Friday, April 4, 2008

Death Valley Day 3: The Break-up of Rodinia

The Neoproterozoic section in and around Death Valley is metamorphosed to the point of being almost occult; there are tantalizing little glimpses of original fabric and textures, and diligent searching can turn up some sedimentary structures here and there, but by and large, the whole thing can be fairly frustrating. I can only imagine that it would take a fair bit of time to get your eyeballs trained to these old, busted-up rocks, and a fair bit of confidence and imagination to get anything out of them.

Regardless, these rocks preserve a record of the break-up of the supercontinent Rodinia, an event which was at least as enigmatic as the break-up of The Pixies in 1993. As if that isn’t enough for you, some workers have found evidence for the ol “Snowball Earth” hypothesis in these units (in the form of diamictite and cap-carbonates).

We spent all day wandering through the impressively beautiful Goler Wash, near the ghost town of Ballarat in the Panamint Valley, outside of the park. The picture below really doesn’t do it justice. This area is somewhat famous (amongst the geological cognoscenti only, I suspect) for having a fairly completes section of the Kingston Peak Fm and its defined members.

The Kingston Peak Fm rests on gneissic basement, and has a basal breccia dominated by angular gneissic, granitic, and quartzite clasts (see below for a picture). Interestingly, some interpretations suggest that this is a weathering horizon, implying a substantial amount of time preserved on that surface.

Above the basal breccia, coarse siliclastics, some limestones, and conglomerate facies make up the Limekiln Spring and Surprise Members of the Fm. Apparently, mapping at a more regional scale shows that the Limekiln and Surprise Mbrs show evidence for syndepositional extension associated with the initiation of rifting and break-up of Rodinia.

Of particular interest is the Wildrose Diamictite, which unconformably overlies the lower members of the Kingston Peak Fm (see below). The dark-colored diamictite is overlain by the Noonday Dolomite, and together, forms the typical “glaciated to cap-carbonate system” that have been interpreted to show evidence for Snowball Earth.

On an aside, I think the term diamictite has been brutally abused for many years, and it really is time for someone to call Nomenclatural Protective Services. It seems that it is often used as a short-hand for “glacially-derived conglomerates”, and more specifically as a short-hand for “glacial dropstone-cgl”. Diamictites are just poorly sorted conglomerates with a wide range of grain-sizes, 25% of which are gravel-sized or coarser. That’s it! No genetic connotation what-so-ever!

Still, these diamictites have been interpreted as glacial-drop stones. Frankly, I don’t think these rocks show any evidence for that interpretation in outcrop (see below for yourself). To my mind, the evidence needed to convincingly demonstrate a dropstone origin requires disrupted bedding (when the rock drops into the substrate) and evidence for syndepositional thinning and draping of overlying sediment. Otherwise, who’s to say that this isn’t a hyperconcentrated flow, or a debris flow, or something similar. In the papers I’ve read, there has not yet been any particularly convincing evidence for a drop-stone origin, in my humble opinion.

Anyway, overlying the diamictite is a thick dolomite unit; similar strata relationships have been observed is less metamorphosed rocks elsewhere, and on the basis of these better preserved units, these have been interpreted as cap-carbonates. In this interpretation, the world-girdling glaciers that cut off the surface of the oceans from the atmosphere must have severely disrupted the CO2 cycle. This would have resulted in a build-up of atmospheric CO2, which would eventually overwhelm the ice albedo, and initiate melting. The sudden input of all the atmospheric CO2 would result in sudden, rapid precipitation of carbonate in the oceans, including some carbonate textures from other capping intervals that have been interpreted as meter-scale authigenic calcite and aragonite fans precipitated on the seafloor. Pretty wiggy stuff, but we didn’t see any of that in Goler Wash, due to its fairly cooked nature.

Anyway, should you find yourself in the vicinity of Ballarat in the Panamint Valley, drop on by Goler Wash; it is a pretty neat section to walk through, though at the end of the day, you might just decide that you are glad you don’t have to work on those rocks (and if you do have to work on those rocks, allow me to offer my sincerest condolences).


Callan Bentley said...

Great post, awesome rocks, and it sounds like a fun trip.

After extended conversation on this topic with Jay Kaufman (UMD College Park), who has worked on the Noonday Dolomite and associated units in Death Valley, here are the terms as I understand them:

DIAMICTITE : Poorly sorted sedimentary rock that is matrix-supported (as opposed to clast-supported -- like the one seen in Chris Rowan's geo-puzzle #10 today). No genetic assumptions made: Diamictites can be glacial in origin, or they can result from landslides or debris flows, too.

TILLITE : A diamictite with clear glacial origins.

DROPSTONE : An individual clast which shows clear evidence of truncating lower finer-grained sedimentary layers, indicating it dropped in from above. Overlying (finer-grained) strata are "draped" atop the dropstone.

A big clast in a poorly sorted rock is not a dropstone. The clasts in a tillite are not even dropstones. The term "dropstone" should be reserved specifically for this situation where we observe the truncation of underlying layers.

People who use "diamictite" when they mean "tillite" are being sloppy. And we shouldn't call a diamictite a tillite just because we want it to be one. We need clear evidence (including, in my mind, dropstones) to interpret a diamictite as a tillite.

That's my two cents! Thanks again.

Callan Bentley said...

An additional thought:

Do any of these cobbles appear faceted, or display glacial scratches?

Eric said...

I agree with Kaufman; the use of Diamictite has been fairly sloppy in the literature (SOME of it, at least; there are certainly folks who use it without any genetic connotation).

There was no evidence for faceted clasts, glacial striations, or any of the usual "glaciogenic" textures listed. Interestingly, there were clear changes in clast types through the section, with some intervals having many more granite clasts than quartzite clasts, etc.

All in all, enigmatic rocks!

Christie Rowe said...

AMEN on the diamictite. Here in South Africa, tillite and diamictite are used pretty much interchangeably and it drives me up the wall. thanks for calling them on it.