Geology of Benson Lk. map published and free | High Sierra Topix  

Geology of Benson Lk. map published and free

A place to explore the natural setting (geology, flora & fauna), people, constructed infrastructure and historical events that play and have played a part in shaping the Sierra Nevada as we know it today.
User avatar

Geology of Benson Lk. map published and free

Postby mokelumnekid » Tue Feb 26, 2013 4:15 pm




User avatar
mokelumnekid
Topix Expert
 
Posts: 446
Joined: Mon Sep 22, 2008 4:45 pm
Location: Seattle
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby rlown » Tue Feb 26, 2013 6:22 pm

Oh great wizard, I've looked at the map. Now explain it to us in layman's terms. I read the notes, but I love personal narration. Is this subduction process in the past and what is Ma?

Does the intense amount of Kyvg to the west mean we've been pretty inactive lately, geologically speaking? Is there a timeline from A to B?

inquiring minds..

Russ

PS: I love these topics. I never know what I'm walking on sometimes..
User avatar
rlown
Topix Junkie
 
Posts: 5325
Joined: Thu Oct 25, 2007 5:00 pm
Location: Petaluma and Wilton, CA
Experience: Level 4 Explorer

User avatar

Re: Geology of Benson Lk. map published and free

Postby mokelumnekid » Tue Feb 26, 2013 9:26 pm

Hi Russ:

As far as not knowing what we are walking in, that goes for all of us! The piece of the map that has all the interesting details around Benson Lk. is older than the "granites" that we are accustomed to. They represent packages of much older sedimentary rocks that may have been translated northward from the Mojave area along a fault system that was subsequently obliterated by the intrusion of all the granitic rocks. So if nothing else, when visiting Benson marvel at how they are different from the typical flavors of granite that we normally see. "Ma" refers to millions of years. These dates are new and based on cutting edge techniques that are much more accurate than the previous methods.

One interesting debate is what the mapped extent granite bodies mean. They represent vast bodies of molten rock, but the issue is was the surface expression of this typical volcanoes like Shasta, or large calderas like Long Valley caldera that extends from east of Mammoth Lakes south to almost Bishop? Or perhaps even more like the huge caldera systems that one sees in the Altiplano-Puna regiuon of northern Chile today? My research has suggested the latter, but there is a lot we don't yet understand.

Most all the Sierra granitic rocks are old, from about 86 to over 100 million years old. The youngest ones are typically at the easternmost, higher part of the range and form distinct suites that all seem to have the diagnostic occurrence of those large pink feldspar crystals (climbers call them chicken heads'). There are at least four that have been called out:

1) The so-called granodiorite of Topaz Lake that has those big pink crystals extending from Topaz Lake well south from Sonora Pass and are well exposed in places in the deep heart of Ebbetts Pass and along eastern side of Monitor Passes and esp along Sonora pass Hwy near the top,

2) The so-called Cathedral Peak granodiorite that is well exposed through Tuolumne Meadows but actually goes all the way from Tower Peak to well south of Tuolumne, it is enormous.

3) Mono recesses, there is another unit in that area with the big pink crystals of feldspar

4) Mt Whitney area, again another enormous contiguous body with the same characteristics.

These are some of the largest distinct bodies of granite in the range and some of the youngest.

So the map from Benson Lake illustrates some of the complexity- a "raft" of older metamorphic rock surrounded by younger granitic bodies of disparate ages. One of the challenges in trying to sort out the detailed history is that the intrusion of the big granite bodies pretty much obliterates all that has gone before....
User avatar
mokelumnekid
Topix Expert
 
Posts: 446
Joined: Mon Sep 22, 2008 4:45 pm
Location: Seattle
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby chrisdiercks » Wed Feb 27, 2013 2:05 pm

I really enjoyed going over your map. Thanks for making it available. I had some questions earlier but was hesitant to ask at the time. Is the anticline easily observed from the lake, or do you really need to know what you are looking for. And I noticed your roof pendant contained andalusite, sillimanite (sometimes after andalusite), and cordierite in the mineral assemblages. I think we used to call this a shallow Abakuma-type metamorphism (I suppose I am showing my old college professors' bias but do they still call it that?), and can you recognize any lower to higher metamorphic grade patterns? After that I won't pester you. My hero back when was Hans Cloose; I really loved the highly deformed rocks. Really cool stuff. Nothing like getting paid to get some exercise.

old wannabe geologist
User avatar
chrisdiercks
Topix Acquainted
 
Posts: 35
Joined: Fri Jan 07, 2011 7:19 am
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby giantbrookie » Wed Feb 27, 2013 7:51 pm

Wow, what a nifty map. Vali, Scott et al. have much better field discipline than I do. A lot of potential field mapping time of mine would have been spent trying to see how large a fish I could catch out of Benson. Perhaps that is why I map in those terrible places with poison oak and ticks and all that. No distractions until I leave the field for those pints of IPA.
Since my fishing (etc.) website is still down, you can be distracted by geology stuff at: http://www.fresnostate.edu/csm/ees/facu ... ayshi.html
User avatar
giantbrookie
Founding Member & Forums Moderator
Founding Member & Forums Moderator
 
Posts: 2439
Joined: Wed Dec 28, 2005 10:22 am
Location: Fresno
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby mokelumnekid » Wed Feb 27, 2013 8:08 pm

GB knows more than he lets on! (BTW GB, Scott has the advantage on us that eh doesn't fish or drink).

These "roof pendants" like the one at Benson Lake have created quite a stir some years ago, as they suggested a previously unknown strike-slip fault that translated packages from (perhaps) as far south as the Mojave northward, although I have heard rumblings lately about some modification to that interpretation. In any event both GB and I can recommend additional reading but it will be of a rather technical nature. Just give a shout if interested.
User avatar
mokelumnekid
Topix Expert
 
Posts: 446
Joined: Mon Sep 22, 2008 4:45 pm
Location: Seattle
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby rlown » Wed Feb 27, 2013 8:56 pm

The more info you post that is free, the more we know and then you can answer our dullard questions. Tried getting to some papers before but they're all locked behind fees. All one can see is half of an abstract, on average.

It was very nice to get some background on what lies beneath our feet. Thanks!

Russ
User avatar
rlown
Topix Junkie
 
Posts: 5325
Joined: Thu Oct 25, 2007 5:00 pm
Location: Petaluma and Wilton, CA
Experience: Level 4 Explorer

User avatar

Re: Geology of Benson Lk. map published and free

Postby chrisdiercks » Thu Feb 28, 2013 7:29 am

I would be very interested in something current discussing metamorphic facies and grades in that area or the Sierra in general. It's funny, I woke up last night thinking amphibolite. I'm with Russ in that I like to know what's under my feet and a bit about it.
User avatar
chrisdiercks
Topix Acquainted
 
Posts: 35
Joined: Fri Jan 07, 2011 7:19 am
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby Troutdog 59 » Thu Feb 28, 2013 9:03 am

Thanks for posting the map Moke. Very nice indeed. I have always enjoyed the different rock types we travel through while backpacking. It was a significant factor in choosing geology as a major in college. While my business card calls me a Geologist, I am very much out of the loop these days with mapping, structural geology, and current lines of geologic thinking as I have been working in water quality for 20 years. That said, I did my undergrad thesis on the roof pendants further south around Bald Mountain in the Sequia National Forest and the Golden Trout Wilderness so I do understand this stuff, just rusty. Lots of calc silcate stuff down that way with some very nice garnets along the contacts and within the pendant itself. My first paying job as a Geo included alot of field mapping in the Santa Monica Mountains and we saw alot of nice fossil assembledges and faulting. While I loved being outside logging borings (downhole) and test pits, I didnt really enjoy the fact that the field work I was doing was leading to the increasing development of the area, so I swithched over to the water quality side of things. Very nice to get back to some real geology. Think I might even bring my hand lens and brunton with me on an inspection I have to do today :D .
If you stand in the light, you get the feel of the night, and the music that plays in your ear......
In your mind you can hear, a voice so sweet and clear, and the music that plays in your head......
As it flows up from the ground, taking all that hear the sound, close your eyes, it’s about to begin.

R. Trower
User avatar
Troutdog 59
Topix Expert
 
Posts: 549
Joined: Thu Dec 23, 2010 12:11 pm
Location: Clovis
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby sekihiker » Wed May 29, 2013 11:00 am

Cool map. Having spent a summer field session mapping part of the Log Cabin Mine pendant, I know how difficult it was putting together something that coherent.
User avatar
sekihiker
Founding Member
 
Posts: 268
Joined: Sun Dec 18, 2005 2:47 pm
Location: Fresno
Experience: Level 4 Explorer

User avatar

Re: Geology of Benson Lk. map published and free

Postby mokelumnekid » Fri May 31, 2013 9:51 pm

So who are you working with to be mapping the LCM pendant? Just curious. Find anything interesting, worth sharing?
User avatar
mokelumnekid
Topix Expert
 
Posts: 446
Joined: Mon Sep 22, 2008 4:45 pm
Location: Seattle
Experience: N/A

User avatar

Re: Geology of Benson Lk. map published and free

Postby sekihiker » Mon Jun 03, 2013 9:01 pm

mokelumnekid wrote:So who are you working with to be mapping the LCM pendant? Just curious. Find anything interesting, worth sharing?

Warren Nokleberg was our professor. He spent a few years at Fresno State before he began full time with the USGS. We started each day at 10,000+ feet elevation and ended it near Lee Vining.
Let's see, isoclinally folded hornfels is about all I remember. I never saw a combination of all of our work. The group of students that had the best time in that session were the ones that ended their descent each day near the entrance to a Lee Vining bar.
User avatar
sekihiker
Founding Member
 
Posts: 268
Joined: Sun Dec 18, 2005 2:47 pm
Location: Fresno
Experience: Level 4 Explorer


Return to Sierra Nevada History / Natural History & Setting



Who is online

Users browsing this forum: No registered users and 1 guest