Site 1227 (redrilled Site 684) was drilled on the upper slope of Peru margin at 430 mbsl. A 150-m-thick middle Miocene to Holocene condensed section of the distal Trujillo Basin was recovered (Fig. F2). Below 12 m of Quaternary diatom and foraminiferal ooze, Pliocene mixed diatomaceous and siliciclastic sediment is present. A 5-m-thick, coarse, pyrite-rich glauconitic sand marks the base of the Pliocene at 50 mbsf. Both the foraminiferal ooze and the glauconite sand are well sorted and coarse grained and seem to be the result of winnowing. Also, both horizons are characterized by low TOC, high natural gamma radiation, and high magnetic susceptibility, which is indicative of condensed sections and may contain erosional surfaces or even major unconformities. These horizons correlate with the Miocene/Pliocene and Pliocene/Pleistocene boundaries. Below the lower horizon, the drilled section comprises Miocene diatom ooze. Horizons of phosphatic pebbles as much as 2 cm across are often intercalated in these sediments. The pebble layers caused poor recovery because they were not washed out by the drilling fluid, but, as the foraminifer and glauconite sand layers, they show high natural gamma radiation and magnetic susceptibility and probably also represent condensation or erosion surfaces. The pebbles consist of hard phosphate and dolomite, which was probably eroded from hardgrounds and/or diagenetically formed layers and were concentrated by winnowing and rounded on the seafloor (cf. Monterey Formation; Garrison and Graham, 1984). These condensation horizons punctuate the reduced stratigraphy at Site 1227.
More than 10 carbonate pieces were logged at Site 1227; most occurred as reworked pebble layers, but in situ layers were also found (Fig. F2). Between 30 and 50 mbsf, massive 2-cm-thick barite layers were found, whereas dolomite layers were mostly found in the underlying Miocene diatom ooze. Pebbles are gray to very light gray or yellowish. In general, they are densely lithified, but some semilithified pieces were also present. At 31 mbsf, a fragment of a 0.5-cm-thick, white calcite layer was observed.
Inhomogeneous microsparitic dolomite (Pl. P1A) consists of micron- to decimicron-scale euhedral rhombs with "stairlike" structures (Pl. P1B), whereas larger xenotopic crystals were observed in cements (Pl. P1C). Abundant, mostly benthic foraminifers are often infilled with coarse fringe cement and a later generation of blocky dolomite cement (Pls. P1C, P1D, P2A). Diatom frustules are also often infilled with dolomite cement (Pl. P2B). Some dolomites are more friable and show a high porosity. Most cavities are elongated and follow lamination. They are surrounded by rims of calcite microsparite or dolomicrosparite (similar to Pl. P6A), suggesting the cavities were already present during the time of cementation. Framboidal pyrite is seen growing in the empty space inside a diatom frustule (Pl. P2C). In a few cases, almost fossil-free detrital quartz was observed being cemented by dolomicrosparite.
XRD mineralogy analysis revealed very well ordered dolomite slightly enriched in calcium (Fig. F3). Using the equation of Lumsden (1979), CaCO3 contents of 52–56 mol% were calculated for the dolomite phase. Calcite is always present in low amounts, probably as a result of marine sedimentary calcite. Luminescence is generally strong orange and reveals a zonation of different generations of isopacheous fringes (Pl. P2D).
A total of 200 m of Pliocene–Holocene sediments was recovered at Site 1228 (redrilled Site 680) in the distal Salaverry Basin on the Peru shelf at 250 mbsl. The uppermost 60 m consists of Pleistocene–Holocene diatom-rich hemipelagic sediments with variable contents of siliciclastic sediment (Fig. F4). Below 60 mbsf, the entire sequence consists of Pliocene diatom-poor siliciclastic material. Glauconite-bearing quartz-rich sand was found in lithostratigraphic Subunit IIB (66–90 mbsf). Phosphate nodules are present throughout the sediment column and are probably related to nondeposition and erosion surfaces.
The uppermost dolomite was sampled at 33 mbsf; however, the dolomite nodules are concentrated in a zone between 40 and 80 mbsf in both Pliocene and Pleistocene sediment. Most dolomite layers are ~5 cm thick and are homogeneous light to yellowish gray (Fig. F4); some are laminated. Friable disseminated dolomite is common around the layers, but it is not continuously distributed throughout the soft sediment.
Dolomite consists of dense, lithified, decimicron-scale idiotopic microsparite (Pl. P3A, P3B). Distinct orange rhombs can be observed with cathode luminescence. Parallel-bedded, stylolite-like veins show fine seams and are partially hollow or filled with sparry cement. The veins are visible even macroscopically (Fig. F4; Section 201-1228B-6H-2). Coarse dolomite cement also forms fringes around irregular cavities (Pl. P3C, P3D), mostly formed by the intraskeletal voids of the foraminifers (Pl. P4A). A few foraminifers and diatoms are filled with sediment forming geopetal structures; however, coarse sparitic cavity fill is rare. Foraminiferal tests at this site often show walls (Pl. P3D) that are thickened by cement. The cement is in crystallographic continuity with the test, as it shows the same direction of extinction under crossed nicols and probably is syntaxial cement. Plate P4C and P4D shows diatoms surrounded and encased by dolomite. At 33 mbsf (Pl. P5A), a siliceous frustule is more strongly affected by dissolution and small dolomite crystals are forming in the pores. The crystals seem to be all in the same crystallographic orientation and might be in continuity with the larger crystal in the back (cf. Bernoulli and Gunzenhauser, 2001; Bernoulli et al., 2004). In all other examples, the surfaces are well preserved, with the dolomite crystals perfectly replicating the porous diatom structure (Pl. P5B, P5C). Some of them are also filled with framboidal pyrite (Pl. P5D). Quartz and feldspar are cemented in some of the layers (Pl. P4B), which demonstrates that the formation of hard lithified dolomite layers is not strictly related to diatomaceous sediments.
All analyzed samples consist of well-ordered dolomite with 50–55 mol% calcium carbonate in the dolomite phase. A slight enrichment in calcium is also indicated by EMPA. The low calcite content in some of the samples is probably a result of the presence of nannofossils or foraminifers.
At Site 1229 (redrilled Site 681; water depth = 153 m) on the Peru shelf, 200 m of Quaternary and Pliocene sediments were recovered in the central Salaverry Basin. The uppermost 130 m consists of Quaternary laminated diatom ooze with a siliciclastic zone between 40 and 100 mbsf. Several condensation horizons and/or erosion surfaces with phosphate nodules were found (Shipboard Scientific Party, 2003), often at the base of 10-m cycles (glacial–interglacial cycles) (Wefer et al., 1990; Meister et al., this volume). Below 130 mbsf, Pliocene and Pleistocene well-sorted feldspar quartz sands are present.
Dolomite layers are most abundant at Site 1229 (Fig. F5), concentrated around 30 and 100 mbsf. Below 130 mbsf, dolomite is rare in the siliciclastic Pliocene–Pleistocene sediments. Dolomite occurrence is correlated with the beginning of upwelling-related sedimentation at ~140 mbsf and occurs generally in zones with high TOC and high bacterial cell numbers (cf. D'Hondt, Jørgensen, Miller, et al., 2003). Dolomite occurs as 3-cm-thick layers, which mostly show flat bottoms and tops and appear to be part of continuous layers cut by advanced piston coring. Lamination parallel to the top and bottom of the layer was observed in most of the samples. Typically, relatively hard and dense laminae are interlayered with soft, porous 0.5-cm-thick laminae. The color is mostly yellowish light gray, but a few layers are light gray. The nodules are surrounded by friable dolomite.
The hard layers consist of a homogeneous microsparitic groundmass of decimicron-scale euhedral dolomite rhombs. The grain size is coarser around cavities and fossils. Veins and irregular cavities oriented parallel to the lamination and surrounded by fine microsparitic seams were found in the friable laminae (Pl. P6A). They are probably a result of incomplete cementation. Large 50-µm crystals showing edged surfaces were found in a dolomite layer at 7.5 mbsf, associated with well-preserved diatoms (Pl. P6B). A few samples are rich in well-sorted angular quartz grains, reflecting the composition of the surrounding sediment.
Foraminiferal tests are rare at Site 1229. They often show a fringe of fibrous calcite cement, which is partially replacing the original test. The cavity is partially filled with blocky dolomite cement, which postdates the formation of the fibrous cement (Pl. P6C, P6D). These generations are clearly revealed by cathode luminescence (Pl. P7A, P7B, P7C). SEM images (Pl. P8A, P8B, P8C, P8D) show the foraminiferal tests to be covered by perfectly euhedral decimicron-scale dolomite rhombs. They are oriented exactly along the surface of the test and replicate the pore pattern of the foraminiferal test. Framboidal pyrite fills the empty pore space between the dolomite rhombs and, therefore, postdates the growth of the rhombs (Pl. P9A, P9B).
Diatoms are very abundant and occur in diverse shapes at Site 1229 (Pl. P9C, P9D). They are well preserved and enveloped by euhedral dolomite rhombs, which replicate the pore pattern of the diatom. Dolomite grows into the round pores of the diatom frustules (Pl. P10A, P10B, P10C, P10D). A siliceous skeletal fragment found at 99 mbsf (Pl. P11A) is strongly corroded and seems to be less stable than the other surrounding diatom frustules, which appear to be very well preserved. A dolomite rhomb grew beneath but did not infill all of the space between the pores and probably stopped growing when the siliceous frustule still was being corroded.
Thin section staining is mostly colorless, reflecting the dolomitic mineralogy of the sample. In all samples, well-ordered dolomite was determined to be the major component by XRD analysis (Fig. F3). Calcite occurs only in trace amounts except in the uppermost 15 m, where higher calcite levels might be due to higher percentages of nannofossils in the sediment. In one sample that contained a centimeter-scale bivalve shell, high-Mg calcite was detected. The Ca content calculated from XRD using the equation of Lumsden (1979) ranges from 50 to 55 mol%. The slight enrichment of Ca in the dolomite phase was also observed by EMPA. Cathode luminescence shows yellow to orange with brighter luminescence in the coarser areas.
On the lower slope of the Peru Trench (5086 mbsl), 280 m of sediment was penetrated (Fig. F6), and >200 m of Pleistocene–Holocene black diatom ooze with variable amounts of siliciclastic material was recovered. This thickness is greater than that in the depocenter on the shelf. Below a major fault at 216 mbsf, Miocene diatom-rich and siliciclastic sediments with low TOC content are present. Sulfate is consumed from the pore water within the uppermost 7 mbsf; below 10 mbsf, signs of degassing (gas bubbles) were recognized. This site shows high methanogenic activity and stability of gas hydrates (D'Hondt, Jørgensen, Miller, et al., 2003).
Sediments from the lower slope of the Peru margin are highly affected by downslope mass movements that caused centimeter-scale high-angle normal faults and ductile slumping (Kemp and Lindsley-Griffin, 1990). Below 200 mbsf, sediments show increased compaction and stiffness and pervasive cleavage. Disruption and brecciation is common. These deformation structures are due to a compressional regime and underthrusting along the accretionary prism (Kemp and Lindsey-Griffin, 1990).
Carbonate precipitates are rare at Site 1230. Several dolomite layers were identified at ~230 mbsf (Fig. F6), but more were found at deeper levels during Leg 112 drilling (Thornburg and Suess, 1990). A 3-cm-thick dolomite layer was also sampled in a diatom ooze at 6 mbsf during Leg 112. At 229 mbsf, an 18-cm-thick hard lithified dolomite breccia layer was recovered at Site 1230. Breccia layers such as this are common at greater depths and are ascribed to the tectonic activity of the accretionary prism. Disseminated dolomite only occurs in the neighborhood of hard, lithified layers.
Dolomite breccias consist of clasts of subhedral dolomicrite (Pl. P11B, P11C). Between the clasts, cement often shows xenotopic 10- to 50-µm texture with the grains developing compromise boundaries (Pl. P12A, P12B, P12C, P12D). A fossil found at 229 mbsf is completely replaced by dolomite (Pl. P13A, P13B). The central cavity is filled with pyrite, which was precipitated after dolomitization. Fossils are rarely found in these hard dolomite layers; only a few angular quartz grains are present.
Cathode luminescence revealed several generations of sediment infill and cementation between the clasts (Pl. P13C, P13D). The clasts are surrounded by a yellow luminescent rim that clearly separates dolomite generations older and younger than brecciation. Dolomitization of the clasts with relatively low luminescence and zonation predates deformation. Each deformation event was followed by a new generation of strongly luminescent cement. In general, later generations are more luminescent. A central cavity is visible in Plate P13D. Precipitates of a dark, fine-grained mineral could not be identified by morphology, but blue staining of thin sections indicates the presence of iron. As iron could not be detected by EMPA in the dolomite phase, it probably represents pyrite precipitated in the cavity during a later phase of diagenesis. Dolomite analyzed by XRD is well ordered and contains 51–53 mol% calcium carbonate (Fig. F3). EMPA confirms the enrichment of calcium in the dolomite phase.