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doi:10.2204/iodp.proc.330.105.2012 SedimentologyAt Site U1374 on Rigil Guyot sediment occurs in (1) the uppermost sedimentary cover of the seamount, (2) three intervals within the top of the volcanic basement, and (3) finer grained interclast infill deposits, thin-bedded sedimentary layers, or peperitic intervals deeper in the volcanic basement in intervals mainly consisting of basalt breccia (volcanic or sedimentary in origin). Five stratigraphic units were defined on the basis of stratigraphic relationships and the compositional and textural characteristics of the sediment at macroscopic and microscopic scales (Fig. F4):
Unit I
Stratigraphic Unit I represents the youngest sedimentary cover found at Site U1374 and extends downhole from the seafloor to the older sediment cover of underlying Unit II at 6.64 mbsf (Fig. F4). This unit’s lower boundary was defined by the occurrence of ferromanganese encrustations on top of underlying Unit II (see below). Its age was constrained by foraminiferal and nannofossil assemblages (see “Paleontology”). Unit I is composed of sandy foraminiferal ooze with local occurrences of reworked glass fragments and pumice (Fig. F5A). Although this unit shows severe drilling disturbance (possible entire loss of original beddings), paleontological observations (see “Paleontology”) indicate that the ooze likely represents a recent winnowing residue of sediment originally richer in finer grained particles and nannofossils. The lowermost part of Unit I (6.49–6.64 mbsf) is composed of drilling breccia that includes pumiceous gravel and poorly consolidated sandy foraminiferal ooze similar in composition to the overlying ooze. Smear slide observations (Samples 330-U1374-1R-1, 75 cm; 1R-2, 75 cm; 1R-3, 75 cm; 1R-4, 75 cm; 1R-5, 15 cm; and 1R-5, 50 cm) showed that the foraminiferal ooze includes some fresh glass, clinopyroxene, plagioclase, and lithic fragments. Unit II
Stratigraphic Unit II represents an older 10.06 m thick sediment cover that extends between 6.64 and 16.70 mbsf. Its lower boundary is defined by the first occurrence downhole of a basalt lava flow in underlying Unit III. Unit II includes a complicated sediment assemblage that ranges in composition from vitric sandstone to foraminiferal bioclast limestone to basalt conglomerate with bioclasts. Distinct sediment compositions, textures, and ages (see “Paleontology”) allowed this unit to be divided into five subunits (Fig. F4). The basalt clast compositions in Units II, IX, and XI indicate that Unit II is distinct from other sediments downhole; Unit II includes basalt clast Types 1–3, whereas Units IX and XI generally lack Type 2 and contain local occurrences of Types 4–11 (Table T2; see also U1374A.DOC in CHAR in SEDIMENT in “Supplementary material”). Estimates of grain size and roundness with depth in Units II, VII, IX, and XI (see “Sedimentology” in the “Methods” chapter [Expedition 330 Scientists, 2012a]) are illustrated in Figure F4 and provided in a supplementary table (see U1374A.XLS in SIZE in SEDIMENT in “Supplementary material”). Subunit IIA
Stratigraphic Subunit IIA is 6.95 m thick and extends between 6.64 and 13.59 mbsf (Fig. F4). Its lower boundary was not recovered, and the Subunit IIA/IIB boundary was defined on the basis of the appearance of bioturbation and bioclasts in underlying Subunit IIB (see below). Subunit IIA is composed of fossil-barren multicolor volcanic sandstone (Fig. F5B). An age of deposition between late Maastrichtian and Pleistocene was based on stratigraphic relationships and paleontological dating of overlying and underlying sediments (see “Paleontology”). The occurrence of ferromanganese encrustations at the top of Subunit IIA indicates existence of a hardground surface. The sandstone of Subunit IIA is essentially composed of altered glass fragments with rare occurrences of fresh glass. It is very well sorted and is characterized by several thin-bedded intervals with subhorizontal layering, normal grading, and cross-bedding structures. The size of the largest volcanic clasts (observed over 10 cm intervals) ranges between very coarse sand and granule size. A fining-upward sequence may occur in the lowermost part of Subunit IIA (Fig. F4). The average roundness of volcanic clasts per 10 cm interval is very angular. Microscopic observations of four thin sections (Samples 330-U1374A-2R-1, 35–38 cm [Thin Section 107]; 2R-2, 106–109 cm [Thin Section 108]; 2R-3, 70–73 cm [Thin Section 109]; and 2R-4, 10–13 cm [Thin Section 110]) indicated that the glass fragments in the sandstone are highly vesiculated. Some are pumiceous and most contain euhedral olivine crystals. The very high angularity of the glass suggests absent, or very limited, reworking. Sample 330-U1374A-2R-2, 106–109 cm (Thin Section 108), includes remnants of fresh glass and olivine, some of which contain fresh, small (<10 µm) melt inclusions. At least two generations of cement (possible zeolite and later calcite) were observed. Subunit IIB
Stratigraphic Subunit IIB is 0.19 m thick and extends between 13.59 and 13.78 mbsf (Fig. F4). Its lower boundary was not recovered, and the Subunit IIB/IIC boundary was based on the lower bioclast content and distinct sedimentary structures in underlying Subunit IIC (see below). Subunit IIB is composed of multicolor volcanic sandstone with common bioclasts. It contains two distinct lithologies. The first consists of cemented darker volcanic sandstone with abundant vesiculated glass fragments devoid of phenocrysts. The second is a micrite-bearing lighter volcanic sandstone with abundant calcitized subeuhedral olivine, minor vesiculated glass fragments devoid of phenocrysts, and rare subrounded basalt grains (Fig. F5C). The lighter micrite-bearing sandstone is abundantly bioturbated, whereas the darker cemented sandstone contains only few boreholes. Because of the limited extension or recovery of Subunit IIB, the stratigraphic relationship between the two sandstones is unclear. However, the composition of burrow infills in the darker sandstone suggests that the olivine-rich sandstone may represent a younger phase of sedimentation. The uppermost part of Subunit IIB composed of darker sandstone possibly represents an intraclast. Both sandstones include shallow-marine bioclasts (gastropods, shell fragments, and echinoderms). The age of Subunit IIB was restricted to the late Maastrichtian on the basis of fossil ages in the darker and lighter sandstones (see “Paleontology”) and inferred stratigraphic relationships with underlying Subunits IIC and IID. Thin section observations (Sample 330-U1374A-2R-4, 27–32 cm [Thin Sections 111, 134, and 134A]) showed that the micrite in the olivine-rich sandstone contains abundant recrystallized radiolarians and foraminifers. Glass fragments in Subunit IIB are entirely altered and very angular, but they exhibit a slightly more rounded habit than the glass fragments of overlying Subunit IIA. Geopetal structures found in gastropods are consistent with the undisturbed (horizontal) orientation of the lower occurrence of darker volcanic sandstone. Pores in this sediment were filled with granular sparite. Subunit IIC
Stratigraphic Subunit IIC is 1.27 m thick and extends from 13.78 to 15.05 mbsf (Fig. F4). Its lower boundary corresponds to an erosional contact (possibly an angular unconformity) with the underlying foraminiferal bioclast limestone of Subunit IID. Subunit IIC is composed of multicolor volcanic sandstone with few shallow-marine and hemipelagic bioclasts (Fig. F5D). The sandstone resembles that of Subunit IIA and is essentially composed of highly vesicular glass fragments with few euhedral olivines. On the other hand, the sedimentary textures in Subunit IIC are distinct from those of Subunit IIA, with the occurrence of poor, diffuse subhorizontal layering and cross-bedding structures. Bioclasts in Subunit IIC are predominantly composed of shell fragments and gastropods, with rare ammonite fragments (see inset in Fig. F5D; see also “Paleontology”). Maximum clast size (very coarse sand to granule size) and clast roundness (angular to very angular) are similar to those of Subunit IIA. The late Maastrichtian age of deposition of Subunit IIC was constrained by nannofossil assemblages found in a burrow infill of underlying Subunit IID (Fig. F6; see also “Paleontology”). Observation of one thin section (Sample 330-U1374A-2R-4, 88–91 cm [Thin Section 112]) showed that the pores of the sandstone in Subunit IIC include three generations of cement, with two fringing fibrous cements (zeolite and calcite) and a later granular cement filling the center of the interstitial spaces. Fossil fragments are entirely replaced by granular cement. Pervasive alteration of glass fragments and olivine was observed. Subunit IID
Stratigraphic Subunit IID is composed of a 0.26 m thick foraminiferal bioclast limestone that extends from 15.05 to 15.31 mbsf (Figs. F4, F5E). Although the contact between Subunits IID and IIE was not observed, the lower boundary of Subunit IID was defined on the basis of the increased occurrence of basalt clasts and the reduced abundance of foraminifers in underlying Subunit IIE (see below). Fossils in Subunit IID are composed of abundant shallow-marine bioclasts (shell fragments, red algae, gastropods, echinoderms, and bryozoans), benthic and planktonic foraminifers, and small ammonite fragments. Most of the larger bioclasts are broken and rounded, whereas the foraminifers do not display evidence of substantial reworking. Well-rounded basalt pebbles were observed to have a maximal size larger than that of volcaniclasts in Subunits IIA, IIB, and IIC and similar to that of basalt clasts in underlying Subunit IIE (Fig. F4). The uppermost part of Subunit IID includes a condensed interval composed of a complicated assemblage of basalt clasts, different types of bioclast-rich limestone, and ferromanganese-phosphate encrustations (Fig. F6). Also, the interval contains burrows filled with the late Maastrichtian glass-rich sandstone of overlying Subunit IIC (“Snd” in Fig. F6). A foraminiferal assemblage found in a limestone sample (“Lm3” in Fig. F6) provided a late Campanian (~75.7–75.2 Ma) age of deposition (see “Paleontology”). Also, preliminary observations of the foraminiferal assemblage indicated that the sediment of Subunit IID was deposited at hemipelagic depths. Sedimentologic and paleontological observations indicate a significant time gap (~6–11 Ma) between emplacement of the volcano-sedimentary sequences below and above Subunit IID. The chemical composition of limestone Sample 330-U1374A-3R-1W, 66.5–70.5 cm (Fig. F6), was measured by qualitative X-ray fluorescence (XRF) analyses using the shipboard Thermo Scientific Niton XL3 Analyzer. XRF analyses showed that Horizons 2 and 3 are characterized by high iron, manganese, and phosphorous contents. These data indicate the occurrence of several ferromanganese-phosphate encrustations, suggesting several hiatuses in the uppermost interval of Subunit IID (Fig. F6). Lm2 in Figure F6 contains high iron, manganese, and phosphorus, which likely reflects phosphatization of the sediment and suggests the presence of phosphatic hardground(s) in Subunit IID (see XL3_EVAL.PDF in XRF in “Supplementary material”). Thin section observations (Sample 330-U1374A-3R-1, 70–72 cm [Thin Sections 113, 128, 128A, and 128B]) showed that the sediment matrix of Subunit IID is predominantly composed of micrite. However, granular, syntaxial, and dogtooth cement as well as vadose silt encountered in interstitial pores or dissolved shell fragments indicate a vadose environment of cementation (Fig. F7A) (Flügel, 1982). Subunit IIE
Stratigraphic Subunit IIE is 1.39 m thick and extends from 15.31 to 16.70 mbsf (Fig. F4). Its lower boundary corresponds to the top of an underlying basalt lava flow in Unit III (see “Igneous petrology and volcanology”). Subunit IIE is composed of grain-supported, poorly sorted grayish basalt conglomerate with bioclasts (Fig. F5F). The intercobble and interboulder spaces, as well as cracks in the underlying basalt lava flow, are filled with finer grained basalt sandstone-conglomerate with abundant shallow-marine bioclasts (e.g., red algae, shell fragments, and annelids), light gray micrite, and cement. The orientation of the shell fragments defines subhorizontal layering throughout Subunit IIE. Volcaniclasts include altered vesicular to nonvesicular glass fragments with plagioclase. These fragments were not observed in overlying Subunits IIA, IIB, or IIC and suggest a distinct volcanic source for the Subunit IIE fragments in the volcanic basement. The maximum clast size in Subunit IIE is slightly larger than that in overlying Subunit IID. The average roundness of clasts in Subunit IIE ranges between very angular and subrounded. A late Campanian or older age of deposition for this subunit was defined on the basis of paleontological determinations in Subunit IID and inferred stratigraphic relationships. Similar to Subunit IID, thin section observations (Sample 330-U1374A-3R-2, 65–68 cm [Thin Section 117]) indicated the occurrence of dripstone cement and geopetal structures defined by vadose silt and sparry calcite cement (Fig. F7B, F7C). These observations suggest that cementation of Subunit IIE occurred in a vadose environment (Flügel, 1982). Unit VII
Stratigraphic Unit VII is a 4.24 m thick sedimentary interval that extends from 37.60 to 41.84 mbsf within a predominantly volcanic sequence. Its upper boundary is defined by a transitional contact with the volcanic basalt breccia of Unit VI. Its lower boundary is defined by the occurrence of volcanic basalt breccia in underlying Unit VIII. Unit VII is composed of volcaniclastic deposits with few bioclasts. A high content of vitric fragments, the occurrence of bioclasts, and a lack of micrite are characteristic features of Unit VII. This unit was divided into three subunits on the basis of distinct sediment compositions and textures (Fig. F4). A late Campanian or older age of deposition was determined on the basis of faunal assemblages found in the sediment above and stratigraphic relationships. Subunit VIIA
Stratigraphic Subunit VIIA is 1.19 m thick and extends from 37.60 to 38.79 mbsf (Fig. F4). Its lower boundary was not recovered, and the Subunit VIIA/VIIB boundary was based on the distinct sedimentary structures and sediment composition of underlying Subunit VIIB (see below). Subunit VIIA is composed of grain-supported, very well sorted multicolor basalt sandstone with few shallow-marine bioclasts (e.g., red algae, shell fragments, annelids, gastropods, and echinoderms) (Fig. F5G). The sediment is predominantly composed of vitric and basalt grains. It is entirely cemented and characterized by pervasive steep layering with possible large-amplitude cross-beds. The maximal clast size of Subunit VIIA ranges from very coarse sand to granule size, smaller than that of the volcanic breccia above and below (Units VI and VIII, respectively). The average clast roundness ranges between very angular and angular. Thin section observations (Sample 330-U1374A-7R-4, 26–30 cm [Thin Section 125]) indicated that the sediment includes at least three generations of cement, with (in order of formation) fibrous zeolite, fibrous calcite, and sparry calcite. Subunit VIIB
Stratigraphic Subunit VIIB is 2.69 m thick and extends from 38.79 to 41.48 mbsf (Fig. F4). Its lower boundary was not recovered, and the Subunit VIIB/VIIC boundary was constrained by the appearance of multicolor basalt breccia with bioclasts in underlying Subunit VIIC (see below). Subunit VIIB is composed of grain-supported cemented multicolor volcanic sandstone with few large basalt clasts (Fig. F5H). The sandstone is predominantly composed of fragments of poorly to moderately vesicular altered glass with abundant plagioclase. A shallow-marine bioclast component occurs in the lower part of Subunit VIIB. The sandstone of Subunit VIIB is characterized by a steeply inclined, consistently layered structure without evidence of large-amplitude cross-beds. The size of the grains is bimodal, with few basalt pebbles embedded in a very well sorted, very coarse sandstone (Fig. F4). The average roundness of the clasts ranges from very angular to subrounded. Thin section observations (Samples 330-U1374A-7R-4, 112–114 cm [Thin Section 126]; 7R-5, 14–18 cm [Thin Section 127]; 8R-1, 53–57 cm [Thin Section 129]; and 8R-2, 111–115 cm [Thin Section 130]) showed that the cement of the sandstone resembles that of the overlying basalt breccia (Subunit VIIA), with occurrences of fibrous zeolite, fibrous calcite, and sparry calcite. Subunit VIIC
Stratigraphic Subunit VIIC is 0.36 m thick and extends from 41.48 to 41.84 mbsf (Fig. F4). Its lower boundary corresponds to a transitional change to the underlying volcanic interval composed of basalt breccia (Unit VIII). Subunit VIIC consists of cemented grain-supported multicolor basalt breccia with bioclasts (Fig. F5I). The bioclasts include a shallow-marine fauna similar in composition to that observed in sediments above (Subunits IIB–IID and VIIA–VIIB). The larger clasts are composed of basalt cobbles, and the average clast roundness is angular to subangular. Thin section observations (Sample 330-U1374A-8R-3, 19–22 cm [Thin Section 13]) showed that the breccia includes rare foraminifers. Interstitial cement is composed of granular calcite. Unit IX
Stratigraphic Unit IX is a 21.03 m thick sedimentary interval that extends between 63.67 and 84.70 mbsf within a predominantly volcanic sequence. Although the upper boundary of Unit IX was not recovered, it is believed to correspond to a lithologic change from reddish monolithic basalt breccia (Unit VIII) to multicolor heterolithic basalt breccia (Unit IX). The lower boundary of Unit IX was defined by the occurrence of a basalt lava flow at the top of underlying Unit X (see “Igneous petrology and volcanology”). Unit IX is essentially composed of volcaniclastic deposits, and its characteristic features include a lack of micrite and fossils and rare occurrences of sediment clasts (vitric sandstone). Late-stage sedimentary infills are common within the coarser breccia deposits (Fig. F8). Distinct sediment compositions and textures allowed Unit IX to be divided into three subunits (Fig. F4; Table T2). Subunit IXA
Stratigraphic Subunit IXA is 11.11 m thick and extends from 63.67 to 74.78 mbsf (Fig. F4). Its lower boundary is marked by a sharp change downhole to the multicolor volcanic sandstone of Subunit IXB (see below). Subunit IXA is composed of heterolithic cemented multicolor volcanic breccia with few altered volcanic fragments (Fig. F5J). The compositional range of the larger basalt clasts is wider than that observed in Unit II (Table T2). The breccia is grain supported, poorly sorted, and devoid of fossils. Its maximal clast size ranges between pebble and cobble size, with very angular to subrounded clasts (Fig. F4). Local changes in grain sorting and the orientation of contact with the underlying finer grained sediment of Subunit IXB outline the occurrence of steep layering throughout Subunit IXA (Fig. F5J). Microscope observations (Samples 330-U1374A-13R-1, 50–53 cm [Thin Section 137]; 14R-1, 8–10 cm [Thin Section 138]; and 14R-1, 57–60 cm [Thin Section 139]) indicated that the breccia includes several generations of cement (fibrous zeolite, fibrous calcite, and sparry calcite). Subunit IXB
Stratigraphic Subunit IXB is 3.42 m thick and extends from 74.78 to 78.20 mbsf (Fig. F4). Although not recovered, its lower boundary was defined on the basis of the occurrence in Subunit IXC of basalt breccia (see below). Subunit IXB is composed of fossil-barren multicolor volcanic sandstone with few larger basalt clasts (Fig. F5K). The sandstone is predominantly composed of fragments of plagioclase-bearing variously vesiculated altered glass. The basalt clasts have a clast type composition distinct from those of overlying Unit II and Subunit IXA (Table T2). Subunit IXB has a bimodal grain size distribution with pebbly, cobbly basalt clasts embedded in a coarse sandstone. The size of the largest basalt clasts is similar to that of other sediments in Unit IX (Fig. F4). Clast roundness in Subunit IXB ranges between very angular and angular. Subunit IXB is characterized by a diffuse, steep layering in the finer grained sediment, which is locally enhanced by a higher concentration of basalt pebbles (Fig. F5K). Thin section observation (Sample 330-U1374A-14R-3, 59–63 cm [Thin Section 140]) suggested that Subunit IXB shares a cementation pattern similar to that of overlying Subunit IXA, with occurrences of at least three types of cement (fibrous zeolite, fibrous calcite, and sparry calcite). Subunit IXC
Stratigraphic Subunit IXC is 6.50 m thick and extends from 78.20 to 84.70 mbsf (Fig. F4). Its lower boundary corresponds to a contact with a basalt lava flow in the upper part of Unit X (see “Igneous petrology and volcanology”). Subunit IXC is composed of multicolor basalt breccia very similar in terms of composition and texture to the sediment observed in Subunit IXA (Fig. F5L). Unit XI
Stratigraphic Unit XI is the last downhole occurrence of a thick (>1 m) sedimentary interval at Site U1374. The unit is 6.58 m thick and extends from 109.87 to 116.45 mbsf. Its upper boundary is gradational with the basalt breccia of Unit X. Its lower boundary is defined by the occurrence of a peperitic basalt in underlying Unit XII (see “Igneous petrology and volcanology”). The composition of basalt clasts found in Unit XI is slightly distinct from those of other sedimentary intervals above (Table T2). Distinct sediment compositions and textures allowed Unit XI to be divided into two subunits (Fig. F4). Subunit XIA
Stratigraphic Subunit XIA is 0.41 m thick and extends from 109.87 to 110.28 mbsf (Fig. F4). Its lower boundary corresponds to a sharp, steep contact with underlying multicolor basalt conglomerate with bioclasts (Subunit XIB; see below). Subunit XIA includes multicolor volcanic sandstone similar in terms of composition and texture to the volcanic sandstone found in Subunit IXB (Fig. F5M). However, the lower part of the sandstone in Subunit XIA includes minor amounts of shallow-marine bioclasts. Microscope observations in Subunit XIA (Sample 330-U1374A-21R-3, 80–84 cm [Thin Section 151]) showed that the sediment is predominantly composed of poorly to moderately vesicular altered glass fragments with plagioclase. Cement precipitated in interstitial pores comprises fibrous zeolite, fibrous calcite, and sparry calcite. These observations further support the high compositional and textural similarity of Subunits XIA and IXB. Subunit XIB
Stratigraphic Subunit XIB is 6.17 m thick and extends from 110.28 to 116.45 mbsf (Fig. F4). Its lower boundary was defined on the basis of the occurrence of a peperitic basalt lava flow in the uppermost part of underlying Unit XII (see “Igneous petrology and volcanology”). Subunit XIB is composed of grayish basalt conglomerate with bioclasts. It is compositionally and texturally similar to the conglomerate found in Subunit IIE (Fig. F5N). However, the occurrence of larger shallow-marine bioclasts in Subunit XIB is indicative of very limited reworking of some of the fossils prior to final deposition. Thin section observations (Samples 330-U1374A-21R-4, 76–80 cm [Thin Section 152]; 22R-1, 2–6 cm [Thin Sections 153 and 165]; 22R-2, 1–5 cm [Thin Section 154]; and 22R-3, 21–25 cm [Thin Section 155]) indicated that the conglomerate includes a bioclast faunal assemblage similar to that of Subunit IIE, with abundant shell fragments and red algae; common bryozoans, annelids, and echinoderms; and rare foraminifers. The sediment in Subunit XIB also includes pendant, meniscus, and dogtooth cement, as well as horizontal geopetal structures (Fig. F7D). Similar to cement patterns found in Subunit IIE, these observations are indicative of a vadose environment of cementation for Subunit XIB. Sediments in volcanic sequencesApart from Stratigraphic Units I, II, VII, IX, and XI, defined above, additional occurrences of sediment were encountered at Site U1374 in peperitic intervals, thin-bedded sedimentary intervals, or interclast infill deposits in the predominantly volcanic sequences. It is important to note that distinguishing between sedimentary and volcanic deposits was very difficult in large intervals of basalt breccia because both types of deposit can have similar compositional and textural characteristics. We present below only the most obvious sediment occurrences that were identified in predominantly volcanic sequences on the basis of their matrix-supported texture and/or finer matrix grain size. Three peperitic intervals were observed at Site U1374:
In addition, several thin-bedded sedimentary intervals were observed in the volcanic basement:
Fine-grained sediments consisting mostly of basalt and vitric sandstones were observed throughout the predominantly volcanic sequence of Hole U1374A as infill deposits between larger clasts of basalt breccia (Fig. F8C–F8E). Locally, geopetal structures were formed by the infill of sediment and later calcite cement (see point 2 in Fig. F8C). Rare shallow-marine bioclasts and micrite were locally found in infill deposits down to 121.28 mbsf (Fig. F4). Interpretation of lithologies and lithofacies at Site U1374Eight lithofacies were defined in Units I, II, VII, IX, and XI, permitting overall characterization of the environment of deposition of the sediment and stratigraphic development at Site U1374 on Rigil Guyot (Figs. F4, F5). Lithofacies 1 corresponds to the grain-supported grayish basalt conglomerate with bioclasts of Subunits IIE and XIB. It is characterized by the occurrence of (1) rounded basalt clasts and bioclasts, (2) abundant shallow-marine bioclasts, and (3) cement textures indicative of a vadose environment. These observations are strong indications that the sediment of Lithofacies 1 was deposited in a shallow-marine, subtidal to intertidal environment. Lithofacies 2 comprises the multicolor basalt breccia of Subunits VIIA and VIIC. This lithofacies is characterized by the occurrence of shallow-marine fossils and fibrous zeolite-calcite cements and the lack of micrite content. These characteristics suggest that Lithofacies 2 reflects emplacement of debris flows in a shallow-marine environment. Lithofacies 3 includes the multicolor volcanic sandstone of Subunits IIB and IIC. It is characterized by (1) the occurrence of hemipelagic to shallow-water fossils; (2) an essentially monomict composition with highly vesicular, partly reworked vitric fragments with olivine; (3) extensive bioturbation; (4) diffuse layering with large-amplitude cross-bedding structures (Subunit IIC only); and (5) fibrous cement. These observations suggest that Lithofacies 3 represents subaerial to shallow-marine volcanic products deposited or reworked in a shallow-marine to hemipelagic environment. More abundant bioclasts and burrows as well as occurrences of micrite were observed in Subunit IIB, which suggests deposition in a low-energy, possibly hemipelagic environment. Lithofacies 4 corresponds to Subunits VIIB and XIA and is characterized by the occurrence of multicolor volcanic sandstone with few bioclasts. The volcanic sandstone of Lithofacies 4 is compositionally and texturally distinct from that of Lithofacies 3 in that (1) it is mostly composed of poorly to moderately vesicular vitric fragments with abundant plagioclase, and (2) its texture is characterized by strong to diffuse layering with few basalt pebbles. The consistency of layering and the absence of cross-beds and scour structures, as well as gradational transitions between well-layered and poorly layered intervals, suggest that sediment in Lithofacies 4 was deposited in a hyperconcentrated flow(s), a process hydrodynamically intermediate between a dilute stream flow (such as turbidites) and a debris flow (Smith and Lowe, 1991). This mode of emplacement possibly reflects dilution of a debris flow progressing in a submarine environment (e.g., Kataoka, 2005). Lithofacies 5 includes Unit IX and is composed of fossil-barren heterolithic basalt breccia and volcanic sandstone. Textural and compositional characteristics of these sediments, as well as the nature of underlying and overlying sedimentary and volcanic intervals (see “Igneous petrology and volcanology”) likely indicate that Lithofacies 5 reflects emplacement of debris flows into a shallow-marine or subaerial environment. Lithofacies 6 includes the foraminiferal bioclast limestone of Subunit IID and corresponds to a condensed section with possibly several hardground surfaces that developed between the late Campanian and late Maastrichtian (see “Paleontology”). Occurrences of keeled forms of planktonic foraminifers suggest that the sediment originally was deposited at hemipelagic depth (see “Paleontology”). On the other hand, the sediment contains abundant shallow-marine bioclasts and rounded basalt pebbles, as well as cement and dissolution textures characteristic of diagenesis in a subtidal to intertidal environment. Also, the upper boundary of Subunit IID corresponds to an erosional surface, and this subunit includes burrows filled with the volcanic sandstone of Subunit IIC (Lithofacies 3). On the basis of these observations, Lithofacies 6 was interpreted to record (1) the initial drowning of a formerly oceanic island at Site U1374, which allowed formation of a condensed interval with several ferromanganese-phosphate encrustations of Subunit IID, and (2) the emergence in a later stage of the previously deposited condensed interval, which is recorded by cement and dissolution textures, an erosional upper boundary, and burrows filled with substantially younger volcanic sandstone. Lithofacies 7 includes the multicolor volcanic sandstone of Subunit IIA. Characteristic features of Lithofacies 7 are occurrences of (1) very angular glass fragments forming the bulk of the sediment, (2) high vesicularity and pumiceous texture of the glass, and (3) several layers with subhorizontal normal grading. On the basis of these observations, we interpret Lithofacies 7 to reflect deposition of possibly air fall or shallow-water volcanic products in a submarine environment with limited postdepositional reworking. Lithofacies 8 includes Unit I and is predominantly composed of sandy foraminiferal ooze. This lithofacies corresponds to the youngest deposit found at Site U1374 (see “Paleontology”) and reflects pelagic sedimentation on the flat-topped Rigil Guyot. Another lithofacies corresponds to the deepest fossil-bearing sedimentary intervals found at Site U1374 (Unit XIV [256.75–257.49 mbsf] and Unit XVI [290.32–291.27 mbsf]). This lithofacies is characterized by the occurrence of micrite-bearing basalt conglomerate with angular fragments of red algae and other shallow-marine biota. It was interpreted to represent shallow-marine sediment deposition on the flank of a former oceanic island. The stratigraphic distribution of the above-described lithofacies and lithologies provides some significant constraints on the development and evolution of Rigil Guyot at Site U1374 (Fig. F9). Although the deepest occurrence of sediment observed in volcanic basement (Unit XVI; 290.32–291.27 mbsf) was deposited in submarine conditions, the fossil content provides clear evidence for the occurrence of nearby subaerial conditions at the time of deposition. Similarly, sediments found in Subunits IIE–XIB (Lithofacies 1, 2, 4, and 5) indicate a nearby subaerial to shallow-marine environment of deposition between 116.45 and 15.31 mbsf. On the basis of these observations and the nature of the volcanic sequences (see “Igneous petrology and volcanology”), we propose that the entire interval from 290.32 to 15.31 mbsf (Units XVI–IIE) records growth of the drilled volcano in subaerial to shallow-submarine conditions. Possibly, the first occurrence of fossil-bearing sediment at 291.27 mbsf may correspond to an unconformity indicative of the shoaling of the island away from Site U1374 and toward the center of this volcanic edifice (see “Igneous petrology and volcanology”). The deepest clear occurrence of subaerial conditions at Site U1374 was found in Subunit XIB (110.28–116.45 mbsf; Lithofacies 1) and possibly records the first emergence of the drilled volcano at Site U1374. Coeval occurrences of shallow-marine bioclasts and volcanic deposits above Subunit XIB uphole to 29.69 mbsf (including the fossil-bearing peperitic interval of Unit III) define an ~87 m thick interval (116.45–29.69 mbsf) that was deposited under more consistent subaerial (and still shallow submarine) conditions. This interval may have resulted from (1) occasional subsidence of the oceanic island during discontinuous supply of sedimentary and volcanic materials at Site U1374, (2) significant eustatic changes, and (3) occurrence of unrecovered or unrecognized erosional surface(s). Significantly, bedding and layering structures below 16.70 mbsf (Unit III and below) at Site U1374 dip consistently with a moderate angle, whereas dips above 16.70 mbsf (Unit II) are mostly subhorizontal (see “Structural geology”). This observation and the deposition of the basalt conglomerate of Subunit IIE in a subtidal to intertidal environment suggest that the Unit II/III boundary at 16.70 mbsf corresponds to a major erosional surface. We interpret this surface to record wave-induced flattening of the drilled volcano at Site U1374. In this interpretation, a thick volcanic sequence was possibly deposited on top of Unit III but completely eroded before emplacement of Subunit IIE. Deposition of Subunit IIE was followed by sedimentation of Subunit IID (Lithofacies 6), which was deposited in a hemipelagic environment between the late Campanian and late Maastrichtian and was probably subjected to subtidal to intertidal conditions in the late Maastrichtian (see above). Although occurrence of a condensed interval in Subunit IID most likely reflects initial drowning of the drilled seamount at Site U1374, the exact cause of possible later emergence in Subunits IIC, IIB, and IIA needs to be constrained by additional dating and correlation with possible similar events elsewhere in the Pacific. Possibly, these repeated emergence/drowning events in Subunit IID occurred in response to some significant eustatic changes in the latest Cretaceous (see “Paleontology”). A second drowning event of the former island is recorded by the volcanic sandstone of Subunits IIB and IIC (Lithofacies 3) that contains shallow-marine fossils, foraminifers, radiolarians, and ammonite fragments. The volcanic sandstone of Subunits IIA–IIC (Lithofacies 3 and 7) is interpreted as a possible record of rejuvenated volcanism on Rigil Guyot on the basis of (1) a significant age span (at least ~6–11 m.y.) between emplacement of lava flows in Unit III (late Campanian or older) and volcanic sandstone in Subunits IIA–IIC (late Maastrichtian); (2) the occurrence of highly vesicular olivine-rich glass fragments in Subunits IIA–IIC, which are clearly distinct from moderately vesicular plagioclase-rich glass fragments found in volcaniclastic deposits in Unit III and below; and (3) the pristine habit of vitric fragments and the occurrence of graded, subhorizontal beds found in Subunit IIA (Lithofacies 7), which possibly indicate the occurrence of volcanic material initially produced in a shallow-marine to subaerial environment. Deposition of Unit II was followed by a condensed interval with ferromanganese-phosphate encrustations at 6.64 mbsf. Finally, deposition of sandy foraminiferal ooze (Lithofacies 8) represents the last record of sedimentation on top of Rigil Guyot, which occurred in a pelagic environment during the Pleistocene to Holocene (see “Paleontology”). In summary, the volcano-sedimentary sequences recovered at Site U1374 (Units I, II, VII, IX, XI, and other sedimentary intervals and infill deposits) represent an extended record of the stratigraphic development of Rigil Guyot. Our preliminary interpretation suggests the following, from oldest to youngest:
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