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doi:10.2204/iodp.proc.324.103.2010 SedimentologyOne hole was cored at Site U1346 on the northeast flank of Shirshov Massif (water depth = 3619 meters below sea level [mbsl]). The primary goal of drilling at this site was to recover the basaltic basement estimated to reside below ~140 mbsf; therefore, sediment coring began only ~39 m above the suspected basement/sediment interface. Despite difficult drilling conditions, ~4 m of sediments were recovered prior to entering basaltic basement at 139.2 mbsf. The first six cores recovered from Hole U1346A contain sedimentary material representing various lithologies and depositional environments, with an average recovery of 10.2% between 100.5 and 141.7 mbsf. The uppermost sedimentary section (Cores 324-U1346A-1W through 3R) yielded only small, isolated, dark-colored chert fragments. Recovery in Cores 324-U1346A-4R through 6R improved because of a reduction in the amount of chert in the formation. This short interval of lithified sediments included a sequence of intermingled basalt and limestone in Section 324-U1346A-4R-1 interpreted as a debris flow. Section 324-U1346A-4R-2 yielded a series of laminated volcaniclastic sequences, grading from very coarse sand to clay, interpreted as turbiditic in origin. The remaining sediments, from the base of Section 324-U1346A-4R-2 to the top of 6R-1, are composed of clay-bearing limestones and calcareous mudstones containing abundant shell fragments and other biogenic components, along with glauconite and altered volcaniclastics. Taken together, these components are suggestive of a relatively shallow marine depositional environment in close proximity to a volcanic source. Unit descriptionsWithin the ~4 m of sediment recovered between 100.5 and 139.2 mbsf in Hole U1436A, four units can be identified (Fig. F4), although the relationship between these units is ambiguous because of the loss of interunit contacts:
Unit IThe large chert piece (16 cm) recovered in Core 324-U1346A-1W is brownish gray, with fine laminations visible in places. A band of shell fragments, originally calcitic in composition and now entirely composed of silica, suggests complete replacement of the original carbonate matrix by cryptocrystalline silica during diagenesis. The additional chert in Cores 324-U1346A-1W through 3R is black with faint laminations and includes rare silica-replaced shell fragments in places. Traces of the softer sediments, which presumably interbed with the indurated chert horizons, are observed only as occasional thin (<1 mm) chalk coatings on the chert fragments in Core 324-U1346A-2R. Because of extensive disturbance and fragmentation introduced by the drilling process, no other soft sediments were recovered. Unit IIThe upper 1.2 m of Core 324-U1346A-4R features highly altered, vesiculated, green basalts intermingled with fine-grained, radiolarian-bearing limestones. The limestones are predominantly composed of fine sparite, probably formed from the recrystallization of nannofossil ooze, as a small percentage of poorly preserved nannofossils now remain as a minor component. Two different colors of limestone are observed in this interval, grayish brown and very dark gray, with higher clay content relative to carbonate in the darker bands. The boundary between these two limestone types is convoluted and deformed, especially where in close proximity to the basaltic lobes. Moderately preserved, unidentified radiolarian fossils are present throughout the limestone but are particularly numerous and well preserved in the darker clay-rich bands (Fig. F5). A large (~3 cm) fossil fragment, possibly representing fish debris, is present in Section 324-U1346A-4R-1. Small pieces of apatite debris and fine shelly remains are present within the limestone, as are small isolated clasts of basalt in some horizons. There is no evidence of bioturbation in the form of burrows; however, the sediment does appear to be homogeneous and well mixed, which is suggestive of an oxic, bioturbated depositional environment. Secondary pyrite is visible as small crystals throughout the limestone, and it is concentrated as fine bands in the darker clay-rich layers. The contact between the basalt and the sediments is convoluted (Fig. F6). The basalt appears to have intruded into the limestone in places, although there is no evidence of chilled margins in the basalt or baked sediment at the contacts. The original bedding in the limestones between the lighter and darker lithologies is contorted, providing evidence of soft-sediment deformation prior to lithification. Unit IIIA 90 cm, finely laminated sand-silt-claystone interval in Section 324-U1346A-4R-2 contains several fining-upward sequences of medium sand grading to clay-sized particles (Fig. F7A). The dominant constituents in this sequence are clay minerals and volcanic glass with varying contributions from radiolarians, iron oxyhydroxides, and glauconite throughout. The volcanic glass fragments are on the order of 100 µm in size and are generally angular in shape (Fig. F7B). The coarser horizons within the graded sequence are predominantly composed of clays, carbonate, and detrital glass but also contain a significant number of moderately preserved radiolarians in some horizons. In some instances the skeleton of the organism is composed of fine-grained secondary silica, allowing delicate features such as wall structures and spines to be observed. In other cases the silica has been entirely replaced by sparite, leading to a corresponding decrease in preservation quality. At the base of the sequence, an isolated piece of calcite-cemented volcanic breccia contains large (0.5–1 mm) altered feldspar laths and fragments of mafic volcanic rock. These fragments are rounded and contain numerous needlelike feldspar laths in a dark glassy matrix (Fig. F7C). There are three major fining-upward sequences, with possibly as many as seven minor graded sequences overall. Each sequence likely represents a discrete depositional event. The laminations are well preserved throughout indicating an absence of bioturbation, except for one claystone layer at the top of the second fining-upward sequence from the bottom (interval 324-U1346A-4R-2, 107–108 cm), where the trace fossil Chondrites can be seen (Fig. F7D). The base of this second sequence has an erosional contact with the fine clays beneath it and a scour mark filled with coarser, medium sand–sized grains. Flame structures and convolute lamina in clay–silt sized material at the top of the uppermost sequence can also be seen (Fig. F7E). Unit IVThis unit extends from the uppermost part of Section 324-U1346A-6R-1 to the base of 4R-2 and is composed predominantly of bioclastic limestones and calcareous mudstones. At the base of the unit, just above the sediment/basement contact, a heavily recrystallized, pale gray limestone piece with steeply inclined laminations contains altered grains of green glauconite. Above this, spanning Sections 324-U1346A-6R-1 and 5R-1, gray indurated limestones containing shell fragments, including the well-preserved outer whorl of an unidentified ammonite, are observed. The suture lines of the ammonite shell are clearly visible in some orientations. These gray limestones are quite "clean" and consist almost exclusively of recrystallized carbonate with only a minor clay component. The remainder of the unit, from Section 324-U1346A-5R-1 to the base of 4R-2, is a mixed sequence of laminated limestones and calcareous mudstones. Apart from a coarse bioclastic piece consisting largely of inoceramid bivalves at interval 324-U1346A-4R-2, 138–142 cm, limestones from the lower part of Core 4R to the upper 6 cm of Section 5R-1 are finely laminated and have less glauconite and fewer shell fragments than the underlying muddy limestones in Section 5R-1. The matrix of the bioclastic limestones is largely composed of micrite with associated clay minerals, although secondary sparite cement is present in some horizons. Soft calcareous mudstones are a common feature of Core 324-U1346A-5R. This lithology has a higher proportion of clay minerals relative to the limestones stratigraphically above and below and is accordingly less well indurated, which led to extensive drilling disturbance in some intervals. Some of the calcareous mudstones are dark brownish gray, suggesting an increased proportion of organic matter, which is supported by weight percent total organic carbon (TOC) measurements taken in this interval (Table T2). A significant spike in the uranium content of the sediments in the calcareous mudstone section of Section 324-U1346A-5R-1, between 50 and 70 cm, also supports this observation (see "Physical properties"). Green glauconite pellets are present throughout this unit and are especially abundant in Core 324-U1346A-5R, where they appear as subrounded to subangular clasts within the carbonate matrix. A concentration of these clasts is present at 129.2 mbsf at the base of a dark grayish brown nannofossil-rich mudstone. Small plagioclase feldspar laths are a common minor component of many of the limestones and are generally subangular in shape. Radiolarians, present throughout the sequence, have often been replaced by calcite and are generally not well preserved. Traces of a diverse epifaunal assemblage including bivalves, gastropods, sponges, benthic foraminifers, and possible echinoderms can be seen in the calcareous mudstones of Core 324-U1346A-5R (Fig. F8). Fragments of inoceramid bivalves are a common component of this unit and in some instances constitute a significant percentage of the carbonate content of the limestone (Fig. F8A). Small dark-colored elongate fragments, tentatively identified as wood, were found in this unit. These elongate fragments are concentrated in the calcareous mudstones in Section 324-U1346A-4R-CC and at the top of 5R-1 but were also identified in the laminated limestones from this interval. Sedimentary carbon contentSix samples were taken from Cores 324-U1346A-4R through 5R to determine total carbon, carbonate (CaCO3), and TOC content in weight percent (Table T2). Prior to analysis, sediment samples were scraped to remove surface contamination then freeze-dried and powdered to remove water and ensure complete homogenization. Carbonate content was determined by acidifying ~10 mg bulk sediment samples with 2 N HCl and measuring the CO2 evolved with a UIC Coulometer. CaCO3 content ranged from 3.5 wt% (±0.2 wt%; 2σ) to 98.4 wt% (±0.8 wt%; 2σ). All of the samples measured contained >25 wt% carbonate with the exception of one taken from the finely laminated sand-silt-claystone at interval 324-U1346A-4R-2, 61–63 cm, composed primarily of volcaniclastics. Total carbon content was determined by combustion of the sample at an initial temperature of 900°C in a Thermo Electron Corporation, Flash EA 1112 Series carbon-hydrogen-nitrogen-sulfur (CHNS) analyzer. This value was used to calculate TOC content by subtracting the percentage carbon as carbonate value obtained from coulometric analysis from the total carbon content value obtained from the CHNS analyzer. TOC contents at Site U1346 were low, averaging 0.5 wt% (±1.1 wt%; 2σ). Only one sample, taken from a dark grayish brown nannofossil-rich calcareous mudstone (interval 324-U1346A-5R-1, 12–13 cm), contained >0.5 wt% organic carbon. InterpretationPoor recovery in the sedimentary section of this site and the loss of contacts between different stratigraphic units makes interpreting the exact sequence of events leading to the deposition of the sediments challenging. In addition, the ages of the different units at this site are not well constrained because of the general lack of age-diagnostic species typically used in biozonation (see "Paleontology"). This also leads to difficulty in estimating diagnostic lithologic features such as sedimentation rates. The available age models, however, do suggest an Early Cretaceous (Berriasian to early Hauterivian) age for sediment deposition in stratigraphic Units I–IV, which correlates well with the estimated basement age for Shirshov Massif from magnetic lineation studies in the region (Nakanishi et al., 1999). In general, the lithology and fossils of the sediments recovered in Units I–IV suggest progressively shallower paleodepths of deposition with increasing depth in the core. This observation is in agreement with the model of a subsiding volcanic edifice, where the oldest sediments above basement are deposited in the shallowest water depth with subsequent younger sediments deposited in progressively deeper marine settings (e.g., Winterer and Sager, 1995). The presence of significant amounts of carbonate throughout Units I–IV precludes deposition below the carbonate compensation depth (CCD). In addition, the presence of numerous lithologic and paleontological water-depth indicators in Unit IV provides further support for a relatively shallow marine depositional environment. The presence of possible wood fragments and numerous reddish rock fragments identified as scoria (oxidized pyroclastics) are suggestive of an emergent, possibly vegetated, volcanic land mass in close proximity to the depositional area. The reconstructed paleoenvironments of Units II and III are more complicated to interpret, as the lithologic features observed could be interpreted in several different ways and may have been influenced by the specific unknown local conditions at this site. The presence of volcanic materials such as glass and feldspar, found throughout the graded sand-silt-claystone sequence, suggests the site was in fairly close proximity to the volcanic source during deposition of Unit III, either directly through eruptions or indirectly through weathering of previously erupted basaltic lavas. Paleogeographic reconstructions place the location of Shatsky Rise in the equatorial paleo-Pacific during the Early Cretaceous, thousands of kilometers away from any terrigenous source of material (e.g., McNutt and Fischer, 1987). This observation suggests that any significant source of nonpelagic sediment, such as the abundant clay–sand sized material found in Unit II, must have had a more proximal, and probably volcanic, source. The origin of Unit II, which includes the mixed basaltic and limestone lithologies, is again puzzling. Despite the intermingling of the basalt with the deformed limestone, the absence of chilled margins where the basalt is in contact with the sediment suggests that the basalt had already cooled and partially solidified when it encountered the calcareous sediments. With regard to the depth of emplacement, the high vesicularity of the basalt might suggest relatively shallow depths (<300 m), whereas the lithology of the intermingled limestone suggests a deeper, more pelagic setting. Unit IThis unit, for which we have very limited core recovery, is thought to represent deposition in the Early to mid-Cretaceous. At Site U1346, Unit I seems to be composed of interbedded soft nannofossil chalks and indurated radiolarian chert beds, which contribute to the poor recovery of this unit. The chert beds of this unit are the result of remobilization of biogenic silica, most likely sourced from radiolarian tests. The occasional nannofossil chalks, which occur as thin coatings on the cherts, suggest an oxic, pelagic environment where deposition occurred well above the CCD. The high abundance of silica and radiolarian "ghosts" suggests the site was close to an area of high productivity during the time of deposition, probably related to equatorial upwelling. Unit IIThe limestone recovered in Section 324-U1346A-4R-1 was likely deposited in a hemipelagic to fully pelagic setting because of the fine texture of the matrix and lack of shelly or sandy material indicative of very shallow marine settings. The high carbonate content (>90 wt%) of these sediments and the presence of nannofossils indicates deposition well above the CCD. Fine laminations, preserved where the bedding has not been disturbed, and the contribution of both radiolarians and nannofossils to the sediments in the absence of large bioclasts suggests quiescent waters certainly deeper than storm wave base and possibly as deep as 700–1000 mbsl. It is possible that the lack of shelly material, however, is caused by the entire carbonate-bearing platform having subsided below the point where shallow-water fauna such as bivalves could thrive in abundance, leading to a lack of such material in adjacent sediments deposited at modest depths (300–400 mbsl). Where the highly vesiculated basalt intermingles with the sediments, bedding features become convoluted and deformed. All of the sediments in this unit are cemented with carbonate, and much of the material in close proximity to the volcanic rock appears to be recrystallized nannofossil ooze, suggesting diagenetic alteration caused by heating or changes in pore water chemistry. The basalt lacks chilled margins, which is evidence that the rock material solidified prior to contact with the soft sediments. The basalt could have been emplaced as a flow from a late-stage flank eruption, whereupon it cooled, solidified, and slumped into an area of carbonate sedimentation. This would have caused some soft-sediment deformation to the carbonates without destroying the original bedding, which is still evident in the limestones. This model also accounts for the presence of basaltic clasts within the sediment package, the seeming disparity in igneous versus sedimentary depositional depths, and the lack of glassy cooling texture in the basaltic lobes where they are in contact with the sediments. Unit IIIUnit III consists of several normally graded, sand to clay fining-upward sequences, which we interpret as representing deposition from subaqueous mass movement events, most likely the action of turbidity currents. The finely laminated sequences and normal grading are indicative of hydraulic sorting processes. The presence of scours and flame structures suggests lateral and rapid emplacement of material from a flow rather than passive settling through the water column. The low carbon content of the sediments and abundant clay and angular glass fragments throughout points to a proximal volcanic source. As the largest grains in the fining-upward sequences are medium sand, the depositional environment most likely represents the distal lobe of a turbidity fan rather than deposition close to the source of the events. The coarsest material in the sequence occurs as an isolated piece at the base of the lowermost graded sequence, which may represent the coarse basal unit of an earlier, more proximal turbidity current event. As the presence of basaltic lava in Unit II clearly indicates active volcanism in the immediate vicinity after the deposition of Unit III, it is not unreasonable to suppose that sediments accumulated on the steep sides of a volcanic plateau where they could be destabilized by successive eruptions and form large debris flows. The isolated bioturbated horizon in the fine claystone at the top of one graded sequence indicates oxic to suboxic conditions and a period of quiescence before the arrival of the next depositional event. Paleodepth is estimated to be >500 mbsl, based on the lack of shallow-water carbonate material, the size of the volcanic material deposited, and the degree of hydraulic sorting exhibited in the sequence. This estimate, however, is uncertain because of the unknown height and incline of the original volcanic platform, which would influence the depositional pattern of this sequence. Unit IVThis unit represents the oldest sedimentary material recovered at Shirshov Massif and is interpreted as being deposited in a shallow marine setting. Water depth may have been 150–200 mbsl or as shallow as 50 mbsl, based on assorted lithologic and biological information, including paleodepth estimates from benthic foraminifer assemblages (see "Paleontology"). The presence of epifaunal biota such as bivalves, gastropods, and possibly echinoderms, all commonly associated with shallow-water marine environments, is strongly suggestive of shallow water depths during deposition of Unit IV. The shelly remains are not preserved in life position and so probably represent the debris from the proximal very shallow water environment where they were living. The authigenic mineral glauconite, which is abundant throughout this unit, is found in the modern world exclusively in marine environments. It is often associated with organic matter, especially within fecal pellets, and often replaces biogenic structures such as foraminifer shells. Glauconite is often considered to require mildly reducing conditions and a reasonably low sedimentation rate in order to precipitate and has been used to infer such conditions for periods of ancient deposition (Harder, 1980). Attempts have also been made to infer paleodepth based on glauconite because of its restriction of formation in the modern oceans to the shelf and upper slope (50–500 mbsl). However, some recent work suggests constraints on modern glauconite formation may not hold true for formation in the past; therefore, glauconite cannot be considered a robust paleodepth indicator (Chafetz and Reid, 2000). Caution should perhaps then be exercised in inferring depth information from its presence in the case of Site U1346, although mildly reducing conditions associated with decaying organic matter at depths from 50 to 500 mbsl throughout the sequence would not be at odds with other lithologic and paleontological information. The presence of suspected wood fragments in Core 324-U1346A-5R leads us to conclude that there may have been emergent vegetated land in the vicinity, probably associated with the earlier volcanic edifice building event. If the grains associated with the basal limestones are indeed volcanic scoria, this may be indicative of subaerial eruptions (see "Igneous petrology"), which lends weight to the interpretation of emergent land close to the area of deposition at Site U1346. |
