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doi:10.2204/iodp.proc.330.103.2012 SedimentologyThe following stratigraphic units and subunits were identified at Site U1372 on the basis of macroscopic and microscopic observations of sediment composition and texture (Fig. F4):
Unit I
Stratigraphic Unit I extends from the seafloor to 13.50 mbsf (Fig. F4). The dominant lithology is light yellowish-brown to brown sandy foraminiferal ooze that contains minor pumice, vesiculated volcanic glass, and mineral fragments (Figs. F5, F6). Well-rounded pumice and subrounded glass fragments colonized by annelids are evidence for reworking of the grains on the ocean floor (Fig. F7). Poor compaction and the small amount of clay-size matrix of the ooze forming the bulk of the unit led to severe internal disturbance and almost complete loss of original bedding and sedimentary structures during drilling and retrieval. The lowermost part of Unit I (11.86–12.06 mbsf) contains distinctive light olive-brown pumiceous gravel with larger grains of black volcanic glass. The gravel is more consolidated than the rest of the sediment in Unit I. Paleontological data suggest this interval contains a mixture of middle Miocene to early Pliocene species and possibly represents a condensed section according to the slow sedimentation rate (see “Paleontology”). In addition, poorly consolidated aggregates of volcanic glass fragments locally occur throughout Unit I. In Sections 330-U1372A-2R-2 and 3R-2, such aggregates are underlain by beds of whitish foraminiferal ooze with in-flow drilling texture along the core liner. Occurrence of these beds suggests that the unit may have experienced less drilling disturbance in these localized sections. Smear slide estimates for Unit I (Samples 330-U1372A-1R-CC; 2R-1, 70 cm; 2R-2, 45 cm; 2R-3, 70 cm; 2R-3, 107 cm; 2R-4, 75 cm; 2R-5, 27 cm; 2R-5, 105 cm; 2R-6, 22 cm; and 2R-CC) show high abundances of foraminiferal tests with minor amounts of nannofossils and silt-size calcite grains (Fig. F6A). Foraminiferal and nannofossil data provide evidence for partial reworking of the sediment forming Unit I (see “Paleontology”). Rare pumiceous grains, volcanic glass, feldspar, pyroxene, and lithic fragments occur throughout Unit I. Grain preservation ranges from severely altered to fresh. Foraminiferal tests filled with nannofossil ooze/calcite and opaque material are white and brown in color, respectively. The whitish foraminiferal ooze that occurs below poorly consolidated aggregates of volcanic glass fragments contains more nannofossils, poorly altered foraminifers, and silt-size calcite grains. Unit II
Stratigraphic Unit II is 32.08 m thick and extends from 13.50 to 45.58 mbsf (Fig. F4). Sediment fracturing during drilling was minor in Unit II. Although the Unit I/II boundary was not recovered, contrasting consolidation states, compositions, textures, and ages between Sections 330-U1372A-3R-CC and 4R-1 support the occurrence of a major stratigraphic boundary at 13.50 mbsf. The base of Unit I is composed of unconsolidated middle Miocene to early Pliocene pumiceous gravel, whereas the uppermost part of Unit II is composed of well-consolidated early Paleocene basalt breccia (see “Paleontology”). The Unit II/III boundary is marked by the first occurrence of a lava flow with a peperitic texture, which forms the uppermost part of the volcanic sequence (Unit III) at 45.58 mbsf (see “Igneous petrology and volcanology”). The bulk of Unit II is composed of poorly sorted grain-supported basalt breccia and conglomerate, including finer grained sediment, cement, and voids in interstitial spaces. A distinctive interbed of foraminiferal limestone (Subunit IIB) with ferromanganese encrustations occurs between 18.00 and 18.34 mbsf. The clasts are predominantly composed of variously altered basalt fragments (as large as boulder size), with rare to minor shallow-water bioclasts that gradually increase in abundance downhole (Figs. F4, F8). Macro- and microscopic observations revealed seven types of basalt clasts in Unit II that exhibit a compositional spectrum similar to that of volcanic deposits encountered in the underlying volcanic sequence of the seamount (Unit III) (see U1372A.DOC in CHAR in SEDIMENT in “Supplementary material”). The nature and occurrence of these clasts are summarized in Table T2. Visual core descriptions and thin section observations in Unit II allowed definition of five stratigraphic subunits based on differences in clast angularity and size and the composition of intercobble and interboulder spaces (Figs. F4, F9; see also U1372A.XLS in SIZE in SEDIMENT in “Supplementary material”). Subunit IIA
Stratigraphic Subunit IIA is 4.50 m thick and extends from 13.50 to 18.00 mbsf (Fig. F4). The dominant lithology of Subunit IIA is poorly sorted grain-supported multicolor basalt breccia (Fig. F9A). Intercobble and interboulder spaces are composed of voids, cement, finer grained volcaniclastic breccia-sandstone, and foraminiferal limestone. The morphology of intergranular cement (i.e., isopachous and fibrous) indicates precipitation in an active zone of marine phreatic environment (Flügel, 1982) (Fig. F10). Abundant normal grading and geopetal structures occur in the intercobble and interboulder spaces (Fig. F11). In the finer grained calcareous volcaniclastic sediment, sedimentary flow and current structures are common and include erosional contacts, laminae, and cross-laminae (Fig. F12). Minor bioturbation and possible liquefaction structures were found in calcareous sediment. A gradual lithologic change was documented throughout the subunit, with an increase in the amount of intercobble and interboulder foraminiferal limestone and an increase in the maximum grain size for every 10 cm uphole (Fig. F4). A foraminifer-rich limestone sample at 13.87 mbsf provided an early Paleocene age of deposition for the subunit (see “Paleontology”). Thin sections (Samples 330-U1372A-4R-1W, 37–41 cm [Thin Section 2], and 4R-3W, 77–80 cm [Thin Section 6]) of finer grained intercobble and interboulder sediment in Subunit IIA show abundant foraminifers and calcispheres, minor grains of altered and fresh basalt, volcanic glass, plagioclase, and pyroxene, as well as rare shallow-water bioclasts. Subunit IIB
Stratigraphic Subunit IIB is 0.34 m thick and extends from 18.00 to 18.34 mbsf (Fig. F4). Although the uppermost and lowermost parts of this subunit were probably not recovered, its boundaries are well constrained by the occurrence of ferromanganese encrustations in its uppermost part and a basalt boulder or lava flow field in the uppermost part of underlying Subunit IIC (see below). Subunit IIB is predominantly composed of foraminiferal limestone with inoceramid shell fragments and basalt clasts (Fig. F9B). Ferromanganese encrustations surround limestone and basalt in the three uppermost pieces of Subunit IIB (18.00–18.13 mbsf). An intraclast of brown basalt conglomerate or a clastic dike was recognized at 18.26 mbsf. This intraclast/dike displays evidence for semiconsolidation at the time of deposition/emplacement, with fractures filled by foraminiferal limestone from the matrix. In Subunit IIB, the maximum clast size has a narrower range of distribution than that of Subunits IIA and IIC, and the grains in Subunit IIB are generally more rounded as well (Fig. F4). Thin section observations (Samples 330-U1372A-5R-1W, 31–34 cm [Thin Section 7], and 5R-1W, 32–34 cm [Thin Section 8]) revealed that the finer grained calcareous sediment contains abundant benthic and planktonic foraminifers and altered fragments of inoceramid shells (Fig. F13). This fossil assemblage defines a latest Cretaceous age of deposition for Subunit IIB (see “Paleontology”). Other minor components encountered in thin sections include biogenic grains (e.g., calcispheres, echinoderms, ostracods, and fish teeth), basalt and volcanic glass fragments, plagioclase, and clinopyroxene. Subunit IIC
Subunit IIC is 7.92 m thick and extends from 18.34 to 26.26 mbsf (Fig. F4). The lithology of Subunit IIC is very similar to that of Subunit IIA and is predominantly composed of poorly sorted grain-supported multicolor basalt breccia with intercobble and interboulder spaces composed of voids, cement, finer grained volcaniclastic breccia-sandstone, and foraminiferal limestone (Fig. F9C). Sedimentary and cementation textures in Subunit IIC are also similar to those in Subunit IIA, with abundant current and infill structures, minor sedimentary flows, and two generations of marine cement (Figs. F10, F11, F12). Subunit IIC slightly differs from Subunit IIA in that it has (1) slightly increased content of bioclasts, (2) lower average clast roundness, and (3) a distinct pattern of maximum clast size (pebble to cobble size) that increases uphole (Fig. F4). The uppermost part of Subunit IIC is composed of a >1 m thick basalt boulder or lava flow that in its lower part includes cracks filled by micritic limestone dissimilar to that found in Subunit IIB above (i.e., limestone that does not include abundant inoceramid shell fragments). Thin section observations in Subunit IIC indicate that the intercobble and interboulder spaces are composed of sediment with a large compositional spectrum that ranges between micritic limestone mostly devoid of volcaniclasts and grain-supported volcanic sediment (sandstone to breccia/conglomerate). The foraminifer-bearing micritic limestone (Samples 330-U1372A-5R-2W, 1–3 cm [Thin Section 9], and 6R-1W, 28–31 cm [Thin Section 13]) is mainly composed of micrite (>85%) with a minor grain component composed of calcispheres, foraminifers, possible sponge spicules, basalt, variously altered volcanic glass, plagioclase, and clinopyroxene. The grain-supported volcaniclastic sediment (Samples 5R-3W, 16–20 cm [Thin Section 10]; 5R-4W, 49–52 cm [Thin Section 11]; 5R-4W, 126–130 cm [Thin Section 12]; and 6R-2W, 53–56 cm [Thin Section 15]) contains various amounts of sparite cement and foraminifer-bearing micrite, which define fine laminae and cross-laminae in the finer grained deposits (Fig. F12C). Isopachous and fibrous submarine cement was observed in more coarsely grained volcaniclastic sediment (Fig. F10B, F10C). Rare biogenic fragments composed of bivalves, calcareous red algae, and annelids were encountered in all Subunit IIC sediment. Other minor components are plagioclase and clinopyroxene. Subunit IID
Stratigraphic Subunit IID is 13.27 m thick and extends from 26.26 to 39.53 mbsf (Fig. F4). A gradual lithologic change occurs between Subunits IIC, IID, and IIE. The upper boundary of Subunit IID is defined by the first occurrence downhole of a large interval of poorly sorted grain-supported conglomerate with cemented interstices. Its lower boundary is defined by the (gradual) appearance of brown micrite replacing cement in pore spaces, the fast disappearance downhole of orange and red basalt grains, and a marked increase downhole in the abundance of shallow-water bioclasts in underlying Subunit IIE (see below). The dominant lithology of Subunit IID is multicolor basalt conglomerate with interstitial spaces composed of abundant voids and cement and minor micrite (Fig. F9D). Grains mostly include basalt and altered volcanic glass, with minor bioclasts (annelids, calcareous red algae, oysters, bryozoans, and echinoderms). A pebbly intraclast of annelid packstone occurs at 36.12 mbsf. The maximum clast size ranges widely from pebble to boulder size, and the grains are significantly rounder than in overlying subunits (Fig. F4). A thin section (Sample 330-U1372A-7R-2W, 105–111 cm [Thin Section 17]) showed that the grains of the conglomerate in Subunit IID predominantly include well-rounded basalt with lesser amounts of altered glass, feldspar, clinopyroxene, and biogenic fragments (bivalves, calcareous red algae, and annelids). Most of the conglomerate is cemented by sparite. A very minor amount of foraminifer-bearing micrite was observed next to larger clasts, which indicates efficient winnowing of the finer grained fraction shortly after deposition of the conglomerate (Fig. F12D). The micritic component preserved few foraminifers, calcispheres, and ostracods. Submarine cement similar to that found in Subunits IIA and IIC was observed. Subunit IIE
Stratigraphic Subunit IIE is 6.05 m thick and extends from 39.53 to 45.58 mbsf (Fig. F4). A gradual change occurs from Subunits IID to IIE in the interval between Sections 330-U1372A-7R-4A, 65 cm, and 7R-5A, 77 cm. The upper boundary of Subunit IIE is based on the quick appearance downhole of brown interstitial micrite, an increased abundance of bioclasts, and the disappearance downhole of orange and red basalt clasts. The dominant lithology of Subunit IIE is poorly sorted grain-supported bluish-gray basalt conglomerate with minor bioclasts (Fig. F9E). The bioclasts are composed of annelid, calcareous red alga, oyster, bryozoan, and echinoderm fragments with larger grain size than that of the bioclasts in overlying Subunit IID. Another distinctive feature of Subunit IIE is the lack of voids/cement encountered elsewhere in Unit II. The maximum clast size ranges from pebble to boulder size. The size of boulder-size clasts increases in Subunit IIE compared with overlying subunits; grains are more angular than those in overlying Subunits IIA–IID (Fig. F4). Thin sections of Subunit IIE (Samples 330-U1372A-8R-1W, 20–23 cm [Thin Section 20], and 8R-2W, 24–28 cm [Thin section 21]) indicate that the finer grained sediment fraction is composed of annelid packstone with basalt clasts. Minor components include variously preserved glass, plagioclase, clinopyroxene, opaque minerals, larger bioclasts (calcareous red algae, bryozoans, bivalves, and echinoderms), and rare broken planktonic foraminifers. The presence of inoceramid shell fragments indicates a Cretaceous age of deposition for Subunit IIE (see “Paleontology”). Sediment in underlying volcanic sequenceSediment in the volcanic sequence at Site U1372 (Unit III and below) is restricted to small enclaves in peperite intervals and minor deposits in hyaloclastite intervals (see “Igneous petrology and volcanology”). This sediment is distinct from that found in overlying Units I and II and consists of (1) dark multicolor basalt siltstone-sandstone devoid of bioclasts and carbonate matrix in the peperites and (2) dark green basalt conglomerate devoid of bioclasts and primary carbonate matrix in hyaloclastite intervals. Interpretation of lithologies and lithofacies at Site U1372Unit I is interpreted as the youngest record of sedimentation on Canopus Guyot, occurring under the influence of strong oceanic currents. The sandy foraminiferal ooze in this upper unit likely represents a winnowed residue of foraminiferal ooze originally rich in nannofossils. This interpretation is supported by the roundness of pumice, volcanic glass, and mineral fragments, as well as observations made on foraminiferal assemblages (see “Paleontology”). The presence of pumice and fresh glass fragments in Unit I suggests that the sediment includes a minor tephra component, most likely derived from the nearby Tonga-Kermadec volcanic arc. Unit II represents an older sedimentary deposit, which we interpret to have formed under shallower neritic to hemipelagic water conditions. The unit is further thought to have been deposited during continuous subsidence of the seamount. This interpretation is supported by gradual decreases in the size of detrital grains (bioclasts and basalt clasts) and the content of shallow-water bioclasts, which suggest a deepening-upward sequence throughout Unit II. The larger bioclasts preserved in Unit II are predominantly composed of reworked annelids and red algae, with minor bryozoan and mollusk shell content. This fossil assemblage suggests a rocky shore environment in the Cretaceous of a previously volcanic island. No rudist-coral limestone was documented at Site U1372. Sedimentary structures and lithofacies in Unit II show that the bulk of the sediment experienced several periods of diachronous deposition, including (1) initial deposition of larger (cobble to boulder size) basalt clasts, which is supported by the occurrence of grain-supported breccia and conglomerate, and (2) repeated events of intercobble and interboulder infill by hemipelagic and volcaniclastic sedimentation, as indicated by widespread occurrences of normal-grading sedimentary sequences overlying larger basalt clasts, geopetal structures, and residual interstitial voids. This pattern of deposition may indicate that the bulk of Unit II formed at the front of a subaerial escarpment or cliff as a talus deposit. The composition and distribution of the basalt clasts indicate that the bulk of the clastic component in Unit II is derived from volcanic deposits similar to those encountered in the underlying volcanic sequence (see “Igneous petrology and volcanology”). Laminae, cross-laminae, and erosional surfaces in Unit II document the existence of ancient oceanic currents along the top and flanks of the seamount and/or wave action in shallow-marine conditions. Subunit IIB is interpreted as a condensed facies deposited during a period of lower clastic input because of the emplacement of a large boulder field or lava flow in the uppermost part of Subunit IIC. This interpretation is supported by the occurrence of ferromanganese encrustations, fewer volcaniclasts, and more abundant inoceramid shell fragments in Section 330-U1372A-5R-1. |
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