LITHOSTRATIGRAPHY

At Site 1236, sediment was cored in three holes to 222.8 mcd (207.7 mbsf) in Hole 1236A, 136.7 mcd (122.8 mbsf) in Hole 1236B, and 180.9 mcd (167.3 mbsf) in Hole 1236C. Basaltic breccia (inferred basement) was reached at 222.3 mcd in Hole 1236A. The sediments at Site 1236 are primarily calcareous, with pelagic oozes predominant in the uppermost ~156 mcd. Farther downhole, nonpelagic grains become increasingly abundant except for the lowermost ~14 mcd, which is pelagic, primarily consisting of chalk.

The sedimentary sequence at Site 1236 was subdivided into four major lithologic units (Units I-IV) with two subunits in Unit II (Table T6; Fig. F11). Unit I contains nannofossil ooze, which is rich in foraminifers, and partly clay bearing. Magnetic susceptibility decreases and lightness increases downhole to ~35 mcd. Unit II sediments primarily consist of nannofossil ooze (Subunit IIA) and unlithified mudstone (Subunit IIB), both partly foraminifer bearing and with numerous intercalations of nonpelagic calcareous sediments. The grain content varies significantly with intervals of unlithified packstone to rudstone and wackestone to floatstone. These lithologies often are present in fining-upward sequences. We distinguished two subunits in Unit II based on the transition from nannofossil-dominated to micrite-dominated sediments. Unit II is characterized by higher GRA bulk density and lightness as well as lower and more variable magnetic susceptibility when compared to Unit I. Coarse-grained nonpelagic calcareous sediments, partly glauconitic, predominate in Unit III. Unit IV includes a multicolored sequence of calcareous chalk with increased amounts of glauconite. Metalliferous sediments with volcanic glass lie directly above the basement, which consists of a basaltic drilling breccia. Magnetic susceptibility increases strongly downhole in Units III and IV, corresponding to decreasing lightness. Bulk density, grain density, and porosity measurements for all units are shown in Figure F12.

Description of Lithologic Units

Lithologic units are primarily defined by visual core description, smear slide examination, GRA bulk density, color reflectance, and magnetic susceptibility measurements. Calcium carbonate concentrations provided additional support.

Unit I

Intervals: Core 202-1236A-1H through Section 12H-5, 0 cm; Core 202-1236B-1H through Section 13H-2, 69 cm; and Core 202-1236C-1H through Section 11H-7, 56 cm
Depths: Unit I: 0-119.5 mcd; Hole 1236A: 0-105.6 mbsf; Hole 1236B); 0-106.0 mbsf; and Hole 1236C) 2.8-106.9 mbsf
Age: Pleistocene to early Miocene (0 to ~17 Ma)

Unit I is characterized by calcareous pelagic oozes of Pleistocene, Pliocene, and Miocene age. The major lithology defining Unit I is nannofossil ooze, which is rich in foraminifers and partly clay bearing (Table T6). Carbonate concentrations are generally >90 wt% (Figs. F11, F13). Sediments of Unit I display little visual variability and few sedimentary structures. Sediment color ranges from very pale brown to white with occasional light gray intervals. Color changes are typically subtle and gradational. Pale brown intervals often display color mottling, which indicates moderate bioturbation. Some of the mottles have dark halos that reveal the presence of finely dispersed iron sulfides.

The biogenic fraction is dominated by nannofossils; discoasters are abundant below 32 mcd. Nannofossil contents are ~50% in the uppermost ~15 mcd, increase to a maximum (>90%) between ~50 and 63 mcd, and gradually decrease to the bottom of Unit I (Fig. F13). Micrite, probably originating from the in situ recrystallization of nannofossils, becomes a significant component of the major lithology below ~90 mcd. Foraminifer contents decrease from >25% in the uppermost ~10 mcd to a few percent around 50 mcd. The lower half of Unit I is characterized by highly variable foraminifer contents averaging ~10%-15% (Fig. F13).

Siliciclastic components are mainly clay minerals, which are variable throughout Unit I. The clay mineral abundance decreases from values of ~10%-20% in the uppermost ~15 mcd to a minimum of negligible content between ~50 and 60 mcd. Farther downhole, the clay mineral abundance varies over a range of ~5%-15% (Fig. F13). Within the siliciclastic component, fine silt-sized feldspar and pyroxene are present in a few samples. Authigenic minerals include trace amounts of pyrite and iron oxides.

Two isolated layers rich in nonpelagic calcareous grains (primarily peloids) at ~65 and 99 mcd (Fig. F14), classified as unlithified wackestone, are a notable minor lithology in Unit I. Additionally, several volcanic ash layers are present, most of which can be correlated across the three holes. The ash layers appear as brownish intervals with a fairly sharp bottom and a gradational top (Fig. F15). They contain primarily brown glass and admixtures of the surrounding sediments. One layer at a depth of ~105 mcd consists of pale glass and minor amounts of biotite, pyroxene, and feldspar. An interval rich in volcanic ash from ~82 to 84 mcd is characterized by increased siliceous microfossil contents, primarily diatoms (see "Diatoms" in "Biostratigraphy"), which are otherwise only occasionally present in trace amounts in Unit I (Fig. F13).

The magnetic susceptibility of Unit I sediments is low (<10 instrument units) (Figs. F11, F16) and decreases from the top of the section to ~35 mcd, roughly paralleling the decrease in siliciclastics and a slight increase in calcium carbonate concentrations (Fig. F13). Likewise, lightness increases downhole within this interval, whereas a* and b* decrease (Figs. F11, F16). In the a*-b* color space, all color measurements of Unit I sediments plot in the "yellow" domain (Fig. F17). The interval above ~35 mcd that is enriched in siliciclastic material is characterized by the highest a* and b* values (Fig. F17B).

Isolated maxima in the magnetic susceptibility mostly correspond to volcanic ash layers. The two layers rich in nonpelagic calcareous materials appear as susceptibility minima. Additional minima are probably disturbed intervals and/or section breaks.

Unit II

Intervals: 202-1236A-12H-5, 0 cm, through 19H-4, 100 cm; 202-1236B-13H-2, 69 cm, through 13H-CC, 18 cm; and 202-1236C-12H-7, 56 cm, through 18H-CC, 21 cm
Depths: Unit II: 119.5-181.0 mcd; Hole 1236A: 105.6-166.1 mbsf; Hole 1236B: 106.0-122.8 mbsf; and Hole 1236C: 106.9-167.3 mbsf
Age: early Miocene to late Oligocene (~17-24 Ma)

Unit II is characterized by two major lithologies, pelagic and nonpelagic calcareous sediments. Pelagic sediments consist of nannofossil ooze and unlithified mudstone, both of which are partly foraminifer bearing. Nonpelagic sediments are present as abundant intercalations of nonskeletal and/or skeletal grain-rich intervals classified as unlithified wackestone, packstone, and grainstone (Table T6; Fig. F18). Grains are primarily coarse silt and sand sized, except for occasional intervals of unlithified floatstone and rudstone in which the grains reach >2 mm (Fig. F18). The transition to the overlying fine-grained calcareous sediments is gradational. The thickness of nonpelagic grain-rich intervals varies from meters to decimeters (Fig. F18). These intervals constitute ~60% of Unit II.

Nonskeletal grains are mostly peloids. Skeletal grains are present as bioclasts, primarily recrystallized benthic foraminifers, bryozoan fragments, and some remains and encrustations of red algae (Fig. F19). Calcium carbonate concentrations are >95 wt% throughout Unit II. Foraminifer contents vary but mostly range from ~10% to 20%. Sediments of Unit II lack siliceous microfossils and contain only minor to trace amounts of clay and extremely rare silt-sized siliciclastic minerals (Fig. F13). Volcanic glass is present in trace amounts, and no distinct ash layers were identified.

Unit II is clearly distinguishable by physical properties. GRA bulk density and lightness are higher in Unit II sediments than in Unit I, reflecting the increase in nonpelagic grain-rich intervals (Fig. F16). These intervals are also characterized by low magnetic susceptibility (Fig. F16). In the a*-b* color space, all color measurements of Unit II sediments plot in the "yellow" domain within a very narrow range (Fig. F17C).

Unit II is subdivided into two subunits, primarily on the basis of the relative abundance of pelagic microfossils vs. micrite. The subdivision is not clearly defined by physical properties.

Subunit IIA
Intervals: 202-1236A-12H-5, 0 cm, through 16H-1, 85 cm; 202-1236B-13H-2, 69 cm, through 13H-CC, 18 cm; and 202-1236C-12H-7, 56 cm, through 15H-5, 0 cm
Depths: Subunit IIA: 119.5-153.3 mcd; Hole 1236A: 105.6-138.5 mbsf; Hole 1236B: 106.8-122.8 mbsf; and Hole 1236C: 106.9-140.3 mbsf
Age: early Miocene (~17-21 Ma)

Subunit IIA is primarily foraminifer-bearing nannofossil ooze (Table T6). Micrite increases downhole as nannofossil abundance decreases. Micrite in Subunit IIA most likely originates from in situ recrystallized nannofossils. The sediment color of the pelagic oozes is primarily very pale brown to white with gradational color changes, except for sporadic mottling indicating intervals of moderate bioturbation. The nonpelagic grain-rich intervals are predominantly white.

Subunit IIB
Intervals: 202-1236A-16H-1, 85 cm, through 19H-4, 100 cm, and 202-1236C-15H-5, 0 cm, through 18H-CC, 21 cm.
Depths: Subunit IIB: 153.3-180.9 mcd; Hole 1236A: 138.5-166.1 mbsf; and Hole 1236B: 140.3-167.3 mbsf.
Age: early Miocene to late Oligocene (~21-24 Ma)

Sediments of Subunit IIB are macroscopically similar to those of Subunit IIA, except that the lower portions of nonpelagic grain-rich intervals more frequently contain nonpelagic grains >2 mm (unlithified rudstone and floatstone) (Figs. F18, F19). The major distinction between Subunits IIA and IIB is the dominance of micrite and the near absence of nannofossils in Subunit IIB. Micrite in the matrix of unlithified mudstones most likely originates from both in situ recrystallization of nannofossils and allochthonous supply together with the nonpelagic grains.

Unit III

Interval: 202-1236A-19H-4, 100 cm, through 22X-CC, 40 cm
Depths: 181.0-207.7 mcd (166.1-192.5 mbsf)
Age: late Oligocene (>24 Ma)

Unit III is characterized by nonpelagic grain-rich calcareous sediments, primarily unlithified packstone and less abundant unlithified rudstone. Unlithified packstones are dominated by well-sorted sand-sized grains (Fig. F20), which are primarily peloids and skeletal grains (recrystallized benthic foraminifers, bryozoan fragments, and some remains and encrustations of red algae). Unlithified rudstones typically fining upward to unlithified packstones. Sediment color varies but is predominantly pale yellow. Unit III sediments lack nannofossils and contain abundant micrite and variable minor amounts of benthic foraminifers (Fig. F13). Siliciclastic components include minor amounts of clay minerals and rare feldspar.

Minor lithologies in Unit III are intervals with pale yellow-brown and greenish gray layers that are increasingly abundant downhole. These are relatively rich in siliciclastic components (up to ~35%, primarily clay mineral and minor amounts of feldspar and pyroxene), volcanic ash, and/or authigenic glauconite. Three volcanic ash layers consisting of up to 80% volcanic glass (partly palagonized) are present near the base of Unit III. Abundant siliceous microfossils (up to 18%) consisting of spicules and diatoms are present in this interval (Fig. F13).

Downhole, carbonate concentrations and lightness decrease significantly and magnetic susceptibility increases (Figs. F11, F16). In the a*-b* color space, all color measurements of Unit III sediments plot in the "yellow" domain (Fig. F17D) with higher variability than in Units I and II. Downhole variations of a* and b* are consistent with increasing abundance of authigenic glauconite, siliciclastic, and volcaniclastic material (Figs. F11, F13, F16).

Unit IV

Interval: 202-1236A-23X-1, 0 cm, through 24X-4, 45 cm
Depths: 207.7-222.3 mcd (192.5-207.2 mbsf)
Age: late Oligocene (>24 Ma)

Unit IV represents a complex succession of sediments directly overlying the basaltic basement at 222.3 mcd. Firm to indurated chalk is the dominant lithology in this unit. The chalk is characterized by abundant nannofossils (partly recrystallized in situ to micrite) and small amounts of foraminifers (Fig. F13). Authigenic glauconite is present throughout the unit and reaches up to ~50%, producing a range of greenish hues within decimeter-scale intervals. Centimeter-scale color banding of different greenish hues is present in Core 202-1236A-23X and the upper sections of Core 24X (Fig. F21). Zoophycos trace fossils were observed, particularly in Core 202-1236A-23X. Sediments directly overlying the basaltic basement are reddish brown and contain abundant iron oxides and reworked basalt fragments, as well as ~10% volcanic glass.

Magnetic susceptibility is high in Unit IV and increases downhole, and lightness decreases strongly over the same interval. In the a*-b* color space, almost all color measurements of Unit IV sediments plot in the "green" domain (Fig. F17E). These results are consistent with a significant siliciclastic and volcaniclastic component and the abundance of glauconite in Unit IV.

Interpretation and Depositional History

The primarily pelagic sedimentary sequence recovered at Site 1236 spans the entire Neogene and the uppermost part of the late Oligocene. The depositional history of the site (Fig. F22) is consistent with its tectonic backtrack path and is related to the evolution of the Nazca Ridge.

The fine-grained sediment, abundant nannofossils, and trace fossils suggest a low-energy pelagic depositional environment for Unit IV directly overlying the basement, without indications of any input of nonpelagic calcareous material. The presence of authigenic glauconite indicates water depths below the typical storm wave base to a maximum of 500 m (Fairbridge, 1967; Porrenga, 1967) during initial deposition. This depth constraint agrees with the range of paleowater depths calculated from the assumed thermal subsidence of the basaltic basement since the late Oligocene (see "Introduction").

After the initial pelagic phase, a major event in the depositional history of Site 1236 was the emergence of volcanic islands in the vicinity of the site (probably represented by a bathymetric high located southeast of the site) (see "Introduction"), which was likely related to a major plate reorganization that resulted in enhanced volcanic activity in the late Oligocene (~25 Ma) (Duncan and Hargraves, 1984). This event is documented by several volcanic ash layers near the base of Unit III and an abrupt lithologic shift to predominantly unlithified nonpelagic grain-rich calcareous sediments in Units II and III. These sediments suggest that the islands were surrounded by carbonate platforms that supplied neritic calcareous material to Site 1236. Although the dominance of well-sorted packstones with skeletal grains indicates the proximity of a nearshore high-energy environment, the presence of authigenic glauconite might also suggest relatively calm water conditions below the storm wave base (Fairbridge, 1967; Porrenga, 1967). Hence, the coarse-grained carbonates at Site 1236 were most likely supplied by gravity currents (supported by occasional fining-upward sequences) originating from nearby carbonate platforms during that time. The uphole decrease in siliciclastic and volcaniclastic constituents and the strong decrease in magnetic susceptibility point to the erosion of the volcanic island(s) during the late Oligocene, when sedimentation rates were highest at Site 1236 (~1 cm/k.y.) (see "Age Model and Mass Accumulation Rates").

At the base of Unit II (~24 Ma), sediments are nearly pure carbonates, indicating already submerged volcanic islands. The downslope supply of neritic calcareous material, however, continues into the Miocene sequence, indicating continued proximity to carbonate platforms, causing slightly higher sedimentation rates in Unit II (~1 cm/k.y.) compared to Unit I (~0.4-0.9 cm/k.y.) (see "Age Model and Mass Accumulation Rates"). Fine-grained calcareous sediments, at least in Subunit IIA, are clearly pelagic, as evidenced by the abundance of nannofossils. These sediments are dominated by micrite in Subunit IIB, which is most likely the product of in situ nannofossil recrystallization. Hence, pelagic carbonate sedimentation was established in the early Miocene.

Pelagic calcareous oozes of Unit I represent conditions similar to modern (i.e., low surface) productivity and resulting low sedimentation rates (~0.4-0.9 cm/k.y.) within the southeastern Pacific subtropical gyre. Two isolated layers containing nonpelagic grains at depths of ~65 and ~98 mcd represent the latest supply of neritic sediments to Site 1236, indicating the final submergence of carbonate platforms in the late Miocene at ~10 Ma. The submergence of the carbonate platforms is most likely primarily controlled by thermal subsidence of the oceanic crust, as the middle Miocene is characterized by long-term sea level lowering (Exon, Kennett, Malone, et al., 2001).

Pliocene and Pleistocene sediments (~0-40 mcd) are slightly enriched in siliciclastic clay as also indicated by uphole increasing magnetic susceptibility and decreasing calcium carbonate concentrations. The minor siliciclastic fraction probably represents eolian supply from the South American Atacama Desert, assuming that prevailing wind directions were similar to the modern conditions. Our record might therefore indicate successive enhancement of aridity in South America and/or increasing wind speeds since the early Pliocene, possibly related to the uplift history of the Andes.

The dominance of brown volcanic glass in the ash layers present in Units I and III indicates a primarily basaltic composition consistent with an oceanic source of the ash layers. This volcanic activity is most likely related to both submarine and subaerial volcanic eruptions within the Nazca Ridge hotspot track. Volcanic glass in Unit IV probably originates from the reworking of basaltic basement. Volcanism might have been decreased or absent during the early Miocene, as Unit II lacks ash layers. One ash layer of intermediate composition, which might originate from the Andes, was recovered. Some ash layers and the surrounding sediment contain abundant siliceous microfossils, which points to selective preservation and/or episodic production of siliceous material (see "Diatoms" in "Biostratigraphy").

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