Friday, August 29, 2008

GC vs Owyhee

Dr. Jerque,

In a rare moment of 9-5 nonbillable geology work I submit:

The four observations you list (added below in italics) as some of the differences between the Owyhee and GC intra-canyon lava dams are some of the exact same ones I note in the manuscript that is collecting dust on my desk at home.
1. Volume of lava vs. volume of water
2. Mode of entry: canyon rim cascade vs. tributary valley route

3. Severity of topography (vertical and lateral trajectory of lava incursion)

4. Proximity of volcanic vent to the canyon; abundance of pyroclastic material.
This is fabulous! I look forward to hearing more about these items and your other GC observations.

Best Owyhee example of lava dam features

The Sand Springs Wash area on the Owyhee retains the best example of a lava dam in the field area. At this site, there are two lava flows from the Saddle Butte vent to the west. The overlying flow is separated from the underlying flow by a spectacularly well-developed lava delta with pillow-laden foresets that dip upvalley. The dam forebay is a turtle-back like feature with a 'carapace' of chilled lava. The surface of the older Saddle Butte lava upstream of the dam is covered with lacustrine mud which is capped with fluvial gravel.

Upstream of this site, the older Saddle Butte lava flow overlies Owyhee River gravel at the site of the Weeping Wall where a spring issues from the contact between the gravel and the lava. Upstream of Weeping Wall, the older Saddle Butte lava contains a sequence of lava deltas and subaerial lavas that suggest a complex interaction with the river.

Downstream, the eastern margin of the younger Saddle Butte flow is wasting away on a grand scale.

First insights from the Grand Canyon

Probably the most striking aspect of my recent trip down the Grand Canyon is the stark contrasts of the intracanyon lava flows and related response of the river there with those of the Owyhee.
  • The lava stratigraphy in the GC is notably more complicated than the Owyhee.
  • In GC the variability in lava textures and the facies variability of related volcaniclastic deposits is much greater. Some GC volcaniclastic deposits are downright bizarre looking to me.
  • In several circumstances, there is strong evidence supporting the occurrence of catastrophic flooding in conjunction with the incursion of lava into the Grand Canyon (note photo above); whereas on the Owyhee, there is no clear evidence of this.
  • In GC, there are few to no really good examples of lava deltas. The Owyhee has world class examples.
  • In GC, the intracanyon flows are not associated with large-scale (post-emplacement) mass wasting processes. The Owyhee has some spectacular post-emplacement landsliding of lava flow remnants and, thus, continue to plague the river for 1000s of years.
Bottom line is that there is great evidence in the Grand Canyon for lava dam failure and related catastrophic flooding. There is weak to no evidence in the Grand Canyon for effective, relatively long-lived damming of the river. In contrast, there is weak to no evidence on the Owyhee River to support catastrophic flooding related to lava dams. There is great evidence on the Owyhee that substantiates effective, relatively long-lived damming of the river by lava.

Obvious questions: Why the profound differences? Why is the Owyhee so much easier to dam with lava than the Colorado?

1. Volume of lava vs. volume of water
2. Mode of entry: canyon rim cascade vs. tributary valley route
3. Severity of topography (vertical and lateral trajectory of lava incursion)
4. Proximity of volcanic vent to the canyon; abundance of pyroclastic material.

I am pretty sure it involves aspect of each of these factors. Stay tuned for more maunderings and some more details on the points above.

Tuesday, August 26, 2008

SHRIMP in the Owyhee?? Fire up the barbee mate!

Dear Dr. Froude et al.:

I concur with Dr. Jerque. He refers to a plethora of field evidence that adds detail to the story presented in Ninad's work. I'd like to add that you may want to check out a paper from the Idaho State crowd of Beranek, Link, and Fanning (see below). This paper came out some time back and offers a few possible numbers for ages of big picture events in the region. If I recall correctly, the most Owyhee-relevant discussion items are based on only a few data points compared to other drainages studied and could be strengthened by additional analyses. Nontheless, it is pretty darn interesting to this spud farmer trapped in the concrete jungle.


P.S. Dr. Jerque: I think we are on the same page about the Rytuba and Vander Meulen work. I did overstate its relevance in reference to the Now Voluminous (Once Dreaded) Rim Gravels but refer to it merely in the context that it provides evidence of a long-occurring interaction between volcanism, the fluvial system, and expansive sedimentation in the region. I wonder if it is even relevant to the latest question because "The inital influx of major fluvial systems into the volcanic field after about 14.5 Ma is reflected...."(see abstract)?

How old is the Owyhee?

Today, Dr. Froude presented the most basic question: How old is the Owyhee River. I pulled some stuff out of the air in a quick response, and ultimately claimed the river developed between about 5-8 Ma and 1.8 Ma. I decided my response was too quick, and too poorly founded, so I started looking for some of Ninad's data. Turns out that Miami Univ puts all of their dissertations online. Very handy. Thus, I present some of Ninad's dissertation below to serve as a reference point for all of us. By combining his data with our collective field observations, I think we can answer the question. For the sake of completeness, here is my response to Froude's query:

Bogus Rim flow and underlying flows fill a surprisingly deep paleovalley that runs along the alignment of the modern canyon below iron point. Gravels are present at the base of the sequence in a few places (including possibly in the Owyhee Breaks area) and there are erosional intervals preserved between some of the flows. Not sure how well constrained the ages of the lowest flow (the 'lower Bogus lavas') are, but somewhere between 5 and 8 comes to mind (without looking anything up). There was a river flowing north before the Bogus lavas were emplaced. This river created the paleotopography in the Grassy Mountain Rhyolite and sediments before the first big barf of basalt flowed north. Based on the thickness of the Bogus Lavas in the Rinehart Canyon area, there were some deeply incised tributaries flowing into the river.

In terms of field evidence, the biggest influx of gravel occurs in conjunction with the end of the damming event caused by the Bogus Rim flow. I believe that evidence is mounting that a very large lake occupied the area upstream of where the Bogus Rim flow would have created a dam somewhere near Iron Point. Thus, the pre Bogus Rim river was probably a nearly full blown Owyhee. Possibly the full blown river developed in conjunction with surmounting the Bogus Rim dam? That should have been sometime after about 1.8 Ma. I recently collected a tephra from the lake sediments that I postulate were deposited into a Bogus Rim dammed lake. Dating that may be of some value.

Here is the geochronology from our area as reported by Ninad Bondre:
Here is a nifty map that places these data in a better context:
Also, Ninad's thesis includes a handy little geologic map:

Some thoughts:

Despite the geochronology, I am thinking that the Owyhee Butte lava must predate the Bogus Bench lava. Field relations suggest that the gravel that pervasively overlies the Owyhee Butte lava is related to damming of the Owyhee River by the Bogus Bench (Rim) lava. I have not seen any gravel below the Owyhee Butte lava (forms the Artillery Rim), but Liz and I noted in July that there are hyaloclastite units in the upper parts of this package. Note also that the Bogus Bench (Rim...Brim?) lava has no gravel on top until you get a few km downstream from Iron Point. There, you find a 5-8 m thick deposit of locally derived, but rounded gravels. (I have shown pictures of this before in a previous post, but may add them again soon for emphasis). We know from multiple locations that the Bogus Rim lava flowed down a channel of the Owyhee River and overlies rounded gravel in various places. The local gravel pile likely relates to decommissioning of the dam.
Thus, the river is at least 1.92 Ma old. Note that the 4569 vent has a date of approximately 5.64 Ma. The one-sided plateau morphology of that vent and flow complex supports the idea that it formed a lake on its south side, much like Bogus Bench. However, I have not seen (nor looked hard for) gravel. My hunch is that this may be related to a precursor drainage. Speculative.
My preliminary conclusion is that the river formed between 5.6 and 1.92 Ma, probably closer to 1.92. Any thoughts?

All figures and the table in this post taken from:

Bondre, N.R., 2006, Field and geochemical investigation of basaltic magmatism in the western United States and India. PhD. Dissertation, Miami University, Miami, OH., 252 p.

Monday, August 4, 2008

Grand Hiatus for Dr Jerque

Fellow Yeehows. Unfortunately I will be spending the next three weeks slogging through the Grand Canyon with various experts on its geology. Special attention will be given to lava flows, lava dams(?), and all sorts of other things. I am hiking down to Phantom Ranch on Wednesday morning with a pack full of all the stuff I forgot to send ahead. Of special note is a new 10 MP Pentax camera that is waterproof and dustproof. This way I hope to get some fearless shots from the river. Recently took it to Tahoe to try it out:

I will certainly provide a synopsis when I return.