doi:10.2204/iodp.proc.324.106.2010
Sedimentology
One hole was drilled at Site U1349 on the summit of Ori Massif (water depth = 3127 mbsl). Sedimentary recovery reached 20%, with 10.06 m of sediment recovered from Cores 324-U1347A-1W through 7R over a stratigraphic interval of 49 m. Cores 1W through 4R are predominately red chert with occasional ooze and porcellanite. Below Core 4R is a friable sand-silt-claystone sequence that also contains granules of highly weathered volcanic material. This section is so disturbed by drilling that further interpretation of the rock is difficult. Most of Cores 5R and 6R contain greenish gray volcaniclastic sandstones and lapillistones, likely deposited by turbiditic flows. A thin yellowish red clay-rich horizon in Section 324-U1349A-6R-CC is interpreted as a paleosol. Basement was reached in Core 7R at 165.1 meters below seafloor (mbsf), where highly vesicular oxidized basaltic flows were first encountered. Downhole, several other thin sedimentary layers are interbedded between basaltic lava sections. The most significant of these is an oolitic limestone in Section 324-U1349A-9R-1 at 173.7 mbsf, which downhole logging data suggest may be stratigraphically as thick as 6 m.
Unit descriptions
The stratigraphy at Site U1349, including basaltic basement divisions and sedimentary interbeds, is shown in Figure F5.
Unit I: red cherts with calcareous porcellanite coatings and pinkish ooze (116.0–135.3 mbsf).
Unit II: sand-silt-claystone and sandstone with volcaniclastics (135.3–144.9 mbsf).
Unit III: volcaniclastic sandstones, breccia, and conglomerate (144.9–165.1 mbsf); divided into two subunits:
Subunit IIIa: volcaniclastic sandstones and volcaniclastic breccia (144.9–156.1 mbsf).
Subunit IIIb: yellowish red clay-rich layer (paleosol?) (156.1–156.3 mbsf) and a brownish polymictic volcaniclastic conglomerate (156.3–165.1 mbsf).
Unit IV: vesicular basaltic lava flows with occasional calcareous sedimentary interbeds (Subunits IVa, IVc, and IVd).
Subunit IVb: oolitic limestone, with a large biogenic component and some volcaniclastics (173.7–173.8 mbsf) occurring as a sedimentary interbed between vesicular reddish brown basalt (see "Igneous petrology").
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Unit V: basaltic flow breccia; autobrecciated basalt and basaltic lava pods (see "Igneous petrology").
Unit I
Red cherts are the dominant lithology of Unit I, which spans Sections 324-U1349A-1W-1 through 4R-1; however, one dark brown chert nodule was recovered at interval 324-U1349-4R-1, 4–6 cm. Several of the red cherts have porcellanite edges, which are partially silicified and contain numerous foraminifers. The foraminifers are closely packed and extend as far as 1.5 cm from the edge of the nodule toward the red chert center. Other chert pieces have thin (1–2 mm) pale pinkish white porcellanite coatings. Radiolarian fossil "ghosts" are present but rare. Core 324-U1349A-2R contains 3 cm of pale pink nannofossil ooze with abundant planktonic and benthic foraminifers (see "Paleontology"). Some well-preserved fish debris is also present in this interval. No other soft sediments were recovered in this unit. Several red chert pieces were recovered in other units (Fig. F5); however, they are unlikely to be in situ and we regard them as "fall-in," as they are likely to have fallen from the borehole wall and were cored out of sequence.
Unit II
The recovery of this unit is limited to ~21 cm of highly brecciated sand-silt-claystone, one small piece of friable sandstone in Section 324-U1349A-4R-1, and two (3–5 cm) pieces of yellowish brown sandstone with volcaniclastics at the top of Section 5R-1. The pale yellow-brown brecciated material is calcareous and contains numerous weathered volcaniclastic fragments. These fragments include rounded granules of light green alteration clays, reddish scoria, and dark gray lithic clasts. Many gravel-sized angular pieces of red chert are mixed with the soft sand-silt-clay pieces, presumably a result of the extensive drilling disturbance in this section. The two yellowish brown sandstone pieces recovered in Core 324-U1349A-5R are similar in composition to the sand-silt-clay but have been lithified (Fig. F6).
Unit III
Unit III is composed of calcite-cemented, lapilli-sized, clast-supported or matrix-supported volcaniclastic breccia, volcaniclastic sandstone, and an oxidized volcanic conglomerate. The volcaniclastic breccia is polymictic and contains granule- to pebble-sized clasts. Unit III is divided into two subunits (IIIa and IIIb).
Subunit IIIa
Subunit IIIa spans interval 324-U1349A-5R-1, 13 cm, to 6R-2, 6 cm. The majority of this unit is gray lapilli-sized volcaniclastic breccia (Fig. F7) alternating with volcaniclastic sandstone. Thin sections reveal a minor biogenic component, including echinoderm and bryozoan fragments (Fig. F8). Rounded volcanic clasts are rare; however, we observed occasional armored clasts (Fig. F9). Secondary calcite is pervasive and observed in thin section with fibrous zeolites displaying low birefringence.
The volcaniclastic breccia is generally structureless and unsorted, although some normal grading and alternating medium-grained volcaniclastic sandstone and volcaniclastic breccia is present (Fig. F8). Clasts are mainly granule to pebble sized and show a variety of colors, such as red, gray, green, black, and brown. They exhibit varying degrees of roundness, mostly subangular to subrounded basalts, and vesicularity. Alteration of the primary volcanic material is common, though some clasts do appear relatively "fresh." The sandstones, and in some cases moderately sorted lapillistones, exhibit both normal and reverse grading. Volcaniclastic sandstones are sometimes laminated and often normally graded. Reverse grading is most common in Section 324-U1349A-5R-1.
Subunit IIIb
Subunit IIIb is mainly composed of altered yellowish red volcaniclastic conglomerate (interval 324-U1349A-7R-1, 6–96 cm). A textural and compositional change occurs at the top of this subunit. Interval 324-U1349A-6R-2, 9–26 cm, is composed of highly weathered sand-silt-clay material. The uppermost part of this crumbly material (interval 324-U1349A-6R-2, 9–14 cm) is gray-brown and appears gradational from the gray volcaniclastic rocks of Subunit IIIa above to the more oxidized materials that characterize Subunit IIIb. Directly below the gray zone, a pink-orange, very crumbly silty sandstone with clay and few granule-sized altered volcaniclastics is observed. This material contains abundant yellow and red Fe oxyhydroxide/oxide minerals and displays a pedogenic texture typical of poorly developed paleosols (Fig. F10). Secondary calcite is present both as cement with micritic texture and as well-formed <1 mm rhombic crystals.
With the exception of interval 324-U1349A-7R-1, 0–6 cm, where pink chert fall-in is present, the remainder of Subunit IIIb is composed of polymictic conglomerates. The majority of the clasts in this subunit are partly to highly oxidized volcaniclastics; however, clasts of rounded mudstones and siltstones are also present. The conglomerate spanning interval 324-U1349A-7R-1, 6–60 cm, is moderately to poorly sorted (Fig. F11). Most clasts are cemented with pale yellow micrite, although light gray recrystallized calcite is also present as cement. It is important to note no zeolites were observed in thin section. Marine fossils make up a minimal component as rounded granules. A few thin (1–2 cm) layers of imbricated clasts are present within this interval.
Below the conglomerate, the lower portion of Section 324-U1349A-7R-1 becomes a structureless, matrix-supported, very poorly sorted volcaniclastic breccia. The matrix is mainly granule sized, and the clasts are mainly pebble sized. A very large (~8 cm) subangular red clast, rich in titanomagnetite, is present at 60–70 cm in this section (Fig. F11) and has the highest magnetic susceptibility measured at Site U1349 (see "Physical properties").
Unit IV
Subunit IVb
A thin (13 cm) interval of pale brownish gray bioclastic limestone was recovered at the top of Section 324-U1349A-9R-1, between igneous Subunits IVa and IVc. This interval, although poorly recovered, may represent a sedimentary interbed as thick as 6 m, based on downhole logging data (see "Downhole Logging"). The limestone is matrix supported and contains abundant biogenic clasts, including shallow-water fauna such as bryozoans, echinoderms, and bivalve fragments (Fig. F12). Palagonite (altered volcanic glass shards) and small feldspar crystals are also found. Ooids with concentric laminated rims are present, and many of the biogenic grains also have cement coatings. The limestone is well sorted; all clasts are subrounded to rounded in shape and show signs of reworking. The matrix is a fine-grained calcite with some secondary calcite cement. The sedimentary contacts with the over- and underlying basalts were lost during coring; however, sediment of a similar type infills vesicles in several sections of the basalts spanning igneous Subunits IVa, IVc, and Unit V (see "Igneous petrology").
Interpretation
The basalts in Unit IV are highly oxidized and vesicular, suggesting emplacement in either a subaerial or very shallow marine setting. The oolitic limestone interbeds between flows at the top of Unit IV indicate a very shallow marine setting. The coarse volcaniclastic deposits that make up Units II and III show evidence of sorting and oxidation, suggestive of volcanism and deposition in a shallow-marine (Units II and III) or subaerial (top of Subunit IIIb) environment. The lithologies present in Unit I are consistent with deposition in a fully pelagic marine environment after volcanic activity on Ori Massif had ceased.
Unit I
The chert, chalk, and calcareous ooze comprising Unit I is suggestive of a fully marine, pelagic depositional environment and were likely deposited once the volcanic edifice had subsided below the photic zone. The dominantly red cherts suggest low sedimentation rates in an oxic environment. The original, well-bioturbated sediment, preserved as porcellanite patches within the silicified cherts, further suggests that the benthic depositional environment was well ventilated. The silica in the cherts is likely sourced from radiolarians, as evidenced by the circular fossil ghosts preserved in some horizons. This unit is similar in composition, recovery, and age to the upper cores from Unit I at Site U1348 and may represent the same stratigraphic unit as seen at Tamu Massif.
Unit II
Poor recovery in Unit II makes interpretation of the depositional environment challenging; however, the material is different enough from that of Unit III to warrant a separate unit division. The reddish brown color of the sediment suggests deposition in an oxic environment, consistent with subaerial or shallow-marine deposition. Clasts in the lithified sandstone (Core 324-U1349A-5R) are clearly of volcanogenic origin. Their subangular shape suggests a short transportation distance and minimal reworking prior to deposition.
Unit III
Unit III consists of coarse volcaniclastic sandstones and conglomerates that range from gray to reddish brown and are mainly composed of polymictic volcanogenic clasts. These clasts are generally subrounded to subangular in shape. This facies is considered epiclastic, as the rocks contain a variety of different types of clasts, presumably sourced from different parent materials. The rocks in this unit are likely to have been deposited in a subaerial to very shallow marine environment.
Subunit IIIa
The volcaniclastic breccias and sandstones form alternating layers of Subunit IIIa, displaying normal and reverse grading and occasional fine laminations. The depositional mode, interpreted as a mixture of low-density turbidites (Bouma, 1962) and high-density turbidites (Lowe, 1982), requires submarine deposition where hydraulic sorting could occur. Thin flow units (3–50 cm) are likely to have been deposited by low-density turbidity currents, whereas the thicker flow units (from 10 cm to 1 m) likely represent high-density turbidites. Flow unit boundaries are gradational and often amalgamated, indicating a high deposition rate. Armored volcanic clasts and red scoria are present, providing evidence for subaerial or explosive, very shallow marine volcanism.
Subunit IIIb
The upper part of Subunit IIIb is composed of highly weathered silty sand with clay and granules of subangular to subrounded altered volcaniclastics. These components, coupled with the highly oxidized nature of the deposit and pedogenic textures, suggest the beginnings of soil development on top of the oxidized conglomerates below. The recovered interval 324-U1349A-6R-2, 18–26 cm, contains several features typically associated with the B horizon in soils. For example, if we assume the conglomerate beneath is the parent material, the sediment in this interval shows obliteration of much of the original rock structure. The material appears to have experienced alteration that allowed the development of clay minerals in addition to mechanical weathering which would have created the abundant silt- and sand-sized grains (Soil Survey Division Staff, 1993). The red coloration is most likely related to the presence of numerous sesquioxides or oxyhydroxide minerals rich in Fe, Al, and Mn. Often, water draining through a developing soil causes displacement of metals that collect at some depth in the subsurface and form oxyhydroxide coatings on the sedimentary grains (Soil Survey Division Staff, 1993). The final evidence that this interval is a paleosol comes from the overall brittleness of the sediment and the blocky structures that characterize its texture (Retallack,1991). Because the material directly below this sediment was not recovered, further work is required to confirm our interpretation of it as a paleosol. A complete assessment of the mineralogy, trace metal composition, and/or oxygen stable isotopes will confirm or disprove our interpretation.
The rest of Subunit IIIb is composed of graded to structureless altered yellowish red volcaniclastic conglomerate and breccia. The clast-supported, often laminated, and graded conglomerates are indicative of deposition in a subaqueous environment. The presence of imbricated clasts in these layers suggests deposition by strong unidirectional currents. The bottom of the subunit is structureless, matrix supported, and poorly sorted and therefore is interpreted as a debris flow deposit (Middleton and Hampton, 1976; Lowe, 1982).
The yellowish red color of the volcaniclastic conglomerate indicates an oxidative, likely subaerial, environment. However, deposition of imbricated clasts requires transport by water. Development of a paleosol requires subaerial exposure of a rock surface; therefore, we suggest that many of the clasts comprising the lower part of Subunit IIIb were sourced from land but deposited in a very shallow water environment. These rocks were subsequently exposed subaerially, which allowed for oxidation and soil development.
Subunit IVb
The limestone bed in Section 324-U1349A-9R-1 lies within the basaltic "basement" but apparently represents a break in lava flow emplacement during which marine sedimentation could occur. The presence of round grains and ooids is suggestive of deposition in a very shallow, agitated marine environment. The dominance of bioclastic components associated with shallow-marine environments (e.g., echinoderms and bryozoans) is consistent with this interpretation. Other sedimentary intervals from Unit IV (e.g., interval 324-U1349A-7R-2, 27–35 cm) suggest successive periods of lava emplacement and quiescence when limited normal marine sedimentation resumed.