Saturday, January 28, 2017

Corundum crystals from Southern California

The mineral corundum, which is second only to diamond in terms of hardness, consists of aluminum oxide (Al2O3). Corundum comes in a variety of colors, depending on the trace amounts of other minerals (e.g., rutile = titanium oxide) it contains.

The color can be red, blue, yellow, brown, green, or purple to violet, and some crystals contain color zones. Pure corundum is white. If the color of corundum is red, it is called rubyIf the color is blue, it is called sapphire.

A friend recently gave me some corundum crystals from Cascade Canyon, San Gabriel Mountains, about 2 miles southwest of Mount Baldy, which is near the town of Upland in Los Angeles County, Southern California

A hand specimen (4 cm wide) containing small, scattered
 crystals of corundum. The color is between ruby and sapphire.
Most collectors would most likely refer to these crystals as ruby.
Close-up of the left-middle side of the hand specimen shown above.
The lenticular crystal in the lower right side is 4 mm long.

The corundum at the Cascade Canyon locality formed when complexly deformed sedimentary rock (of Paleozoic age) was contact metamorphosed (heated up) by small granitic intrusions (of Cretaceous age). 

If you want to see outcrop pictures and more information about this locality, just Google the phrase:  Cascade Canyon ruby

Sunday, January 15, 2017

A middle Eocene heart urchin

Heart urchins, also called spatangoids, are echinoderms (sand dollars, sea stars, etc.), which are generally characterized by having 5-rayed (pentameral) symmetry. This post focuses on a middle Eocene heart urchin known as Schizaster diabloensis Kew, 1920. It was named for its occurrence in sedimentary layers near Mount Diablo, just east of San Francisco.

A hand specimen of siltstone rock from the Llajas Formation has three specimens of
S. diabloensis on the same bedding plane. The hand specimen is 5 cm (2 in.) wide.
This species of heart urchin was common in northern and southern California during the middle Eocene (approx. 47 million years ago). The specimens shown here are from the Llajas Formation in Simi Valley, California. This formation was deposited in shallow-marine, warm-water conditions. The entire geologic time range for this species is late Paleocene through middle Eocene.

Five specimens of S. diabloensis from the Llajas Formation. The largest specimens are
  2 cm (0.8 in.) wide. All are top-side up.
Echinoderms, past and present, are strongly gregarious and can occur in great numbers on the ocean floor. Spatangoids have a fossil record extending back to the Cretaceous. They are burrowers and living below the surface provides protection against predators. During the Cretaceous, many new forms of predators evolved, which, which gave the force for some echinoderms (like spatangoids) to adapt to these adverse conditions by becoming infaunal (i.e., burrowers), mainly in fine-grained deposits, like siltstone.

You can readily see the five-rayed symmetry of the feeding grooves on the dorsal (top) surface of each specimen. The central groove, called ambulacrum III, is the longest and is sunken on most spatangoids, whereas the two posterior grooves are smaller. 

Monday, January 2, 2017

A Late Cretaceous stalked crinoid stem

Crinoid remains are extremely rare in the Late Cretaceous fossil record of California. A friend recently donated a stalked crinoid-stem fossil collected from Upper Cretaceous rocks in the Santa Ana Mountains, Orange County, Southern California. I have seen many fossils from these rocks but never a crinoid. Its geologic age is Turonian (about 90 million years old). The genus of this fossil is unknown.

This specimen is 8 cm long and 3 mm wide. I also put a modern-day crinoid "stem" (from Cuba) alongside, for comparison; it is 6.5 cm long and nearly 3 mm wide. You can definitely see that the fossil is, indeed, a crinoid.

Crinoids are echinoderms. Some other examples are sea stars (starfish), brittle stars, sea urchins, and sand dollars. Crinoids were very common in Paleozoic faunas, and their remains have contributed substantially to Paleozoic limestones. Crinoids today are less abundant than they once were, but at the present time there are approximately 25 stalked genera (all attached to the ocean floor and restricted to depths greater than 100 m). There are also about 90 or so unstalked genera, and these are able to swim about when they are adults.

This drawing shows the main morphologic parts of a stalked crinoid (i.e., having a column or "stem"). The "stem" was originally somewhat flexible during life and could sway slightly with the prevailing water currents.

Both the fossil and modern-day columns shown above in the photo are missing their calyx (where the stomach was located) and their arms.