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doi:10.2204/iodp.proc.313.201.2014

Results

Types of grain-size frequency curves

The 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 types

Figure 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:

• Types 1 and 2: the mode generally shows symmetrical pyramidal shapes on the fine silt class, but Type 1 has a smaller amount of finer clay fractions. Both types have gently concave left (finer) and concave right (coarser) lower slopes. Although Type 1 is the finest among 16 types, both Types 1 and 2 can be classified as clayey silt. Both of the types are recognized in three samples from Site M0027. Three samples from Site M0028 are Type 1, and one sample from Site M0029 is Type 2.

• Type 3: this type is recognized only in one sample from Site M0029; it is characterized by a bimodal shape with the higher mode on fine silt and the lower on very fine sand. This type is classified as very fine sandy silt.

• Type 4: the mode has a nearly symmetrical, triangular shape with a gently concave right lower slope. This type is classified as silt and is recognized in two samples from both Sites M0027 and M0029.

• Type 5: though similar to Type 6 in its general mode curve shape, Type 5 has an acute peak between fine to medium silt with a more concave and long left slope. Type 5 is classified as silt and is observed in four samples from Site M0027 and in one sample from Site M0029.

• Type 6: this type is the most frequent type found in silty sediment samples from all three of the sites. It reaches 53.5% and 49.3% at Sites M0027 and M0028, respectively, but decreases to 20.8% at Site M0029. This type has a leftward skewed asymmetric shape with a mode on medium silt and gently concave slopes on both sides. Type 6 is classified as silt as well as Types 4 and 5.

• Type 7: the mode has a pyramidal symmetrical form, both flat sides and a peak at medium silt. It includes <20% of very fine to fine sand. Thus, this type is classified as slightly sandy silt. This is the third most common type at the three sites and occurs adjacent to Type 6 horizons.

• Type 8: the mode is somewhat similar in shape to Type 7 except for the convex-up coarser side on coarse silt and very fine sand. This type is classified as sandy silt and is observed in 1–3 samples at each site.

• Type 9: the mode forms a round-top asymmetrical shape with a peak at medium to coarse silt and includes >20%–30% very fine to medium sand. Type 9 is classified as very sandy silt and is observed in one to four samples of the three sites.

• Type 10: the mode is similar in its asymmetrical triangular general shape to Type 6, but its peak is situated on medium to coarse silt. Though Type 10 is also similar to Type 9, it has an angular mode with flat left and slightly concave right sides. Type 10 is the second most common type found at all the three sites. This type is classified as slightly sandy silt. Four types (Types 7–10) can be generally classified as sandy silt.

• Type 11: although classified as poorly sorted sandy silt as well as Type 9, this type is characterized by a mode with convex-up left side and a distinct peak at coarse silt to very fine sand. The mode is somewhat similar to Type 9 in general shape and the flat lower part of the both sides, though the top surfaces differ. Type 11 is recognized in one sample from Site M0027 and in five samples from Site M0029.

• Type 12: the mode has an acute peak on coarse silt to very fine sand with a skewed toward finer, long left lower slope. This can be classified as silty very fine sand. Type 12 was found only in five samples at Site M0029.

• Types 13 and 14: these types are classified as sandy silt, have modes with similar shapes with peaks at medium silt, and have shoulderlike convexity in very fine and fine sand classes. As Type 14 includes more coarser components, its mode has a conspicuous convex form in the right side. This means that Type 14 is coarser than Type 13 as a whole. Type 13 is recognized in two samples at Site M0027 and in six samples at Site M0028, and Type 14 is seen in two samples at Site M0027 and in three samples at Site M0029.

• Types 15 and 16: these types are characteristic bimodal shapes within a wide range of grain size, indicating poorly sorted silt to sand sediments: the mode for Type 15 has a set of left lower and right higher peaks at fine silt and very fine sand, respectively. Type 15 very sandy silt is only detected in three samples at Site M0029. Type 16 mode has two similarly high peaks situated on the coarser side, namely medium silt and fine sand. Furthermore, Type 16 contains common medium sand grains. Type 16 very silty fine sand is recognized in two samples at Site M0027 and in four samples at Site M0029.

Sandy sediment types

Figure 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.

• Type A differs from Types B and C; the mode is skewed toward finer and the shape is asymmetrical, with the gentle concave left slope finer than medium sand. Type A medium to coarse sand is recognized in two samples from each site.

• Type B is characterized by a mode with symmetrical shape with an acute and high (leptokurtic) peak and both steep concave slopes, indicating well-sorted medium to coarse sand deposits. Type B is common at all three sites.

• Type C has the left side steep and leftward-continuing low skirt, showing a small amount of finer components than fine sand. The right half is similar to Type B in the rightward concave shape. Type C (medium to coarse sand deposits) is abundant at Site M0027 but was not found at Site M0029.

• Type D is characterized by a mode with a central high peak on medium to coarse sand and both sides smooth and flat over a wide range of grain sizes, indicating that this sediment is very poorly sorted. Type D also includes very coarse sand and granule gravel.

• Type E has a trapezoidal shape with a gently rightward-inclined flat top (fine to coarse sand) and symmetrically inclined sides.

• Type F is characterized by a mode with left-side steep slope and right-side gentle slope and a peak between medium and coarse sand. It includes granules and a small amount of small pebbles. Therefore, Type F granule-bearing medium to coarse sand is a very poorly sorted sediment similar to Types D and G.

• Type G has a conspicuous bimodal shape with the left peak on fine sand and the right peak between coarse and very coarse sand. Type G poorly sorted fine to very coarse sand includes a small amount of granules and small pebbles.

• Type H is a medium to very coarse sand with granules. Its mode peaks at medium sand, though it includes a large amount of coarse to very coarse sand. The mode for Type H has a very steep left slope side and a gentle right slope.

Stratigraphic trends of average grain-size variations

The 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 M0027

Figure 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 M0028

Mean 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 M0029

Though 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 distribution

Types 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 M0027

Among 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 M0028

As 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 M0029

Curves 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.

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