25 June 2018

Mafic volcanics atop felsic instrusives: Sonora Pass, California

Posted by Callan Bentley

As mentioned the other day, I’ve been in the Sierra Nevada this past week, which is full of geological delights. Today I’d like to show you a pretty profound unconformity in the region of Sonora Pass, with ~11 Ma volcanic rocks overlying ~89 Ma plutonic rocks.

I’d like to first examine the two major rock units, exploring their varieties, and then look at the contact between them. So our plan is:

  1. The granite (granodiorite) of Topaz Lake
  2. The overlying volcanics (lava flows and lahars)
  3. The unconformity that separates them

Let’s begin…

1) The granite

The granodiorite of Topaz Lake is ~89 million years old, and is distinctive on several levels.

It includes microgranular mafic enclaves (MMEs) which tend to be, as their name implies, finer-grained and darker in color than the granitoid in which they are hosted. These are interpreted to be small blobs of immiscible mafic magma within a larger felsic-composition magma body. They tend to be round, though they can be elongated by magma flow (prior to crystallization) or by post-crystallization deformation. The first would be a primary magmatic structure; the second would be a secondary tectonic structure. A few examples:

Here is one that shows internal spheroidal weathering, something I had not previously seen:

I also found a single example of a xenolith, a chunk of pre-magma host rock that was broken off and inforporated into the magma chamber prior to cooling and crystallization:

However, the most striking thing about the granodiorite of Topaz Lake are its eye-catching huge potassium feldspar megacrysts:

In places, these make up a significant volume of the rock:

In places, these huge feldspars weather out and can be collected from among the grus-like soil:

My son shows off his find, along with my friends Jason Westfall of Sonora High School, and Laura Hollister:

2.) The volcanics

The next of our two units is a sequence of mafic lava flows and lahar deposits that erupted from the Little Walker Caldera sometime around 10 or 11 million years ago. This is the caldera itself (on the east side of Sonora Pass):

The rocks which emerged as either lava or mudflows from this (today) scenic caldera look like this:

The lahars include logs of petrified wood, representing the ~10 Ma forest that used to exist here. It’s a distinctive orange color:

Lavas with flow-aligned feldspar phenocrysts:

Higher up in the sequence, I found beautiful almond-sized and -shaped amygdules:

On the banks of the stream, we found plenty of charismatic volcanic breccia cobbles – bits of lahar subsequently tumbled by the headwaters of the Stanislaus River:

3) The unconformity:

So now we can discuss the relationship between these two units:

At the right (western) edge of the Little Walker Caldera, the same situation applies: You can make out the layered volcanics directly overlying the spheroidally-weathered granite beneath:

The rocks below this surface (traced out as a black line on the annotated photos above) is an ancient erosional surface, an unconformity. The rocks below that surface cooled deep underground around 89 million years ago. The rocks above it formed on the surface of the planet as layered deposits, around 10 or 11 million years ago. This contact is a nonconformity, a type of unconformity where the rocks below the erosional surface are igneous or metamorphic (as opposed to sedimentary).

When you’re just above the unconformity surface in the lowermost volcanic strata, you can find granite cobbles included within the lahars:

As you can see, the shape of those granite cobbles varies between a bit more angular and quite well-rounded. These are interpreted to be cobbles tumbled in the 11 Ma ancestor to the Stanislaus River, the river that carved the paleo-canyon into which the Little Walker Caldera vomited its lava and lahars. Downstream (to the west), you can see some lovely well-rounded boulder gravels, some of which show clear imbrication, as Ryan Hollister demonstrates here at a roadcut on Highway 108 west of Cold Springs:

A wee bit to the west, at the gas station that marks Cold Springs, you can see these river gravels in contact with overlying lahar deposits: Here, the gravels remain in between the other two units; they have not been removed.


Overall, the thought is that with such significant (paleo-)topographic relief on the nonconformity surface in the Sonora Pass region, we’re looking here at a scene ~11 million years ago with relief not too different than today: an ancient river had cut into uplifted Sierra Nevada plutons, tumbling boulders it derived from those granites downstream. Then the Little Walker Caldera roared to life and poured all kinds of volcanic slurry down the ancient valley gradients. In the upper reaches of the (paleo-)drainage, the volcanic deposits scoured out most of the fluvial deposits, but lower down in the drainage, relief may have been gentler, or the volcanic flows had lost a lot of their scouring power, and the lahars were laid down atop the fluvial gravels. The paleo-canyon was filled up with volcanic debris. Later, erosion began anew, and cut into both rock types to carve a new canyon that partially coincided with the old one. This newer version of the canyon was enhanced by Pleistocene glaciation.

Amazingly, the story doesn’t stop there, but includes a surprising denouement: the eruption of another batch of lava into the newer version of the canyon. It was this most recent volcanism that built the Columns of the Giants, a basaltic feature about mid-way down the drainage, and the subject of Ryan Hollister’s acclaimed virtual field trip for Science Friday. The volcanism there occurred after glaciation had happened at least once, and was succeeded by at least one more episode of glaciation.

All told, I’d say the Sonora Pass region is an extraordinary example of a landscape with a beautifully repetitive series of igneous and erosional events through the past 90 million years.