Koza river is a 40 km-long river which originates from the southeastern foot of the Daitou Mountain located on the boundary between Koza Town and Tanabe City in Wakayama Prefecture. It runs southerly with merging Hiraigawa and Soénogawa rivers at Nanakawa reservoir. Then it runs to the southeast to the Kumanonada (Pacific Ocean) with merging some more minor tributaries. The coverage is 356 km².

Koza River rocks

Geology of the Koza valley mainly consists of the Tertiary Shimanto accretionary complex and the overlying Kumano Group sedimentary rocks. In contrast to the Kumano river, distribution of the Kumano acidic rocks is very limited, which only exits as an E-W striking dike and the associated small dikes striking perpendicular to the main dike. However, cobbles of the Kumano acidic rocks are relatively common in the sampling location maybe because the location is close the dike body.

Sandstone of the Kumano Group
The main constitution mineral of this sandstone is quartz, plagioclase and K-feldspar with minor micas. Feldspars give dusty looking due to alteration whereas quartz gives transparent looking because it is not altered by weathering. Plagioclase and K-feldspar can often be distinguished by twinning: the former gives a parallel black-and-white stripe pattern under crossed polars, whereas the latter gives a black-and white check pattern. This rock also contains various kinds of rock fragment. A chert fragment is found in the left of the photograph. Matrix is very small in this rock.

Bulk sand

Quartz occupies more than 60 %, and the sum of quartz and feldspars occupies almost 100 %. Among feldspars, albite (Ca-free plagioclase) is relatively abundant, which might be derived from the weakly metamorphosed Shimanto accretionary complex.

Heavy Minerals

Heavy mineral modal composition in the Koza river sand is very simple. Ilmenite occupies about 90 % and the rest is occupied mostly by zircon. Both of these minerals are common in felsic igneous rocks such as granite.

Orthopyroxene in the Koza river sand is euhedral and considered to be of tephra origin. Distribution of magnesium number (Mg# = Mg / (Fe²⁺ + Mg)) of the orthopyroxene shows a normal distribution with a mode at Mg# = 66-68, except for two iron-rich grains. This infers that these orthopyroxenes are derived by a single volcanic eruption. Compositions of two exceptional orthopyroxenes are in the range of Mg# 44-48, which corresponds to the mode of the Kumano orthopyroxenes