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doi:10.2204/iodp.proc.324.107.2010 SedimentologyOne hole was drilled at Site U1350 on the flank of Ori Massif, Shatsky Rise (water depth = 4056 mbsl). Only one purely sedimentary unit was defined at this site (stratigraphic Unit I) that spanned Cores 324-U1350A-1W through 6R. Although it is likely Unit I is composed of both chert and calcareous ooze or chalk, only chert (predominantly black and, less commonly, red-brown) with minor amounts of porcellanite coating was recovered. Well-preserved radiolarians, concentrated around relict burrow features, are common in Unit I cherts. Igneous basement was reached in Core 6R at 143.1 mbsf, but additional sediments interbedded with the basaltic strata in stratigraphic Units II and IV were encountered. These sediments, predominantly fine-grained carbonates with radiolarians and volcaniclastics, contained varying quantities of bivalve and brachiopod fossils. Sedimentary interbeds were especially prevalent in Unit IV (Cores 25R through 26R), where small pillow basalts appear to have intruded carbonate-rich ooze prior to lithification. Unit descriptionsThe stratigraphy at Site U1350, including basaltic basement divisions and sedimentary interbeds, is shown in Figure F4.
Unit IThe recovery of Unit I at Site U1350 was poor and restricted to small isolated pieces of chert in Cores 324-U1350A-1W and 3R through 6R. No material was recovered in Core 2R. Black cherts are the predominant lithology; however, some reddish brown chert is present at the top of Core 1W. Many of the pieces contain relict bedding and traces of bioturbation, preserved as gray porcellanite patches (Fig. F5). Pyrite is occasionally seen replacing burrow structures in the black chert but is absent in the red. Many chert pieces in this unit are coated by thin (1–2 mm) pale brownish white porcellanite. Radiolarian fossils are abundant, well preserved in many of the cherts, and often concentrated around relict burrow structures. Sedimentary interbeds with igneous basement (Units II–IV)Several small sedimentary layers were recovered interbedded with the igneous strata at Site U1350 but were not extensive enough to warrant unit divisions of their own. See Figure F4 for location of these interbeds, which are denoted by yellow lines in the diagram. Unit IISubunit IIaSubunit IIa is predominantly composed of massive flow basalts (see "Igneous petrology"); however, small sedimentary interbeds were found in Cores 324-U1350A-8R, 9R, 14R, and 15R. Many of these interbeds have blackened outer rims and display a waxy texture. Sediments are predominantly limestone with altered volcaniclastics. Some of the volcaniclastic grains altered to clay minerals, which are more susceptible to weathering and often leave small cavities within the limestone matrix. Pyrite is occasionally associated with the weathered volcanic clasts but is not present elsewhere in the limestone. Pale blue zeolitic minerals were observed in both hand specimen (Fig. F6) and thin section. Sedimentary rocks in this subunit often contain secondary calcite as cement or in the form of thin veins. The few radiolarians observed in thin section were nearly completely replaced by calcite and poorly preserved. One small rock fragment (1.5 cm x 2 cm) in Core 324-U1350A-9R contained two different lithologies distinguished by a sharp color boundary and minor textural change (Fig. F7). Approximately 60% of the rock is volcaniclastic silty sandstone and the remainder volcaniclastic limestone. Subunit IIbThis subunit represents the transitional phase between the aphyric pillow succession in Subunit IIc and the massive flows of Subunit IIa and spans Cores 324-U1350A-17R through 19R (see "Igneous petrology"). Sedimentary interbeds present in Cores 17R and 18R consist of greenish gray volcaniclastic limestones and dark gray calcite-cemented volcaniclastics. Some of the rocks in this subunit contain volcanic glass shards, and alteration is apparent and likely from contact with hot basaltic material as it was emplaced nearby. Intervals 324-U1350A-17R-2, 120–124 cm, and 18R-1, 11.5–20.5 cm, contain brownish gray altered volcaniclastic limestones composed of calcite-cemented palagonitized glass shards. These rocks are friable, contain clay, and can be easily scratched with a fingernail. Subunit IIcFew sedimentary beds were recovered from Subunit IIc, which is an aphyric pillow basalt succession (see "Igneous petrology"). Gray volcaniclastic limestones found in Cores 324-U1350A-20R and 21R have blackened and cracked margins. Thin sections reveal that these limestones are primarily micritic and contain radiolarians that have been replaced by calcite. Preservation of the microfossils is only poor to moderate; however, delicate netlike structures are still visible in some specimens. (Fig. F8). It is important to note that sedimentary interbeds were not recovered from the lower part of Subunit IIc (~265–290 mbsf). Unit IIIUnit III is predominantly composed of hyaloclastite and brecciated basalt and spans the short interval between the base of Section 324-U1350A-24R-3 and the end of Section 24R-4 (see "Igneous petrology"). No sedimentary interbeds were recovered from this unit. Unit IVMany calcareous sedimentary beds were recovered intercalated with the plagioclase phyric pillow basalts in Unit IV (see "Igneous petrology"). These sediments are exclusively limestone with one exception. Volcaniclastic sandstone composed of dark gray, laminated, altered hyaloclastite was recovered spanning 126–142 cm in Section 324-U1350A-25R-1. The rock appears to be composed of grains of different sizes; however, in thin section it is apparent that the glass shards in the "fine-grained" portion are more compacted and have less pore space with calcitic cement infilling than those in the "coarse-grained" portion (Fig. F9). This 16 cm long piece shows signs of postdepositional brittle deformation in the form of small en echelon microfaults (Fig. F10). The remainder of the sedimentary material recovered from Unit IV is micritic limestones, often with a volcaniclastic component. Many of the limestones have blackened baked margins and slightly waxy textures where they are in contact with the pillow basalts. The pillows themselves often have black glassy rims and fractured margins, sometimes infilled with sediment. Poorly to moderately preserved radiolarian fossils and radiolarian "ghosts" are common in the limestones in this unit. Most have been replaced by calcite or clay but with their delicate structures preserved; a few siliceous radiolarians were observed in thin section. Shell fragments are also present, though not highly abundant (Fig. F11). Clay minerals are present throughout the rocks, most likely formed from alteration of volcaniclastic material. Sand-sized rounded carbonate grains are present in thin sections from Section 324-U1350A-26R-3. Four small brachiopod shells (~1.5 cm) were observed in this section (26R-3, 69–78 cm). Their shells are thin and have been completely recrystallized with a coating of small sparry calcite crystals growing inside. InterpretationUnit IThe black cherts interbedded with softer calcareous sediments at Site U1350 probably represent the same broad unit as the stratigraphic Unit I division at both Sites U1346 and U1347 on Shirshov and Tamu massifs, respectively. In comparison to Sites U1348 and U1349 and biostratigraphic indicators (see "Paleontology"), the red and brown chert pieces retrieved from the wash core (324-U1350A-1W) were likely formed during the mid-Cretaceous, whereas the black chert in Cores 3R through 6R is Early Cretaceous in age. This unit was probably deposited in a relatively deep pelagic setting (but above carbonate compensation depth) where the dominant primary sediment was likely to have been radiolarian-bearing nannofossil oozes. Remobilization of biogenic silica associated with diagenesis is the most reasonable explanation for chert development within the sediments. The degree of mottling and relict burrow structures suggest an oxic depositional environment with an active benthic community. However, the subsequent pyritization of these biogenic traces indicates reducing conditions and postdepositional alteration. Sedimentary interbeds within igneous basement (Units II–IV)A significant quantity of sedimentary material was recovered within the basaltic basement. All of these sediments have compositions and structures consistent with submarine deposition. The textural features in the surrounding igneous strata support this interpretation (see "Igneous petrology"). The majority of the sediments are calcareous, mostly limestones or volcaniclastic limestones. The volcaniclastic component in some limestones appears highly altered, especially in Subunit IIa. Clusters of framboidal pyrite present within the limestone matrix always occur in association with weathered volcanic grains and are not present elsewhere in the rock. This suggests the pyritization took place within the igneous rocks themselves shortly after the formation of the basaltic material and prior to its incorporation into the sediments. The presence of high amounts of micritic carbonate indicates a relatively shallow emplacement depth for both the sediments and the surrounding igneous units. The maximum depth of deposition must have been above the lysocline. The consistently high proportion of moderately to poorly preserved calcite-replaced radiolarians within the limestones suggests a high-productivity zone, probably associated with paleoequatorial upwelling. The paleodepth and depositional environment is further constrained by the presence of macrofossils, such as brachiopods and bivalves (especially at the base of Unit IV); these fossils are associated with shallow-water oxic conditions. The proportion of intercalated sediments to basalt is highest in Unit IV, where the presence of lithified limestone between friable pillows has evidently aided drilling recovery. Distortion of the limestones where they contact igneous rocks suggests the basaltic pillows were extruded into a calcareous ooze prior to lithification, where the lavas subsequently chilled. This is also evidenced by the many chilled (glassy) margins on the pillow edges, which can be seen throughout this unit. The intercalated sediments often have cracked, blackened margins and waxy textures presumably caused by dehydration and baking upon close contact with the still-cooling basalt. Other nonlimestone sedimentary rocks are present in Units II and IV. Although the recovery of these is generally restricted to isolated pieces, they are typical of materials deposited near submarine volcanic eruptions, as the primary composition of the grains in these calcite-cemented sandstones is volcanic glass, altered hyaloclasts, or fine ash. | |||
