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doi:10.2204/iodp.proc.313.201.2014 ResultsTypes of grain-size frequency curvesThe grain-size frequency curves for the samples from the three sites drawn by the installed software in the LA-300 and CAMSIZER are divided into 16 silt and 8 sand types on the basis of their curve shape, mode position, grain size range, and volume of skewed coarser or finer grain components. These curves reflect altogether the nature of the source area, the erosion processes, and sediment transportation history. Silt sediment typesFigure F2 shows the representative histograms and cumulative volume curves of 16 types (Types 1–16) of grain-size frequency distributions in 220 silt samples finer than fine sand. The three most frequent types, namely Types 6, 10, and 7, reach 45%, 14.5%, and 10%, respectively, as a whole and occupy ~70% of the total (Table T4). No other type exceeds 4%. The frequency of types varies depending on the site. Among the three sites, Site M0029 is more variable, though the number of measured samples is the smallest. The following is a brief description of each curve type:
Sandy sediment typesFigure F3 shows the representative histograms and cumulative volume curves of eight types (A–H) of grain-size frequency distributions in a total of 55 sandy samples from the three sites measured by the CAMSIZER. Three curve types, namely Types A, B, and C, show monomodal simple curves and occupy 85% of the total measured samples (Table T4). No other type exceeds 6% of the total. Their frequency varies depending on sites, though the numbers of measured samples are smaller than in silty sediments. Each type is briefly described below. Types A–C have monomodal shapes with peaks in medium sand and both upper steep and lower concave slopes, showing relatively well sorted sand sediments. Types D–H are observed only in one or two samples at Site M0027, except for Type F, which is found in a sample at Site M0028. These types are poorly sorted sediments and show characteristic grain-size frequency curves different from Types A–C.
Stratigraphic trends of average grain-size variationsThe four parameters of mean, median, mode, and standard deviation calculated from the measured grain-size data generally show similar stratigraphic trends at each site. Average grain-size (arithmetic mean) data are representatively plotted on the measured intervals of geologic columnar sections in Figures F4, F5, and F6. Mean grain size of a total of 275 measured sediment samples ranges from silt to coarse sand. Silty sediments dominate as a whole. Clayey silt was recognized only in a few samples from 207 to 196 mcd at Site M0027. Clay-dominated horizons are not detected in our grain-size analyses, though visual core descriptions (VCD) describe the common occurrence of clay sediments (see the “Expedition 313 summary” chapter [Expedition 313 Scientists, 2010a]). It is worth noting that the mean grain size of measured samples from the three sites is differentiated as fine–medium silt and medium–coarse sand due to the limited occurrence of coarse silt to fine sand. Site M0027Figure F4 shows the stratigraphic trends of average grain-size variations for 135 measured sediment samples from the interval Cores 313-M0027A-170R to 66X (489–195 mcd). In the intervals 475–415 and 336–197 mcd, medium to coarse silt is largely dominant. The sediments between 208 and 197 mcd are composed of fine silt to clayey silt and are the finest among measured horizons of the three sites. Conspicuous stratigraphic changes in mean grain size are not observed within these silty intervals, but a few subtle fining-upward trends from 30–40 to 10–20 µm can be recognized in the intervals 478–463, 314–295 and 250–240 mcd. Small-scale oscillations and kinks within the 336–197 mcd interval seem to occur near the sequence boundary horizons or to reflect the stacking patterns of sequences described by Miller et al. (2013a). Furthermore, the coarsening-upward trend from 22 to 36 µm can be recognized in the lowest interval of 487–478 mcd. In sandy intervals 357–336 mcd and 408–366 mcd, medium to coarse sand ranging from 0.250 to 0.800 mm is dominant, except for a few intercalated silt laminae/thin layers. The most conspicuous coarsening-upward trend is recognized from 415 to 396 mcd, from medium silt to coarse sand (31–564 µm). The coarsening-upward trend continues until 366 mcd, though slight grain size changes from medium to coarse sand occur. This trend corresponds to the upward-shallowing lithostratigraphic changes described in the “Site M0027” chapter (Expedition 313 Scientists, 2010b) and Miller et al. (2013a) Above the poorly recovered horizon at 366–357 mcd, a few oscillations within the 360–806 µm range (medium to coarse sand) are recognizable in the 357–336 mcd interval, similar in shape to the equivalent horizon of total gamma ray curves (see the “Site M0027”chapter [Expedition 313 Scientists, 2010b]). Site M0028Mean grain size in silty sediments from measured intervals 523–415 and 323–270 mcd narrowly ranges between 20 and 50 µm within medium to coarse silt, showing only subtle stratigraphic changes (Fig. F5). Within the interval 450–415 mcd, two coarsening-upward trends are detectable at 450–435 and 432–416 mcd. These seem to correspond to lithology to some extent. It is notable that the interval 441–415 mcd shows finer measured results due to transitional sediments from very fine/fine sandy silt to silty very fine sand, because the same interval was mainly defined as very fine/fine sand in the lithology column by VCD (see the “Site M0028” chapter [Expedition 313 Scientists, 2010c]). As sandy sediments are measured only in seven samples from Site M0028, stratigraphic changes are not well defined. Site M0029Though there are fewer measured samples at this site, silty sediments at Site M0029 are coarser than at Sites M0027 and M0028, ranging from 15 to 100 µm (Fig. F5). Mean grain size shows more distinctive stratigraphic changes between medium and very fine sand at the intervals 522–475 mcd and especially 378–310 mcd, generally corresponding to lithology and sequence stratigraphy (see the “Site M0029” chapter [Expedition 313 Scientists, 2010d]). On the other hand, sandy sediments are slightly finer than at Site M0027. Stratigraphic trends of types of grain-size distributionTypes of grain-size distribution are not necessarily quantitative characteristics; however, grain size changes at all three sites are considerably concordant to lithology, average grain-size curves, and sequence stratigraphy (Figs. F4, F5, and F6). Site M0027Among the measured silty sediment samples, Type 6 is common and is associated with Types 5 and 7–10. The coarsening-upward trend in mean grain size from 415 to 396 mcd appears to be parallel to the change from Type 9 to Type 16. Type C is abundant in the sandy sediment interval 409–366 mcd, except for the lowest three samples, which are Types A and B, and for one sample at 378.24 mcd, which is Type H. In the upper sandy interval 357–336 mcd, frequent changes among seven types (A–G) generally seem to correspond to a few small-scale oscillations in average grain size, suggesting some relation to the glauconite-bearing lithology (see the “Site M0027” chapter [Expedition 313 Scientists, 2010b]). Site M0028As silty sediments dominate among the measured samples, Type 6 is common and is associated with Types 7–10 as at Site M0027. In the interval 440–424 mcd, the upper part of Sequence m5.34, Type 13 is common. The interval 450–415 mcd shows two subtle coarsening-upward trends in mean grain size, whereas types of grain-size distributions also change twice concordantly in general. Seven measured sandy samples are divided into two samples each of Type A, B, and C and one sample of Type F. Site M0029Curves of both mean grain size and type of grain-size distribution show nearly the same trends, especially above 360 mcd, also broadly corresponding to lithology and sequence stratigraphy. Some turnoff points in the type of grain-size distribution and mean grain size in some cases correspond to sequence boundaries (e.g., Sequences m4.2 and m.4.1). Therefore, this means that both parameters are closely related to each other. |