18 replies to this topic

[Lighthawk]

The granite boulders mentioned in my report are an interesting puzzle.  They seem to be above one layer of basalt and beneath another.  The lighter colored alluvial fans are a high percentage of granite.  

 

 

 

 

 

 

[ski3pin]

Andy, thanks for providing these photos! Yup, a very interesting puzzle.

[Foy]

I wouldn't overlook spheroidal weathering as contributing to the roundness of the granite boulders.  The top pic makes me wonder if the lower basalt was laid down as a flow which ceased extrusion from the vent for long enough for an alluvial fan emerging from higher ground to develop on top of it, with such deposition then interrupted by a resumption of basalt extrusion flowing over it.  The top of the bouldery layer directly behind Ms. Lighthawk's head appears to have tan clayey material between the boulders.  I wonder if that represents a soil horizon developed at the surface atop the alluvial fan before the younger basalt flow covered it up.  The bottom pic looks a lot like a soil horizon (a paleosol) in that it appears to be clayey (directly behind the rock hammer handle) and sits directly atop a bouldery horizon and is capped by more basalt which covered and preserved the otherwise easily eroded soil horizon.  If you were to carefully pick around in the clayey material you might find root casts and if you did, the alluvial sheet deposit over a basalt flow and itself overlain by another basalt flow might be the answer.

 

Foy

Edited by Foy, 13 January 2022 - 10:39 PM.

[Lighthawk]

I wouldn't overlook spheroidal weathering as contributing to the roundness of the granite boulders.  The top pic makes me wonder if the lower basalt was laid down as a flow which ceased extrusion from the vent for long enough for an alluvial fan emerging from higher ground to develop on top of it, with such deposition then interrupted by a resumption of basalt extrusion flowing over it.  The top of the bouldery layer directly behind Ms. Lighthawk's head appears to have tan clayey material between the boulders.  I wonder if that represents a soil horizon developed at the surface atop the alluvial fan before the younger basalt flow covered it up.  The bottom pic looks a lot like a soil horizon (a paleosol) in that it appears to be clayey (directly behind the rock hammer handle) and sits directly atop a bouldery horizon and is capped by more basalt which covered and preserved the otherwise easily eroded soil horizon.  If you were to carefully pick around in the clayey material you might find root casts and if you did, the alluvial sheet deposit over a basalt flow and itself overlain by another basalt flow might be the answer.

 

Foy

 

 

Thank for weighing in, Foy!   I was hoping you would comment.

 

What you say makes sense to me.   Clearly the granite boulders are both on top of basalt and beneath another flow.

There are are areas where the surface is composed of a mixture of volcanic and granitic material, which are much lighter than the surrounding basalt flow.  The geologist who took us there explained the granite weathers more quickly, perhaps leaving quartzite (if I recall correctly) behind in larger pieces.

 

 

The question that seemed to be most perplexing was how these fairly large weathered boulders were transported here.  

You can see the size of some of the feldspar crystals.

 

 

 

We'll get back there next winter, if not sooner and poke around looking for root castings.  There have been other castings found in lake bottom sediments nearby, but those have a different history I believe.   Fascinating stuff!

 

Here's a 4k sized image with a bit more detail:

 

Edited by Lighthawk, 14 January 2022 - 03:36 PM.

[ski3pin]

Andy, did GeoMitch mention the source of the granite, where did it come from?

[Lighthawk]

That's still a big part of the puzzle

[ski3pin]

Ahhhh................

[Foy]

I can't stress more strongly that someone like GeoMitch, who has not only done graduate work in the area but has probably worked the section professionally since his training, is the true resource in matters such as this, not yours truly. I feel absolutely inadequate to do anything other than SWAG when it comes to a setting as historically and presently as dynamic as Death Valley.  But what the heck, it ain't like I'm preaching "settled science" for political gain, so here goes:

 

From all of < 30 minutes skimming of some USGS pubs from the 1960s and a couple of other more recent sources, the pluton/batholith sized granitic intrusions generally mapped as either the Paiute Monument or the Pat Keyes intrusive are huge in aerial extent and fairly old by DV standards--mid- to late Mesozoic (mid Jurassic to late Cretaceous).  With the extraordinary degree of horst and graben development from the late Cretaceous through last week, it seems that with source rock areas the size of batholiths (generally 40 square miles or more in aerial extent), we might expect extensive near-surface spheroidal weathering within the outcrop area of  the plutons and mass wasting to combine to form widespread interlocking alluvial fans containing extensive granite cobble and boulder content which could cover many square miles of area away from and off of the flank of the batholiths, over then-recent basalt flows forming mainly in the valleys,, only to be covered by younger basalt flows.  If we were to envision the exact area photographed as having been near the toe of alluvial fans, we'd expect some mix of smaller boulders and cobbles and finer fan sediments,  including clays.  We'll remember that the extensive feldspars within the granites to also chemically weather into clay minerals, also, such that the toe of the fan could represent a more gently sloped area on which a paleosol might develop.  Then, turn on the faucet of more basaltic flows from a nearby vent, and the relatively thin veneer of "toe of the fan" deposits are sandwiched and somewhat protected from erosion by the basalt "cap" and thus preserved until Andy and Ms. Lighthawk arrive on the scene with a great deal of curiosity and expertise with the camera. 

 

The USGS pubs mention outcrops of the Paiute Monument and Pat Keyes plutons including boulders up to 50' wide partially buried in a soup of quartz and feldspar sand, so perhaps that development within an alluvial fan deposit miles away from the main outcrop area continues after the initial transport from the outcrop area of the granites into the deposits of the fan and have the look and feel of a quartzite interfingered with clayey material which had been either actual sedimentary clays or weathered feldspars.

 

One thing I have the hardest time envisioning is the extensive very young and ongoing uplift and down-drop (horsts and grabens) of the present day DV area ranges and valleys, and to a great degree, all over the Mountain West  I'm much more wired to expect little to no Cenozoic tectonic activity (though much to the surprise of many, some very recent detailed geologic mapping in the Blue Ridge Mountains has led to the inference of Neogene uplift in the form of vertical faults with movement the order of tens to > 100 meters right in the heart of the Southern Appalachians in Boone, NC).  So much for "settled science", eh?

Anyway,  what you found and photographed is absolutely fascinating to this old geologist, and I hope to  learn more about it in the future, especially when some geologists with a far better understanding of the area than I have can weigh in on the matter.

 

Foy

[Lighthawk]

Thanks for laying it out, Foy.  I think I can follow 80% of what you're saying     

 

I'm fascinated by the intricacies involved it the transport of non-native materiel and uplift/graben-horst dynamics.  Saline Valley is a major DV graben.  Whew, all of this makes for a complex 3D timeline and makes my head hurt.  This same area has numerous faults with travertine deposits from active springs and chunks of lake bottom sediment deposits strewn about.   You would love it.  It's a winter location, unless you like heat.

 

This isn't my discovery and I don't want to speak for GeoMitch.   It was generous of him to take us out there and show us his conundrum.  If I learn more, I'll certainly report back.  Glad to get another "young at heart" geologist's interest piqued. 

Edited by Lighthawk, 15 January 2022 - 04:57 AM.