Geopic of the week: Turkey fat smithsonite

 

Turkey fat smithsonite is the common name for the variety of zinc-ore mineral pictured above.  It derives its name from its yellow color and globular crystal habit (called botryoidal which is Greek for bunch of grapes).  The yellow is due to the presence of cadmium.  Early miners likened its appearance to the fat of a delicious turkey.

Smithsonite, along with other lead- and zinc-bearing minerals, was mined in the lead/zinc districts of the northern Ozark Plateaus and the western Ouachita Mountains.  Production started in 1857 and ended in 1962, with the peak occurring during WWI.  Though mining has ceased, it’s estimated that 110,000 short tons of shallow resources remain, and significant deeper deposits may, as of yet, have gone undiscovered.

Happy Thanksgiving from the Arkansas Geological Survey!

Geopic of the week: Asymmetrical anticline with intrusion

This photo is of an asymmetrical anticline in the Stanley Formation.  It’s asymmetrical because the right limb of the fold is dipping at a steeper angle than the left limb.  This type of fold is common in the Ouachita Mountains, however, this one has a small igneous intrusion on the left limb (lower left, dark gray).  The intrusion consists of a dike, which split several of the lower beds at nearly a right angle, and a sill emplaced parallel to the bedding.

From this picture, and basic geologic principals, we can tell the history of these rocks.  Sediment was first deposited in horizontal layers (principal of horizontality).  Later, the layers cemented to form solid rock – the layers must have been firm before they were deformed because they maintained their shape.   Next, tectonic forces in the earth bent the rock into an anticline and, after it was folded, the igneous intrusion was forced into the rock. We know the intrusion was last because it cut across the rock layers and the fold (principal of cross-cutting relationships).

One of the most challenging aspects of geology is interpreting a lot from a little information.  It’s also part of what makes it so interesting!