Friday, June 17, 2016

Friday Field Photo: Cannonball Chert

Back in my eastern Nevada field days, I was lucky enough to go on a stratigraphy tour that took me to several good examples of chert nodules in the mostly Pennsylvanian Ely Limestone.
Nicely spherical chert nodules in the Ely Limestone, with 2.5 lb sledge for scale.
In the 1950s and 1960s, R. A. Breitrick and J. E. Welsh described the detailed stratigraphy of the Ely Limestone in the area of Ely and the Robinson mining district (with Breitrick, at least, continuing to work in the area to this day). They divided the formation into mappable units, W through C, bottom to top. (Their units A and B have since been placed in the overlying Permian Riepe Spring Limestone.) The only summary of this stratigraphy I could find online is shown here (pdf; Maher, 1995, p. 22).

Several units within the Ely, notably units S, R, P, L, and F, contain cannonball cherts (although I'm not sure about the details of units T, U, V, and W: I can't find my field sheets in the mess my office is currently in!). Theses photos are most likely from S, R, or P (my thoughts).
Fairly large, sub-spherical chert nodule.
This is the best example I have of a chert nodule that is approaching "cannonball"  in size and shape. Apparently, cannonball chert nodules can weather out and end up looking a lot like loose cannonballs. An excellent example of a loose cannonball nodule can be seen here [pdf] on page 27 (Maher, 1995, Fig. 7C).

These photos were taken in the spring of 2007 on the northeastern slopes of Rib Hill, a location that might or might not be accessible from Highway 50 or Route 6 via a side road formerly known as S.R. 44 or S.R. 485 (the location might be behind a locked gate).


John Spoden said...

I'll bet you walk over some nice points.
Ever been in the mountain tops north of round mountain... 10 or so miles to the north? The reason I ask is for some reason they still draw my attention in a mystical kinda way, the lure of gold kinda but more on an ancient nomatic habitat level. I know a little about the moutain tops north of Belmont and their ancient lure intices me to go camping this summer in the BS, my grandson loves spencer hot springs and is looking forward to a soak (2nd year Stanford). Anyway, thanks for the geology lessons.

Silver Fox said...

I've been near the top of Mt. Jefferson, which is about 10 miles north of Round Mountain -- hiked up there when the Alta Toquima archaeological dig was in progress. It's a pretty unusual area.

Inyo said...

An excellent photograph of chertified Chaetites sponges (the chert nodules--originally thought to represent a variety of coral) in the Pennsylvanian Ely Limestone. Identical silicified Chaetites sponge blobs occur in the Pennsylvanian Bird Spring Formation of southern Nevada (Las Vegas district).

Silver Fox said...

Thanks for the info on the nodules! I just added your page to my sidebar. :-)