This Earthcache brings you to the Serene Garden, a quiet little corner of the Gardens by the Bay modelled after a Japanese Garden. The Garden features Japanese rock gardens, with rocks imported from the Kochi Prefecture on the Japanese island of Shikoku. These rocks are primarily a type of sedimentary rock known as Chert which is the subject of this Earthcache.
What is Chert?
Fig. 1: A sample of chert
Chert is a sedimentary rock (see Fig. 1) that is composed mainly of microcrystalline quartz, which is a variety of the mineral silica (SiO2). It forms through the accumulation of silica-rich remains of marine microorganisms, such as diatoms and radiolarians (also known as biogenic chert), as well as through the chemical precipitation of silica from groundwater. Chert can also develop through replacement of pre-existing rocks by silica.
Fig. 2: Diatoms and Radiolarians
Biogenic chert often forms from the silica-rich skeletons of microscopic marine organisms, particularly diatoms and radiolarians (see Fig. 2). When these organisms die, their silica skeletons settle on the ocean floor and accumulate over time. The silica from the skeletons undergoes the process of diagenesis, which involves the gradual transformation of the original silica into a more compact and crystalline form. This process can result in the formation of a gel-like substance, which eventually hardens to become chert.
Chert can also form through the chemical precipitation of silica from groundwater. Silica-rich fluids percolate through sedimentary rocks, and when the conditions are right, silica precipitates out of the solution and forms chert. The source of silica in these cases can be volcanic rocks, organic matter, or pre-existing chert beds.
In some instances, chert can form through the replacement of pre-existing rocks, such as limestone, by silica. Silica-rich fluids replace the minerals in the original rock with chert, preserving the original structure to some extent.
Types of Chert
There are two main types of occurrences of chert, bedded chert and nodular chert:
Fig. 3: Bedded chert
Bedded chert (Fig. 3) forms in distinct layers or beds within sedimentary rocks. Bedded chert is often associated with ancient oceanic environments and can contain fossilized remains of microorganisms.
Fig. 4: Nodular Chert
Nodular chert (Fig. 4) occurs in discrete, irregularly shaped nodules within sedimentary rocks. These nodules often display a concentric layering, suggesting incremental growth over time.
Chert from the Niyodo River
Fig. 5: The blue waters of the Niyodo River
Niyodo Chert is a type of rock that is particularly notable for its occurrence in the Niyodo River area of Kochi Prefecture on the island of Shikoku, Japan. The Niyodo River itself is renowned for its clear cobalt blue colour, and it is a major tourist attraction for Kochi Prefecture.
The Niyodo Chert is renowned for its unique colors, ranging from reds and yellows to blue-grey, making it highly prized for ornamental purposes. The vibrant colors of Niyodo Chert are believed to be a result of the presence of various mineral impurities, such as iron oxides and hydroxides, within the chert matrix Niyodo Chert is primarily found in the Niyodo River basin and is associated with sedimentary rocks of the Chichibu Belt.
Fig. 6: Map of the extent of the Chichibu Belt
The Chichibu Belt (Fig. 6) is a geological formation located in central Japan, forming part of the south-eastern section Japanese island of Honshu and extending through parts of the Saitama, Yamanashi, and Nagano Prefectures. The belt continues in a south-westerly direction, forming the central part of the island of Shikoku and a section of the island of Kyushu.
The Chichibu Belt is associated with the convergence and collision of tectonic plates. Specifically, it represents the suture zone where the North American Plate has collided with and is subducting beneath the Asian Plate. This collision led to the closure of an ancient ocean, the Tethys Sea, during the Mesozoic era. The Niyodo Chert is formed from the sedimentary rocks made from the marine deposits that was once at the bottom of the Tethys Sea which was uplifted into mountains during this plate collision.
The Earthcache Tasks
Make your way to the published coordinates, where you will find a rock garden filled with large chert boulders. To begin on your tasks, position yourself in front of the electric box shown in the picture below, and look straight ahead.
You should see a chert boulder with a white quartz vein through its side. This quartz vein is probably an inclusion due to silica-rich water filling a crack in the rock formed during a tectonic event and then crystallising. Study this rock and answer the following questions:
- Describe the colour and the structure of the chert rock here (what most of the boulder is made from). Are the grain sizes visible in the chert? If they are, estimate the grain size.
- Based on the structure of the chert rock and the information provided about the Chichibu Belt above, suggest how this chert was formed (biogenic, chemical or replacement), and whether this is bedded or nodular chert.
- Measure the thickness of the quartz vein in the rock. In addition, using information in the listing above, suggest why even though chert and quartz are made of the same mineral (silicate), there is a difference in their colours.
Finally, make your way to WP2, where you will find a small waterfall lined with a different type of chert from Niyodo.
- Take a photo of yourself, your GPS device, or a personal item at this waterfall as proof of your visit.
As with all Earthcaches, please send your responses to me via email or the Message Centre. There is no need to get all the answers perfectly right as long as a conscious and honest effort is made.
Logs that do not meet the task requirements may be deleted.
I hope you enjoyed your geology field trip here, and happy caching!