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Volcaniclastic sediments

One of the unexpected results of Expedition 310 is the recovery of relatively large amounts of volcaniclastic sediments. The entire ~36 m rock sequence drilled in Tiarei marginal Hole M0008A consists of volcaniclastic sediments. Volcaniclastic sediments were also recovered at several other sites, but the amounts are highly variable, from mere sand and silt impurities in the carbonate rock units to minor components (<50 vol%) in carbonate sand units to major components (50 vol%) in sand/​silt (or sandstone/​siltstone) interbedded with carbonate units beds. As discussed earlier (see “Sedimentology and biological assemblages” in the “Methods” chapter), despite their clear volcanogenic origin, volcanic lithic fragments were considered epiclastics, and the lithologic names used were therefore based on grain size (see Table T1 in the “Methods” chapter) instead of lithologic names for pyroclastic deposits (e.g., Fisher and Schmincke, 1984). Using this nomenclature, the predominant type of volcaniclastic sedimentary units drilled during Expedition 310 are mixed volcanic sand/​silt or volcaniclastic sandstone/​siltstone units. Individual volcanic granules, pebbles, and minor cobbles are major components in the rubble between some rock units and occur as inclusions in the reefal carbonates.

Volcaniclastic sediments consist of seven principal components:

  1. Silt- to cobble-size lithic volcaniclasts,
  2. Various “carbonate fragments,”
  3. Crystal fragments (pyroxenes and undifferentiated feldspars/​feldspathoids [foids]/quartz),
  4. Clays (mud),
  5. Iron oxides/​oxyhydroxides that occur either as coatings around grains or as individual grains,
  6. Matrix and/or cement in lithified rocks, and
  7. Occasional reworked or rip-up clasts of carbonate sedimentary rocks.

Rare wood fragments and fine, delicate plant roots were observed at Site M0008 only. Two texturally very similar rock pieces recovered in Section 310-M0008A-7R-CC and interval 8R-1, 0–80 cm, were considered parts of a single, large (65 cm recovered) basalt “boulder.” Together, the pieces do not show systematic grain-size variation (e.g., coarse-grained interior grading to fine-grained to glassy top and/or bottom) and other textural features (e.g., horizontal flow or trachytic texture; vesicle concentration at the top) indicative of a lava flow, and their upper and lower contacts, which may have indicated baking/​chilling of the country rocks, were not recovered. Consequently, the two pieces were considered a single, unlithified volcaniclastic component, and together with the underlying round and polished granules and pebbles and a cobble they compose a relatively thick rubble unit. However, it must be noted that the chemical compositions of the pore waters above and below this particular rubble unit are different, which necessitates the presence of an impervious layer (e.g., a dense lava flow) above or in the rubble section (see “Geochemistry”). Moreover, seismic data indicate a continuous and distinct reflective layer at this particular drilled horizon that is consistent with a prominent lava flow (see Fig. F5 in the “Site surveys related to Expedition 310” chapter). Therefore, it is possible that the boulder is part of a lava flow unit overlying a thinner rubble unit.

As noted earlier, the most distinctive volcaniclastic sedimentary intervals are unlithified mixed sand and silt. It is important to note, however, that the sandstone and siltstone are only weakly to moderately lithified, and thus some of the sand and silt intervals could be drilling artifacts. This phenomenon is clearly illustrated by the presence of sand and silt in some of the core catchers immediately below lithified sandstone and siltstone units (e.g., Sections 310-M0008A-8R-1, 8R-CC, 10R-1, and 10R-CC). Structurally, the mixed sand/​silt and sandstone/​siltstone intervals are massive, although faint stratifications indicated by relatively more sandy layers alternating with more silty layers were observed. Moreover, elongate carbonate fragments (e.g., shell and coral fragments) are occasionally weakly imbricated. Sharp layer boundaries within units and distinct cross-stratifications were not observed. Boundaries, however, were generally not recovered intact; the majority of the observed boundaries in volcaniclastic intervals are defined by the abrupt occurrences of rubble beds.

Texturally, sand and silt components range in shape from angular to rarely round but are predominantly angular to subangular. In contrast, the larger-sized granules, pebbles, and cobbles are subround to round. Grains are unsorted, and in general, size grading within individual units is rarely observed. As will be described in detail below, however, pronounced downhole variation in the color and composition of the volcanic sand/​silt units was observed at Site M0008.

Compositionally, volcanic lithic components are several varieties of basalt and occasional hawaiite/​alkalic andesite. Alkalic diorite pebbles were also observed. No detailed examination of silts, sands, and granules was done, but visual and hand lens/​binocular microscope observation of the boulder, cobbles, and representative pebbles show that lithic clasts are fine grained or essentially aphanitic (grain size = <1 mm) basalts. Grain-size variation within a fine-grained basalt clast is imparted by the presence of phenocrysts, which vary in size from ~1 to >5 mm (i.e., from aphanitic basalt to fine- to coarse-grained basalt). Phenocrysts also vary in abundance from <1 to ~20 vol%, giving aphyric to highly phyric textured basalts. The main phenocryst phases are subhedral to euhedral pyroxenes (many are augite) and olivine. Many of the pyroxenes show sieve texture, possibly due to either postmagmatic mineral reaction or alteration of glass/​mineral inclusions. Many of the olivine crystals are altered along margins and fractures to brown clay (iddingsite), but a few are surprisingly fresh. Feldspars and/or feldspathoids also occur as phenocrysts but are not as dominant and distinctive as the ferromagnesian minerals. Some phenocrysts cluster together or are glomerophyric, whereas others are oriented subparallel, indicating flow direction, or are trachytic.

The majority of the lithic basalt clasts are nonvesicular, but some are slightly to highly vesicular. In fact, a few basalt pebbles appear scoriaceous, but they are too dense to be classified as scoria. A few vesicles are lined with amygdules of iron oxides and carbonates, but as a whole the vesicular variety of the basalt clasts is nonamygdaloidal. The unfilled nature of the vesicles and the fresh-looking nature of some olivine phenocrysts suggest that many of the basalt cobbles and pebbles are only slightly altered. Visual observation suggests that many of these larger sized basalt clasts are not heavily weathered and altered. In contrast, some of the basalt granules and particularly the sand and silt grains are angular to subangular fragments intermixed with clays and iron oxides/​oxyhydroxides.