LACUSTRINE CHEMICAL CHERT
Cherts are siliceous sedimentary rocks of biogenic or chemical origin. Many primary cherts beds are marine. Many chert nodules are secondary, diagenetic features formed by remobilization & localized precipitation of siliceous materials.
Here's an unusual form of chert. It formed in a rift basin lake environment in eastern Africa. This chert formed principally by inorganic chemical means, but microbes have been shown to catalyze some of the chemical reactions involved.
The first two photos below are cross-section views. The 3rd photo is the sole (bottom bedding plane) of the rock. They show that this rock consists of a gray layer between two pale greenish layers. This is an interbedded lacustrine chemical chert & erionite siltstone. The gray material in the middle is the chert (microcrystalline quartz - SiO2). The pale greenish layers are the erionite siltstone. Erionite is one of the zeolite minerals, and has the formula Ca3K2Na2(Al10Si26O72)á30H2O.
Stratigraphy & Age: bedded cherts of the upper Green Beds (a.k.a. lower High Magadi Beds; a.k.a. upper Chert Series), upper Upper Pleistocene, 40 k.y.
Locality: shoreline outcrop along the southeastern shore of Lake Magadi, southwestern Kenya, East African Rift Valley (vicinity of 1¼ 57Õ South, 36¼ 16Õ East).
Lacustrine chemical chert (gray) between two layers of erionite siltstone (pale greenish) (above & below; cross-section views; above: 11.6 cm across; below: 6.4 cm across).
Erionite siltsone (view of bottom bedding plane of rock shown above; 12.0 cm across at its widest) - some of the disturbed areas on this bedding plane represent crystal casts of gaylussite (Na2Ca(CO3)2á5H2O) or calcite (CaCO3).
So, how did this rock form? It's a bit of a long story. First of all, this rock wouldnÕt exist if it werenÕt for the Afar Hotspot ripping eastern Africa apart. Volcanic activity and volcanic rocks are common along the East African Rift Valley. In the Lake Magadi area, hydrothermal fluids below the lake floor percolate through the volcanic bedrock. The common lithology here is trachyte lava - an alkaline, intermediate, extrusive igneous rock. The trachytes have had some of their components leached out by the hydrothermal fluids. This typically includes sodium and silica.
As these fluids, rich in dissolved sodium and silica, enter Lake Magadi, precipitation occurs, resulting in sodium-silicate gels on the lake bed. Magadiite and eugsterite precipitate onto the lake floor. These two minerals are ephemeral (unstable). Magadiite is a hydrous sodium hydroxy-silicate (NaSi7O13(OH)3á3H2O) and eugsterite is a hydrous sodium calcium sulfate (Na4Ca(SO4)3á2H2O). The precipitation of these two minerals appears to be strongly catalyzed by cyanobacterial activity. Upon exposure to air, the ephemeral minerals dehydrate into chert beds. For the long, detailed version of this story, see Behr (2002).
- Tony Peterson (pers. comm.)
- Behr (2002 - SEPM Special Publication 73: 257-273).