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doi:10.2204/iodp.proc.334.104.2012 Lithostratigraphy and petrologySite U1379 was drilled to investigate the lithostratigraphy of the upper slope sequence and the uppermost portions of the basement imaged with multichannel seismic reflection data. A primary goal was to determine the composition of the basement material. Coring two holes from the seafloor to 949 mbsf recovered sediment and sedimentary and igneous rocks. Hole U1379C is predominantly composed of a monotonous sequence of silty clay to clay that alternates with widely interspersed decimeter-scale sandy layers. Hardened concretions of carbonate mud are present in the interval between Cores 334-U1379C-16H and 40X (~79–305 mbsf). Shell fragments are widely dispersed throughout the monotonous clay. A sequence of 53 tephra horizons is observed below 177.75 mbsf. This predominantly clayey unit is interrupted by sand between 334 and 651 mbsf. The sand changes character in Core 71X (~622 mbsf), gradually becoming progressively coarser with increasing depth beneath the seafloor. The interval between Cores 78X and 102X is composed of fine- to medium olive-green sand with graded beds and disseminated volcanic glass shards. The contact of the basement (as defined in the seismic profiles) with the overlying sandstone in Core 334-U1379C-103X at 881.75 mbsf is not exposed but marked by the first appearance of aphyric to phyric basalt clasts. The lithostratigraphic record of the basement cores and especially of the igneous units is incomplete because of lower recovery rates in Cores 103X through 118X. The basalt contains sparsely distributed plagioclase, pyroxene, or olivine phenocrysts. The phenocrysts are intercalated with dominantly fine-grained sandstones and mafic breccia (i.e., mainly basalt clasts). Below 906.72 mbsf until total depth of the hole at 949 mbsf, no additional igneous rocks were recovered other than mafic clasts occurring within sandstones, breccia, and conglomerates. Other clasts are composed of limestone and mudstone. The remaining part of the core is dominated by sandy and clayey silt. Description of unitsCores recovered at Site U1379 are assigned to five lithostratigraphic units (Fig. F3; Table T2). The 881.75 m sequence above the contact with the basal unit is divided into four units on the basis of lithologic attributes. Intervals cored at Site U1379 include 0–9.5 mbsf in Hole U1379B (Cores 334-U1379B-1H through 2H) and 0–947.52 mbsf in Hole U1379C (Cores 334-U1379C-1H through 118X). Core recovery was excellent (86%). Core from Hole U1379B was collected exclusively for microbiological and geochemical analyses. Lithostratigraphic definitions for Site U1379 are based exclusively on core recovered from Hole U1379C. Unit I
Unit I consists mainly of a medium- to coarse-grained buff sand with disarticulated shell fragments (Fig. F4). The sediment is loose. The unit is generally massive with few centimeter-scale bedding surfaces. Unit II
Unit II consists mainly of olive-green clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay, clayey silt(stone), and tephra. The sediment is firm and well consolidated from 0.93 mbsf (Section 334-U1379C-1H-1) and contains tephra layers that remain unlithified to 650 mbsf (Section 70X-1). In general, Unit II is massive with minor changes in the proportions of clay, silt, and sand grain sizes occurring gradually over <1 m intervals. Few original sedimentary contacts or primary structures were observed because of bioturbation. Clayey silt(stone), sandy silt(stone), carbonate mud sediment, and light gray calcareous clasts and interbeds are more common in the following intervals:
Trace abundances of framboidal pyrite were observed in many of the smear slides throughout Unit II, although locally greater abundances are common. Subunit IIASubunit IIA is a clay-rich sequence that varies from dark greenish black silty clay(stone) to greenish black clayey silt(stone). None of the cored material is truly soft sediment; Cores 334-U1379C-1H through 78X are firm to very firm. This subunit is homogeneous with thick intervals of silt(stone) and clay(stone) (Fig. F5). Small burrows are the most commonly observed sedimentary structure. At the top of Subunit IIA, evidence of slumping is present. The main components of Subunit IIA are terrigenous, dominantly clay. Smear slides indicate that the most common accessory grains in the silt(stone) and clay(stone) include feldspar and lithic (sedimentary and magmatic) fragments. Components present in trace abundances include amphibole, calcite, biotite, chlorite, glass shards, and opaque minerals. Trace abundances of pyrite framboids are widespread. Biogenic components that are sparse but ubiquitous throughout the subunit include nannofossils, diatoms, and foraminifers. Rare radiolarians and phosphatic fish remains were observed throughout the subunit. Wood or plant material is found throughout the subunit, although abundances decrease with depth below the seafloor. Subunit IIBThe top of this subunit is placed at the first occurrence of carbonate concretions. Subunit IIB consists of a clay-rich sequence that varies from dark greenish black silty clay(stone) to greenish black clayey silt(stone) that is distinguished from Subunit IIA by the presence of carbonate (both calcareous and dolomitic) concretions, carbonate mud sediment, and reduced abundances of shell fragments (Fig. F6). Little to no bioturbation is observed in Subunit IIB. Nannofossil abundance is variable in Subunit IIB, ranging from abundant to barren. Subunit IIB has upper (84–126 mbsf) and lower (190–305 mbsf) sequences that are characterized by enriched intervals of hardened carbonate. The subunits are separated by a more monotonous sedimentation of silty clay(stone)–clay(stone) and clayey silt(stone)–sand(stone) that also contain two tephra layers at 161 and 177 mbsf. Smear slides indicate that after the calcareous matrix, the most common accessory grains in the matrix of the carbonate mud are chert fragments, feldspar, chlorite, and volcanic glass and some rare amphiboles, pyroxenes, glauconites, and opaque minerals. Trace abundances of pyrite framboids are common throughout the subunit. Biogenic components that are sparse, but ubiquitous, throughout the subunit include nannofossils, diatoms, and foraminifers. Subunit IICSubunit IIC consists of olive-green silty sand and sandstone (Fig. F7). The sands are fining- and coarsening-upward decimeter-scale sequences. Three intervals with accumulated tephra layers are observed in this subunit. Bioturbation is variable throughout Subunit IIC but generally decreases with depth. Framboidal pyrite was observed in trace abundances in many smear slides throughout Subunit IIC, although locally greater abundances are common. Small disseminated woody debris and plant detritus is recognized within the subunit. Nannofossil abundances in Subunit IIC are variable, ranging from barren to rare. Subunit IIC is interspersed throughout Unit II in the following intervals:
The main components in the sandstone of Subunit IIC are lithic (sediment > magmatic) clasts and feldspar crystals. Smear slides suggest that the abundance of magmatic grains increases toward the base of Subunit IIC, whereas the abundance of lithic fragments decreases with depth. The accessory grains in the sandstone vary as a function of depth; chlorite decreases with depth, whereas amphibole increases with depth. Interestingly, the abundance of volcanic glass is greatest in the middle of Subunit IIC. Additionally, calcite, biotite, pyroxene, and opaque minerals are present without any significant change throughout the subunit. Biogenic components are rare but include nannofossils, diatoms, and foraminifers. Unit III
Unit III consists mainly of olive-green silty sands and sandstone and shows fining- and coarsening-upward decimeter-scale sequences of sand with erosive bases (Fig. F8). Three phases of predominantly sandy deposits alternate with finer hemipelagic sediments and accumulate to a total thickness of 146 m. The coarse sandy deposits are 64% of the total unit thickness. The three sandy intervals are between
Larger (millimeter scale), well-rounded to subrounded sedimentary lithic clasts and shell fragments compose the coarser portions of the sandstones. Rip-up clasts, millimeter-scale laminations, convolute bedding, and chaotic mixing is observed throughout the coarser sequences but is restricted to the partly eroded basal parts of the fining-upward sequences. Tephra layers are less common in Unit III and are concentrated in one interval within the upper part of the unit. Bioturbation is variable throughout the unit but generally decreases with depth. Framboidal pyrite in trace abundance was observed in many smear slides, although locally greater abundances are common. Small, disseminated wood and plant debris are recognized throughout coarser horizons in Unit III. Smear slides indicate that the sandstones are dominated by lithic clasts that are primarily composed of magmatic rock fragments and feldspar minerals, indicating that the main components of Unit III are terrigenous. Chlorite is the most abundant accessory mineral and is followed, in order of abundance, by volcanic glass, opaque minerals, and amphibole. Trace abundances of calcite, pyroxene, quartz, and opaque minerals are present. Biogenic components are rare but include nannofossils and foraminifers. Unit IV
Unit IV consists of carbonate-cemented medium- to coarse-grained sand with well-rounded, lithic pebble-sized clasts and thick-walled shell shards. Interbeds of centimeter-thick shell layers and localized well-rounded pebble conglomerate are present in the basal meter of the unit (Fig. F9). Unit V
Unit V is composed of matrix-supported breccia with clasts of limestone, basalt, and mudstone in a fine sandy matrix intercalated with basalt in the upper part (881.75–906.72 mbsf; Fig. F10) and a sequence of variably sandy and clayey silt matrix in the lower portion (916.40–947.52 mbsf). After shipboard analysis, the nature of Unit V remains enigmatic. Competing hypotheses suggest that this material is either a weathered veneer overlying pristine backstop material or material similar to the subaerially exposed Osa mélange that is younger and less deformed. BasaltThe basalt in Unit V ranges from aphyric to moderately phyric composition containing plagioclase, pyroxene, and olivine phenocrysts (Fig. F11). These phenocrysts are euhedral to subhedral and ≤5 mm. The groundmass of the basalt is fine grained and varies from light to dark gray. Mineral compositions are summarized in thin sections in “Core descriptions.” Plagioclase and pyroxene-phyric basalts are overprinted by a crosscutting vein network. The veins are preferentially straight with some irregularities. Offsets along the vein boundaries indicate that they were emplaced along faults (shear veins). Some of the veins do not show any displacement parallel to the vein boundaries, indicating that some of them were emplaced in Mode I fractures. In the recovered core fragments, subvertical green veins of unknown mineralogy represent the oldest generation, followed by subhorizontal white veins and subvertical white veins. Some alteration is evident within the basalts, although this alteration is localized to the margins of the veins, open fractures, surfaces of single rock pieces, and phenocrysts. Along the margins of the green veins, host rock alteration is recognized as narrow (0.5 mm) margins of dark green mantles extending into the basalt. Along the subhorizontal and subvertical white veins, the surrounding groundmass is partly altered to light green. Alterations adjacent to the open fractures are characterized by a brownish green color. Tephra layersCores collected in Hole U1379C contain 53 tephra layers intercalated with background sediment of the different units. Individual tephra layers range from 2 to 45 cm thick. Only two tephra layers are recognized in the uppermost 324 m of sediment, at 161 and 176 mbsf. Both are intercalated in the carbonate mud–rich Subunit IIB within the phase dominated by fine background sedimentation. In the subjacent Subunit IIC, an upper sequence of 19 tephra layers is recognized within the uppermost 60 m of sediment (309–381 mbsf; Sections 334-U1379C-42X-3 to 48X-2). The upper tephra sequence is separated from second tephra-rich sequence by a 70 m sequence devoid of tephra. The second tephra-rich sequence that is composed of 10 tephra layers extends from 448 to 500 mbsf (Section 58X-3 to 60X-7). A third, 85 m thick tephra-rich sequence consisting of 14 discrete tephra horizons extends from 562 to 647 mbsf (Section 67X-4 to 70X-6). Between 647 mbsf and the next following sequence of more frequent tephra layers at 751 mbsf, two tephra layers at 682 and 683 mbsf (Sections 81X-2 and 81X-3) are recognized. A fourth interval of five tephra layers occurs between 751 and 773 mbsf (Sections 89X-1 and 91X-CC). The lowermost tephra layer was recognized at 875 mbsf (Section 102X-5). In general, tephra is most common in Unit II, more specifically within Subunit IIB, and is less abundant within Unit III. The composition of the 53 identified tephra layers is variable. Twenty three are light gray to pinkish/brownish white felsic ash, 19 are pinkish gray/brown layers, and 11 are pinkish/greenish black layers. Dark black tephra beds account for ~20% of the tephra bed assemblage in Hole U1379C. Tephra layers are generally soft, but the tephra layers in Core 334-U1379C-60X are indurated and are consequently classified as tuff. Unconformable and/or inclined bedding of tephra beds is rare in the uppermost units (0–651 mbsf). Inclined tephra horizons are first recognized in lithostratigraphic Unit III. In some cores, bioturbation occurred at the top of the tephra layers. In general, tephra layers have a sharp basal contact to underlying terrigenous sediment but a gradual transition contact with overlying ash-bearing pelagic sediment and many are normally graded in grain size and well sorted (Fig. F12). The felsic white ash is mainly (>90 vol%) composed of clear, colorless glass shards varying from angular blocky, cuspate, flat, and Y-shaped shards with nearly no bubbles to highly vesicular pumiceous textures with many elongated bubbles. The transparent glass shards of the felsic tephras have no visible signs of alteration until 500 mbsf (Core 334-U1379C-60X). Below this depth, devitrification structures increase with depth within the glass shards, reflecting differing levels of alteration. Grain size ranges from very fine to coarse ash (up to millimeter size). The mineral assemblages consist of plagioclase, pyroxene, hornblende, and biotite. Although plagioclase is the dominant phenocryst phase in this core, some of the individual tephras are dominated by amphibole and biotite. An increase in the abundance of amphibole and biotite phenocrysts is commonly interpreted to reflecting increasingly evolved felsic magma. Dark gray mafic ash layers consist predominantly of dark to light brown glass shards. Most of the glass shards have blocky shapes and are medium to poorly vesicular and show strong signs of alteration, especially with increasing depth within the hole. The mineral assemblages of the mafic tephras include plagioclase, pyroxene, and spinel. In contrast to felsic tephras, mafic tephras contain more crystals. Carbonate fragments and concretionsHard fragments of limestone and dolomitic limestone were recovered beginning at ~84 mbsf (Core 334-U1379C-14H) and continue intermittently to 748 mbsf. Because most of these fragments were found in the rubble at the tops of cores, their stratigraphic position is uncertain. Most of the in situ calcareous concretions are concentrated within Subunit IIB. Intact limestone concretions and calcareous claystone/mudclasts were observed embedded in clayey siltstone in some cores. Most of the calcareous rocks consist of micrite plus nannofossils. Many contain benthic foraminifers and sand or silt grains (e.g., quartz and feldspar). Dolomite is more abundant than calcite with increasing depth. Calcareous clasts are present within the brecciated horizons of Unit IV. X-ray diffraction analysisPreliminary X-ray diffraction analysis of the sediment suggests that there is little variation in composition among lithostratigraphic Units I–IV. X-ray diffractograms indicate that the major mineral components are clay minerals including illite, smectite, kaolinite, and attapulgite, as well as primary minerals including calcite, anorthite, and quartz. Amphibole (hornblende and richterite), chlorite, pyroxene (augite and hypersthene), olivine, and pyrite peaks are also recognized. (Fig. F13A). Mineral composition variability is greatest in sandy sediments that were collected from Unit I and the uppermost section of Unit II, where shell fragments are more common. Samples from these intervals produce spectra dominated by calcite, Mg-rich calcite, and aragonite (Fig. F13B). Depositional environmentThe cover sequence recovered from Hole U1379C is a terrestrially sourced shelf and upper slope sequence that is consistent with high sediment accumulation rates and increasing water depth through the depositional interval (see also “Paleontology and biostratigraphy”). The basal portions of the sequence are consistent with an interval of erosion separating well-indurated sands from calcareous cemented breccia. Clast compositions in the breccia include limestone, basalt, and sandstone. This sequence is capped by a <1 m thick accumulation of medium- to coarse-grained well-rounded Holocene sand. |
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