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doi:10.2204/iodp.proc.330.107.2012 SedimentologySediment at Site U1376 was restricted to the sedimentary cover of Burton Guyot, with no sedimentary interbeds occurring in the igneous basement. Two stratigraphic units were defined on the basis of compositional and textural characteristics of the sediment at macroscopic and microscopic scales (Fig. F4):
Unit I
Stratigraphic Unit I represents a younger sedimentary cover that extends between the seafloor and 23.45 mbsf. It is composed of several layers of multicolor volcanic breccia; multicolor sandy volcanic breccia; multicolor layered, laminated, or bioturbated volcanic sandstone; and bioturbated brown volcanic tuff. The distinct compositions of the volcaniclastic deposits and sedimentary textures were used to divide Unit I into four subunits (Fig. F4). Estimates of grain size and roundness with depth (see “Sedimentology” in the “Methods” chapter [Expedition 330 Scientists, 2012a]) are illustrated in Figure F4 and provided in a supplementary table (see U1376A.XLS in SIZE in SEDIMENT in “Supplementary material”). Subunit IA
Stratigraphic Subunit IA extends from the seafloor to 1.52 mbsf (Fig. F4). Its lower boundary is defined by an erosional surface and the disappearance of rounded basalt pebbles in underlying Subunit IB (see below). Subunit IA is composed of grain-supported multicolor volcanic breccia with two interbeds of bioturbated brown volcanic sandstone (Fig. F5A). The top of the subunit is capped by a ~3 cm thick ferromanganese encrustation. Three other ferromanganese encrustations were observed at the interface between the breccia and sandstone at 0.29, 0.43, and 0.76 mbsf. The crusts and sediment infills in burrow holes contain minor amounts of nannofossil-foraminiferal chalk. Nannofossils found between 0.41 and 0.76 mbsf provide a possible Miocene age of deposition for some parts of Subunit IA; the ages of the uppermost (<0.41 mbsf) and lowermost (>0.76 mbsf) Subunit IA are unconstrained and may be younger or older than Miocene, respectively (see “Paleontology”). The volcanic breccia is cemented by calcite and zeolite and has a layered texture. The clasts are composed of abundant sand-size to pebbly grains of basalt and altered volcanic glass. Rare partly fresh olivine-pyroxene aggregates were observed as grains in the breccia or enclaves in some basalt clasts. The largest clasts per 10 cm interval are composed of basalt cobbles. Average clast roundness ranges between very angular and subangular. Some of the largest basalt grains are rounded to well rounded. Microscope observations (Sample 330-U1376A-1R-1, 30–33 cm [Thin Section 231]) showed that the brown bioturbated volcanic sandstone is composed of abundant reworked vitric fragments and minor basalt grains. Basalt grains are more abundant toward the base of the sandstone. These observations indicate that the volcanic sandstone layers of Subunit IA likely represent turbidites. Microscope observations of a layer of volcanic breccia (Sample 330-U1376A-1R-1, 103–107 cm [Thin Section 232]) indicated that the sediment is mostly composed of a heterogeneous assemblage of basalt grains and altered glass, with some large pyroxene, olivine, plagioclase, and amphibole. Subunit IB
Stratigraphic Subunit IB is 2.98 m thick and extends from 1.52 to 4.50 mbsf (Fig. F4). Its lower boundary corresponds to the appearance downhole of monolithic layered sandy volcanic breccia and volcanic breccia of Subunit IC (see below). Subunit IB is predominantly composed of heterolithic layered multicolor volcanic breccia, with some laminated brown and gray volcanic sandstone (Fig. F5B). The age of deposition of Subunit IB is indirectly constrained between the later Cretaceous(?) and Miocene on the basis of stratigraphic relationships and paleontological ages obtained in Subunits IA and IIB (see “Paleontology”). The bulk of the sediment of Subunit IB is composed of fragments of basalt and volcanic glass. The largest clast size ranges between coarse sand and pebble, and the average clast roundness is very angular to subangular. Clast size increases uphole throughout Subunit IB (Fig. F4). An interval of volcanic sandstone (Section 330-U1376A-1R-3, 69–100 cm) was interpreted as a turbidite on the basis of an erosional surface at the bottom of the interval, normal grading in the lowermost part of the sandstone, and fine laminae in most of the interval. Thin section observation (Sample 330-U1376A-1R-3, 75–78 cm [Thin Section 233]) showed that the turbidite in Section 330-U1376A-1R-3 is composed of abundant glass; minor clinopyroxene and feldspar; and rare amphibole, biotite, olivine, and algal fragments. The sediment of Subunit IB is cemented by zeolite and calcite. Subunit IC
Stratigraphic Subunit IC is 16.98 m thick and extends from 4.50 to 21.48 mbsf (Fig. F4). Its lower boundary corresponds to the appearance downhole of heterolithic multicolor volcanic breccia in Subunit ID (see below). Similar to Subunit IB above, the age of deposition of Subunit IC is indirectly constrained between the later Cretaceous(?) and Miocene on the basis of stratigraphic relationships and paleontological ages obtained in Subunits IA and IIB (see “Paleontology”). Subunit IC is predominantly composed of monolithic multicolor sandy volcanic breccia and volcanic breccia and rare monolithic multicolor volcanic sandstone (Fig. F5C). The breccia is layered, with a few occurrences of erosional surfaces and cross-bedded structures. The clasts in the breccia are mostly composed of altered fragments of glass and fine-grained (aphyric) basalt. Similar to Subunit IA, partly fresh olivine-pyroxene aggregates were found as grains in the breccia and enclaves in some basalt clasts. The largest clast size is consistent throughout Subunit IC and ranges between granule and pebble, with a small increase and decrease in grain size at the top and bottom, respectively, of the subunit (Fig. F4). The average roundness of the clasts is very angular to subangular. Microscope observations (Samples 330-U1376A-1R-4, 39–42 cm [Thin Section 234]; 2R-4, 33–36 cm [Thin Section 235]; and 2R-6, 31–35 cm [Thin Section 236]) showed that the volcaniclastic deposits of Subunit IC are essentially composed of vesiculated altered glass fragments with olivine and fine-grained basalt clasts. Some glass fragments are pumiceous. The basalt clasts occur both as grains in volcaniclastic deposits or, more rarely, as enclaves in larger glass fragments. There is a lack of evidence for reworking of the clasts in the volcaniclastic deposits of Subunit IC. Subunit ID
Stratigraphic Subunit ID is 1.97 m thick and extends from 21.48 to 23.45 mbsf (Fig. F4). Although not recovered, its lower boundary is defined by the appearance of carbonate sediment in underlying Unit II (see below). The deposition age of Subunit ID is constrained between the later Cretaceous(?) and Miocene on the basis of stratigraphic relationships and paleontological ages obtained in Subunits IA and IIB (see “Paleontology”). Subunit ID is composed of heterolithic multicolor volcanic breccia with a few bioclasts and monolithic brown coarse tuff (Fig. F5D). These volcaniclastic deposits are normally graded (volcanic breccia) or layered and bioturbated (coarse tuff). Compaction structures (load casts and a synsedimentary fault) were observed at the interfaces between the tuff and breccia (Points 1 and 2 in Fig. F5D). The maximum clast size of the breccia ranges between very coarse sand and pebble, with very angular to subangular grain roundness. Microscope observations (Sample 330-U1376A-3R-3, 81–84 cm [Thin Section 237]) indicated that the volcanic breccia is cemented by calcite and zeolite. Bioclasts in this sediment are mostly composed of red algae. A sample of coarse tuff (Sample 330-U1376A-3R-3, 113–117 cm [Thin Section 238]) is composed of vesiculated partly pumiceous glass fragments with a pristine texture suggestive of a volcanic (pyroclastic) origin. Unit II
Stratigraphic Unit II is an 18.48 m thick older sedimentary cover that extends between 23.45 and 41.93 mbsf on top of the volcanic basement of Burton Guyot. Occurrences of white algal limestone and dark gray basalt conglomerate allowed this unit to be divided into two subunits (Fig. F4). Subunit IIA
Stratigraphic Subunit IIA is 15.15 m thick and extends from 23.45 to 38.60 mbsf (Fig. F4). Its lower boundary is defined by the occurrence of dark gray basalt conglomerate in underlying Subunit IIB (see below). The age of deposition of Subunit IIA is constrained between the later Cretaceous(?) and Miocene on the basis of stratigraphic relationships and paleontological data from Subunits IA and IIB (see “Paleontology”). Subunit IIA is mainly composed of white coralline algal boundstone with some minor white algal rudstone (Fig. F5E, F5F). The composition of the limestone evolves downhole from pure white carbonate sediment devoid of volcanic grains at the top to yellowish-red carbonate sediment with a few basalt clasts at the base. This compositional pattern defines a transition to the basalt conglomerate with bioclasts of underlying Subunit IIB. The largest basalt clasts of the lower part of Subunit IIA range in size from coarse sand to pebble, with a gradual increase in size toward the base of Subunit IIA (Fig. F4). The average basalt clast roundness is angular to well rounded. The bulk of the limestone of Subunit IIA (boundstone) represents a biogenic framework related to in situ growth of algae. Encrusting forms of algae were found throughout the subunit (Fig. F5F), whereas branching forms of algae predominantly occur in the upper part (in intervals devoid of volcanic clasts) (Fig. F5E). The boundstone also includes some minor bryozoans, annelids, and encrusting bivalves (e.g., oysters). Minor amounts of nonencrusting bivalves, gastropods, echinoderms, solitary coral, and basalt clasts predominantly occur in the rudstone that represents moderately reworked bioclast-rich limestone. Bivalve boreholes and dissolution features in the algal framework, fossil-bearing micrite, and shell fragments are frequent in Subunit IIA. Dissolution voids are often partly or entirely filled with fossil-barren micrite (Fig. F6A). This micrite probably consists of vadose silt, which indicates a subtidal to intertidal environment of cementation (Flügel, 1982). Similarly, frequent dissolution features and cementation patterns suggestive of a vadose environment were also observed in thin section (Samples 330-U1376A-3R-4, 129–133 cm [Thin Section 239]; 3R-5, 99–103 cm [Thin Section 240]; 4R-1, 45–49 cm [Thin Section 241]; 4R-2, 55–58 cm [Thin Section 242]; and 4R-3, 65–68 cm [Thin Section 243]) (Fig. F6B). Subunit IIB
Stratigraphic Subunit IIB is 3.33 m thick and extends from 38.60 to 41.93 mbsf (Fig. F4). Its lower boundary corresponds to an erosional surface that marks the transition downhole to the volcanic basement of Burton Guyot (see “Igneous petrology and volcanology”). A later Cretaceous(?) age of deposition of Subunit IIB is constrained by the occurrence of possible rudist fossils (see “Paleontology”). Subunit IIB is composed of a dark gray basalt conglomerate with a few shallow-marine bioclasts (e.g., red algae, annelids, and bivalves) (Fig. F5G, F5H). Three erosional surfaces were recognized in the subunit at 39.74, 41.07, and 41.88 mbsf. The largest grain size ranges between very coarse sand and boulder, and the average grain roundness is very angular to well rounded. Microscope observations (Samples 330-U1376A-5R-1, 65–67 cm [Thin Section 244], and 5R-2, 80–83 cm [Thin Section 245]) indicated that the basalt conglomerate is predominantly composed of fragments of fine-grained basalt and algae, with a minor amount of bryozoans, bivalves, annelids, echinoderms, shell fragments, and clinopyroxene. In Thin Section 244, interstitial spaces of the conglomerate include dogtooth cement and geopetal structures defined by vadose silt and sparry calcite cement (Fig. F6C). These features are indicative of a vadose environment of cementation (Flügel, 1982). Two generations of calcite cement (fibrous and granular) were recognized in Thin Section 245 (Fig. F6D). These observations are indicative of a shallow-marine subtidal to intertidal environment of deposition for Subunit IIB. Interpretation of sediment and volcaniclastic deposits at Site U1376Units I and II at Site U1376 represent the sedimentary cover of Burton Guyot, which probably was deposited after erosional flattening of the drilled volcanic edifice. Unit I is interpreted to have been deposited in a hemipelagic or pelagic environment. The ferromanganese encrustations of Subunit IA suggest a lower sedimentation rate in the uppermost part of the drilled sequence. The chalk included in the ferromanganese crusts or burrows of Subunit IA contains nannofossils with a middle Miocene to late Miocene age (see “Paleontology”). These ages may correspond to the age of sediment deposition or postdepositional percolation of fine-grained sediment in cracks or voids. Occurrences in Subunits IA and IB of turbidites and layered volcanic breccia that possibly represent hyperconcentrated flow deposits (Smith and Lowe, 1991) document the existence of highly dynamic depositional processes on top of Burton Guyot. The 16.98 m thick monolithic layered deposits of Subunit IC are interpreted to represent volcanic products formed during the later Cretaceous and Miocene. These deposits were possibly emplaced during a rejuvenated volcanic stage of Burton Guyot (see “Igneous petrology and volcanology”). Subunit ID, which is composed of layered heterolithic volcanic breccia and coarse tuff, is interpreted to consist of turbidites, hyperconcentrated flow deposits, and volcanic deposits emplaced in a hemipelagic environment. Similar to deposits of Subunit IC above, the tuffs of Subunit ID also record a possible rejuvenated volcanic stage of Burton Guyot between the later Cretaceous and Miocene. Unit II is interpreted as a transgression sequence deposited on top of Burton Guyot in the later Cretaceous (see “Paleontology”) and possibly younger times. The composition, grain roundness, and cement textures of Unit II suggest deposition in a high-energy environment in the zone of wave influence, with possibly frequent emergence events. These observations and the deposition of Unit I in a hemipelagic to pelagic setting suggest that the Unit II–I transition likely reflects subsidence of the drilled seamount. Boundstone and rudstone of Subunit IIA record the formation of a 15.15 m thick algal reef that developed (under very shallow marine conditions) probably shortly after flattening of the drilled seamount by erosional processes. Subunit IIB, which is composed of basalt conglomerate, is interpreted to have been deposited in a shallow-marine subtidal to intertidal environment on the basis of basalt clast roundness, the occurrence of shallow-marine bioclasts, and cement textures. The erosional surface recognized at the base of this subunit (~41.93 mbsf) corresponds to the boundary between the sedimentary cover and volcanic basement at Site U1376. Possibly, this boundary reflects a major erosional event associated with the flattening of Burton Guyot. |
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