Friday, April 11, 2008

Death Valley Day 5: Lost Burro Section

A major component of the geology in the west is made up of the Paleozoic miogeocline; this wedge of sediment thickened seaward, and represents the tectonically quiescent passive margin of the western edge of the continent. There are some phenomenal sections in Death Valley that expose portions of this wedge, and one of the profs leading the trip has a sweet-tooth for Paleozoic miogeoclinal strata; as such, our fifth day in Death Valley was spent wandering through the Devonian Lost Burro Fm, near the Lost Burro Mine. The mine is up fairly high (3000+ meters, or so), and has that classic western geology look to it, all scrubby vegetation and outcrop.

This section has been interpreted as a large carbonate ramp, and is chock full of stromatoporoid sponge mounds (such as in the picture below). These sponges were the dominant reef-building critters in the Devonian.

There were also some sections dominated by fragments of a wispy, thin-walled Amphipora, which is also a stromatoporoid (thanks Shanan!) that must have been getting beat-up and sloshed around before its final deposition (below):

Thankfully, there were SOME bedforms to be seen in amongst the carbonates; the picture below shows some calciclactic (ooidal) HCS and SCS; this high energy lithofacies always capped off the shallowing-upward cycles (parasequences) preserved in this overall shallowing upward succession.

Further up the canyon, we were able to follow these carbonates all the way up to a pretty slick surface. The picture below shows a fairly important transition in the geological history of the world; the tan-orange stuff to the right in all shore-face sands (trough and planar x-beds, HCS and SCS, ripples, and shelly detritus), whereas the darker stuff to the left is all siliciclastic mud interbedded with thin carbonates.

It is pretty clearly a flooding surface, putting deeper-water units overtop of shallower, coarser units. But what is really nifty is the paleo story preserved here. Below that contact, there were stromatoporoid reefs; above that contact, there are only crinoids. Turns out, this is the Frasnian-Fammenian boundary, the smallest of the “big-5” extinction events. The stromatoporoids (and trilobites) got whacked pretty hard during this "event", and were replaced by the more familiar post-Devonian faunas and reefs (such as crinoid-reefs).


Silver Fox said...

I like the way you are posting about your D.V. field trip - a post for each day. I might try that with one of my recent trips. I thought of inserting them into the blog on the day they happened (by using previous dates, the dates of each field trip day), but I like the way yours are coming out.

Oh, you used that recently "debated" word - miogeocline!

Eric said...

As you well know, the geology out there in Death Valley is just too great; I ended up with so many pictures, that I felt that I had to split em up.

And I shall name first born "Miogeocline".

cambro said...

The "spaghetti rock" with the wispy fossil fragments is made up of Amphipora, which is also a stromatoporoid.

Eric said...

Thanks for the correction, Shanan!