Abekawa is a 53.3 km-long river that originated from the ridge bounding Shizuoka City, Minobu Town and Hayakawa Town. It merges with Warashinagawa-river at the 5 km upstream from its mouth and flows into the Suruga Bay, with total coverage of 567 km2. The Abekawa-river is rather steep because of high elevation rate of the Southern Fossa Magna, an orogenic belt formed by a collision of the Izu Peninsula to the Honshu Island. For this reason, the usual volume of the water is small unlike for its width, which abruptly changes into a violent muddy stream on stormy days. A heavy rain causes a supply of voluminous amount of landslide materials from the Oya-kuzure, one of the three greatest landslides in Japan, near the origin of the Abekawa-river. This makes a characteristic looking of the Abekawa-river banks covered by relatively large-sized cobbles and gravels even in the vicinity of its mouth.

Geology of the Abekawa-river
Both of main stream of the Abekawa-river and the Warashinagawa-river are covered chiefly by the Shimanto Supergroup, a Tertiary accretionary complex. In the middle reaches of the main stream, serpentinite and magnesite-bearing rocks are characteristically distributed. In the uppermost stream, granitoids (mostly granodiorite) intrusions are distributed in the Shimanto Supergroup. With an intrusion of granitoids, gold deposit was formed around them. There used to numbers of gold mines in this area, and still some gold dusts are found in the Abekawa-river banks as well as Fujikawa-river.

See textbook for geologic maps (PDF)

Cobbles in the Abekawa-bank rivers are divided into three groups by their origins.

Group 1: Deep sea origin)
Peridotite, a basal rock of the oceanic plate (now converted to serpentinite by oceanfloor alteration) and basalt (now converted to greenstone), and the overlying deposits of chert and red mudstone.

Group 2: Trench-fill sediment origin
Sandstones and conglomerates of terrigenous origin.

Group 3: Younger igneous rocks
Granitoids (granodiorite and diorite) and volcanics erupted on-land (now mostly altered to greenstones).

Serpentinite
A metamorphosed peridotite composed mostly of serpentine (minor spinel is also included).
Upper) open polar, lower) crossed polars.

Quartz and albite (Ca-free plagioclase) are the dominant minerals. Modal proportion of heavy minerals are generally low and most grains are considered to be sandstone or low-grade metamorphic rocks origin. A major difference from the Fujikawa-river sand is lack of plagioclase (excluding albite). This is because the absence of young volcanics like Mt. Fuji in the valley of the Abekawa-river.

Heavy Mineral Modal Proportion

Clinopyroxene, ilmenite and spinel are the dominant heavy minerals followed by amphiboles, epidote, zircon and pyrite. The sparseness of orthopyroxene infers little distribution of ashfall in this region. Clinopyroxene and spinel are originated from greenstone (altered basalt) and serpentinite (altered peridotite), respectively.

Although very rare, orthopyroxene compositions show a peak of Mg number at 70, which is clearly different from that of the Fujikawa-river, suggesting a different ash for their source.

Age of monazite makes two clusters: < 300 Ma and 1800–2000 Ma. Granitoids along the Abekawa-river have ages about 10 Ma. Monazite from sand is much older than them and considered to be derived from sandstones of the Shimanto Supergroup. This pattern is similar to those observed in many sandstones in Japan. There is no formation nor rock older than 1800 Ma in Japan, hence these monazite grains are derived from the Eurasian continent.