Friday, October 21, 2016

Ancient wave-formed ripple marks

I have always enjoyed finding ancient examples of sedimentary structures that help a geologist determine depositional environments. The example I show here are ripple marks made by waves in a middle Miocene (about 13 million years old) lacustrine (lake) deposit in the Mint Canyon Formation, northern Los Angeles County, southern California.

The above picture is a vertical look at the ripple marks. The rock is siltstone, and these ripple marks were found in a section of muddy rocks containing very small freshwater gastropods.

This picture is a side view of the rock slab shown above. The up-current side of each ripple mark is low angle, whereas the down-current side is high angle. So, as viewed in this photograph, the wave current moved from right to left.

Saturday, October 15, 2016

Howlite, an evaporite mineral from southern California

This post concerns an interesting mineral which can be found not too far from where I live in southern California. The mineral is howlite (also called “white turquoise” by some collectors). It is a borate mineral (calcium borosilicate hydroxide) that is found in evaporite deposits. In my area, these kind of deposits occur in the middle Miocene Mint Canyon Formation in Tick Canyon, Soledad basin. The howlite found there occurs in its most common form, as nodules. This locality is known for its high-quality howlite, including some rarely found crystals of this mineral. To see pictures of these crystals (I do not have any), go to Wikipedia and type in the word “howlite.”

Nodule of howlite, 5 cm width.
The nodules are white with fine gray or black veins, which create an erratic weblike patterns. The nodules are used in jewelry. They are easily dyed to imitate other minerals, especially turquoise, which also commonly has a veining pattern.

This picture shows some of the variation found in nodules of howlite. Nodule is 5 cm width.

In Tick Canyon, howlite was mined for a time, along with other evaporate minerals. The mining operation has long since closed down.