Kumano river is a 183 km-long river which originates from the Omine Mountains in Tenkawa Village, Nara Prefecture. It runs southerly through Gojo City and Totsukawa Village, then flows along the boundary between Wakayama and Mie Prefectures to the Kumanonada (Pacific Ocean). The river is called as Totsukawa river in the upstream in Nara Prefecture, and changes its name to Kumano river in the Wakayama Prefecture after merging Kitayamagawa river. The Kumano river is meandering everywhere reflecting complex canyon topography. The coverage is 2,360 km².

Kumano River rocks

Geology of the Kumano valley is mainly consists of the Cretaceous to Tertiary Shimanto accretionary complex and the overlying Tertiary Kumano Group sedimentary rocks, which are overlain and/or intruded by the Tertiary Kumano acidic rocks. Shimanto accretionary rocks often exhibit intensely deformed structures that were formed during their formation under strong shear stress at subduction zone. For Kumano acidic rocks, rhyolite (volcanic rock, erupted magma), quartz porphyry (hypabyssal rock, crystallised at shallow depth) and welded tuff (remelted tuff after deposition by its own heat) are found.

Quartz porphyry (Kumano acidic rocks)
A sort of felsic hypabyssal rock. It resembles to granite, but crystallised from the source magma at shallower depth. In a plutonic rock such as granite, all constituent minerals exhibit almost equal grain size (equigranular texture). In contrast, quartz porphyry exhibits porphyritic texture that consists of large-grained phenocrysts (mostly quartz and feldspars) surrounded by finer-grained quartz.
(top: plain-polarised light, bottom: crossed-polarised light)

Mélange (Shimanto belt)
A rock that consists of a mixture of more than one lithology is called mélange. Transparent domains in the upper photograph used to be sandstones, which are now transformed into aggregates of finer-grained quartz due to recrystallisation during deformation (see enlarged photograph in the bottom). Less transparent domains that occupies the majority of the rock used to be mudstones. This rock might originally be deposited on the seafloor as a sand / mud alternation, which was complexly mixed with each other due to strong stress during subduction and/or exhumation processes.

Bulk sand

Quartz occupies about 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

Ilmenite occupies about 90 % and minor amount of epidote, TiO₂ minerals (rutile or anatase), zircon occupy the rest. These minerals are common in felsic igneous rocks such as granite.

Orthopyroxene in the Kumano river sand is euhedral and considered to be of tephra origin. Distribution of magnesium number (Mg# = Mg / (Fe²⁺ + Mg)) of the orthopyroxene shows a concentration in a narrow range at Mg# = 44-52 (mode at 46-48), suggesting a single volcanic eruption origin. There are also some orthopyroxenes of Mg# 64-70, which corresponds to the mode of the Koza orthopyroxenes

Zircon U-Pb ages
Zircons of very young age (< 25 Ma) are dominant. These are derived probably from the Kumano acidic rocks that erupted / intruded about 15 Ma. Minor peaks are also found around 100 Ma and 200 Ma. Although few, there are some grains older than 1800 Ma. These features are common with sandstones in accretionary complex in Japan, so these zircons are most likely derived from sandstones of the Shimanto belt.