TR: Fun from Hell For Sure to Little Lucifer, with Davis, Wanda, Goddard, and Gunsight Too! 9/21 - 9/30/ 2023

[Sam R]

That roof pendant stuff gives me the shivers! Where are my White Granite Slabs Forever Sierras?

[Harlen]

Sam R wrote:

That roof pendant stuff gives me the shivers! Where are my White Granite Slabs Forever Sierras?

I too get shivers of excitement when I am in the colorful metamorphic rock of the Kaweahs, McGee Creek, or the Ritter Range. Sam, you reminded me of one of the most baffling geologic facts (I hope it's a fact) regarding the incredible depth of some of the Sierra metamorphic formations. We've all noticed relatively thin metamorphic septa, and roof pendants, but what blew my mind was the following quote from Stephen Whitney's Naturalist Guide book:

"Roof pendants of both Paleozoic and Mesozoic age are scattered as well through the eastern part of the Sierra, forming such prominent peaks as the Sierra Buttes, Mt. Tallac, Mt. Dana, and many others. The spectacular Ritter Range... is but the exposed tip of a roof pendant consisting of 30,000 ft. of metavolcanic of Mezozoic age. ...the rocks composing the peaks near Convict Creek are part of a section of metasediments estimated to be 50,000 ft. thick, and up to 475 million years old."

When I first read that, I was amazed by the depth of "roof pendant" rock. My next question to myself was, "How can scientists measure such great depths?!" I am sure they aren't drilling core samples anywhere near that depth. This seems to me more like a great core of the ancertral Sierra Range thrust back to mountainous heights. At that thickness it is hard to view these formations as "roof pendants," isn't it? John/@giantbrookie, I think you have some geologic explaining to do-- I would sure appreciate it.

Thanks Sam, for the inspiration to revisit this conundrum.

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McGee Creek from above Pioneer Basin.

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This would be a part of that 50,000 foot metasedimentary formation.

[Sam R]

Well, I may have misused the "roof pendant" term in this case, but that black rock around Ionian Basin looks brooding to me, even a bit spooky--not nearly so amiable as the granodiorite we all know and love. Kim Stanley Robinson calls one of the entrances to the Ionian "the gates of Mordor," and I can start to see why.

I love that Stephen Whitney book. At one point he says the Red Fir forest, dimly lit as it can often be, has a "sepulchral quality." I thought, on first reading that, Now here's a guy with some style.

[Wandering Daisy]

The dark rock in Ionian Basin is also hard on feet with its block shapes and sharp edges! The large lingering snowfields in a high-snowpack year actually made travel easier since I would rather walk on snow (with traction devices) than sharp talus. Thankfully the basin is very compact, so you do not have to go far between each lake. It is very "cheery" in my opinion, when sunny with amazing blue skies. It does get a bit depressing in rain or overcast. It is far from trailheads, so most people, including myself, add it to a longer trip. But after the trip, have always wished I had simply focused on Ionian Basin and spent more time. On a trip in the late 1990's in August of a very high snowpack year, I did a day-hike/bivouac loop down the Enchanted Gorge from Chasm Lake and up Goddard Canyon. That is a great loop if you want to see Enchanted Gorge but avoid the more difficult drop to Simpson Meadow. There were still snow bridges over many creeks and actually belly-rolled across one.

[giantbrookie]

Sam R wrote:

That roof pendant stuff gives me the shivers! Where are my White Granite Slabs Forever Sierras?

"Roof pendants of both Paleozoic and Mesozoic age are scattered as well through the eastern part of the Sierra, forming such prominent peaks as the Sierra Buttes, Mt. Tallac, Mt. Dana, and many others. The spectacular Ritter Range... is but the exposed tip of a roof pendant consisting of 30,000 ft. of metavolcanic of Mezozoic age. ...the rocks composing the peaks near Convict Creek are part of a section of metasediments estimated to be 50,000 ft. thick, and up to 475 million years old."

When I first read that, I was amazed by the depth of "roof pendant" rock. My next question to myself was, "How can scientists measure such great depths?!" I am sure they aren't drilling core samples anywhere near that depth. This seems to me more like a great core of the ancertral Sierra Range thrust back to mountainous heights. At that thickness it is hard to view these formations as "roof pendants," isn't it? John/@giantbrookie, I think you have some geologic explaining to do-- I would sure appreciate it.

I don't know the origin of the 30k feet deep estimate, but I can speak to the 50 k feet thick section estimate. A stratigraphic thickness is measured perpendicular to the layering given that deposition of sedimentary rocks would be in layers that were originally more or less flat. So 50 k feet thick usually does not refer to vertical thickness in present reference frames because these layers have been steeply tilted in the long expanse of geologic time since the layers were deposited. So a stratigraphic section measurement is usually determined by surface measurements made in traverses perpendicular to the layering. Stratigraphic thicknesses are not determined by direct drilling and sampling except regions of petroleum extraction (but the wells do not get drilled to 50k foot depth).

In much of the high Sierra, the geology in 3D is dominated by granitic rocks that are expected (based on geophysical data such as seismic velocities) to extend far beneath the surface, but not the entire thickness of the crust which is a bit thinner than expected in the Sierra (say 30 km or so) given that the lower part of the crust is expected to be made up of more gabbroic rocks. In the northern Sierra the belt of granitic rocks sort of tracks northward into Nevada whereas the relative volume of metamorphic rocks is much greater (than the southern Sierra). In such areas metamorphic rocks may be expected to extend from the surface to the base of the crust, which would be a lot deeper than 30 k feet (again ~30 km is more likely). This is the part of the Sierra where I do the bedrock-oriented part of my research because my goals are to understand the plate tectonic history recorded in the metamorphic rocks.

Getting back to the hiking side, I like the metamorphic or mafic rock scenery, but as WD points out such rocks tend to be more splintery and more difficult to walk over. They also make for crummier climbing owing to loose rock and less reliable holds. Interesting about the Kim Robinson "Mordor" reference. By coincidence I've called the Kaweahs "Mordor" ever since a trip I did in 2002. The "Mordor" impression was enhanced by this sickly brown wildfire smoke---you can get the picture.

[Harlen]

John/giantbrookie wrote:

... I can speak to the 50 k feet thick section estimate. A stratigraphic thickness is measured perpendicular to the layering given that deposition of sedimentary rocks would be in layers that were originally more or less flat. So 50 k feet thick usually does not refer to vertical thickness in present reference frames because these layers have been steeply tilted in the long expanse of geologic time since the layers were deposited. So a stratigraphic section measurement is usually determined by surface measurements made in traverses perpendicular to the layering.

Thanks John. When you refer to" measurements made in traverses perpendicular to the layering," Are you saying that that 50,000' thickness of the Convict Creek metasedimentary "roof pendant" might be measured not vertically down, but at an oblique angle, perhaps from McGee Creek north through Convict Creek, and on past Laurel Creek toward the Sherwins, or wherever 50,000' is arrived at? You also reminded us about the method of determining the composition of subsurface elements through seismographic readings. In Arctic Alaska, the oil companies had a technique using great "thumping" devices that would generate seismic data used to determine likely oil-bearing layers at great depths. Might geologists be getting their subsurface data via this technique?

Thanks for your thoughtful reply above; hope you don't mind having your day job matters mixed in with your Sierra fun here at HST.

[giantbrookie]

John/giantbrookie wrote:

... I can speak to the 50 k feet thick section estimate. A stratigraphic thickness is measured perpendicular to the layering given that deposition of sedimentary rocks would be in layers that were originally more or less flat. So 50 k feet thick usually does not refer to vertical thickness in present reference frames because these layers have been steeply tilted in the long expanse of geologic time since the layers were deposited. So a stratigraphic section measurement is usually determined by surface measurements made in traverses perpendicular to the layering.

Thanks John. When you refer to" measurements made in traverses perpendicular to the layering," Are you saying that that 50,000' thickness of the Convict Creek metasedimentary "roof pendant" might be measured not vertically down, but at an oblique angle, perhaps from McGee Creek north through Convict Creek, and on past Laurel Creek toward the Sherwins, or wherever 50,000' is arrived at? You also reminded us about the method of determining the composition of subsurface elements through seismographic readings. In Arctic Alaska, the oil companies had a technique using great "thumping" devices that would generate seismic data used to determine likely oil-bearing layers at great depths. Might geologists be getting their subsurface data via this technique?

Yes measurements of layer thickness are done obliquely on the surface. Regarding the seismic data, that is in fact how seismic data is acquired (ie what you note about petroleum exploration)--the thumping generates the seismic waves (essentially sound waves through rock) that are recorded up by seismometers placed at various locations. The comparison of the arrival times of the seismic waves at the various seismometers is used to determine the seismic velocity structure of the rocks at depth

[Harlen]

Got it John. I love learning about, and trying to imagine the "Ancestral Sierra." Is it fair to say that looking at the dark and jagged Ritter Range/Minarets, and the "Black Divide," allows us a somewhat anachronistic view at what the Ancestral Sierra might have looked like? Thanks again for your professional insights giantbrookie... you are so much more thn just a fisherman.