My Catch in Togawa Park I, from Oyama Subgroup: Picking up Stones in Tanzawa III

 

A hem. So, this week I show off my catch from Mizunashi River 水無川 in Tokawa Park 戸川公園 this summer. First, let me repeat the map I showed you last week.

Let’s begin with the stones from Oyama Subgroup 大山亜層群, colored blue on the above map. Yeah, Tokawa Park is in Hadano Basin, but these stones have tumbled down from deeper inside Tanzawa 丹沢. I tell you why we thought so one by one. Oyama Subgroup is nearest to the Park. The first is this.

Don’t you think it has greenish hue? I think it’s green tuff. They were originally volcanic ash and magma spewed out from undersea volcanos that were very active some 17million years ago on Philippines Plate. Very hot sea water with high pressure at the bottom of the ancient ocean created chlorite from the silica-containing magma that gave the rock green pigment. A microscopic view for its surface is this.

Many kinds of granules there. The stone does not change color when we wet it. It means these small pieces stick together so tightly that water does not steep in easily. It’s an evidence how hot and with heavy pressure were applied when they became tuff. We can find the layer of green tuff in Oyama Subgroup area next to Togawa Park. The average thickness of Oyama Subgroup is about 4000m. Lots of green tuff made of ash is there and a mass of cooled magma also became a part of 4000m. Oyama Subgroup has exa-tons of basalt, like

In large rocks in Tanzawa, basalts were often trapped in hardened volcanic ash which became tuff. When a rock of tuff was crumbling into piece, the confined remnants of volcanic magma like these were released. The white dot in the first stone here may be zeolite and/or calcite from ground water that filled holes after volcanic gas was released when magma was cooling. Also, as Oyama Subgroup was made from underwater volcanic activities millions of years ago, the area has fossils of oceanic creatures buried under the pile of volcanic ash. Here is an example:

Fossils? This one? Oh yes. This piece has lots of fossils of forams. Could you recognize a sort of flattened tiny rugby balls inside? Here’s its microscopic photo.

The stone containing these fossils was cut from “above” to show their rugby ball form. I polised it by sandpapers. The most famous form of forams is star shaped, but many forams have pancake shapes. Here we’re observing fossils of pancake-like Nephrolepidina lived some 15 million years ago 240km offshore from Tokawa Park. When we cut them half in the middle and see them from side, they look like a flattened rugby ball. Forams are tiny protists which have calcareous test (“shell”). Especially for those floating in the ocean, the speed of evolution is very fast and they diversify themselves in numerous forms. Such speedy change gives us a good indicator of time for the formation of a rock when they were entrapped. These squashed rugby balls lived in the ancient Pacific Ocean some 15 million years ago, i.e. the formation of this rock was around that time. They were carried in the rock from offshore, crushed with Eurasian Plate, and raised from the bottom of the sea to the ASL 290m (at Okura Bus Terminal 大倉バス停) and up for Oyama Subgroup. Oh, one thing. Now in Tokawa Park, we cannot find fossils. The above stone was collected from another point of Oyama Subgroup. There are several locations in the Subgroup where we’re still able to find fossils easily. I don’t tell you where in this blog. Sorry. 😉

I also found this in Togawa Park. A mixture of hard epidote and tonalite.

With microscopic view, we can enjoy granules of greenish epidote and black-white tonalite whose part is oxidized and turned brownish.

Beautiful, isn’t it? GoogleGemini said this stone could be a metamorphic tonalite. Deeper down Tanzawa Group, there is a huge mass of tonalite made of magma (more to it next week). It came there by intruding into the crustal-shortened underwater volcanic ashes et al on Philippines Plate. The tonalite received terrific heat and pressure from the tectonic clash. Especially under Oyama Subgroup, the piling up magma and ash contain lots of iron and calcium that are dissolved in the underground water. These elements met with tonalite, chemically reacted and produced epidote. This one also could have the same history. But it’s more closer to tonalite.

Tokawa Park is certainly on the border between Hadano Basin and Oyama Subgroup. But Mizunashi River and its system can carry another types of stones from neighboring subgroups. I show you next week my catch travelled from these areas to Hadano Basin. Please stay tuned!

At the entrance of Tokawa Park, there is Tanzawa Visitor Centre. The contact address is

Hadano Visitor Center for Tanzawa-Oyama Quasi-national Park
丹沢大山国定公園・自然公園秦野ビジターセンター

1513 Horiyamashita, Hadano, Kanagawa Prefecture, 259-1304
〒259-1304 秦野市堀山下1513

Phone: 0463-87-9300
https://www.kanagawa-park.or.jp/tanzawavc/