unit II contains a smaller scale cyclicity, as indicated by textural change from wackestone to grainstone and by varying abundance of blackened grains. These cycles may be related to Milankovitch-driven rhythmicity, which controls sea-level change, climate, and oceanographic dynamics. These factors are considered to significantly influence the offshelf transport and productivity of the cool-water shelf carbonates. Detailed postcruise analyses should clarify the mechanisms involved in the development of this thick prograding wedge.
One of the most remarkable aspects of Unit II is the occurrence of four nannofossil chalk intervals. Similar chalk intervals are present in Unit II at Site 1127 but are absent at Site 1131, which is located between both sites (see "Lithostratigraphy" in the "Site 1127" chapter and "Lithostratigraphy," in the "Site 1131" chapter). The reason for the absence of chalk at Site 1131 is unknown at present.
Unit I consists of five subunits, each representing development of a bryozoan mound. The bryozoan mound development reached a climax in Subunit IB, with the thickest floatstone to rudstone interval containing highly diverse bryozoan growth forms. Furthermore, different orders of cyclicity within the coarsening-upward succession are recognized, as is the case at Site 1132 (see "Lithostratigraphy" in the "Site 1132" chapter). The larger cycles represent development of major mound systems, including establishment, aggradation, and lateral migration stages of the mounds. The smaller cycles may reflect Milankovitch-driven rhythmicity in either productivity or current velocity.
Subunit IA resembles the lower parts of other subunits, with a dominance of packstone and grainstone with rare floatstone. Seismic evidence shows that the mound complexes are overlapping and laterally migrating with time. This subunit probably represents an intermittent stage or initial stage in mound development.
The main subunit boundaries within the bryozoan mound complex appear to correlate with seismic reflectors seen in dip sections across the shelf-slope break. Therefore, a synthesis of internal structure of the mound succession and geometric distribution of seismic reflectors will give important information on the dynamic architecture and temporal evolution of the bryozoan mound complex as a whole.