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doi:10.2204/iodp.proc.324.101.2010

Synthesis

Sediments

Despite the fact that the recovery of igneous basement was the main objective of Expedition 324, >100 m of sedimentary material was recovered from the five sites drilled on Shatsky Rise. The variety of different sediments, ranging in age from Early Cretaceous to middle Cenozoic, represents several different depositional environments. As anticipated, the dominant stratigraphic facies in the uppermost cores at all sites is chert interbedded with chalk and calcareous oozes. At every site, these sediments make up Unit I (Fig. F8). The combination of chert and soft chalks undoubtedly contributed to the consistently poor recovery of this unit, which was often limited to one or two chert fragments per core. The sediments comprising Unit I are indicative of pelagic carbonate and silica-rich deposition at each of the massifs, initiated once that portion of the volcanic platform had subsided out of the photic zone. Age constraints within this facies are somewhat difficult to determine, but generally range from Berriasian through middle Cenozoic, suggesting that open-marine sedimentation was the dominant mode of deposition for the majority of Shatsky Rise's posteruptive history. The persistently high radiolarian component present in many of the stratigraphic units is testimony to high productivity, probably associated with the position of Shatsky Rise near the paleoequator during much of the Early to mid-Cretaceous.

Other (nonpelagic) sedimentary material was also recovered from many of the sites, including bioclastic limestones, radiolarian-rich siliciclastics, and thick volcaniclastic sequences (Fig. F8). This unexpected diversity of basal material may help to illuminate the complex history of sedimentation and subsidence at Shatsky Rise, after the main volcanic-edifice building phase had ceased but before fully pelagic sedimentation was initiated. Interestingly, sediments were found interbedded with igneous material comprising the basaltic basement at several of the sites. These sediments, largely undated thus far, may yet yield invaluable information about the relative duration of eruptive events and the environment into which these igneous units were emplaced.

The discovery of shallow-water sediments at many sites, and even subaerial basal sediments at one site, suggests that Shatsky Rise may have been a semi-emergent archipelego rather than a purely submarine edifice during the Early to mid-Cretaceous. Lithologic and biologic indicators for shallow marine depositional environments include carbonate-rich sediments, shallow-water fossil assemblages, and the occurrence of small possible wood fragments, glauconite, and sedimentary structures suggestive of shallow water deposition.

Tamu Massif, at the southern end of Shatsky Rise, is thought to represent the oldest part of Shatsky Rise. At the southernmost site drilled during Expedition 324, Hole U1347A on the eastern flank of the massif, Early Cretaceous sediments (Berriasian to late Valanginian) were found overlying basement, confirming age estimates from previous studies. These sediments include a ~60 m sequence of graded and laminated radiolarian-rich sand-silt-claystones (Fig. F8), which contain diverse trace fossils and occasional body fossils, such as the ammonite fragments found in Core 324-U1347A-8R. The sedimentary material is fine grained, probably of volcanic origin, and contains bedding structures indicative of deposition by turbidity currents. Glauconitic radiolarites and silicified limestones overlie these laminated siltstones and contain preserved radiolarian tests, replaced in many cases by green glauconite minerals. Additionally, the material is cross-bedded in a style indicative of very shallow marine conditions, at or above wave base, suggesting a shallowing-upward sequence prior to subsidence and deposition of the pelagic chert-rich facies.

Material recovered from the second Tamu Massif site (U1348), on the northern flank, sharply contrasts with that recovered from Site U1347 and represents sedimentation in a large range of depositional environments. Basaltic basement was not reached at this site despite drilling to >320 mbsf, although a large proportion of the material at this site is volcanic in origin and probably represents redeposited hyaloclastic (volcanic glass) material from submarine eruptions (Fig. F8). The sedimentary structures in this succession are suggestive of hydraulic sorting and include laminations and graded bedding (both reverse and normal), as well as erosional contacts, making deposition by the action of turbidites or debris flows probable. Shallow-water fauna such as well-preserved crinoids and gastropods are present within some of these volcaniclastic units, indicating the environment was relatively shallow and marine. Unfortunately, age constraints for these unique volcaniclastic units prove elusive, although a tentative Early Cretaceous age can be assigned based on radiolarian assemblages and the biostratigraphic ages of overlying sediments. Other highlights from Site U1348 include the recovery in Core 324-U1348A-12R of green zeolitic and celadonite-rich clays thought to represent altered ash beds, and sandstones predominantly formed of tabular coral fragments in Core 11R. A 25 cm section of early–middle Aptian foraminifer-bearing calcareous ooze was recovered in Core 10R and may provide an invaluable record of Pacific paleoceanography during this crucial mid-Cretaceous transition. Another remarkably well preserved section (1 m long) of nannofossil ooze, thought to span the Late Cretaceous Santonian to Campanian transition, was also recovered in Core 324-U1348A-2R. A small piece of gray ooze at the top of this core and the contents of the wash core (324-U1348A-1W) above are middle Cenozoic in age, suggesting either contamination from material much higher in the borehole or a genuine hiatus between the Late Cretaceous and middle Cenozoic at this site.

Ori Massif, north of Tamu Massif, is thought to represent either a later phase of volcanism or a rifted segment of the larger edifice. Two sites were drilled on Ori Massif; the first (Site U1349) is centered on the summit and the second (Site U1350), ~70 km away, is on the eastern flank of the massif. The basaltic basement at Site U1349 was heavily altered and covered by a sequence of volcaniclastic conglomerate, breccia, and sandstones (Fig. F8). A weathered, highly oxidized horizon in Core 324-U1349A-6R is interpreted as a paleosol, providing direct evidence for subaerial exposure. Evidence for very shallow water emplacement of the uppermost lava flows at Site U1349 comes from interbedded oolitic bioclastic limestones in Cores 324-U1349A-7R and 9R. Only sparse sedimentary material, in the form of black chert, was recovered above basement at Site U1350 (Fig. F8), but an interesting sequence of limestones intruded small pillow lavas was recovered in the lowest part of the hole within Cores 324-U1350A-25R and 26R.

Northeast of Ori Massif lies Shirshov Massif, presumed to be the youngest of the volcanic edifices at Shatsky Rise on the basis of magnetic lineation data. Site U1346 was drilled on Shirshov Massif on the northern rim of the summit. The basaltic basement at this site was reached at 139.2 mbsf, underlying a largely pelagic sedimentary sequence. Only the lower portion of these sediments was cored, beginning at 100.5 mbsf. Although sedimentary recovery averaged only ~10%, the diversity and sequential changes within this material provide a record of the subsidence of the volcanic platform. Directly overlying the basement, shallow-water biogenic limestones were recovered (Fig. F8), consisting primarily of a fine-grained carbonate matrix with abundant shell fragments, foraminifers, radiolarians, echinoids, and glauconite. Oxidized fragments of scoriaceous material as well as small (millimeter size) possible wood fragments were identified in these limestones, suggesting that there was a landmass nearby. Upward in the sequence the biogenic component of the limestones decreases, develops laminated bedding structures, and contains larger amounts of siliciclastic material in the silt and clay size range. A large spike in U content was observed in the NGR data toward the top of the limestone succession, within Core 324-U1346A-5R. A fine-grained volcaniclastic turbidite deposit was recovered above the limestones (Fig. F8); it is similar to the finer hyaloclastic material recovered at Site U1348 on Tamu Massif. Present within this meter of laminated mud and fine sandstones are at least three major fining-upward sequences bounded by erosive contacts. The sediments are mostly composed of clays and altered volcanic glass with varying amounts of iron oxyhydroxide minerals, glauconite, and radiolarians. Another limestone package rests above the turbidite; it is interbedded with altered vesicular basalt. The carbonates in this unit are fine grained, occasionally sparry (indicating recrystallization of the calcite), and laminated and contain several dark clay-rich bands. These carbonates are interpreted to represent deeper water sedimentation. The basalt/sediment contacts are convolute, indicating that the basalt was likely emplaced while the sediment was still soft. However, there is no evidence of chilled margins, which suggests the basalt had already cooled and solidified before being mixed with the sedimentary material by slumping or debris flow.

Biostratigraphy

Shipboard studies of calcareous microfossils (calcareous nannofossils, planktonic foraminifers, and benthic foraminifers) allow new constraints to be placed on the evolutionary history of Shatsky Rise. At Sites U1346 and U1347, Early Cretaceous calcareous nannofossils and benthic foraminifers were retrieved from calcareous and/or siliciclastic sediments immediately above the basaltic basement. The youngest estimated basement ages, based on calcareous nannofossils, are Berriasian–Hauterivian for Site U1346 and Berriasian–Valanginian for Site U1347. These estimates compare well with the ages delineated by magnetic lineations (Fig. F3). Benthic foraminifers comprise a unique monofamily assemblage, which has not been reported from any other Cretaceous Pacific deep-sea sections and gives paleowater depths of <500 m for Site U1346 and <200 m for Site U1347 (Fig. F26). This finding provides an independent line of evidence for shallow-water or subaerial eruption of Shirshov and Tamu massifs, complementing other sedimentological and physical volcanological observations.

At Site U1348, the pelagic sediment cover of Tamu Massif (northern flank) yields diverse, abundant planktonic foraminifers ranging in age from the mid- to Late Cretaceous (Aptian–Campanian). The nearly linear age-depth relationship shows the absence of a major sedimentation gap. The recovery of 120 Ma ooze (early Aptian), probably the oldest known fresh pelagic sediment, is not noteworthy, and guarantees stable isotopic study of individual foraminifer species. Future progress in taxonomic, paleoecological, and paleoceanographic studies of foraminifers is anticipated.

Physical volcanology and igneous petrology

The volcanology of Shatsky Rise is more complex than initially anticipated, with each hole exhibiting different characteristics and combinations of units (Fig. F9). The typical igneous units are, in order of increasing size: (1) pillow lavas (~0.2–1.0 m diameter) displaying typical glassy rinds, chilled zones, internal vesicle distributions, and joint patterns; (2) pillowlike inflation units (~0.8–2.0 m thick) displaying glassy rinds or contacts, chilled zones, vesicular tops, and degassed interiors with pipe vesicles; (3) small massive flow units (~2–4 m thick) with vesicular tops, homogeneous, coarser grained interiors with pipe vesicles and segregation features, and narrow basal chilled zones; and finally (4) large massive flow units (~5–23 m thick) with narrow, glassy upper contacts, vesicular tops grading down into vertically extensive, nonvesicular, degassed interiors with sparse, well-developed pipe vesicles and segregation features, a coarser crystal groundmass, and a narrow basal chilled zone. Overall, the vesicularity of the igneous units was high, and most sites are characterized by a high proportion of smaller sized inflation units. However, although the large massive flow units are less common, they can constitute a significant proportion of the core. Often the basalts are intercalated with or directly overlain by relatively to very shallow marine sediments, indicating that most lavas erupted at shallow water depths and low hydrostatic pressures. In one instance, at Site U1349, several reddened flow tops occur, probably indicative of subaerial eruption, or at least a period of emergence.

The five volcanic basement sites may be summarized as follows:

• Site U1346 on Shirshov Massif consists of ~53 m of highly vesicular pillow lavas and larger pillowlike inflation units of aphyric micro- to cryptocrystalline basalt. These are interpreted as part of a single pillow lava stack. The nature of the sedimentary intercalations within the succession indicates that the environment deepened progressively from nearshore to offshore marine conditions throughout the eruptive period.

• Site U1347 is on the southeastern flank of Tamu Massif (~200 km northeast of Hole 1213B). The ~160 m thick basement succession consists of massive basalt flows and pillow inflation units intercalated with volcaniclastic sedimentary layers. The basalts include (1) an upper series of four massive lava flows (~8–19 m thick); (2) a ~75 m lava stack with more massive (~3–6 m thick) basaltic flows passing upward to pillowlike inflation units (1–2 m thick) and pillow basalts (<1 m thick), which likely represent successive eruptive pulses during which lava effusion rates diminished; and (3) two massive, internally homogeneous lava flows consisting of a very thick (~23 m) upper massive (tabular) flow overlying a second (partially penetrated) flow. The frequent recovery of thick, often fresh, glassy rinds within the pillow stack indicates that alteration was essentially buffered in these rocks.

• Site U1348, on a basement high on the north flank of Tamu Massif, is unusual in that it provided ~120 m of volcaniclastic sediments, including ~90 m of hyaloclastite. This material initially proved difficult to interpret because of pervasive alteration, which masks both the original texture and structure of this lithology. However, chemical composition, measured from a few large basaltic clasts, is consistent with that of the lavas found at the other sites. The predominance of altered glass shards is indicative of substantial submarine volcanism nearby.

• Site U1349 was drilled near the top of Ori Massif. The upper sedimentary section is the usual chert-chalk sequence, which changes downhole to volcaniclastic materials containing fragments of basalt. Beneath the volcaniclastics is a weathered volcanogenic conglomerate that lies unconformably upon red-brown basaltic flows. These thin altered lavas have extremely vesicular flow tops, many of which are deeply reddened, probably as a result of subaerial weathering, and are intercalated with multiple limestone (oolitic) horizons indicative of periodic marine incursions. Increasingly thicker lava inflation units occur toward the bottom of the lava section before passing into an underlying submarine succession of lava flow breccias, hyaloclastite fragments, and more massive, irregularly shaped lava pods. Accordingly, this volcanic sequence appears to have developed in progressively shallower to emergent conditions, followed by submergence after volcanism ceased.

• Site U1350 is 70 km east of Site U1349 on the eastern flank of Ori Massif, where the seafloor is ~800 m deeper. Drilling penetrated ~173 m into volcanic basement and yielded (1) a series of massive basalt flows passing downhole into (2) a transitional zone, (3) aphyric to sparsely plagioclase-phyric pillow lavas, (4) a thin layer of hyaloclastite, and (5) a succession of well-preserved plagioclase-phyric pillow lavas set in a matrix of originally unconsolidated or fluidized micritic limestone. The inflation units above the hyaloclastite are complex, containing a mixture of massive flow and pillowlike units sparsely interspersed with thin sedimentary horizons. Beneath the hyaloclastite, the high core recovery preserved in great detail an intricate stack of small pillow lavas (~0.1–0.5 m) and numerous pillow/sediment baked contacts. Alteration was minimal toward the top of this hole, comparable with that observed in Hole U1347A.

From a petrological point of view, Shatsky Rise presents a basaltic province in which plagioclase-clinopyroxene intergrowths are well developed, from the early crystallization of mottles with ophitic textures (Hole U1349A) to the formation of sparse phenocrysts and glomerocrysts (Holes 1213B, U1347A, and U1350A) to the quench stage and the stage of coarser grained crystallization in the interiors of flows and pillows (all holes). This petrographic attribute is evident even among the rare and tiny quench crystals in the one glassy sample recovered among the hyaloclastites of Hole U1348A. The basalts of Holes U1346A and especially U1349A are more primitive than at other sites, carrying fairly abundant olivine and Cr spinel, and yet the plagioclase-clinopyroxene intergrowths occur even in the altered glass margins of these rocks. The above observations indicate that, at all stages of crystallization, the temperature interval between the onset of plagioclase crystallization and that of clinopyroxene crystallization was negligible. Experimentally, this is not a usual feature of MORB at low pressure, with the exception of melts containing significant dissolved water. A range of temperatures may have existed, and that will be determined by onshore mineral studies, but in all cases plagioclase and clinopyroxene crystallized cotectically.

Some of the basalt in Hole U1349A can be termed picritic; however, these rocks differ from the only well-documented picrite found on modern spreading ridges. More evolved olivine tholeiite from drill sites on the Mid-Atlantic Ridge still has an obvious gap in crystallization temperature at all cooling rates between plagioclase and later clinopyroxene. As MORB-like as Shatsky Rise basalt may be in its geochemistry, in this one respect it is different.

Alteration

Basaltic rocks forming the Shatsky Rise have all undergone pervasive fluid-rock interaction, with lateral and vertical variations in degree and conditions of alteration (i.e., temperature, redox, and fluid composition). Overall, alteration at Shatsky Rise occurred under relatively low temperature and relatively oxidizing conditions, resulting in a slight to complete replacement of primary phases (glassy mesostasis, olivine, plagioclase, and pyroxene) by clay minerals (i.e., smectites). Calcite and pyrite are also common secondary minerals, replacing primary phases and filling vesicles, veins, and voids. Other alteration minerals at Shatsky Rise commonly include zeolites, rare hematite, and very rare chlorite.

Predominant alteration of Shatsky Rise basalts is a gray alteration, resulting from interaction of basalts with seawater-derived fluids under anoxic-suboxic conditions at low-temperature and low water-rock ratios. This alteration type was encountered at Sites U1346, U1347, and U1350 and affected the basalts to various degrees (from slight to high). Primary phases (olivine and glassy mesostasis) are commonly replaced by clay minerals (i.e., montmorillonite, nontronite, and saponite), whereas pyroxene and plagioclase are generally well preserved. Fresh titanomagnetite is also present in lava flow interiors at these sites when alteration degree is <50%. Significant fresh glass was commonly observed on flow and pillow margins of Holes U1347A and U1350A and more rarely in Hole U1346A. A predominance of calcite and pyrite at these sites also suggests interaction of the basalts with CO₂- and S-rich fluids.

Basalts recovered at Sites U1348 and U1349 differ in degrees and types of alteration. Hole U1348A recovered hyaloclastites in which glassy clasts were altered to palagonite and cemented by calcite and/or zeolite (i.e., phillipsite). Basalts recovered in Hole U1349A have been affected by extensive water-rock interaction, with variations in temperature and redox conditions with depth. Secondary low-temperature and oxidizing mineral assemblages, dominated by iddingsite, hematite, and calcite in the upper 205 mbsf (Unit IV), change to relatively higher temperature secondary mineral assemblages composed of clay minerals, zeolite, minor chlorite, and rare serpentine below 205 mbsf. Unit IV in Hole U1349A shows evidence of subaerial weathering and of lavas being in very shallow water (e.g., hematite, nine horizons of intense red alteration interpreted as subaerially exposed flow tops, and thin interbedded oolitic limestone). This suggests that prior to typical low-temperature oxidizing seawater-rock interaction, basalts in Hole U1349A were first altered under subaerial oxidative tropical conditions.

Geochemistry

Alteration has modified the major and trace element compositions of virtually all igneous rocks analyzed during Expedition 324. Compositions of samples from Sites U1347 and U1350 have been affected the least overall (e.g., Fig. F11A). Interelement relationships indicate that the Site U1347 and U1350 rocks are variably evolved tholeiitic basalts. Similar to basalts from Site 1213 on the southern Tamu Massif, the compositions of the Site U1347 and U1350 lavas show some overlap with data for the OJP but are more similar in many elements to ocean-ridge basalts. More specifically, the Site U1347 lavas and many of the Site U1350 basalts resemble E-MORB. Some Site U1350 samples, particularly from the deeper part of the basement section, have relatively low Zr/Ti ratios similar to those of normal ocean-ridge basalts. A broad generalization is that results for these sites on the whole suggest a source slightly richer in the more incompatible elements than normal ocean-ridge mantle and/or that the majority of magmas formed by slightly smaller amounts of partial melting than normal ridge basalts and possibly in the presence of residual garnet. Compared to the Site 1213 basalts, some of the Site U1347 and U1350 lavas show slightly greater enrichment in the more incompatible elements.

Seeing through the effects of alteration is a much greater challenge with samples from Sites U1346, U1348, and U1349. However, alteration-resistant elements, particularly Zr and Ti, provide the following insights. All of these rocks are (or were originally) tholeiitic basalts. Those from Shirshov Massif Site U1346 are similar to the Site 1213 basalts, whereas the Site U1348 volcaniclastic rocks from the northern Tamu Massif are broadly similar to the lavas of Site U1347 (Fig. F11B). In addition to severe alteration, the compositions of basalts from Site U1349 atop the Ori Massif have been modified by variable accumulation of olivine and possibly clinopyroxene. Allowing for these effects, the Site U1349 samples appear to represent significantly less differentiated magmas than rocks from the other sites; they show compositional similarities with primitive ocean-ridge and Ontong Java basalts.

Structure

Primary structural elements observed in Expedition 324 cores include sediment bedding, conjugate joints, veins, breccias, and microfaults. In addition, synmagmatic structures, such as amygdules, vesicles, pipe vesicles, sheet flow structures, and interpillow and intrapillow structures, are also examined. Most veins are postmagmatic and were filled after lavas cooled. Most horizontal veins show syntaxial growth, cutting veins with other orientations, suggesting that horizontal contraction produced the syntaxial veins at a late stage of formation. Syntaxial veins in Holes U1349A and U1350A are more common than those in other holes, possibly implying that Ori Massif experienced more contractional stress than Tamu and Shirshov massifs. The spatial distribution of joints and veins is complex in Shatsky Rise lavas. In general, dip angles of both veins and joints become gradually steeper downhole, except for Hole U1350A. The population of joints and/or veins possibly depends on the physical properties of the rocks. In all the holes, regular veins are commonly well developed in basalts with fine-grained groundmass and low vesicularity. Two primary synmagmatic structures, pillow and sheet flow structures, are common in all holes except for U1348A. At the other four sites, sequences of stacked pillow lavas whose sizes range from ~20–200 cm or stacked sheet flows with sizes of up to 23 m were observed. Only the upper part of Hole U1349A implies subaerial eruption; rocks in all other cored sections are products of submarine eruptions.

Paleomagnetism

The goal of paleomagnetic studies during Expedition 324 was to characterize magnetic remanence of recovered igneous rocks. The majority of basalt samples showed low coercivity and blocking temperatures characteristic of titanomagnetite (-maghemite) with a range of Ti content. Overall, it was found that stable characteristic magnetization inclinations are shallow and mostly negative. However, several atypical behaviors were encountered and need to be addressed during postcruise research before interpreting the results. For example, in some stratigraphic units, many samples displayed irregular behavior during demagnetization experiments. Additionally, samples from Hole U1349A may show evidence of a chemical remanent magnetization (carried by hematite) instead of a thermoremanent magnetization (carried by titanomagnetite). In some samples, partial self-reversal of the magnetization may have occurred during the thermal demagnetization procedure. However, the overall results show shallow average inclinations at all sites, supporting the hypothesis that Shatsky Rise was formed near the paleomagnetic equator. Furthermore, the low scatter in stratigraphic-average inclination groups implies that at most sites, especially Sites U1347 (Tamu Massif) and U1346 (Shirshov Massif), little time passed during the eruption of igneous units (Fig. F27).

The only sedimentary measurements made during Expedition 324 were on the volcaniclastic sediments at Site U1348. Samples from this site show higher coercivity, consistent with single-domain titanomagnetite. The average stable magnetization inclinations from these samples are also low, implying low paleolatitude. Because these rocks are volcaniclastic sediments, the inclinations are from a depositional remanent magnetization, rather than a thermal remanent magnetization as at other sites, that requires further measurements and interpretation.

Physical properties

Expedition 324 recovered igneous material that had experienced minimal to near-complete alteration, which naturally affected physical properties. Site U1346, atop the Shirshov Massif, and Site U1349, at the summit of the Ori Massif, exhibit the most extreme and variable alteration, which is reflected in their physical properties. Natural gamma radiation counts from ⁴⁰K decay are three to four times higher in the altered material recovered at these sites versus the comparatively fresher material at Sites U1347 and U1350 (Fig. F28). Magnetic susceptibility displays distinct drops associated with oxidative alteration at both Sites U1346 and U1349. Cores from Hole U1349A are remarkable for the extent of oxidative alteration, suggested by the presence of hematite in Sections 324-U1349-7R-1 through 10R-5. In this interval, the magnetic susceptibility drops to levels ubiquitously <1000 x 10^–5 SI (Fig. F29). Below the oxidative front, where hematite is no longer present, magnetic susceptibility is higher. An analogous relationship is seen in Hole U1346A, where the gray alteration has a higher magnetic susceptibility than the oxidative brown alteration. Another notable physical property in Hole U1346A is a distinct excursion in NGR from ²³⁸U decay to values of almost 200 cps at 129.6–129.9 mbsf (Section 324-U1346-5R-1) in sedimentary material (Fig. F28).

In stark contrast to the altered material of Holes U1346A and U1349A, the comparatively fresh igneous rocks recovered in Holes U1347A and U1350A have low NGR counts (<5 cps) (Fig. F28). Coring in both Holes U1347A and U1350A recovered alternating pillow lavas and massive inflation flows with broadly similar physical properties. Discrete measurements yield densities of 2.32–3.23 g/cm³, porosities of 0.17%–29.47%, and P-wave velocities (with negligible anisotropy) of 3.04–7.08 km/s. Notable intervals include a massive flow unit (Cores 324-U1347A-26R to 29R) at the bottom of Hole U1347A. This unit displayed the highest consistent readings of magnetic susceptibility, as well as cyclic magnetic susceptibility variations that might be used to decipher the chemical stratigraphy of the flow. Site U1350 was the most consistent with respect to downhole physical properties. Very little variation in density and only a slight downhole decrease in magnetic susceptibility was observed. Sedimentary interbeds were commonly observed in both Hole U1347A and U1350A cores and readily distinguished by their lower GRA bulk densities and magnetic susceptibility. The variations in density, porosity, and P-wave velocity measured on discrete samples were dominantly related to sampling bias (e.g., vesicularity) rather than lithologic differences.

Hole U1348A was quite different from the other sites in terms of its physical properties. Magnetic susceptibility values were <110 x 10^–5 SI and GRA densities were lower compared to igneous material from other sites (Figs. F29, F30). GRA density was also notable for its smooth, gradual increase in individual stratigraphic units, plausibly related to compaction. The P-wave velocities were the lowest seen at any of the sites, with values ranging from 2.09 to 3.32 km/s with an average of 2.78 ± 0.79 km/s (2σ; N = 13). The volcanic rocks are also notable for their high porosity (all between 20% and 40%).

Downhole logging

Downhole logging data obtained during Expedition 324 included natural and spectral gamma ray, density, neutron porosity, PEF, electrical resistivity, and sonic measurements, as well as borehole formation–oriented images from the FMS. Interpretation of gamma ray and electrical resistivity downhole logs was used to identify logging units at four sites. Data were recorded in sections covered by the BHA, in the sedimentary sequences in the open hole interval, and in the basaltic basement. The sedimentary sequences throughout the Shatsky Rise sites show prominent gamma ray anomalies associated with U and K enrichments. Some of the most prominent anomalies are found at the sediment/basement interface and in the most altered sites. The anomalies may be indicative of focused hydrothermal fluid flow, whereas shallower anomalies recorded through the BHA may correlate to oceanic anoxic events previously interpreted in this area. In several of the holes, high K content in the basement section also indicates a high degree of alteration compared to upper crustal sections previously drilled in other oceanic localities (Fig. F31). Electrical resistivity measurements in the basaltic basement show distinctive high-resistivity zones that likely represent massive flows and pillow flow units, interspersed with low-resistivity zones that mark sediment interbeds and highly altered zones. FMS images show zones of distinctive pillow lavas; zones with high fracture and vein densities; vesicular, brecciated, and volcaniclastic intervals; and intervals that represent massive lava flows, flow contacts, and variably dipping beds throughout the Shatsky Rise sites.

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