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doi:10.2204/iodp.proc.344.106.2013

Lithostratigraphy and petrology

Hole U1380C was drilled to investigate the lithostratigraphy and structural geology of the lower portions of the upper slope sequence and the uppermost portions of the basement, as interpreted in multichannel seismic reflection data. This hole deepens Site U1380, drilled during Expedition 334, and complements adjacent Site U1378, where only the upper part (539.9 mbsf) of the slope sediment was drilled during Expedition 334. After deploying casing and washing down to 438 mbsf, we reached the target depth at 800 mbsf and recovered 362 m of sedimentary rocks (Cores 344-U1380C-2R through 52R). Overall core recovery was moderate at 56%. The majority of the sedimentary sequence consists of clayey siltstone and silty claystone (~59% and ~24.5%, respectively), with common interlayered centimeter- to decimeter-sized beds of fine- to medium-grained sandstone (~14.5%). The siltstone sedimentary sequence is disrupted by coarse-grained, shell-rich sandstone; two conglomerates (~1%); and 25 tuff layers (<1%) dispersed throughout the sedimentary sequence. Three lithostratigraphic units are distinguished based on lithologic compositional changes (Fig. F5; Table T2).

Unit I extends from 438 to 552.72 mbsf (Sections 344-U1380C-2R-1 to 13R-7, 55 cm) and is characterized by massive dark greenish gray silty clay and three sandstone-rich horizons with centimeter- to decimeter-sized sandy layers. These sandy layers increase in abundance, thickness, and grain size with depth. Matrix components comprise lithic fragments (sedimentary and magmatic), feldspar, glass shards, and amphibole. Biogenic components include rare nannofossils, diatoms, and foraminifers.

Unit II extends from 552.72 to 771.62 mbsf (Sections 344-U1380C-13R-7, 55 cm, to 47R-3, 106 cm) and is visually defined by a relatively sharp lithologic change to greenish gray clayey siltstone with intercalated sandstone and conglomerate layers. Unit II is further divided into two subunits based on compositional and depositional variations. Subunit IIA is characterized by poorly to weakly consolidated sand beds that contain abundant to common shell fragments. The matrix also contains abundant lithic fragments, feldspar, and zeolites, but the main mineralogical variation is the higher abundance of amphibole in this subunit. Low recovery hinders the precise measurement of the thickness of Subunit IIA (~11.38 m), but we assume that the missing material is composed of the same loose sand as the recovered sections. The lithologic boundary between Subunit IIA and IIB in Core 344-U1380C-15R (564.10 mbsf) was not recovered. Subunit IIB is very dark greenish gray clayey siltstone characterized by two fining-upward sequences of centimeter- to decimeter-thick medium- to coarse-grained sandstones and fine conglomerates. Matrix and sandstone components are predominantly terrigenous lithic fragments (magmatic more common than sedimentary), feldspar, glass, and rare heavy minerals. Biogenic material is mostly absent, and none was observed toward the bottom of the unit. Highly altered tuff layers in the lower parts of Cores 344-U1380C-30R (688.04 mbsf) and 47R (770.56 mbsf) are characterized by color changes from dark greenish gray clayey siltstone to reddish brown calcareous siltstone and by variations in the matrix mineralogy.

Both subunits are characterized by normally graded sandstone beds that are often laminated, with abundant sapropel fragments that form several centimeter-thick conspicuous horizons between the laminae. The conglomerates are very thickly bedded and contain poorly sorted, pebble-sized, matrix-supported lithic fragments. Upper contacts between fine- and coarse-grained sediments, when observed, are mostly gradational, whereas the lower contacts are erosional and commonly contain rip-up clasts, load casts, and sand lenses.

Unit III extends from 781.75 to 800 mbsf (Cores 344-U1380C-47R through 52R) and is a fine-grained silty claystone with rare but thick (as thick as 1.20 m) intercalated fine- to coarse-grained sandstones. The matrix contains mostly terrigenous material dominated by lithic fragments and feldspar but is nearly devoid of biogenic material.

Description of units

The 362 m long sedimentary sequence (Cores 344-U1380C-2R through 52R) recovered in Hole U1380C is assigned to three lithostratigraphic units (Fig. F5; Table T2).

Unit I

• Interval: Sections 344-U1380C-2R-1, 0 cm, to 13R-7, 55 cm
• Thickness: 114.72 m
• Depth: 438.00–552.72 mbsf
• Age: late? Pliocene to late Pleistocene
• Lithology: silty clay with fine sandstone

Unit I is dark greenish gray silty clay with minor sandstone layers and four well-lithified (0.5–2 cm thick) tuff layers disseminated between Cores 344-U1380C-7R and 13R (Fig. F6). The clay sediment is moderately to highly bioturbated and well consolidated. The sandy layers are calcite cemented and occur throughout the unit as centimeter- to decimeter-thick beds that increase in abundance, thickness, and grain size with depth. Three sandstone horizons occur between 470 and 480 mbsf (Cores 344-U1380C-5R and 6R), 495 and 510 mbsf (Cores 8R and 9R), and 548 and 553 mbsf (Cores 12R and 13R). In these horizons, many sandstone beds exhibit rounded rip-up clay clasts at their base, features commonly associated with frequent erosional contacts at their base. Framboidal pyrite is macro- and microscopically ubiquitous throughout Unit I.

The main matrix components of Unit I are terrigenous and dominated by clay. Smear slides indicate that the most common accessory grains in the silty clay include feldspar and lithic (sedimentary and magmatic) fragments. Trace components include amphibole, calcite, biotite, chlorite, glass shards, and opaque minerals. The sandstone horizons are composed of feldspar and lithic fragments; however, unlike the silty clay, the sedimentary lithic component is much larger than the igneous component. Glass, amphibole, and pyroxene are rare. Rare biogenic components include nannofossils, diatoms, and foraminifer fragments.

Unit II

• Interval: Sections 344-U1380C-13R-7, 55 cm, to 47R-3, 106 cm
• Thickness: 218.9 m
• Depth: 552.72–771.62 mbsf
• Age: late Pliocene to early Pleistocene
• Lithology: clayey siltstone with medium to coarse sandstone

Unit II is clayey siltstone with interlayered sandstone and minor conglomerates. Unit II is divided into two subunits based on compositional changes. Subunit IIA contains abundant amphibole and feldspar, whereas clay is mostly absent. The sandstone layers contain abundant shell fragments and are very poorly consolidated. Subunit IIB lacks amphibole, and feldspar abundance is much diminished relative to Subunit IIA. Zeolite abundance, in contrast, increases substantially.

Unit II contains 21 disseminated tuff layers. A horizon between Cores 344-U1380C-29R and 31R contains 16 layers that range from 1 to 14 cm thick. Five more layers ranging from 2 to 47 cm thick are found in Core 344-U1380C-47R. These two tuff intervals, characterized by a reddish brown color, occur between 688.04 and 695.10 mbsf and 770.56 and 771.62 mbsf. Similarity of color and composition (with anorthite-rich feldspar and pyroxene) within these intervals imply that these are highly altered tuff layers. Physical properties and paleomagnetic data are consistent with this observation (see “Physical properties” and “Paleomagnetism”).

Subunit IIA (552.72–564.10 mbsf)

Subunit IIA consists of very dark greenish gray medium- to coarse-grained sandstone that contains abundant shell fragments (Fig. F7) intercalated with minor well-lithified and moderately bioturbated clayey siltstone layers. Occasional thin sandstone horizons are well lithified by calcite cement; however, most are poorly consolidated. The sand layers are commonly massive (as thick as several decimeters), but some beds exhibit millimeter-scale lamination. Shells, shell fragments, and lithic fragments are abundant and conspicuous components within the sandstone.

Smear slides indicate the sandstone components of Subunit IIA are terrigenous with abundant lithic fragments (sedimentary and magmatic), feldspar, amphibole, glass shards, and chlorite. Trace mineral phases include pyroxene, calcite, and opaque minerals. The siltstone matrix composition is similar to that of the sandstone, albeit with minor abundance variations of the major mineral phases. Biogenic components include common nannofossils, rare diatoms, and rare foraminifers; however, overall abundance in the sand is lower than in the siltstone.

Subunit IIB (564.10–771.62 mbsf)

Subunit IIB consists of two fining-upward sequences (564.1–687.5 and 695.1–770.5 mbsf) of very dark greenish gray clayey siltstone that is moderately bioturbated and intercalated with sandstone and minor conglomerate beds (Fig. F8). The sandstone layers are abundant and massive (as thick as 80 cm) and are normally graded; they become thicker and coarser with depth, and many are also laminated. The siltstone, sandstone, and conglomerate layers are moderately well lithified by calcite cement. Bioturbation is moderate in the siltstone lithologies and is not observed in the sandstone. This unit is moderately to highly fractured by drilling.

In general, the fine sandstone and siltstone layers are well sorted, whereas the coarse-grained sandstone and conglomerate layers are poorly sorted. The clayey siltstone often contains reworked sandstone lenses and, occasionally, sedimentary dikes that cut across bedding planes. Load casts and convolute bedding, due to synsedimentary liquefaction, were observed at the transition between the conglomerate and siltstone layers (Fig. F9). The sandstone beds exhibit erosive contacts at their base and mostly gradational contacts into the overlying sediments. Toward the bottom of the sequence, rip-up siltstone clasts and fragmented (as large as 8 cm) calcite-cemented breccia are more frequent. The abundance of shell fragments in Subunit IIB is reduced toward the base of the unit. In contrast, sporadic wood fragments and sapropel increase with depth and form thin horizons along the sandstone bedding planes (Fig. F10). Conglomerate layers contain subrounded to subangular pebble-sized clasts composed of volcanoclastic and plutonic material, sedimentary clasts, and rip-up siltstone clasts (Fig. F11).

Siltstone and sandstone smear slides indicate that the dominant matrix composition is volcanogenic and sedimentary lithic fragments, along with common feldspar and glass. Accessory phases include calcite, chlorite, amphibole, and pyroxene. The biogenic components decline precipitously and are absent in the lowermost part of Subunit IIB. The conglomerate beds are matrix supported and contain the same clast and mineral assemblage as the siltstone and sandstone.

Unit III

• Interval: Sections 344-U1380C-47R-CC, 0 cm, to 52R-CC, 19 cm
• Thickness: 18.25 m
• Depth: 771.62–800 mbsf
• Age: late Pliocene
• Lithology: silty claystone

Unit III is massive, well-lithified, very dark greenish gray silty claystone with intercalated sandstone layers (Fig. F12). The silty claystone contains common calcite-cemented horizons. Sandstone beds in Unit III are less common than in Units I and II but are generally thicker (as thick as 1.20 m).

Smear slides indicate that the most abundant components in the siltstone and sandstone layers are lithic fragments (sedimentary and magmatic) and feldspar. Zeolites are common, whereas amphibole, chlorite, calcite, and opaque minerals are present as accessory minerals. Microfossil components continue to be very rare to nearly absent.

X-ray diffraction analyses

Preliminary X-ray diffraction analysis of sediment samples from Hole U1380C suggests that there is little variation in composition within each lithostratigraphic unit (Fig. F13).

X-ray diffractograms of Unit I indicate that the major mineral components are phyllosilicates, including chlorite and smectite, quartz, plagioclase, and calcite. Zeolites (laumontite and heulandite) and pyrite are also present in all samples, whereas amphibole (hornblende) peaks are sporadically observed.

Stronger variation in mineral composition is noticed between silty clay samples that were collected from Unit I and Unit II sediment. Smectite and pyrite peaks are not observed in the upper part of Unit II, and plagioclase is less apparent. Hornblende is present in significant amounts in all samples from the upper part of Unit II. Analcime, a zeolite-like mineral, is present as a major mineral in Unit II, whereas it is not observed in Unit I.

Samples from Subunit IIA produced spectra dominated by zeolites (analcime, laumontite, and heulandite), calcite, quartz, plagioclase, hornblende, and chlorite.

X-ray diffractograms of samples from Subunit IIB indicate that the major mineral components are zeolites (analcime and heulandite), quartz, calcite, plagioclase, and chlorite. Laumontite is present in the upper part of Subunit IIB but decreases with depth and is absent below 710 mbsf. Smectite and pyrite are minor components. Tuff layers in Subunit IIB present different general patterns with respect to the rest of the sediment; they contain more zeolite (with laumontite events in both horizons), a different plagioclase composition, and an enrichment in hornblende.

X-ray diffractograms of Unit III indicate that the major mineral components are quartz, plagioclase, analcime, calcite, and chlorite. Heulandite and pyrite are minor phases.

Depositional environment

Lithostratigraphic Unit I is a continuation of the sediment cored in Hole U1380A during Expedition 334 and resamples the same lithologic features. Lithologic comparison to sites from Expedition 334 reveals a good correlation between Unit I from Hole U1380C (Expedition 344) and Unit II from Hole U1378B (Expedition 334). This correlation is based on the similar occurrence of general lithology (e.g., sandy layers increasing in thickness and abundance toward the bottom of units), as well as the similar terrigenous composition of the sediment as observed in smear slides.

The sedimentary succession recovered from Hole U1380C is probably an alternating terrestrially sourced, turbiditic upper slope (Units I and III) to shelf (Unit II) sequence, eventually being influenced by deltaic-derived sediments. This system has fed the depositional area with coarser, organic-rich (wood) sediment, alternating with fine-grained sediment during undisturbed sedimentation conditions (i.e., overbank facies). The frequent erosional bottom contacts of the sandstone and conglomerates, the rip-up clasts observed at sandstone to siltstone contacts, the subrounded to subangular shape of the clasts, and the occasional fragments of interbedded coal attest to the proximity of the source area. The dynamic depositional environment is also well documented by the varying grain sizes that range as large as pebbled sized conglomerates, especially in Unit II, and the cyclic increasing abundances of sandstone throughout the sedimentary sequence.

Two large-scale fining-upward cycles (100 m thick sediment packages) in Unit II may be evidence for relative tectonically or climatically controlled sea level changes and/or the repeated migration of the depositional area into the active or deactivated channel systems typical of deltaic depositional areas. Unit III seems to represent a return to an upper slope system but is too thin to provide more detailed constraints and/or interpretations.

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