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A Bad Day for Geology

Tesuque Creek in "Springtime" runoff

Perhaps I should modify that to say “a bad day for doing geology”, as in, it’s really hard to find subtle contacts in the rock record when the ground is covered in two inches of fresh snow, and the trail is still crusty with old snow (which I thought might be gone by now) and the critical exposures in the creek are covered by floods of ice water. I was pushing my luck, I admit, by driving up to the aspen belt and hoping for an idyllic stroll, but the weather this weekend put the icing on the cake – or rather, the mountains – with a spring storm that simply wouldn’t move along.

After last week’s expedition to the higher parts of Hyde Memorial State Park, I’ve been anxious to examine the rocks even higher up the hill. NM 475 takes on a whole new character as you drive out of the park. It begins its ascent to Ski Santa Fe along a series of looping curves that hug the contours of the Santa Fe Range, bringing you some beautiful views and access to some classic local hikes, before delivering you to the spruce-bound parking area in Aspen Basin, 11,000 feet above sea level.

Not long after you leave Hyde Park, you might notice that the orangy-pink color of the roadcuts have changed to a subtle grey (unless you are keeping your eye on the road as you are supposed to do). As always, there’s a reason for this: you’ve crossed into the true crystalline heart of the range, the rocks that support the highest peaks above Santa Fe. The creek beds begin to take on this appearance:

Grey boulders of tonalite in upper Tesuque Creek

A closer look at these outcroppings reveals a rock of great beauty:

A closer look at the hornblende tonalite. Penny for scale.

It looks like something you’d find in Yosemite National Park. Or on your kitchen countertop, if you’re lucky. The old name for this rock – the one I would have applied after Dr. Shea’s petrology class back in 1978 – is quartz diorite, but a more accurate classification makes it a tonalite. It’s rich in quartz, like all the plutonic rocks up in the Santa Fe range, which puts it squarely in the granite family, but instead of being dominated by pink feldspar, it has nothing but white ones. The metals hidden in this waxy white mineral are calcium and sodium, rather than the potassium that characterizes the pink feldspars.

As you can see, this ┬árock also has a high percentage of black minerals – enough to give the boulders a rather dark complexion. These are crystals of hornblende and the shiny black mica called biotite. Since each of these minerals contains a proportion of the metals iron and magnesium – and since they are so abundant in the rock – the tonalite fits into another important category of the igneous rocks: it’s mafic.

Mafic is a modifier compounded out of the words “magnesium” and “ferric” (iron-bearing). The complementary term is felsic, compounded out of the words “feldspar” and “silica” (quartz). Granite is a felsic rock. Tonalite is usually considered a felsic rock too, but our outcroppings are unusually rich in iron minerals. Before your eyes roll back in your head, just relax and bear in mind that these chemical distinctions are fundamental to geologists trying to work out how the crust of the Earth is formed.

My goal was more modest: I just wanted to see if I could figure out which rock got here first. Did the pink granite engulf the grey tonalite, as some maps indicate, or did the tonalite intrude itself into the granite, as more recent work shows? To do this, I needed to find a place where the two rocks meet, and establish their cross-cutting relationship.

I was unsuccessful.

I figured I could find a meeting place – ┬áthe contact – somewhere along Tesuque Creek just below the Big Tesuque Campground, where it is easy to park, and where what little information I could scare up from the literature showed that there was some sort of change down the hill.

You are clearly in the tonalite at Big Tesuque Campground. You can even see the place where it mingles with an overlying glob of very high-grade gneiss, right in the parking lot:

Banded gneiss (above glove) engulfed in grey tonalite

But as I started down the Big Tesuque Trail – in completely unsuitable running shoes – I realized that today was not a good day for making subtle distinctions among the rocks. My shoes were soon soaked with new wet snow. Instead of revealing secrets, the creek was doing its best to hide them:

Cold water on my theories

In some places it looked like the two rock types might have mingled, themselves:

Pink and grey together: a boulder of granodiorite

That wasn’t any help.

When I finally staggered down far enough down the creek to find an indisputably in-place outcrop of pink granite, I was dismayed to see that its face was disfigured by slickensides:

Those angled scratches on the granite are slickensides

Slickensides are evidence that rocks have slid past one another, along a fault, leaving scratches. This is fantastic information if you are trying to work out which way a fault moved in the past, but all it told me was, there was every possibility that the granite and the tonalite had slid past one another long after each had crystallized, and that I would not be able to tell which one intruded into the other. Snow and water were hiding other exposures in the creek bed, so at this point I bagged it and headed back up the hill.

I’ll go back in June and have another look.

My gut feeling is that the grey tonalite is younger than the granite. It looks fresher. It’s less foliated, so it hasn’t been pushed around as much as the granite. On the other hand, its mafic quality makes it a more primitive rock than granite, chemically speaking. So I’ll need to find that indisputable, in-place contact. On a better day for geology.
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