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doi:10.2204/iodp.proc.330.101.2012

Principal results

Site U1372

Canopus Guyot is one of the oldest seamounts in the Louisville Seamount Trail, with an estimated age of ~75–77 Ma (Table T1). Its volcanic edifice is located at 26.5°S, 174.7°W and, with a second guyot at 26.6°S, forms two coalesced volcanic centers that span ~60 km. Site U1372 was placed on the summit plain of the northern volcanic center, close to its southern shelf edge at 1957.6 m water depth (Fig. F11). A ~14 m package of soft pelagic sediment was cored using a gravity-push approach with little or no rotation of the rotary core barrel assembly, followed by standard coring into ~32 m of basaltic breccia and coarser conglomerate materials and ~187 m into the igneous basement. Unfortunately, the targeted depth of 350 m into basement could not be reached because the drill string became irretrievably stuck in rubbly volcaniclastic breccia with cobble-size fragments of basaltic lava. This required the hole to be abandoned at 232.9 meters below seafloor (mbsf) with no attempted downhole logging.

The recovered sequence of volcanic and sedimentary rocks (Fig. F12) covers the latter part of the constructional phase of the seamount at Site U1372, a brief subaerial phase, and its final subsidence below sea level, as evidenced by the overlying sedimentary rocks. From the bottom up, the sequence starts with a hyaloclastite-rich sequence, indicative of submarine explosive volcanism of alkalic basalt composition. Over time this changed to a shallow-marine and subaerial eruption environment, as evidenced by oxidized red-brown peperitic flow tops and the more massive lava flows on top of the volcanic basement. An erosional unconformity was found on top of the volcanic basement, overlain by a bluish-gray basalt conglomerate likely deposited in a nearby rocky shore environment of a volcanic island emerging in the Cretaceous. This early sedimentary cover predominantly consists of basalt breccia and contains a single horizon of foraminiferal limestone of late Campanian to early Maastrichtian age. This breccia likely formed in neritic to hemipelagic shallow-water conditions and is itself unconformably overlain by early Pliocene to Holocene foraminiferal ooze, which is evidence for sedimentation in a deep pelagic environment.

Lithostratigraphy and biostratigraphy

Two stratigraphic units predominantly composed of sedimentary deposits were recognized at the top of Hole U1372A (Fig. F12A). Stratigraphic Unit I extends to 13.50 mbsf and represents the youngest record of pelagic sedimentation on top of the seamount. Its lower boundary is defined by the first occurrence downhole of consolidated basalt breccia in Unit II. Unit I is composed of unconsolidated sandy foraminiferal ooze with local occurrences of reworked glass and pumice fragments. Analysis of grain morphology and paleontological observations indicate the influence of strong oceanic currents during deposition of the unit. The occurrence of pumice and fresh glass fragments suggests that the sandy foraminiferal ooze includes a minor tephra component, most likely derived from the nearby Tonga-Kermadec volcanic arc. Calcareous nannofossils and planktonic foraminiferal assemblages from Unit I are typically late Pleistocene–Holocene, mid-Pleistocene, and late Miocene–early Pleistocene in age.

Unit II extends from 13.50 to 45.58 mbsf and represents an older sedimentary deposit, which we interpret to have formed under relatively shallow neritic to hemipelagic water conditions. Its lower boundary is defined by the first downhole occurrence of lava flows in the volcanic basement. Unit II is composed of basalt breccia (Fig. F13A) and conglomerate, with a minor interval of foraminiferal limestone that includes a small number of ferromanganese encrustations and inoceramid shell fragments. Petrographic description shows that the composition of the basalt clasts is similar to that of volcanic deposits in the underlying volcanic basement. Within Unit II five stratigraphic subunits were defined on the basis of clast angularity and composition of the intercobble and interboulder matrix. From top to bottom, Unit II is composed of multicolor basalt breccia (Subunit IIA); foraminiferal limestone with basalt clasts, ferromanganese encrustations, and abundant inoceramid shell fragments (Subunit IIB); multicolor basalt breccia (Subunit IIC); multicolor basalt conglomerate (Subunit IID); and a boulder-bearing bluish-gray basalt conglomerate deposited on top of volcanic basement (Subunit IIE). Bioclasts of shallow-water origin (calcareous algae and annelids; Fig. F14A) and basalt clasts occur throughout Unit II. However, clast roundness and the amount of shallow-water bioclasts increase with depth into the sequence, whereas the amount of planktonic fossils (foraminifers and calcispheres) decreases. Repeated occurrences of inoceramid shell fragments and foraminifers in the consolidated micrites and limestone of Unit II provide preliminary age estimates ranging from early Paleogene to Cretaceous (Fig. F14B). Because the breccia and conglomerates are dominantly of igneous origin and are mostly composed of aphyric and olivine-phyric basaltic cobbles and boulders, Unit II is surmised to represent a nearby rocky shore environment of a Cretaceous volcanic island. Interestingly, the entire sedimentary sequence of the late Paleogene and early Neogene is missing in Hole U1372A; Unit II therefore is unconformably overlain by Unit I.

Igneous petrology

Below the sedimentary succession of Units I and II, 187.3 m of volcanic rocks was penetrated from 45.6 mbsf to the bottom of Hole U1372A at 232.9 mbsf (Fig. F12B). The igneous basement section of the hole can be divided broadly into an upper (83 m thick) part consisting of lava flows and a lower (104 m thick) part mostly composed of volcaniclastic rocks. The lava flows of Units III–X range from aphyric to highly olivine-phyric. Flows in the topmost 27.2 m of the succession (to 63.7 mbsf) have peperitic tops, implying interaction between lava and carbonate mud, whereas those in the following 28.7 m (to 92.4 mbsf) have scoriaceous tops. Together with the occurrence of oxidized flow tops, these lava flows likely erupted subaerially or in very shallow marine conditions.

The first downhole appearance of hyaloclastic material in Unit VI, which contains altered glass between two lava flows at 92.4 mbsf, marks a change to submarine conditions deeper in the seamount sequence. Volcaniclastic rocks dominate the succession from 128.9 to 228.4 mbsf in Units XI–XVI. The volcaniclastic deposits can be divided into seven individual eruptive packages on the basis of the phenocryst content of the basaltic clasts. Three of these packages (Units XII, XIV, and XV) were separated from their underlying packages by short intervals (0.13, 0.16, and 3.3 m in thickness, respectively) of vitric-lithic volcanic sandstone. In all cases the sandstone was inferred to form the basal part of the overlying package, either through similarity of phenocryst abundance (Units XIV and XV) or through a graded contact (Unit XII). Within the volcaniclastic units no unequivocal pillow lava or lava lobes were recovered, but several intervals included basalt fragments with curved surfaces, glassy selvages, and a hint of radial vesicle trains. In a few cases small pods were recovered that have delicate lobate lava contacts, indicating that they may have been emplaced in situ within the volcaniclastic deposits and were not displaced (much) afterward (Fig. F13B). Drilling terminated in Unit XVII, a thick (4.3 m penetrated), massive, and essentially unaltered olivine-augite-plagioclase-phyric basalt lava flow.

The igneous rocks in Hole U1372A typically have phenocryst assemblages of olivine, olivine + plagioclase + augite, or plagioclase + augite (Fig. F15). Olivine phenocrysts were found throughout all basement units and also in the basaltic clasts of the breccia and conglomerates of Unit II. Pyroxene phenocrysts and microphenocrysts are always titaniferous (Fig. F15D). Olivine is present in the groundmass in several of the upper series of lava flows but not in those from the lower part of the succession, implying an increase in alkalinity with time. The presence of titanaugite shows that these rocks are not tholeiitic.

Alteration petrology

The entire igneous section in Hole U1372A has undergone various degrees of secondary alteration by low-temperature water-rock interaction and weathering, ranging from slight to complete alteration. Two main intervals showing different dominant colors of alteration (Fig. F16) can be directly related to the oxidation state of the alteration processes. Down to 90 mbsf the volcanic basement has a dominantly reddish alteration color, pointing toward an oxidizing environment under likely subaerial or shallow submarine conditions. From 90 to 232.9 mbsf the alteration becomes more greenish, pointing toward more reducing conditions related to a deeper submarine environment.

Typically, basaltic lava flow units are more fresh and only moderately altered. In many cases fresh olivine phenocrysts (Fig. F15A–F15C) were encountered, as were zones with fresh volcanic glass, particularly in the hyaloclastites of Units VI and XII (Fig. F15B, F15C). Primary magmatic plagioclase and augite are generally well preserved, both as phenocrysts and in the groundmass. Plagioclase shows only minor alteration to sericite or illite in some rocks but is characteristically fresh. Augite is almost always unaltered (Fig. F15D). Usually, olivine is completely altered to iddingsite and Fe oxyhydroxide in the uppermost 90 m of the hole, except for a few intervals where fresh to moderately altered olivine was recovered. Olivine is absent in Units VII–XV from 90 to ~200 mbsf, where mostly aphyric basalt was recovered. From ~200 to 233 mbsf the original olivine phenocrysts largely have been replaced by green clay, serpentine, Fe oxyhydroxide, and carbonates (calcite/magnesite). Throughout Hole U1372A three main groups of alteration phases were distinguished: carbonates (Mg calcite and siderite), clay minerals (saponite, nontronite, glauconite, montmorillonite, and celadonite), and other secondary phases (e.g., zeolite, iddingsite, glauconite, Fe oxyhydroxide, and pyrite). Although these rocks have high porosity and often open void spaces, vesicles (if filled) and veins are variously filled with carbonates, clay minerals, and zeolites.

Structural geology

Structural features at Site U1372 are veins, vein networks, joints, fractures, aligned vesicles, and geopetal structures. Fractures and veins are relatively common in the upper lava flow Units III–X but are rare to absent in volcaniclastic units lower in the succession that must have deformed relatively uniformly via compaction. Veins that are present within the volcaniclastic units are concentrated along unit boundaries. Several of the lava flows (particularly in the subaerial portion above 92 mbsf) display moderate to strong magmatic flow alignment in an approximately horizontal direction, including the elongation and alignment of titanomagnetite. Geopetal structures in the upper part of the sequence are all horizontal, indicating that the drilled succession has not been tilted by partial flank collapse or as a result of the incipient subduction of Canopus Guyot into the Tonga-Kermadec Trench.

Geochemistry

Major and trace element analysis of igneous rocks by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) indicates that alteration has not significantly obscured magmatic signatures, with the exception of K₂O in some samples and CaO in two samples. The data indicate that most samples are alkalic basalt, although several are transitional basalt. Their compositions overlap those measured for dredge samples from other sites along the Louisville Seamount Trail but cover a smaller range of variation. Nevertheless, a sizeable range of compositions is present, with Mg numbers ranging from 34.9 to 73.5, K₂O abundances ranging from 0.46 to 1.94 wt% (Fig. F17A), and Ni abundances ranging from 46 to 472 ppm (Fig. F17C). Zr/Ti ratios, on the other hand, lack significant downhole variation (Fig. F17B). Two samples appear to contain excess olivine phenocrysts. Much of the chemical variation in the other samples appears to be explainable as a product of (rather large) variable amounts of crystal fractionation involving olivine and lesser amounts of clinopyroxene and plagioclase. In general, the Site U1372 basalt is similar to oceanic island lava elsewhere and dredge samples from the Louisville Seamount Trail (Fig. F18). No distinction between shield and postshield stages of volcanism can be made on the basis of ICP-AES results for Site U1372.

Physical properties

Physical property data sets correlate well with the primary distinctions between the sedimentary sequence (Units I–II), units dominated by lava flows (Units III–X and XVII), and volcaniclastic intervals dominated by hyaloclastites (Units XI–XVI). The transition from oxidative alteration in the uppermost 90 m of Hole U1372A to dominantly green alteration indicative of more reducing submarine conditions is reflected in a gradual change from positive a* color reflectance values to negative values downhole (Fig. F16A). Volcaniclastic Units XII and XV, in particular, show markedly lower density, P-wave velocity, and magnetic susceptibility and higher porosity than the surrounding units. Unit XV is also distinctive, with larger a* and b* color reflectance values and an uphole-increasing trend in natural gamma ray counts that contrasts with very low values in Unit XVI, below.

Paleomagnetism

The natural remanent magnetization (NRM) of archive halves from Cores 330-U1372A-4R through 38R was measured using the cryogenic magnetometer at 2 cm intervals (Fig. F19). The intensity measured spans a very broad range, from 3 × 10^–5 A/m to 39 A/m (median = 1.7 A/m), with the lowest values associated with volcaniclastic units. Most of the lava flows and a few volcaniclastic sequences also have relatively high magnetic coercivities, as reflected in high median destructive field (MDF′) values, indicating that a strong peak alternating field is required to demagnetize these samples. This in turn suggests that the majority of the lithologies have retained their original remanent magnetization. Pronounced susceptibility variations mainly reflect variations in magnetic mineral content between various lava flows, volcanic breccia, and finer grained volcaniclastic sediments. The consequent use of nonmagnetic core barrels during Expedition 330 had a positive influence on data quality because there is evidence of only minimal drilling-induced magnetic overprinting on the core samples.

The remanent magnetization directions were calculated and filtered using an automated procedure consisting of typically 6–8 demagnetization steps for each of the 3393 measured 2 cm intervals on the archive halves. This automatic technique selected the best-fit direction on the basis of scatter in the data, the percentage of remanence used in the calculation, and whether the resulting direction is likely to represent the primary magnetization. This fitting procedure allowed us to identify 1364 intervals with the most reliable magnetization directions. The results show a pattern of negative (normal polarity) inclinations in multiple lava flows (Fig. F20). These inclinations are usually consistent with stepwise alternating-field (AF) and thermal demagnetization results from 100 discrete samples (Fig. F21).

Documentation of the paleolatitude of Site U1372 at Canopus Guyot requires a sufficient number of flow units to provide a robust estimate of the time-averaged geomagnetic field at the site. In addition, these units must also be in situ; otherwise, the effects of later tilting or reorientation must be quantifiable from independent information. The presence of predominantly volcaniclastic material in Units XI–XVI and the limited core recovery in this interval make the recognition of in situ lava flow units challenging. In total, 81 lithologic units were defined for the basement sequence in Hole U1372A on the basis of shipboard descriptions. Approximately 20 of these units are in situ cooling units. Even though directions from the conglomerate, breccia, and hyaloclastite units are more scattered in their inclinations, reflecting the fact that some of the basalt pieces recovered from these intervals are random breccia or conglomerate clasts, several intervals may represent in situ lava flows and pods. This would further increase the number of flow units that can be used for determining the paleolatitude of Canopus Guyot around 75–77 Ma. Shore-based studies may provide additional constraints for whether these intercalated units are indeed in situ, time-independent cooling units.

Microbiology

Fifteen whole-round samples were collected for microbiological analysis from more altered rocks and rocks with higher porosities, including unconsolidated sediments (2), volcaniclastic breccia (3), and basaltic lava flows (10). These samples cover nearly all lithologic units recovered from Hole U1372A. All samples were preserved for shore-based DNA analysis, cell counting analysis, and δ³⁴S and δ¹³C isotope analysis. Four samples were used to inoculate culturing experiments with up to 10 different types of cultivation media, and one sample was collected for shipboard analysis to test for possible contamination via fluorescent microsphere analysis.

Site U1373

Rigil Guyot is one of the older seamounts in the Louisville Seamount Trail and has an interpolated age of ~72–73 Ma, only a few million years younger than Canopus and Osbourn Guyots to the northwest (Fig. F1). This seamount is located at 28.6°S, 173.3°W and is part of a small cluster of two guyots and one small seamount to the south (Fig. F22). Rigil Guyot itself consists of a single volcanic center with two small topographic highs, possibly representing eruption of posterosional lavas on the western portion of its summit. During Expedition 330 two sites were drilled on the summit platform of Rigil Guyot: Site U1373 was placed close to the northern shelf edge at 1447.0 m water depth and Site U1374 was placed near the western rift zone at 1559.0 m water depth (Fig. F22). No soft sediment was present at Site U1373, and a hardground entry was made using the rotary core barrel assembly, followed by coring into a ~34 m cover of consolidated sediments and late-stage lava flows and then 31.8 m into igneous basement. Because reentry using a free-fall funnel failed, Hole U1373A had to be abandoned at only 65.7 mbsf. No downhole logging was carried out.

The short sequence of volcanic and sedimentary rocks recovered at Site U1373 (Fig. F23) is part of the subaerial phase in the life cycle of Rigil Guyot, characterized by late-stage volcanism and erosion in a shallow-marine or beach environment. In contrast to Site U1372 on Canopus Guyot and Site U1374 on the western flank of Rigil Guyot, the absence of volcaniclastic deposits containing submarine hyaloclastites reinforces the observation that all Site U1373 lava flow units formed during a later subaerial period following the main constructional phase of Rigil Guyot. From the bottom up, the sequence begins with a massive inflated lava flow of basaltic composition at least 22 m thick, followed by a series of thinner lava flows, all of which have blocky peperitic or brecciated flow tops. Only a thin cover of sediment overlies the igneous basement, with sedimentation being interrupted by the eruption of three autobrecciated basalt lava flows. The conglomerate and breccia that make up this cover are likely formed by “catastrophic” emplacement of two debris flow deposits during a late-stage volcanic period. No evidence for subsidence or any other eustatic change was found at Site U1373, and no significant sequence of soft pelagic sediment was encountered. Only a small sample of sand- to granule-size cuttings with remnants of modern nannofossil and foraminiferal fauna was recovered, which suggests that all pelagic sediment deposited since the Cretaceous has been removed from the seamount summit plain by strong subbottom ocean currents.

Lithostratigraphy and biostratigraphy

Stratigraphic Units I and III at the top of Hole U1373A consist of sediments, whereas the intermediary Unit II is predominantly composed of autobrecciated basalt lava flows with two minor sedimentary interbeds (Fig. F23A). Unit I extends to 9.60 mbsf and is divided into three subunits. Subunit IA is composed of a 15 cm thick multicolor polymict bioclast basalt conglomerate. Cement textures and bioclast composition (planktonic foraminifers, calcispheres, sponge spicules, echinoderms, annelids, algae, bryozoans, and bivalves) indicate that deposition took place in a shallow-marine environment. Subunit IB is composed of a 2.51 m thick matrix-supported brown basalt breccia with a few shallow-marine bioclasts (Fig. F24A). The conglomerate has a heterogeneous clast composition and a calcareous-clayey matrix, interpreted as a mudflow deposit emplaced in a marine shallow-water environment. Subunit IC is a 0.39 cm thick multicolor bioclast basalt conglomerate similar to Subunit IA.

Unit II is predominantly a late-stage volcanic interval that is 6.10 m thick and includes two <50 cm thick sedimentary deposits between three lava flows. The sedimentary deposits consist of fossil-free, heterolithic, multicolor basalt breccia and are interpreted as proximal debris flows.

Unit III is another sedimentary interval that was divided into four subunits. Subunit IIIA is a 1.37 m thick multicolor bioclast-rich basalt conglomerate similar in terms of composition and environment of deposition to Subunits IA and IC. Subunit IIIB is composed of a 6.73 m thick well-sorted multicolor bioclast-rich basalt conglomerate with distinctive cross-bedding and bedding structures and a heterogeneous assemblage of basalt clasts. Cement textures, fossil assemblages, and sedimentary structures indicate that Subunit IIIB was likely deposited in a beach environment. Subunit IIIC is composed of a 5.37 m thick multicolor bioclast basalt conglomerate and a bluish-gray basalt conglomerate. This subunit is believed to have been deposited in a shallow-marine environment. Subunit IIID includes a 4.73 m thick matrix-supported dark multicolor basalt breccia devoid of bioclasts, which we interpreted as a matrix-supported debris flow deposit. Similar sediments were found as interbeds in the underlying volcanic basement sequence.

Thin section investigations of microfossils were conducted on consolidated samples from Units I–III, but no age-diagnostic species could be identified (Fig. F25). Nonetheless, macrofossils of Flemingostrea sp. were found in Subunit IIIB (Fig. F25A), leading to a preliminary age for Subunit IIIB of latest Cretaceous to Miocene.

Igneous petrology

Hole U1373A penetrated a total of 37.9 m of igneous rocks comprising a 6.1 m thick sequence of volcanic breccia (including three autobrecciated volcanic flows) that makes up Unit II in the sedimentary cover and 31.8 m of igneous basement from the base of the sedimentary succession at 33.9 mbsf to the bottom of the hole at 65.7 mbsf (Fig. F23B). Sedimentary Units I and III are breccia and conglomerates composed largely of pebble- to boulder-size basaltic clasts in a sandy matrix. Some basaltic clasts in Unit I have lobate margins with delicate protrusions and therefore cannot have been transported far from their source of origin. They may indicate syndepositional interaction of lava and sediment (i.e., peperite), implying a later phase of volcanism simultaneous with the formation of Unit I (Fig. F24B). Coarse-clastic sedimentation was interrupted by the emplacement of aphyric to olivine-phyric lava flows of Unit II. These flows are almost entirely brecciated, but in places the fragments appear to fit together in a jigsaw-fit texture, which is a common feature of “blocky peperites” and is widely thought to reflect in situ quench fragmentation. The top of the volcanic basement in Unit IV consists of subaerial lava flows of highly olivine-titanaugite-phyric basalt with well-preserved olivine phenocrysts. Unit V consists of aphyric basalt that was also erupted in a subaerial environment, but the base of the lowest flow shows peperite mingling with sediment, as do the flows of Unit VI and the top of Unit VII. Drilling stopped within a >22 m thick inflated sheet flow in Unit VII composed of fine-grained aphyric basalt (Fig. F26A).

The volcanological features of the igneous sequence drilled at Site U1373 suggest lava flowing into an area where water or water-saturated sediment is present but not fully submarine. The emplacement of lava flows in an intertidal or fluvial environment provides a plausible scenario that is consistent with sedimentologic observations. The presence of titanaugite and olivine-titanaugite phenocryst assemblages (Fig. F26B–26D) is characteristic of alkalic basalt.

Alteration petrology

The igneous basement section recovered from Hole U1373A has undergone secondary alteration by low-temperature water-rock interactions or weathering. The alteration of the volcanic rocks, including basalt and hyaloclastite deposits, ranges from slight to high (between 10% and 95%). The >22 m thick massive basaltic lava flow (Unit VII) is relatively well preserved (10% alteration). Two main intervals showing different dominant colors of alteration can be identified and directly related to the oxidation state during the alteration processes (Fig. F27). From the top of Hole U1373A to ~45 mbsf the sequence has a dominantly reddish alteration color, pointing toward an oxidizing environment under likely subaerial conditions. From ~45 to 66.2 mbsf the nearly fresh basalt is faintly greenish, pointing to more reducing conditions related to the submarine environment of lava flow emplacement.

Primary magmatic plagioclase and augite are generally well preserved, both as phenocrysts and in the groundmass. Plagioclase shows minor alteration to sericite/illite in some rocks, but is generally well preserved. Augite is almost always unaltered. Olivine is typically completely altered to iddingsite, hematite, and Fe oxyhydroxide in the uppermost 35 m of hole. From ~35 to 65.7 mbsf the original olivine phenocrysts are largely replaced by green clay, Fe oxyhydroxide, or carbonates (calcite/magnesite). Fresh olivine was found in a clast in Unit I, and moderately fresh olivine phenocrysts occur in Units IV and V. No fresh volcanic glass was encountered in Hole U1373A. Throughout Hole U1373A three main groups of alteration phases can be distinguished: carbonates (Mg calcite and aragonite), clay minerals (saponite, nontronite, glauconite, montmorillonite, and celadonite), and other secondary phases (e.g., zeolites, iddingsite, Fe oxyhydroxide, goethite, and pyrite/chalcopyrite). Numerous vesicles and veins were observed; many are filled with carbonates and clay minerals and other secondary minerals.

Structural geology

Structural features observed at Site U1373 are generally similar to those at Sites U1372 and U1374 in terms of number and types of fractures, veins, magmatic foliations, and geopetals. Geopetal structures are horizontal, indicating that this part of the seamount has not tilted since its formation. Fractures and veins are common in the basaltic lava flows. Fractures are especially abundant in the lowermost Unit VII, a >22 m thick massive lava flow with up to 11 fractures per meter, more than twice the density observed in other fractured rocks at Sites U1372, U1373, and U1374. Unit VII also has moderate to strong macro- and microscopic magmatic foliation, with directions ranging from subhorizontal to subvertical, indicating that this thick flow likely underwent several episodes of lava injection and flow inflation (Fig. F26A).

Geochemistry

Igneous samples from Site U1373 that were analyzed chemically are closely similar in both major and trace element composition to basalt from Site U1372. With the exception of one highly altered lava flow in Unit II, Site U1373 samples are only moderately altered, with weight loss on ignition (LOI) values of <2.40 wt%. Total alkalis (Na₂O + K₂O) and SiO₂ concentrations indicate that all Site U1373 samples are alkalic basalt, except one sample classified as transitional basalt. This sample is a highly olivine-augite-phyric basalt clast from Unit I with high MgO and Ni and likely hosting excess olivine phenocrysts (Fig. F28A, F28B). Its liquid composition thus may have been slightly more alkalic than the composition of the bulk rock. In addition, two other samples of high-MgO, highly olivine-augite-phyric basalt were analyzed (both from Unit IV), and they too appear to contain excess olivine. Evidence that all three high-MgO samples contain excess augite is seen in the variation of Sc with MgO and in their CaO contents and CaO/Al₂O₃ ratios, which are higher than those of their high-MgO counterparts at Site U1372. Overall, Site U1373 data largely overlap with the data arrays of Site U1372 in diagrams of MgO vs. Al₂O₃, Na₂O, and K₂O and in diagrams plotting incompatible elements TiO₂ vs. Sr and Y (Fig. F29). Likewise, the Zr/Ti ratio is within the same restricted range as that found at Site U1372 (Fig. F28C). Despite the general compositional similarity of Site U1373 and U1372 basalt, the two high-MgO samples from Unit IV at Site U1373 have relatively high Sr and Ba contents for their TiO₂ values (Fig. F29). This characteristic does not appear to be caused by alteration, and thus these samples may represent a slightly different magma type than that represented by Site U1372 basalt and the majority of Site U1373 basalt.

Physical properties

The different physical property data sets from Site U1373 samples are mutually consistent and tend to correlate primarily with distinctions between conglomerate units, brecciated lava flows, peperitic basalt, and massive basalt. The brecciated lava flows of Unit II and the peperitic basalt found in Units V, VI, and VII exhibit similar, more consistent physical property values and trends, whereas conglomeratic Units I and III exhibit varying properties depending on the proportion of matrix and clasts. The massive basalt flows of Units IV and VII have consistently high densities, P-wave velocities, porosities, and natural gamma radiation (NGR) but show moderate internal variation in magnetic susceptibility and color reflectance. The variation in color reflectance agrees well with observed alteration colors across all units (Fig. F27A).

Paleomagnetism

The NRM of archive halves from Hole U1373A cores was measured using the cryogenic magnetometer at 2 cm intervals (Fig. F30). The intensities range from 0.08 to 20.46 A/m (median = 2.99 A/m), with the highest values exclusively associated with stratigraphic Unit VII at the base of the hole. Compared to Site U1372 on Canopus Guyot, the basaltic rocks of Site U1373 have more variable MDF′ values, indicating that in some cases (e.g., lithologic Unit 15 in stratigraphic Unit VII) only a small peak alternating field was required to demagnetize these rock types. Other lava flows, however, are expected to have retained their original remanent magnetization, in particular in Units IV–VI (lithologic Units 6–13) that have MDF′ values of 34.9 mT and higher. The more stable nature of these remanent magnetizations is evident in the demagnetization data of both archive-half pieces and discrete samples (Figs. F31, F32). Low-coercivity components are unlikely to be related to drilling-induced magnetic overprinting, which would result in a subvertical component of magnetization, suggesting that the use of a nonmagnetic core barrel during Expedition 330 had a positive influence on data quality. Pronounced susceptibility variations seem to reflect variations in magnetic mineral content between lava flows, volcanic breccia, and also within individual peperitic lava flow tops, as shown for Unit VI and the uppermost part of Unit VII, in particular (Fig. F30C).

Best-fit remanent magnetization directions were calculated for 1436 measured 2 cm intervals on the archive-half cores using an automated procedure. This automatic technique selected the best-fit direction (typically 6–8 demagnetization steps) on the basis of scatter of the data, the percentage of the remanence used in the calculation, and whether the resulting direction was likely to represent the primary magnetization. This fitting procedure allowed us to identify 576 intervals with the most reliable magnetization directions (Fig. F30). These best-fit directions are consistent with stepwise AF and thermal demagnetization results from 34 discrete samples (Fig. F32). The range of inclinations recorded in Hole U1373A is broader and generally shallower than that found in Hole U1372A, with a downhole pattern of mostly negative (normal polarity) inclinations. The dispersed inclination distribution for Hole U1373A may result from including a large number of positive inclinations (reversed polarity) recorded in the uppermost units, which comprise sediments and volcanic breccia, but also by including measurements from Unit VII, which is characterized by lower magnetic stability.

Good agreement between archive-half core and discrete sample data suggests it might be possible to obtain reliable inclinations for 10 in situ cooling units. Augmentation of this data set with results from Site U1374 on the western rift zone of Rigil Guyot is needed to increase the number of in situ cooling units to average out secular variation and achieve a more precise inclination estimate for this ~72–73 Ma seamount.

Microbiology

Five whole-round samples (5–10 cm long) were collected from Site U1373 for microbiological analysis, including samples from sedimentary conglomerate (one), basaltic breccia (one), and aphyric basaltic lava flows (three). All samples were preserved for shore-based cell counting, and four were preserved for shore-based DNA analyses and δ³⁴S and δ¹³C analyses. One sample was used to inoculate culturing experiments with six different types of cultivation media, one sample was collected for shipboard analysis of contamination via fluorescent microsphere analysis, and one sample was used to set up a stable isotope addition bioassay. Fluorescent microsphere counts were practically zero, indicating that the microspheres were not able to penetrate the core and therefore that the chance for microbial contamination is low.

Site U1374

Site U1374 was the third site drilled during Expedition 330 (Table T3) and the second of two sites drilled on Rigil Guyot (Sites U1373 and U1374). This site targeted one of the older seamounts with an interpolated age of ~72–73 Ma, likely only a few million years younger than Site U1372 on Canopus Guyot to the northwest. This drill site was placed near the western rift zone of this guyot, about 10.3 km west of Site U1373 and west of two small, possibly posterosional topographic highs on the western portion of its summit (Fig. F22). Site U1374 was drilled at 1559.0 mbsl. About ~6.6 m of sandy foraminiferal ooze was present, and drilling at Site U1374 was started using a gravity-push approach with little or no rotation of the rotary core barrel assembly. Following careful coring of the soft sedimentary cover, the rotary coring continued into an older ~10.1 m thick sediment layer consisting of consolidated volcanic sandstone, a thin layer of limestone, and grayish basalt conglomerate. Following drilling through this sedimentary cover, 505.3 m of igneous basement was cored. Coring was particularly successful, with a record-breaking 88% average recovery in the igneous basement. Operations were completed with a downhole logging series consisting of a triple combination (triple combo) run, two runs with the Formation MicroScanner (FMS)-sonic tool string, one run with the Ultrasonic Borehole Imager (UBI), and two runs with the third-party Göttingen Borehole Magnetometer (GBM).

As at Site U1373, the upper sedimentary and volcanic sequence at Site U1374 (Fig. F33) is part of a subaerial phase in the evolution of Rigil Guyot. Both sites are characterized by late-stage volcanism, strong erosion, and sedimentation in a shallow-marine or beach environment. Because of the deep penetration at Site U1374, a deeper submarine series of sediment and volcanic rock was recovered on this western rift zone location. From the bottom up, the sequence starts with submarine volcanism, producing a predominant series of volcanic breccia units with an increasing number of in situ lava lobes and more massive flows upward in the sequence. This series is occasionally interrupted by the deposition of fine-grained volcanic sandstone, particularly at the top of the volcanic breccia units, and in the bottom 186 m it is frequently interrupted by a series of intrusive sheets or dikes of mainly aphyric basalt. The entire lower sequence has a normal magnetic polarity, which above ~45 mbsf is overlain by a package of reversely magnetized lava flows and volcaniclastics. This difference in magnetic polarity may indicate a break in volcanic activity and a simultaneous progression to a shallow-marine environment and, in its later stages, a subaerial eruptive environment. This progression is particularly evident in the various breccia types recovered at this site, which range from green hyaloclastite breccia with frothy basaltic clasts (marine) through blocky breccia (shallower marine) to scoriaceous (near sea level or subaerial). A dramatic increase in the thickness of the sedimentary intervals above ~150 mbsf likely indicates the point at which parts of Rigil Guyot started to emerge above sea level and erosion proceeded more rapidly. Both petrography and geochemistry show that the magma that erupted at Site U1374 was alkalic throughout the entire drilled interval.

Lithostratigraphy and biostratigraphy

Sediment at Site U1374 occurs in the uppermost sedimentary cover of Rigil Guyot, in three intervals in a predominantly volcanic basement, and in various basalt breccias (volcanic or sedimentary in origin) as finer grained interclast infill deposits, thin-bedded sedimentary layers, or peperitic intervals. In total, 14 stratigraphic units and subunits were defined in the uppermost 116.45 m of the succession, and five sediment interbeds were identified in the lower igneous basement sequence on the basis of macro- and microscopic observations (Fig. F33).

The uppermost part of the seamount (0–6.64 mbsf) includes a very young sedimentary cover (Unit I) composed of sandy foraminiferal ooze that was deposited in a pelagic environment on the flat-topped seamount. Identification of calcareous nannofossils and planktonic foraminifers shows that this unit formed in the Pleistocene–Holocene. An older sedimentary cover showing subhorizontal bedding (Unit II; 6.64–16.70 mbsf) includes five subunits from top to bottom. Subunit IIA is a multicolor volcanic sandstone with ferromanganese-phosphate encrustations at ~6.64 mbsf (Fig. F34A). Subunit IIB (6.64–13.59 mbsf) is a layered monomict volcanic sandstone without fossils. Subunit IIC (13.59–15.05 mbsf) consists of late Maastrichtian bioturbated volcanic sandstone with abundant gastropods and shell fragments and rare (possible) ammonite fragments. Subunit IID (15.05–15.31 mbsf) is characterized by highly condensed upper Campanian bioclast foraminiferal limestone with ferromanganese-phosphate encrustations, some ammonite fragments, and burrows filled with upper Maastrichtian volcanic sandstone (probably from Subunit IIC). And, finally, Subunit IIE (15.31–16.70 mbsf) comprises upper Campanian (or older) basalt conglomerate with shallow-marine bioclasts (e.g., shell fragments, calcareous algae, and bryozoans) (Fig. F34B).

The underlying volcanic sequence, starting at 16.70 mbsf, is composed of minor basalt lava flows and abundant basalt breccia. The interclast spaces in the basalt breccia are partly filled with finer grained basalt and volcanic sandstone with a local shallow-marine bioclast component. Three thick-bedded sedimentary intervals were identified between 37.60 and 116.45 mbsf. Unit VII (37.60–41.84 mbsf) is the first such interval and is composed of polymict basalt sandstone with abundant vitric fragments and only a few shallow-marine bioclasts, layered volcanic sandstone with rare fossils, and monomict basalt breccia with larger shallow-marine bioclasts. Unit IX (63.67–84.70 mbsf) is the second sedimentary interval and is devoid of fossils. It includes two samples of polymict basalt breccia and an intermediary single interval of volcanic sandstone. The third sedimentary interval occurs at Unit XI (109.87–116.45 mbsf). The upper portion of this interval is volcanic sandstone with few bioclasts, and the lower portion is grayish basalt conglomerate with abundant shallow-marine fossils.

Volcanic deposits below 116.45 mbsf include only minor occurrences of thin-bedded layers of grain-supported, poorly sorted basalt sandstone breccia interpreted as sedimentary intervals. The last occurrences of shallow-water fossils in Hole U1374A were found in two intervals of sedimentary basalt breccia at 256.75–257.49 and 290.32–291.27 mbsf, respectively. These intervals correlate with downhole changes in the nature of the volcanic deposits to decidedly submarine. Contrary to the subhorizontal bedding in the sedimentary cover, bedding orientations in the volcanic basement are locally characterized by moderately dipping ~20°–25° values (Fig. F34C). Because of the small core size it is not obvious whether these dips reflect initial sedimentary dips (e.g., moderate scale cross-bedding) or tilting; the presence of horizontal geopetal structures deeper in the volcanic basement indicates that these deeper layers were not tilted.

Eight lithofacies were defined in the sedimentary cover and thicker sedimentary intervals of the volcanic basement, which permits overall characterization of the environment of deposition at Site U1374. The volcanic basement below 116.45 mbsf is interpreted to have been deposited in a submarine environment on the slope of a former oceanic island. Within this basement the lowermost occurrence of fossil-bearing sediment at 291.27 mbsf possibly corresponds to the first-time shoaling of the island representing today’s Rigil Guyot. Higher in the sequence the volcanic interval between 116.45 and 16.70 mbsf is interpreted to have been deposited in a shallow-marine to subaerial environment on the slope of this former island. A major erosional surface likely occurs at 16.70 mbsf at the base of Unit II, as suggested by changes in the dip of sediment bedding between the volcanic basement and sedimentary cover (see above), and is interpreted to be the result of summit erosion and the original flattening of the drilled guyot. The erosional surface is capped by a shallow-marine basalt conglomerate between 16.70 and 15.31 mbsf and a condensed interval with ferromanganese encrustations from 15.31 to 15.05 mbsf. The age of limestone in this condensed interval of Subunit IID was assigned to the late Campanian and is interpreted to record the initial drowning of Rigil Guyot during the Maastrichtian (Fig. F35). The volcaniclastic sediment deposited on top of this limestone likely represents a record of posterosional volcanism in the latest Cretaceous, as suggested by the occurrence of ammonoid specimen fragments. A second (undated) condensed interval occurs at ~6.64 mbsf and is capped by much younger Pleistocene pelagic sediment. This latter unconformity represents at least 50 m.y. of missing sediment deposition.

Igneous petrology

Hole U1374A on Rigil Guyot penetrated 505.3 m of igneous basement comprising a succession of volcaniclastic breccia capped by lava flows and intruded, in its lower part, by a suite of intrusive sheets or dikes. The igneous sequence (Fig. F33) was divided into 148 lithologic units, which were grouped into 15 stratigraphic units (Units III–VI, VIII, X, and XII–XIX). The basement succession also includes three 4.24, 21.03, and 6.59 m thick sedimentary intervals (stratigraphic Units VII, IX, and XI, respectively). Magmatism recorded at Site U1374 started in a submarine environment and progressed to a shallow-marine and then subaerial environment. This progression is clearly seen in the various breccia types recovered at this site (Fig. F34D–F34G). Abundant blocky breccia probably accumulated as talus deposits through the transportation downslope of volcaniclastic debris that was shed from the fronts of submarine lava flows. In other intervals scoriaceous breccia is probably the product of hydrovolcanic eruptions resulting from the interaction of magma with water (in a shallow submarine environment) or wet sediment, whereas toward the top of Hole U1374A the number of lava flows increases and peperites are found at both the bottom and top margins of these lava flow units. Distinct eruptive packages are often separated by intervals of background sedimentation, five of which were identified at Site U1374. A dramatic increase in the thickness of these intervals in Unit XI indicates the point in time at which parts of the seamount near Site U1374 emerged above sea level and erosion proceeded more rapidly, forming more extensive deposits of basalt conglomerate, basalt breccia, and volcanic sandstone. The phenocryst assemblage in the breccia and lava flows changed from plagioclase-dominated in the lower part of the succession (Units XIV–XIX) to olivine-dominated in the upper part (Units III–XIV), suggesting that the magmas became generally more basic with time (Fig. F36). The magma erupted at Site U1374 was alkalic throughout the drilled interval. The lower 186 m of the succession drilled in Hole U1374A was intruded by sheets of aphyric basalt, which we interpret as dikes (Fig. F34H). Similar rocks were not encountered at higher levels, so these dike units could have extended to levels higher than the present guyot surface, where they were eroded away, or they have been truncated at a yet-unrecognized erosion surface higher in the succession.

Alteration petrology

The entire section of Hole U1374A has undergone secondary alteration by low-temperature water-rock interactions or weathering. The alteration of the volcanic rocks ranges from slight to high (5%–95%), whereas several basaltic lava flows and intrusive sheets are relatively well preserved (10% or less). Two main but overlapping alteration intervals were identified on the basis of different dominant alteration colors, which mainly are related to oxidation state during the alteration processes. From the top of Hole U1374A to ~300 mbsf the sequence has dominantly reddish or brown alteration colors, indicative of oxidizing conditions in subaerial to transitional shallow-marine environments (Fig. F37). Deeper than 370 mbsf the basalt ranges from slightly to highly altered and is predominantly greenish in color, indicative of more reducing conditions related to a more submarine eruptive environment (Fig. F37). Occurrences of gray and relatively unaltered basalt were encountered throughout Hole U1374A.

Plagioclase and augite are generally well preserved as phenocrysts and in the groundmass throughout the entire igneous portion of the core. Plagioclase shows minor alteration to sericite/illite in some rocks but is generally well preserved. Augite is almost always unaltered. Olivine is typically completely altered to iddingsite, hematite, carbonates, and Fe oxyhydroxide, but some sections in the core contain slightly to moderately altered olivine. Some olivine in altered rocks below ~370 mbsf is replaced by green clay, Fe oxyhydroxide, or carbonates (calcite/magnesite). Throughout Hole U1374A three main groups of alteration phases can be distinguished: carbonates (Mg calcite), clay minerals (saponite, nontronite, and celadonite), and zeolites and other secondary phases (iddingsite, Fe oxyhydroxides, goethite, pyrite/chalcopyrite, and thaumasite). The types of zeolite vary from phillipsite in the upper portion of the hole to analcite and gmelinite at depth, indicating a possible thermal alteration gradient. Vesicles, veins, and voids are mainly filled with carbonates and clay minerals below 300 mbsf and with zeolites below 380 mbsf.

Structural geology

Structural features at Site U1374 are dominated by veins (N = 1229), vein networks (N = 515, with 3225 individual veinlets), and fractures (N = 356). Veins are found mostly within lava flows, although veins also occur in larger fragments within volcanic breccia units. The maximum vein width is 25 mm, but most are considerably smaller, with average widths of 0.8 mm. Fractures are also most common in lava units, especially the lowermost 14 m of the hole, with >14 fractures per meter recorded. Structural measurements were also undertaken for intervals with sedimentary bedding (N = 46), 35 geopetals, 18 igneous contacts, 35 vesicle bands, and 80 instances of magmatic flow textures. The orientation of the sedimentary bedding changes significantly downhole from subhorizontal in Units I–II to dips of up to 20°–25° below 16.70 mbsf. Although inclined sedimentary layers are present, all geopetal structures are horizontal (Fig. F35B), indicating that this part of Rigil Guyot has not been tilted since deposition of the geopetal infilling material. Excellent examples of baked contacts and chilled margins were recorded from ~335 to 500 mbsf from a series of steeply dipping sheet intrusions or subvertical dikes. These dikes also contain steeply inclined vesicle bands or flow textures, which also indicate mostly near-vertical magma flow.

Geochemistry

Major and trace element data for igneous samples from Site U1374 overlap considerably with data for Sites U1372 and U1373. However, Site U1374 samples tend to have slightly lower SiO₂ at similar total alkali (Na₂O + K₂O) contents and thus are slightly more alkalic as a group. A diagram of total alkalis vs. SiO₂ shows that most Site U1374 samples are classified as alkalic basalt, but nearly one-third of the samples are basanite or tephrite. No transitional compositions were found, in contrast to Sites U1372 and U1373. Most of the Site U1374 samples are relatively evolved (Fig. F38), with MgO concentrations between 2.78 and 8.54 wt%. Major element and Sc variations indicate that olivine and clinopyroxene were the main mineralogical controls on magmatic differentiation. Incompatible element concentrations display somewhat greater overall variability relative to TiO₂ than that seen for Sites U1372 and U1373, consistent with greater variability in the amount of partial melting or in source composition at Site U1374 (Fig. F39). Despite the compositional overlap and close proximity of Sites U1373 and U1374 (which are located just 10.4 km apart on Rigil Guyot), the rocks from the two sites cannot be correlated and probably represent distinct eruptive events. Likewise, the intrusive sheets occurring in the deeper submarine interval at Site U1374 cannot be correlated with any specific lava flow or eruption package higher up in the drilled sequence.

Physical properties

Physical property characterizations for samples recovered from Site U1374 show clear contrasts between unconsolidated sediments, massive basalt, and volcanic breccia. The intrusive sheets or dikes recovered in the lowermost 186 m have a characteristic physical property signature, distinct from the majority of the basalt flows, lobes, and clasts, and are marked by high NGR, magnetic susceptibility, density, and P-wave velocity and low porosity. More subtle contrasts between olivine- and plagioclase-dominated units are observed in NGR and magnetic susceptibility. The downhole appearance of hyaloclastites at 327 mbsf is marked by a subtle decrease in L* (lightness) and a more marked decrease in P-wave velocity and an increase in porosity. The changes in both P-wave velocity and porosity are more pronounced below 470 mbsf, where a shift in color reflectance (Fig. F37) toward more green and yellow spectra is also observed, correlating with the occurrence of a high proportion of fragments of “frothy” basalt glass.

Paleomagnetism

The NRM of archive-half cores from Hole U1374A was measured using the cryogenic magnetometer at 2 cm intervals (Figs. F40, F41). NRM intensities range from 10^–3 to ~20 A/m (geometric mean = 0.82 A/m), with the highest values associated with lava flows, intrusive sheets or dikes, and basalt clasts in the volcanic breccia/conglomerate units. Similar to Site U1372 on Canopus Guyot but different from the basaltic rocks of Site U1373, the paleomagnetic measurements from archive-half cores at Site U1374 show typically high MDF′ values, indicating that the magnetizations were largely stable and a large peak alternating field was required to demagnetize these rocks. The stable nature of these remanent magnetizations is evident in the demagnetization data of both archive-half pieces and discrete samples, which show little evidence for drilling-induced remanent magnetization, likely because a nonmagnetic core barrel was used. Several lithologic units (e.g., dikes or lava flows) and stratigraphic units are associated with changes in NRM intensity or magnetic susceptibility, and some of these changes (e.g., stratigraphic Unit XV) are also evident from the GBM logs.

Best-fit remanent magnetization directions were calculated for 13,704 measured 2 cm intervals on the archive-half cores using an automated procedure. This automatic technique selected the best-fit direction (typically 6–8 demagnetization steps) on the basis of the scatter of the data, the percentage of the remanence used in the calculation, and whether the resulting direction trends toward the origin of the Zijderveld demagnetization diagrams. This fitting procedure allowed us to identify 5496 intervals with the most reliable magnetization directions. These best-fit directions are consistent with stepwise AF and thermal demagnetization results from 236 discrete samples (Fig. F42). Both archive-half core and discrete sample data reveal a normal polarity zone from ~45 to 522 mbsf, which is remarkably consistent for long intervals containing a wide range of lithologies including basaltic lava flows, dikes, and volcanic breccia. Both data sets also reveal a small interval of reversed polarity magnetization in the uppermost ~45 m of the hole. Interestingly, the range of inclinations recorded in Hole U1374A is narrower and generally steeper than that found in Hole U1373A on Rigil Guyot, though possibly statistically indistinguishable pending further investigation. Both sites also show a group of positive inclinations in their uppermost units, suggesting that sediments and volcanic breccia formed during the latest-stage volcanism and deposition on Rigil Guyot recorded a reversed polarity, in contrast to the normal polarity recorded during the earlier constructional phase of this seamount edifice.

Nineteen in situ cooling units were recognized at Site U1374, providing a consistent estimate of inclination during the formation of Rigil Guyot. Importantly, similar consistency was observed in many of the volcaniclastic units, especially in the lower half of the cored igneous basement, which provided inclination values similar to intercalated lava flows or lobes. In addition, many of the intrusive sheets intruding these volcaniclastic units also have comparable negative inclinations. The good agreement between both archive-half and discrete sample data and among the lava flows, volcanic breccia, and intrusive sheets at Site U1374 suggests that a reliable inclination estimate for Site U1374 is obtainable, particularly when combining these results with the 10 in situ and (likely) contemporary lava flows recovered at Site U1373. Further augmentation of this data set with inclination data from Site U1372 on Canopus Guyot may be considered necessary to increase the number of cooling units sampled in order to better average out secular variations.

Microbiology

Twenty-nine whole-round samples (5–13 cm long) were collected for microbiological analysis at Site U1374. The lithologies of the collected samples include unconsolidated sediments (1), sedimentary conglomerate (2), volcanic breccia (24), and aphyric basaltic lava flows (2). All samples were preserved for shore-based cell counting, DNA analyses, and δ³⁴S and δ¹³C analyses. Eleven samples were used to inoculate culturing experiments with up to seven different types of cultivation media. Growth was detected in samples as deep as 400 mbsf, with media targeting sulfur-oxidizing bacteria and general heterotrophs. Five samples were used to set up stable isotope addition bioassays to determine rates of carbon and nitrogen utilization by subsurface microbes at Rigil Guyot. Two cores were seeded with fluorescent microspheres. Samples from these cores were collected for shipboard analysis of contamination via fluorescent microsphere counts, which revealed that microspheres are released into the drill fluid. However, all counts were reduced to zero after the three sterile seawater rinses to which all microbiology whole-round samples were subjected. This indicates that the microspheres were not able to penetrate the whole-round samples, and therefore the chance for microbial contamination is low in these samples.

Downhole logging

Four tool strings were deployed in Hole U1374A on Rigil Guyot. Three of these tool strings took measurements of natural gamma ray radioactivity, density, neutron porosity, elastic wave velocity, and acoustic and resistivity images of the borehole. The fourth specialized tool string, the third-party GBM, measured three-component magnetic field that may allow determination of the in situ inclination and declination of the eruptive units in the drilled seamount formation. Measurement depths were adjusted to obtain a common wireline log matched depth below seafloor (WMSF) scale across different logging runs. The logged depth interval for Hole U1374A was 128.1–520 m WMSF.

The downhole log measurements (resistivity, density, velocity, and neutron porosity) were used to identify nine log units in Hole U1374A: two in the section covered by the bottom-hole assembly and seven in the volcanic sequences in the open hole interval (Fig. F43). Log Unit I (0–20 m WMSF) shows a spike in gamma ray coinciding with the sedimentary cover in the drilled sequence. Log Unit II (20–128.1 m WMSF) has generally low gamma ray values while still logging inside the drill pipe. Log Unit III (128.1–240 m WMSF) exhibits fluctuating values for density, resistivity, porosity, and velocity and is the first unit below the end of the drill pipe. Log Unit IV (240–278 m WMSF) shows more consistent values for density, velocity, porosity, and resistivity. Log Unit V (278–358 m WMSF) is characterized at its top by a dramatic decrease in resistivity and velocity and an increase in porosity. The uppermost ~12 m of this unit also is characterized by one of the strongest magnetic anomaly patterns recorded with the GBM in both horizontal and vertical field components. Log Unit VI (358–380 m WMSF) exhibits a marked decrease in density, resistivity, and velocity. Log Unit VII (380–469 m WMSF) has relatively consistent density values, higher stable resistivity values, lower porosity values, and higher velocity values. At the bottom of this unit the highest GBM magnetic anomaly was observed over a short interval of ~5 m, with an uncorrected vertical magnetic field anomaly of ~14,000 nT. Toward the bottom of the hole, log Unit VIII (469–490 m WMSF) is characterized by a significant decrease in resistivity, density, and velocity and an increase in porosity, and the lowest log Unit IX (490–507 m WMSF) shows a marked increase in density, velocity, and resistivity and a decrease in porosity. The magnetic field intensities observed with the GBM are the lowest for these bottom two log units.

The GBM was run twice in Hole U1374A, collecting high-quality three-component magnetic data in conjunction with the tool’s rotation history using three optical gyros mounted in the top of the instrument. Negative influences from strong magnetic sources higher up in the tool string were successfully minimized by inserting a truly nonmagnetic aluminum sinker bar immediately above the GBM. At the onset of each deployment the initial orientation of the tool string relative to the Earth’s rotation axis was determined by aligning the GBM with the R/V JOIDES Resolution and then determining the ship’s heading using a newly installed GPS system. With the recorded rotation history of the GBM, the measured vector components can be reoriented (postexpedition) and translated into geographic coordinates, allowing for determination of in situ inclinations and declinations in the seamount formation through modeling of the remanent magnetization. The uncorrected data appear to correlate very well with changes downhole in lithology, as observed in the recovered core and in the standard downhole logging data (Fig. F44). Examples of these are stratigraphic Unit XV, which can be clearly separated from the surrounding intervals in the magnetic data, and Unit XIX, where the magnetic readings of the vertical component drop considerably at ~460 mbsf, coinciding with a change from an interval of hyaloclastites with a high abundance of scoriaceous “frothy” glass fragments to an interval of large jigsaw-fit clasts of aphyric basalt. Both uplog and downlog data are consistent with each other and between two independent runs. Further detailed investigations of the GBM data collected will focus on the separation and identification of the magnetic signals of the different in situ flow units and the determination of both inclination and declination of their NRM, with the intention of estimating the paleolatitude of the Louisville hotspot and its past virtual geomagnetic pole positions.

To conclude, lithologic and structural features were successfully imaged with both the FMS-sonic and UBI tool strings. By combining the FMS and UBI data sets a complete picture of the borehole wall in terms of fractures, clast distribution, amount of alteration, and contrasts in both resistivity and “hardness” can be obtained. The FMS and UBI images are of high quality throughout the borehole, where they accurately reproduce breccia patterns and the outlines of the more solid and massive lava flow units. The images also highlight more clast-rich breccia areas compared to more matrix-dominated zones. Moreover, they show lithologic boundaries when such contacts are not recovered in the core. Because the FMS images can be oriented with respect to north (using GPIT data collected with the same tool string) and because important structural information on key boundaries, fractures, and other features of interest can be deciphered, postexpedition research will allow individual recovered rock pieces (used for paleomagnetic analyses on discrete samples) to also be oriented back to geographic coordinates. This is a key exercise in deciphering the past motion of the Louisville hotspot between 80 and 50 Ma.

Site U1375

Site U1375 on Achernar Guyot (Fig. F45) is estimated to have an age of ~59–63 Ma, and compared to Rigil and Canopus Guyots to the northwest this guyot is relatively small, only 29 km long and 27 km wide. Achernar Guyot is part of a trail of seven small guyots and seamounts that starts with Burton Guyot at the northern end. Knowledge about this extinct volcano will fill an important gap in the age versus distance relationship of the Louisville Seamount Trail and therefore will provide key information to reconstruct past plate motion and the motion of the Louisville hotspot. Site U1375 (1258.0 m water depth) was targeted in the middle of this small edifice away from the guyot’s shelf edges and the thick packages of dipping volcaniclastics on its flanks, in contrast to Sites U1372, U1373, and U1374, which targeted the flanks of these guyots. After Hole U1375A was spudded in ~10 m of soft sediment, drilling became problematic because of instabilities in the uppermost sedimentary cover, most likely caused by the presence of unconsolidated cobble deposits. After Hole U1375A had to be abandoned, Hole U1375B was spudded ~350 m to the northwest; however, severe hole instabilities were also encountered there. To avoid further delays, Hole U1375B was abandoned and a new site was established on Burton Guyot, 91 nmi to the northwest along the Louisville Seamount Trail. In total, three cores were recovered from Holes U1375A and U1375B before the holes were abandoned. The recovered material included ~1.5 m of carbonate-cemented volcanic breccia from Hole U1375A and a 57 cm thick interval of microgabbro (dolerite) from Hole U1375B. No downhole logging could be carried out, and no microbiology samples were taken.

Foraminiferal ooze and volcanic breccia cored in Hole U1375A represent the pelagic cap and an older sedimentary cover at Achernar Guyot, similar to the sedimentary covers at all other sites in the Louisville Seamount Trail. The moderately olivine-augite-phyric microgabbro (dolerite) recovered from Hole U1375B is likely from a large boulder that is part of the sedimentary cover as well, even though no upper and lower contacts were recovered.

Lithostratigraphy and biostratigraphy

Sediment at Site U1375 was mostly restricted to Hole U1375A and represents a pelagic cap and an older sedimentary cover of Achernar Guyot. Two stratigraphic units were defined on the basis of compositional and textural characteristics of the sediment at macro- and microscopic scales. The uppermost part of Hole U1375A (Unit I) was retrieved with only poor (~2%) recovery and a few cuttings retrieved in the core catcher (CC) of Core 330-U1375A-1R. Calcareous nannofossils and planktonic foraminifers observed in the sandy foraminiferal ooze of Unit I display an age range of latest Miocene–Holocene. This young sediment resembles foraminiferal ooze recovered in the uppermost parts of Sites U1372 on Canopus Guyot and U1374 on Rigil Guyot and is interpreted to represent a pelagic cap on top of the drowned seamount. An older sedimentary cover (Unit II) in Hole U1375A occurs between 8.50 and 10.11 mbsf (Fig. F46). From top to bottom this stratigraphic unit includes a ferromanganese-phosphate encrustation at ~8.50 mbsf; a lower to middle Paleocene grain-supported, poorly sorted multicolor basalt conglomerate between ~8.50 and 9.34 mbsf; and an altered monolithic, matrix-supported, poorly sorted multicolor basalt breccia between 9.34 and 10.11 mbsf. The composition and texture of the sediment suggest that Unit II in Hole U1375A includes a hemipelagic interval (Subunit IIA) that was probably deposited after the drowning of Achernar Guyot on top of an older debris flow deposit (Subunit IIB; Fig. F46B). Calcareous nannofossils and planktonic foraminifers observed in Section 330-U1375A-2R-1 (Fig. F47) give a preliminary age of Paleocene for the Subunit IIA breccia, indicating a >55 m.y. interval represented by the unconformity between Units I and II.

Igneous petrology

Hole U1375A was drilled to 11.5 mbsf and recovered 1.5 m of sedimentary rocks containing five types of volcanic clasts. The clast types found in Subunits IIA and IIB include aphyric basalt, moderately olivine-augite-phyric basalt, moderately augite-olivine-plagioclase-phyric basalt, and highly olivine-augite-phyric basalt. Hole U1375B was drilled to 8.5 mbsf and recovered 57 cm of igneous rock. Unit I, the only unit to be defined for Hole U1375B, is composed of moderately olivine-augite-phyric microgabbro (dolerite) with olivine and augite phenocrysts that are >10 mm (Fig. F48).

Alteration petrology

The entire succession recovered from Holes U1375A and U1375B has undergone secondary alteration by low-temperature water-rock interactions or weathering. The overall alteration of the volcanic clasts in sedimentary units from Hole U1375A ranges from slight to high (10%–60%), whereas the moderately olivine-augite-phyric microgabbro (dolerite) from Hole U1375B varies from moderate to high (55%). Plagioclase and augite are generally well preserved as phenocrysts and in the groundmass throughout the entire igneous portion of the core. Olivine is typically completely altered to iddingsite, hematite, Fe oxyhydroxides, and carbonates. Alteration phases are mostly carbonates (Mg calcite), brown clay minerals, and other secondary phases (iddingsite, Fe oxyhydroxides, and goethite). Additionally, the microgabbro from Hole U1375B is characterized by millimeter-thick veins of goethite.

Structural geology

Structural features in Hole U1375A are veins and vein networks in sedimentary clasts and geopetal structures in the surrounding sediments. The geopetals are horizontal, indicating that this part of Achernar Guyot has not been tilted since deposition of the geopetal infilling material. Veins and vein networks are common within the clasts and are as wide as 8 mm, although they are typically much thinner. In Hole U1375B several veins are present in the microgabbro (dolerite). Most of these veins are steeply dipping, with thinner conjugate veins at shallow dips.

Geochemistry

One sample of the Unit I microgabbro from Hole U1375B was analyzed chemically. It is moderately altered and highly evolved and represents one of the most alkalic rocks recovered during Expedition 330. Data for the sample lie in the field of basanite and tephrite in a diagram of total alkalis (Na₂O + K₂O) vs. SiO₂. The microgabbro appears to be the product of crystal fractionation dominated by olivine and, to a lesser extent, augite. It has slightly lower Zr and Y for its TiO₂ content than do igneous rocks from Sites U1372–U1374 and U1376, suggesting that it may represent a different magma type.

Physical properties

Physical property characterization was conducted for material recovered from Holes U1375A and U1375B. The data sets are mutually consistent and fall within the ranges expected, based on the identified lithologies. In Hole U1375A, magnetic susceptibility, bulk density, and NGR all moderately decrease downhole, likely because of a reduction in basaltic clasts, though they may also be affected by the fragmented nature of the recovered material. The 57 cm of microgabbro recovered in Hole U1375B generally has higher magnetic susceptibility and bulk density values and similar NGR values compared to those observed in Hole U1375A. Material from both holes shows overall color reflectance characteristics that are more yellow than blue. In terms of redness versus greenness, the sedimentary rocks of Hole U1375A are consistently more red than green, whereas the single igneous unit from Hole U1375B has a more neutral color.

Paleomagnetism

Although there are no hints that the single ~57 cm long microgabbro unit from Hole U1375B may be an in situ unit, it was nevertheless measured for remanent magnetization. The obtained average inclination for this single rock is 36.3° ± 1.6° (determined from archive-half core data using Fisher statistics). A single discrete sample taken from the same unit was interpreted as having a broadly consistent (reversed polarity) direction.

Site U1376

Site U1376 on Burton Guyot was the fifth site drilled during Expedition 330 (Table T3). This site and seamount have an estimated age of ~64 Ma, slightly older than Site U1375 on Achernar Guyot. Similar to Site U1375, new age data from Burton Guyot will fill an important gap in the age versus distance relationship of the Louisville Seamount Trail, providing key information to reconstruct past plate motion and the motion of the Louisville hotspot. This seamount is one of the smallest volcanoes in the Louisville Seamount Trail, having a base diameter of <30 km. Site U1376 was targeted in the middle of this small edifice (Fig. F49), away from its shelf edges and any packages of dipping volcaniclastics on its flanks; the latter sequences were specifically targeted at Sites U1372, U1373, and U1374. The summit of Burton Guyot is characterized by two small topographic highs on its eastern end. Site U1376 was placed between these highs at ~1503 m water depth. Burton Guyot shows no evidence of tilting.

Hole U1376A was spudded into a sequence of consolidated sediment representing an almost complete section of the sedimentary cover of Burton Guyot, which is ~42 m thick and formed toward the end of the Cretaceous. This ancient cover formed very late in the life cycle of this volcanic island and includes evidence of posterosional or rejuvenated volcanic activity. The sedimentary cover begins from the top with a sequence of volcanic sandstone and breccia with multiple ferromanganese and chalk encrustations that overlies a remarkable ~15 m thick white algal boundstone. This sediment must have formed after the volcanic island was planed off and started to slowly subside. At its base, the sedimentary cover is composed of basalt conglomerate unconformably overlying igneous basement, into which drilling continued for 140.9 m. The igneous basement is composed of a series of submarine volcanic products of predominantly alkalic basalt composition. Average recovery in igneous basement was remarkably high (76%). Operations were completed after a series of downhole logging runs, including a triple combo run, one run with the third-party GBM tool, and two runs with the FMS-sonic tool string.

Record of a posterosional or rejuvenation phase of magmatism at Site U1376 is provided by the volcanic sand and breccia of sedimentary Unit I (Fig. F50). Clasts in these sediments imply at least two magma types. First, some of the sand layers contain fragments of hornblende and biotite, implying the eruption of magma more evolved than that represented by the more basic basement succession. Second, Subunit IC contains olivine-pyroxene aggregates that may be mantle xenoliths. This is supported by the occurrence in Subunit IC of partly resorbed orthopyroxene xenocrysts in basalt clasts with clear reaction coronas, suggesting that the rejuvenated stage magmas were strongly alkaline.

Drilling Hole U1376A into the center of Burton Guyot provided direct access to a deeper portion of the seamount, explaining why no subaerial eruptive products were encountered and why, instead, a sequence of submarine pillow basalts, hyaloclastites, and autobrecciated lava flows was cored (Fig. F50). The presence of olivine and augite phenocrysts in this basalt and the complete absence of plagioclase phenocrysts suggest that the seamount magma was alkaline and more basic than that at Sites U1372, U1373, and U1374, as indicated by most of the drilled volcanic rocks. Geochemical analyses show that the basalt at Site U1376 is slightly less alkaline, although it is still classified as transitional basalt rather than tholeiitic because it contains titanaugite or olivine in the groundmass throughout the entire drilled interval.

Lithostratigraphy and biostratigraphy

Sediments overlying the igneous basement at Site U1376 were divided into two stratigraphic units defined on the basis of compositional and textural characteristics at macro- and microscopic scales (Figs. F50A, F51A–F51D). Unit I represents a younger sedimentary cover that extends from the seafloor to 23.45 mbsf. This cover is mostly composed of monolithic juvenile volcaniclastic deposits that extend from 4.50 to 21.48 mbsf (Fig. F52A). These deposits are interpreted as a possible record of a rejuvenated volcanic stage of Burton Guyot in a hemipelagic or pelagic environment. Other deposits of Unit I include layered volcanic breccia and sandstone, which are interpreted as turbidites and possible hyperconcentrated flow deposits. Four thin (<3 cm thick) ferromanganese crusts occur in the uppermost part of the drilled sequence, which also yielded a minor amount of nannofossil- and foraminifer-bearing chalk (Fig. F51A). Nannofossils found in the upper part of Subunit IA indicate a preliminary age of middle to late Miocene.

Unit II represents an older sedimentary cover of Burton Guyot that extends between 23.45 and 41.93 mbsf. Subunit IIA comprises a 15.15 m thick interval of limestone (classified as boundstone-rudstone; Fig. F51C) composed of abundant red algae and minor amounts of other shallow-marine fossils. This interval is interpreted to represent an algal reef that developed in very shallow marine conditions during subsidence of the drilled seamount. Subunit IIB between 38.60 and 41.93 mbsf is composed of basalt conglomerate with few shallow-marine bioclasts (Figs. F51D, F52B). The conglomerate was emplaced on top of an erosional surface that marks the boundary between the sedimentary cover and underlying volcanic basement of Burton Guyot. No age-diagnostic microfossils were identified in Subunit IB through Unit IV, but molluscan fossils may indicate a later Cretaceous age for Subunit IIB.

Igneous petrology

The 140.9 m basement section cored at Site U1376 on Burton Guyot comprises a succession of basaltic breccia, pillow lava, and massive lava flows. Two stratigraphic units were defined on the basis of different phenocryst content. Unit IV contains mostly olivine phenocrysts and is overlain by Unit III, which contains olivine and augite phenocrysts (Figs. F50B, F51E–F51G). From the bottom up the succession starts with 13.1 m of mostly olivine-phyric basalt breccia, but an interval of highly vesicular aphyric basalt (166.5–167.2 mbsf) on top of this breccia heralds the arrival of a second magma type producing aphyric basalt. The next 31.7 m comprises heterolithic breccia with olivine-phyric and aphyric basalt clasts and thin flows of aphyric basalt. This interval records a period when two types of magma were erupted in the area at the same time. The upper part of this interval (lithologic Unit 26) contains a number of highly vesicular fragments of aphyric and olivine-phyric basalt, which also are oxidized and may provide evidence for a period of shallow-water or subaerial volcanism. Two thin flows of aphyric basalt separated by a 24 cm thick interval of olivine-phyric basalt breccia (lithologic Units 22–24) mark the highest occurrence of aphyric basalt in the recovered eruptive succession (127.57 mbsf).

The uppermost 17.35 m interval of Unit IV is composed of olivine-phyric hyaloclastite breccia containing a high proportion of fresh glass (Figs. F51F, F53C). Unit IV ends at an erosion surface that marks the beginning of Unit III and a change from magma crystallizing olivine alone to a slightly more evolved one that crystallized olivine and augite. Unit III includes a 33.11 m thick massive lava flow (lithologic Unit 15; Fig. F53A, F53B). The presence of pillow lava high in the Unit III succession suggests that most, if not all, of the basement section was erupted in a submarine environment.

Intrusive sheets (dikes) cutting Unit IV represent the last magmatic event recorded in the basement section of Burton Guyot (Fig. F51G). They were not seen in Unit III but may have extended through it. Because there are no aphyric basalt units in Unit III, these dikes cannot be linked directly to any of the recovered volcanic units above its highest occurrence in the top of Unit IV. It is possible, however, that the dikes penetrated Unit III and fed lava flows that have since been removed by erosion. Alternatively, they may have fed aphyric lava flows similar to those in Unit IV and have been truncated at a possible erosion surface between Units III and IV. The presence of what looks like a fragment of a dike at the postulated erosion surface between Units IV and III supports the latter hypothesis.

Unit I, on top of the drilled sequence, comprises four subunits of volcanic sand and breccia representing the products of a phase of volcanism that postdates the erosion and submergence of Burton Guyot. Clasts from Unit I, therefore, are the only certain materials from a posterosional or rejuvenation phase of magmatism that were recovered during Expedition 330. The clasts are mostly composed of fragments of basalt, but some layers are dominated by angular fragments of glass (Fig. F52A) with subordinate grains of plagioclase, augite, altered olivine, hornblende, and rare biotite. Basaltic clasts from Subunit IC contain anhedral crystals of orthopyroxene surrounded by coronas composed of tiny clinopyroxene crystals (Fig. F53D). These are most likely xenocrysts derived from disrupted mantle xenoliths.

Alteration petrology

Overall, the cored succession from Hole U1376A has undergone some degree of secondary alteration by low-temperature water-rock interactions or weathering, but large intervals are only slightly altered. The alteration of the volcanic rocks, consisting of basaltic flows, basaltic breccia, and hyaloclastite deposits, ranges from slight to high (2%–95%). Several basaltic lava flows are relatively well preserved (10% or less).

Core descriptions and thin section observations show that rocks in Hole U1376A are defined by a single overall alteration type typical for submarine environments. From the top of Hole U1376A to the bottom, the sequence displays a greenish color indicative of reducing conditions related to the submarine emplacement environment (Fig. F54). Only minor and sporadic intervals in the uppermost 60 m of core show some reddish/brown alteration. Augite is well preserved as phenocrysts and in the groundmass throughout the entire igneous portion of the core. Some olivine is completely altered to iddingsite, hematite, and Fe oxyhydroxide near the top of the core, but large portions of core contain fresh to slightly altered olivine. Some olivine in the greenish altered rocks is replaced by green clay and carbonates (calcite/magnesite). Overall, three main groups of alteration phases could be distinguished, largely dominated by carbonates (Mg calcite, aragonite, and siderite) and clay minerals (saponite and nontronite). Other secondary phases (iddingsite, Fe oxyhydroxides, hematite, and goethite) are present, and zeolites constitute only a minor amount of the alteration assemblage. Additionally, numerous vesicles and veins were observed that are mainly filled with carbonates and clay minerals.

Structural geology

Structural features at Site U1376 are dominated by veins (N = 1190, with 1489 individual features) and vein networks (N = 280, with 1995 individual veinlets). This site has the highest vein density of any Louisville site, with a maximum density of 39 veins per meter. Veins are also commonly wider than previously observed, with numerous veins between 5 and 10 mm wide, up to a maximum of 30 mm, indicating higher fluid flow than at previous sites. In further contrast to other Louisville sites, veins and vein networks are abundant in breccia and hyaloclastites in addition to rheologically hard lava units, with the highest vein density actually occurring in hyaloclastite units at ~62 and 170 mbsf. The veins are dominantly shallowly dipping and often have subvertical fibrous mineral infills, both of which may indicate subvertical tension within this part of the seamount. The few fractures that are unfilled (N = 55), are present mostly in the rheologically hard lava flows and intrusive sheets. Geopetal structures (N = 26) are overall horizontal, indicating that this part of the seamount has not been tilted since deposition of the geopetal infilling material. Structural measurements were also undertaken for intervals with sedimentary bedding (N = 153) and seven igneous contacts.

Geochemistry

Major and trace element compositions of 13 igneous samples from Site U1376 on Burton Guyot show general similarities to those from Site U1372 on Canopus Guyot and, to a lesser extent, Sites U1373 and U1374 on Rigil Guyot (Figs. F55, F56). However, as a group, the Site U1376 rocks are less alkalic. Alteration is generally moderate, and on a diagram of total alkalis (Na₂O + K₂O) vs. SiO₂ most of the Site U1376 samples are classified as alkalic basalt, whereas four are transitional basalt. Olivine appears to have been the principal control on magmatic evolution. Seven highly olivine-phyric samples have high MgO (between 11.85 and 16.79 wt%) and high Ni contents, likely because they contain excess olivine crystals. However, this high-MgO basalt is distinct from other high-MgO rocks recovered during Expedition 330 in having somewhat lower Al₂O₃ concentrations. In addition, downhole chemical variations indicate that three different magma types are represented among the lava samples from Site U1376, which is generally consistent with the petrographically defined stratigraphic units (Fig. F55). One of the two aphyric dikes encountered in Unit IV is chemically similar to samples of low-MgO olivine-phyric lava from Unit IV, but the other dike has a significantly higher Ba/Y ratio than other Site U1376 basalt.

Physical properties

Physical property characterizations show clear contrasts between lithified volcanic sandstone and conglomerate, carbonates, lava flows, and pervasive volcanic hyaloclastites and breccia. In particular, the boundstone of Subunit IIA is characterized by very low magnetic susceptibility and NGR. These carbonate samples have higher P-wave velocities and lower porosities than predicted on the basis of their densities, reflecting their different chemical compositions when compared to basaltic lithologies. The 33 m thick massive lava flow in Unit III is characterized by a downhole increase in magnetic susceptibility, P-wave velocity, and density within the unit.

Paleomagnetism

The NRM intensity of archive-half core samples from Hole U1376A ranges from 10^–4 to ~10 A/m (geometric mean = 0.4 A/m). The lowest values are associated with the white algal boundstone in Subunit IIA and the hyaloclastite in Unit IV, and the highest values are from the lava flows, intrusive sheets, and basalt clasts in the volcanic breccia of Units III and IV (Fig. F57). The most abrupt variations in NRM intensity and magnetic susceptibility occur at the boundary of Unit I and Subunit IIA, where heterolithic volcanic breccia and volcanic sandstone at the base of Unit I are characterized by NRM intensities and susceptibilities that are roughly three orders of magnitude higher than for the algal boundstone of Subunit IIA. Both NRM intensity and magnetic susceptibility increase systematically with depth through the algal boundstone and into the heterolithic conglomerate of Subunit IIB. Significant NRM and susceptibility variations also occur in the volcanic basement (Units III and IV), with lava flows and dikes typically having NRM intensities in the range of 1–10 A/m. This same range of variations is also apparent in the 33 m thick massive lava flow at the bottom of Unit III (lithologic Unit 15; 72.21–105.32 mbsf). However, magnetization values for the volcanic breccia are quite variable, with the hyaloclastite breccia of lithologic Unit 21 (~114–127 mbsf) having NRM values ranging from >1 to low values of ~10^–3 A/m, similar to that of the weakly magnetized limestone recovered at this site.

Best-fit remanent magnetization directions were calculated for 3947 measured 2 cm intervals on the archive-half cores using an automated procedure (Fig. F58). This automatic technique selected the best-fit direction (typically 6–8 demagnetization steps) on the basis of the scatter of the data, the percentage of the remanence used in the calculation, and whether the resulting direction trends toward the origin of the Zijderveld demagnetization diagrams. This fitting procedure allowed us to identify 1580 intervals with reliable magnetization directions. These best-fit directions are consistent with stepwise AF and thermal demagnetization results from 99 discrete samples (Fig. F59). Both data sets reveal very consistent reversed polarity magnetization throughout the hole, with <1% having negative inclinations.

Eleven in situ cooling units were recognized at Site U1376. Importantly, as at Sites U1373 and U1374 on Rigil Guyot, very similar inclinations were measured for volcaniclastic and sedimentary units at Site U1376 on Burton Guyot, as well as for two intrusive dikes intruding Unit IV. Therefore, the good agreement between archive-half and discrete sample data and among lava flows, volcaniclastic materials, and dikes suggests it is possible to obtain a reliable paleolatitude for Site U1376. Detailed postexpedition measurements are required to better understand the processes causing a consistent magnetization across different lithologies and across the entire recovered seamount sequence.

Downhole logging

Three tool strings were deployed in Hole U1376A at Burton Guyot. Two tool strings took measurements of natural gamma ray radioactivity, density, neutron porosity, and elastic wave velocity and collected borehole resistivity images. The third tool string, containing the third-party GBM, measured the three-component magnetic field of the drilled seamount formation. Measurement depths were adjusted to match across different logging runs, obtaining a WMSF depth scale. The logged depth interval for Hole U1376A was 80.4–182.3 m WMSF. Resistivity, density, compressional wave velocity, and neutron porosity derived from downhole logging measurements were used to identify 13 log units in Hole U1376A: 3 in the section covered by the bottom-hole assembly and 10 in the volcanic sequences in the open hole interval (Fig. F60). These log units correlate with changes from massive basalt flows (stratigraphic Unit III) to units dominated by volcanic breccia and interlayered aphyric and olivine-phyric flow units (stratigraphic Unit IV).

The GBM was run once in Hole U1376A, collecting high-quality three-component magnetic data in conjunction with tool rotation history using three optical gyros mounted in the top of this instrument (Fig. F61). However, GBM data also show that the massive lava flow at the bottom of stratigraphic Unit III is not as homogeneous as it appears in the paleomagnetic data obtained from the recovered cores in Hole U1376A. Additionally, in the unrecovered section of hole between ~130 and 140 mbsf the GBM shows strong variations. During postexpedition work the measured horizontal magnetic component will be split into north and east components, which should provide further insight into these observed variations. Further detailed investigations of the GBM data collected will focus on separation and identification of the magnetic signals of the different in situ flow units and determination of both the inclination and declination of their NRM, with the intention of accurately determining the paleolatitude of the Louisville hotspot and its past virtual geomagnetic pole positions.

Finally, many lithologic and structural features were imaged with the FMS-sonic tool string, in particular highlighting fractures, clast size, and the shape and distribution of massive basalt versus brecciated material. Of particular importance is FMS coverage of the unrecovered section between ~130 and 140 mbsf because this coverage will provide valuable information for reconstructing the lithology for this interval. Because FMS images can be oriented with respect to north and because important structural information on key boundaries, fractures, and other features of interest can be deciphered, postexpedition research will allow individual recovered rock pieces (used for paleomagnetic analyses) to be oriented back to geographic coordinates. This is a key exercise in deciphering the past motion of the Louisville hotspot between 80 and 50 Ma.

Microbiology

Eleven whole-round samples (5–13 cm long) were collected for microbiological analysis. The sample lithologies collected were volcanic sandstone (2), boundstone (2), volcanic breccia (3), and basaltic lava flows (4). All samples were preserved for shore-based cell counting, DNA analyses, and δ³⁴S and δ¹³C analyses. Five samples were used to inoculate culturing experiments with up to 10 different types of cultivation media. Media targeting sulfur-oxidizing bacteria and general heterotrophic bacteria were the most successful, and growth was detected in samples from as deep as 174 mbsf. Two samples were used to set up stable isotope addition bioassays to determine rates of carbon and nitrogen utilization by subsurface microbes at Burton Guyot. One core was seeded with fluorescent microspheres, from which samples were collected for shipboard analysis of contamination via fluorescent microsphere counts. No microspheres were detected on the outside or inside of the whole-round sample, indicating a low likelihood of microbial contamination.

Site U1377

Site U1377 at Hadar Guyot was the sixth and last site drilled during Expedition 330, and with a measured ⁴⁰Ar/³⁹Ar age of 50.1 Ma (Koppers et al., 2011) it is the youngest Louisville seamount drilled and is similar in age to Koko Seamount in the Hawaiian-Emperor Seamount Trail (Fig. F1). Hadar Guyot shows no evidence of tilting and was formed just to the west of the Wishbone Scarp that marks the oceanic crust in that region of the Pacific plate. Hadar Guyot is the smallest seamount cored during Expedition 330, consisting of a single volcanic center with an approximate base diameter of 25 km, and like all of the seamounts drilled it has a flat summit. Site U1377 was placed near the middle of this small edifice (Fig. F62), away from its shelf edges and any packages of dipping volcaniclastics on its flanks. This approach is similar to that used at Sites U1375 and U1376 but contrary to that used at Sites U1372, U1373, and U1374, which targeted the volcaniclastic rocks from the flank sequences in particular. Our two attempts at drilling Site U1377 suffered from instabilities in the uppermost part of the seamount formation. As with drilling at Site U1375 on Achernar Guyot, these instabilities were caused by the predominance of more or less unconsolidated volcanic breccia in the sedimentary cover. As a result, drilling reached only 53.3 mbsf in Hole U1377A (16% recovery) and 37 mbsf in Hole U1377B (39% recovery). No logging could be carried out in these shallow holes.

Despite low recovery, Holes U1377A and U1377B provided enough core material to allow at least a partial reconstruction of the sedimentary cover on top of Hadar Guyot. This cover overlies an igneous basement of trachybasalt, a more evolved lithology than the alkalic basalt drilled at the other sites in the Louisville Seamount Trail (Fig. F63). Both holes start out with nannofossil foraminiferal ooze, followed by an older sedimentary cover including pelagic limestone and volcanic breccia. This middle–upper Eocene foraminiferal limestone contains abundant planktonic foraminifers, ferromanganese encrustations, and rare shallow-marine bioclasts and was likely deposited in a shallow-marine to hemipelagic–pelagic environment. The trachybasalt units in the igneous basement exhibit intervals of pronounced flow banding, suggesting that they formed as massive lava flows or smaller lobate flows. Lower in the sequence of Hole U1377B, however, much smaller individual cooling units with well-preserved curved glassy margins were encountered. These margins are diagnostic of small lobate flows or pillows and emplacement in a submarine environment.

Lithostratigraphy and biostratigraphy

Two units were recognized on the basis of macro- and microscopic observations of the sediment (Fig. F63A). Unit I represents the uppermost sediment of Hadar Guyot and was recovered in Holes U1377A and U1377B. The sediment is composed of nannofossil foraminiferal ooze, which strongly resembles the soft sediment recovered in the uppermost part of Sites U1372 on Canopus Guyot, U1374 on Rigil Guyot, and U1375 on Achernar Guyot. This ooze is considered to reflect recent pelagic sedimentation on top of the drilled seamount. Unit II corresponds to a few cuttings recovered in Section 330-U1377A-3R-CC and 10 small (<20 cm thick) pieces recovered by drilling in situ sediment (in both Holes U1377A and U1377B) and the out-of-sequence material (caused by the partial collapse of Hole U1377B) of Core 330-U1377A-3G. This unit also includes foraminiferal limestone with abundant planktonic foraminifers, some ferromanganese encrustations, and rare shallow-marine bioclasts (e.g., echinoderm fragments) (Figs. F64A, F65) that has been dated as middle–late Eocene. This limestone then seems to overlie an uppermost Paleocene to lower Eocene heterolithic multicolor basalt conglomerate, also with a few ferromanganese encrustations. The matrix of the conglomerate is composed of foraminiferal limestone with abundant planktonic foraminifers and sparse shallow-marine fossils (e.g., echinoderm fragments, larger foraminifers, shell fragments, and a gastropod). Faunal assemblages and sedimentary textures indicate that Unit II at Site U1377 represents one (or several) condensed section(s) likely to have been deposited in a shallow-marine to hemipelagic–pelagic environment on top of Hadar Guyot. On the basis of preliminary age estimates, an unconformity representing ~30 m.y. exists between Units I and II.

Igneous petrology

Holes U1377A and U1377B penetrated 38.2 m and 27.9 m of igneous rocks after entering igneous basement at 15.1 mbsf and 9.1 mbsf, respectively (Fig. F63B). The igneous sequences were divided into 6 lithologic units in Hole U1377A and 18 lithologic units in Hole U1377B. The similarity of the rocks in both sequences led to the definition of only one stratigraphic unit in each hole (Unit III in both cases). The lithology in these two holes is broadly similar, consisting largely of aphyric trachybasalt, in places olivine-rich bands and, in Hole U1377B, intervals containing plagioclase-augite glomerocrysts. In Hole U1377A and the upper part of Hole U1377B the trachybasalt exhibits intervals of pronounced flow banding, suggesting that these parts of the successions formed as massive lava flows or smaller lobate flows (Fig. F64B, F64C). The lower part of the succession in Hole U1377B, however, consists of much smaller (7 cm to 2.08 m) individual cooling units with well-preserved curved glassy margins, diagnostic of small lobate flows or pillows and emplacement in a submarine environment. A curious feature of these margins is that in several instances the volcanic glass between adjacent pillows is seen to connect with the more massive interior of the unit below. It appears that lava in the still-molten interior of a pillow has broken out as a protrusion that filled the space between overlying pillows (Fig. F64D). Alternatively, magma may have been injected into a stack of pillows, but the similarity in appearance between the injected and pillow trachybasalt suggests that, in either case, both were part of the same eruptive event. The presence of glassy pillow margins that are distinct from the glass in the protrusions shows that the pillows must already have had glassy crusts when lava from below protruded into the space between them. In one case, fragments of the pillows are incorporated into the protrusion. Based on the very limited shipboard petrographic and geochemical investigations possible at this late stage of the expedition, it seems likely that the magma represented by the rocks recovered at this site was generally alkalic and intermediate in composition. If postcruise petrographic and analytical studies confirm this, then the rocks recovered at Site U1377 will have the most evolved composition of all rocks drilled during Expedition 330.

Alteration petrology

The rocks recovered from Holes U1377A and U1377B have undergone secondary alteration by low-temperature water-rock interactions and perhaps weathering (Fig. F66). The overall alteration of the volcanic rocks from Hole U1377A ranges from slight to high (10%–75%), whereas the rocks from Hole U1377B vary from moderate to complete (30%–100%). Brown to reddish-brown alteration indicates the prevalence of oxidizing conditions in both holes. Olivine is typically completely altered to white clay minerals in Hole U1377A and to iddingsite and Fe oxyhydroxides in Hole U1377B. Plagioclase is well preserved as phenocrysts and in the groundmass in several lithologic units in Hole U1377B. A few bands of relatively fresh volcanic glass are also present in margins of pillows or lava lobes toward the bottom of Hole U1377B. Alteration phases for both holes are primarily carbonates (Mg calcite, siderite, and ankerite), white clay minerals, and Fe oxyhydroxides (goethite). Fe oxyhydroxides are also particularly abundant in veins in the rocks from both holes.

Structural geology

Structural features at Site U1377 are dominated by veins, vein networks, and vesicle bands. The highest vein density in Hole U1377A is at 15.1–16.6 mbsf (lithologic Unit 1), with 34 veins per meter, whereas most of the recovered intervals have 5–25 veins per meter. In Hole U1377B the maximum vein density is 26 veins per meter, which occurs at 20–21 mbsf (lithologic Unit 2), whereas other recovered intervals typically have lower values between 10 and 24 veins per meter. The veins in both holes have moderate to steep dips, and a single horizontal geopetal structure at 0.85 mbsf in Hole U1377B is evidence that this part of Hadar Guyot has not been tilted since deposition of the geopetal infilling material. Vesicle bands and chilled contacts in Hole U1377B are moderate to steep, with dips ranging from 45° to 90°. Similarly, vesicle bands with either moderate or subhorizontal dips were recovered in Hole U1377A.

Geochemistry

Major and trace element concentrations were measured for one altered (LOI = 3.0 wt%) igneous sample from Unit III of Hole U1377A. This sample is the most siliceous rock analyzed during Expedition 330, with 55.00 wt% SiO₂. It also has the highest Al₂O₃, lowest Fe₂O₃^T (total iron as Fe₂O₃), and second-lowest MgO and CaO concentrations. In a diagram of total alkalis (Na₂O + K₂O) vs. SiO₂, data for the sample fall in the field of basaltic trachyandesite, near the dividing line between alkalic and tholeiitic compositions. However, alteration may have modified the rock’s K₂O and perhaps Na₂O concentrations, whereas incompatible elements less susceptible to alteration are near the high end of values measured for other Expedition 330 rocks. This suggests that the sample represents highly differentiated transitional to alkalic magma that possibly evolved from a composition rather similar to that of most Expedition 330 basalt. However, other characteristics of the sample complicate any simple explanation of its origin. In particular, concentrations of the compatible trace elements Cr, Ni, and Co are anomalously high (686, 421, and 122 ppm, respectively). This unusual combination of characteristics suggests that the sample may represent an evolved magma that was contaminated by a small amount of olivine-rich material from a mush zone or ultramafic wall rock during ascent.

Physical properties

Physical property characterization was conducted for material recovered from Holes U1377A and U1377B. The data sets are mutually consistent and fall within the ranges expected on the basis of identified lithologies. Several distinct intervals of high magnetic susceptibility in excess of 2.5 × 10^–2 SI were observed in the aphyric trachybasalt. In addition, the level of NGR is higher in both Holes U1377A and U1377B than at other Expedition 330 sites. This likely results from a combination of increased alteration and more evolved magma composition at this site. Increased alteration is also seen in the strongly red and yellow color reflectance spectrum in Hole U1377B. Densities and P-wave velocities are consistently lower at this site than they are for lava at earlier sites, possibly reflecting the more evolved composition.

Paleomagnetism

The NRM intensity of archive-half cores from Site U1377 is typically <1 A/m, notably lower than that of other guyots sampled during Expedition 330. This lower value presumably reflects the higher degree of alteration observed. Both holes at Site U1377 had only shallow penetration and, particularly for Hole U1377A, poor core recovery. Nonetheless, samples from both holes appear to have positive inclinations, indicating Southern Hemisphere reversed polarity. Shipboard sampling at these holes was limited because of the short time remaining for shipboard analysis at the end of the expedition (Fig. F67).

Microbiology

Two whole-round samples (8–11 cm long) were collected for microbiological analysis: moderately olivine-phyric trachybasalt from Hole U1377A and aphyric trachybasalt from Hole U1377B. Both samples were preserved for shore-based cell counting, DNA analyses, and δ³⁴S and δ¹³C analyses. The sample from Hole U1377A was used to inoculate culturing experiments with nine different types of cultivation media targeting sulfur- and iron-cycling microbes and general heterotrophic bacteria. Both samples were used to set up stable isotope addition bioassays to determine rates of carbon and nitrogen utilization by subsurface microbes at Hadar Guyot.

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