Site 1126 is located at a water depth of 783.8 m on the eastern Eyre Terrace. It provided a record of middle Eocene-Pleistocene sedimentation in a middle to upper slope depositional setting.
Two major sedimentary packages form the succession. A lower package consists of siliciclastic sediments of unknown age (Fig. F3). These sediments were probably deposited in a marginal marine setting, as indicated by occurrences of bioturbation. Comparison of the general lithologic succession at Site 1126 with stratigraphically equivalent deposits recovered from borehole Jerboa-1 (Bein and Taylor, 1981) indicates that these siliciclastics are probably Cretaceous (Cenomanian) in age.
The upper sedimentary package at Site 1126 consists of middle Eocene-Pleistocene (see "Biostratigraphy") marine pelagic carbonates. These sediments consist of calcareous ooze, silicified calcareous chalk, silicified limestone, and slumped calcareous ooze.
Based on major sediment types, compositional changes, and sediment deformation structures, five lithostratigraphic units are differentiated. Because the core recovery at Site 1126 was good within the uppermost 160 m, the level of confidence for the placement of these boundaries in this part of the succession is high. Below 160 mbsf, however, core recovery was so poor that it was not feasible to differentiate separate units; therefore, sediments between 160 mbsf and the top of the sandstones (Fig. F3) were placed into one unit. Further division of this Unit IV into at least three subunits is indicated by variations in the downhole logs (see "Downhole Measurements").
Unit I consists of ooze with varying amounts of calcareous nannofossils and planktonic foraminifers. Unit I is divided into two subunits on the basis of textural changes, different patterns of sediment alternation, and the presence of a slightly indurated layer in Cores 182-1126B-5H and 182-1126C-5H.
The calcareous ooze of Subunit IA is characterized by a textural alternation between wackestone and packstone. Mud-supported intervals are light gray in color, whereas grain-supported parts are slightly darker and more greenish.
Matrix is dominated by calcareous nannofossils, accessory tunicate spicules, and sponge spicules. Components in the >63-µm fraction consist of abundant planktonic foraminifers, frequent benthic foraminifers, bioclasts, echinoid spines, blackened grains, and rare ostracodes. Glauconite occurs infilling planktonic foraminifers and as isolated grains. There are no major compositional differences in coarse fraction composition between mud- and grain-supported intervals. Macrofossils are rare, represented by large pteropods and scaphopods as well as minor gastropods.
The major part of the subunit consists of matrix-supported sediment with seven major grain-supported intervals, each as thick as 1.50 m (Fig. F3). Textural changes between matrix- and mud-supported intervals are gradational throughout the entire subunit. Comparison of the natural gamma-ray curve (see "Physical Properties") with the succession seen in the cores shows that the darker, coarser layers correlate with high gamma-ray values.
These sediments are moderately to strongly bioturbated throughout. Bioturbation is manifest as lighter to darker gray color mottling. Burrows of the latter color are best expressed at the transition from darker matrix-supported to lighter grain-supported intervals (e.g., interval 182-1126C-1H-4, 85-110 cm). These burrows, having diameters as large as 4 cm, are interpreted as Thallasinoides traces.
The contact between Subunits IA and IB is at the sharp contact between a lower white gray nannoplankton ooze to chalk and an upper light gray nannoplankton ooze. This boundary is recognized in both drill holes. The fact that the deposits underlying this boundary are slightly more lithified may indicate that this layer represents an early diagenetically consolidated horizon. Texture of the deposits is dominantly matrix supported (wackestone texture), with only minor grain-supported (packstone) intervals.
As in Subunit IA, the major components of the matrix are calcareous nannoplankton, together with tunicate and sponge spicules. Isolated dolomite rhombs are present in some of the smear slides (see "Site 1126 Smear Slides" in PDF format). Components of the >63-µm fraction are abundant planktonic foraminifers, frequent echinoid spines, sponge spicules, blackened grains, and minor benthic foraminifers. Glauconite occurs as infill of planktonic foraminifers, but also as isolated grains. As visually estimated, glauconite may represent as much as 20% of the coarse-grained fraction. Macrofossils are rare throughout the subunit: a solitary azooxanthellate coral is present in interval 182-1126B-5H-5, 134-147 cm (Fig. F4). Other macrofossils are large echinoid spines and plates, small gastropods, pteropods, and serpulids.
Contacts between the whitish gray matrix-supported sediments and the light gray grain-supported intervals are gradational. Changes in color and texture, however, occur over shorter intervals than in Subunit IA.
The sediments of Subunit IB are moderately to strongly bioturbated. Burrowing occurs as diffuse color mottling, but burrows of the youngest tiers are better expressed in transitional areas between lithologies. Sediment in these burrows is coarser grained than the surrounding sediment. Such infilling sediments have a packstone texture, and many of the burrows are rich in glauconitic grains.
Unit II consists of ooze with varying amounts of calcareous nannofossils and planktonic foraminifers. The top and the base of the unit are defined by sediment deformation structures interpreted as slumps (Fig. F5). The position of these bodies (Fig. F3) provides a minimum estimate of thickness, as only the slumped parts with inclined bedding can be unequivocally recognized in the cores. The interpretation of the inclined layers as slumps is corroborated by the poor overall correlation of the deposits of Unit II between Holes 1126B and 1126C, suggesting significant lateral inhomogeneity (see "Composite Depths"). Division of Unit II into two subunits relies on the first downhole occurrence of a silicified layer.
The calcareous ooze of Subunit IIA has a wackestone to packstone texture. As in the overlying deposits, matrix-supported intervals are white gray in color, whereas grain-supported deposits are light gray.
The matrix of the sediment consists of calcareous nannofossils together with traces of quartz and clay. A high proportion of siliciclastic components in the matrix (quartz, mica, pyroxene, and clay minerals) occurs in Section 182-1126C-7H-3, 20 cm. The >63-µm sediment fraction in Subunit IIA consists of planktonic foraminifers, 5%-10% glauconite, and minor benthic foraminifers. The only macrofossils recorded in this subunit are mollusk debris in the upper 20 cm of Section 182-1126B-7H-6. As in the overlying lithostratigraphic units, white-gray matrix-supported layers alternate with light gray grain-supported intervals.
Bioturbation is moderate to strong throughout. It occurs as color mottling with some mottles surrounded by green to dark gray/black reduction halos. Burrows of the uppermost tier are represented by minor Chondrites and indeterminate large subvertical round burrows.
Two slumped layers are present. They are best represented in Hole 1126B. The upper layer is in Core 182-1126B-7H; the lower one is in Core 182-1126B-9H (Fig. F5). Texture and composition of the slumped sediments are similar to overlying and underlying sediments, pointing to local provenance of the slumped deposits.
A distinct type of sediment occurs in Section 182-1126B-7H-5, 102 cm, through 8H-1, 66 cm. These deposits have a wackestone to packstone texture and contain lithified chalky lumps as large as 1.5 cm in size. Such lumps are subrounded to very irregular nannofossil chalk. The lumps float in the nannofossil ooze matrix, suggesting a mass-flow origin for these deposits.
The upper boundary of this subunit is characterized by the first downhole occurrence of silicified layers and the presence of a firmground. The first in situ porcellanite/chert is in interval 182-1126B-10H, 93-104 cm. Silicified rock fragments, however, are also present as downhole contamination in the topmost 10 cm of the core, suggesting the presence of uphole porcellanite/chert. As such, the upper limit of Subunit IIB in Hole 1126B was placed at the bottom of the core catcher of Core 182-1126B-9H. In Hole 1126C, the first silicified layer occurs in interval 182-1126C-9H, 0-11 cm. A firmground is present 1.69 m above this layer in Section 182-1126C-9H-5, 85 cm. The position of this firmground defines the upper limit of Subunit IIB in Hole 1126C.
The deposits of Subunit IIB are calcareous oozes with interbedded silicified layers. Silicified intervals are irregular to nodular porcellanites. The matrix of the oozes is dominated by calcareous nannofossils with minor sponge spicules. The >63-µm fraction contains abundant planktonic foraminifers, benthic foraminifers, sponge spicules, pyrite grains, and glauconite. As in the overlying deposits, lighter and darker intervals alternate throughout this subunit. No macrofossils were recorded in the cores.
The deposits of Subunit IIB are moderately to strongly bioturbated throughout. Burrows appear as color mottling and as distinct traces. Zoophycos and Chondrites burrows are mostly present in the lower part of the cycles, whereas Thallasinoides traces are in the uppermost part. Other traces are Planolites and Terrebelina, specifically at the top of a slump body in interval 182-1126C-10X-1, 90-94 cm. Some of the burrows in Subunit IIB are pyritized (e.g., Section 182-1126B-10H-5, 60 cm).
One distinctive postdepositional feature of the sediment is a series of planar color laminations as thick as 1.5 cm. Laminae have a greenish to dark gray or black color and were probably formed by reduction of ferruginous particles in the sediment. Analyses of smear slides and of the fraction >63 µm show that no sediment compositional changes occur in these layers.
Deposits of Unit III consist of poorly differentiated matrix-supported calcareous oozes with interbedded silicified layers. The matrix of the oozes is dominated by calcareous nannofossils. It also contains abundant sponge spicules and organic debris. The >63-µm fraction consists of abundant sponge spicules and common bioclastic debris, as well as small planktonic and benthic foraminifers. Glauconite occurs as infill of the planktonic foraminifers and as isolated grains. No macrofossils were recorded in the deposits of Unit III.
As in the overlying units, the succession consists of an alternation of lighter and darker intervals. Textural differences between both sediment types in Unit III, however, are minor. Both have a mudstone to wackestone texture with darker intervals containing slightly more sponge spicules and bioclasts. Changes between both lithologies are gradational.
The sediments are moderately to strongly bioturbated throughout. Burrowing appears as color mottling but in some cases also occurs as well-defined traces, such as Chondrites burrows in Section 182-1126C-17X-1 or as burrows with backfills (probably Zoophycos) in interval 182-1126B-19H-4, 60-75 cm. As in the deposits of the overlying unit, postdepositional features are represented by green and black laminae as thick as 1.5 cm.
Four lithologies were recovered in this interval: (1) poorly differentiated nannofossil chalk, (2) poorly differentiated spiculitic nannofossil ooze, (3) silicified calcareous pelagic limestone, and (4) porcellanite and chert.
The nannofossil chalk, nannofossil ooze, and the spiculitic nannofossil ooze are whitish to light gray in color. Throughout the unit, they are matrix supported with calcareous nannofossils dominating. Components >63 µm are rare and consist of sponge spicules, with minor small planktonic foraminifers, and bioclasts.
The chalks and oozes are pervasively bioturbated throughout. Burrows appear as mottles and as well-defined traces. This is especially the case in Cores 182-1126C-24R, 25R, and 26R where frequent Zoophycos and Chondrites traces are present. As in the deposits of the overlying units, postdepositional green and black laminae occur in these sediments.
Silicified pelagic limestone is light gray to green in color. Thin-section analysis shows that the abundance of planktonic foraminifers and of sponge spicules varies between the samples (See "Site 1126 Smear Slides" in PDF format).
The porcellanite and chert have a greenish gray color. In some cases they form large nodules as large as 5 cm in diameter, but in other cases silicification may also occur as stratiform features affecting entire layers. Porcellanite and chert pieces contain large (as much as 1.5 cm), round vugs filled in with ooze and chalk. These vugs probably represent burrows that were not affected by silicification. Some of the calcareous nannofossil assemblages used to date the sediments of Unit IV come from these vugs (see "Biostratigraphy").
One layer of a minor lithology is present at the top of Unit IV (Sample 182-1126D-3R-CC, 12-15 cm). It contains bioclastic packstones with frequent planktonic and benthic foraminifers, frequent ostracodes, and echinoderm debris. Nonbiogenic components are angular quartz, glauconite, and mica.
The upper boundary of this unit is defined by the contact between the overlying chalk and underlying siliciclastics in Core 27R. The lower boundary was not drilled at this site.
Subunit VA is a medium brown sandstone, locally cross-laminated, with a lower thin yellowish brown sandstone to claystone. An irregular cavity in the sandstone is centimeter sized and has a thin, laminated iron oxide lining. The cavity is filled with a yellow to brown calcareous sandstone. Terrigenous sand grains have a conspicuous limonitic coating. Carbonate is fine grained. Grains are possibly bioclasts such as minor gastropods. Small open vugs in the calcareous sandstone are lined with blocky calcite crystals.
Sediments consist of alternating black to green siltstone, sandstone, and microconglomerate. Samples analyzed by X-ray diffraction indicate spheroidal limonite, hornblende, feldspar, garnets, and cordierite, as well as accessory muscovite and tourmaline. Grains are angular to subangular.
Fining-upward cycles occur in different cores (e.g., Sections 28R-1 and 33R-1). Cycles have planar, low-angle planar, and low-angle trough cross-bedding in the lower part (sandstones to microconglomerates). Upper parts consist of silty sandstone, mudstone, and claystone with subplanar laminations. Bioturbation is absent to moderate throughout and comprises indeterminate burrows, especially in the fine-grained intervals.
Thin-section analysis (see "Site 1126 Thin Sections," in PDF format, for position of the samples) indicates that the coarse fraction of the sandstone is dominated by lithic grains with an oolitic coating of iron-rich clays. Quartz is common in the fine sand to silt-sized fraction, and medium-sized lobate pelloidal glaucony grains are abundant in some laminae. Some quartz grains show vesicles suggesting volcanic origin. Only very few biogenic grains are present (e.g., a small fragment of silicified wood in Sample 182-1126D-28R-3, 12-14 cm).
The occurrence of bioturbation in the deposits of Unit VB may indicate that they formed in a marine setting. However, no other positive evidence for a marine origin of these sediments could be found. The oldest marine deposits drilled in the borehole Jerboa-1 are Albian in age (Bein and Taylor, 1981). Thus, it seems reasonable to propose a post-Albian age for the siliciclastics at Site 1126. An interpretation of the siliciclastic depositional system is not straightforward. The sedimentary structures and textural changes allow for two interpretations. The deposits could be either part of a storm-dominated tempestite depositional system or could belong to a deeper water turbidite depositional system.
The nature of the unconformity between the Mesozoic and the Cenozoic sequences could not be observed at Site 1126 because of the low core recovery in the corresponding interval. The sequence boundary could correspond to the alteration affecting the siliciclastics in Subunit VA. Coated cavities in the sandstone would thus represent some kind of exposure surface with an irregular infill of younger, Eocene? calcareous sandstone. These sediments can possibly be attributed to seismic Sequence 7 (see "Seismic Stratigraphy"). Overlying sediments of Units IV and III, which roughly correspond to the depositional phase B of the western Great Australian Bight (Feary and James, 1998, reprinted as Chap. 2), record a marine pelagic setting. The sedimentary succession at Site 1126 does not show any major changes throughout these units, with the exception of minor fluctuations of sponge spicule abundance and the presence of chert layers (Fig. F3). The late early Miocene-early middle Miocene warming of the surface waters observed in the onshore sedimentary succession (Feary and James, 1998, reprinted as Chap. 2) could not be identified in the sedimentary succession at Site 1126.
An interruption of the monotonous pelagic sedimentation is reflected by the slumps of Unit II. It is unresolved whether Unit II consists of one major slump or a sequence of different slumps. We favor the interpretation of several episodes of slumping because of the presence of relatively thick undisturbed sedimentary intervals between individual soft-sediment deformed units.
Relatively monotonous pelagic sedimentation at Site 1126 began again during the Pliocene (Unit I). The succession consists of an alternation between layers rich in planktonic foraminifers and layers dominated by calcareous nannofossils. The layers with more planktonic foraminifers have a higher glauconite content, which correlates with peaks in the natural gamma-ray record. The benthic foraminiferal assemblages in these layers (see "Biostratigraphy") do not show any evidence of reworking. Thus, the alternation may reflect (1) changes in paleoceanography (i.e., productivity) or (2) changes in the bottom current regime leading to phases of mud winnowing. Additional postcruise investigations are required to address this question.
The results of downhole logging (see "Downhole Measurements") permitted pinpointing of the depths of the seismic reflections (Feary and James, 1998, reprinted as Chap. 2) (see "Seismic Stratigraphy") and comparison of these data with lithostratigraphic information. There is a relatively good coincidence between the depth of the reflection at the base of Sequence 3 and the position of the boundary between Units IV and III. The reflection at the top of Sequence 6A could not be recognized as a major sedimentary break, although a firmground occurs at a similar depth in Core 182-1126D-9R (Fig. F3).
The boundaries between Unit III and Subunit IIB, Subunits IIB and IIA, and Subunit IIA and Unit I show only a poor correlation with sequence-bounding reflections. This discrepancy may arise in part from the fact that the slump deposits are laterally discontinuous and that the seismic facies of these units are in part chaotic (see "Seismic Stratigraphy"). Finally, there is a reasonably good correlation between the position of the seismic reflection at the base of Sequence 2 and the boundary between Subunits IA and IB.