Category Archives: Southern Rocky Mountains

Strain

One of my absolutely favorite outcroppings in the mountains above Santa Fe is this little stream-polished window into the depths of the Earth’s crust, along Little Tesuque Creek, not far above the Bishop’s Lodge Resort:

Hornblende schist - a kind of amphibolite - along Little Tesuque Creek, Santa Fe, New Mexico

Hornblende schist – a kind of amphibolite – along Little Tesuque Creek, above Santa Fe, New Mexico

The strain displayed by this dark schist is hard to miss. Outcroppings of metamorphic rocks are abundant in the ancient crystalline core of the Santa Fe Range, but few of them exhibit such dramatic stretching as this example. Here’s a close up of a structure known as boudinage:

Boudins of more quartz-rich layers in the schist

Boudins of more quartz-rich layers in the schist

These stony strings of sausages are the result of differing competencies among the mineral components of the schist. The smooth borders attest to the overall ductility of the rock as it was being slowly sheared, deep in the plutonic realm of the middle crust.

These schists are sturdy, and rounded cobbles of this rock are common throughout the foothills and stream beds west of Santa Fe. Most of them glitter in a dark way, as tiny prismatic crystals of hornblende, aligned by shearing strain, send back a little light. Harder to see is their matrix of white plagioclase feldspar, which probably makes up over half the rock’s mineral content.

Santa Fe and Northern New Mexico

Rowe Mesa above the Glorieta Pass into Santa Fe

Rowe Mesa above the Glorieta Pass into Santa Fe

I’ve been leading a few guided hikes lately, in the countryside around Santa Fe, and while I’ve lived and hiked here for many years, I’m still amazed at the diversity of natural features we enjoy in this corner of the Southwest. Finding a suitable walk for guests with geological interests is never a problem. And when you add in the rich overlay of human cultures in New Mexico, almost any walk becomes a dream-like journey though times past, from symbols with which we can resonate, to artifacts of an almost alien world.

Inscriptions on El Morro

Inscriptions on El Morro, a cliff made up of the ancient dunes of a Jurassic desert

Four great provinces of the American West come together near Santa Fe, to account for this diversity. We sit at the foot of the southernmost range of the Southern Rockies, a group of mountains bordered on the east by the Great Plains, and buttressed on the west by the Colorado Plateau. A rift valley bisects these regions from north to south, bringing a prong of the fourth province, the Basin and Range, into our mountain setting.

All of these regions stand far above sea level, basking in the sharp light and dry air of their high altitude settings. The Colorado Plateau averages 2 km above sea level, and a few peaks in the Sangre de Cristo Mountains reach 4 km. Even Albuquerque, in its basin along the Rio Grande, is 1.6 km above the sea. Rocks are well exposed in this high and dry country, and getting out to see them is always a pleasure.

And the variety! All four provinces host young Cenozoic volcanic features: lava flows, ash-flow tuffs, volcanic cones and domes, as well as good exposures of sub-volcanic structures such as dikes, laccoliths, necks, and stocks.

A Pliocene basalt flow on La Bajada Mesa

A Pliocene basalt flow on La Bajada Mesa

Ancient Precambrian metamorphic and plutonic rocks are extensively exposed in the cores of our mountain uplifts.

A boulder of migmatite high in the Sangre de Cristo Mountains

A boulder of migmatite high in the Sangre de Cristo

In each province sedimentary rocks form a colorful blanket, carrying a record of environmental change that ultimately spans the late Paleozoic, Mesozoic, and Cenozoic Eras.

Permian red beds

Permian red beds

A visit to Santa Fe and Northern New Mexico is an invitation to explore a vast and varied natural history with only a little time and effort. Immerse yourself in Deep Time and you will find your travels here enriched in ways you never expected.

 

 

 

 

 

 

 

The Pennsylvanian System in New Mexico

In the last post I discussed an episode of crustal disturbance that created a system of uplifts, basins, and mountains centered in Colorado, but connecting with related disturbances in Oklahoma and North Texas, during the late Paleozoic Era. In Oklahoma the structural features related to this tectonic activity are called the Wichita System. In Colorado they are called the Colorado System, or, much more commonly, the Ancestral Rocky Mountains, because of the remarkable coincidence of the main uplifts with the modern uplifts and mountain ranges of the Southern Rocky Mountains, which we admire today.

This tectonic activity started during the Mississippian Period and died away in the Permian Period, moving vaguely from east to west over time. In Northern New Mexico, the largest impact was during the Pennsylvanian Period and the early part of the following Permian. Because so little geologic activity had occurred in the area prior to the Ancestral Rocky Mountain orogeny, the effects of the disturbance are striking in the sedimentary record here.

Nearly 60 percent of New Mexico was covered with sediments deposited in the Pennsylvanian System as shallow seas and sediment-shedding uplifts rippled up and down an area where, formerly, nothing but ancient granite, schist and a few thin outcrops of Mississippian limestone baked in the Paleozoic sun. Basins sagged on the continental crust and made space for mud, sand, and gravel weathered and eroded from the Ancestral Rocky Mountains, which were now practically rotting in the heavy rainfall and tropical weather of the time. Exotic plants and insects thrived and coal swamps darkened the margins of the basins. In times when sea level stood high, the warm seas clarified and marine organisms multiplied, secreting lime and leaving hard parts.  Limestone is particularly abundant in the shallow marine shelves that surrounded the basins in New Mexico.

This was the Age of Coal. In Europe and Asia the Mississippian and Pennsylvanian Systems are lumped together as the Carboniferous System. Although the ‘carbo’ refers to coal, it might as well refer to carbon, because the creation of enormous amounts of limestone requires the extraction of enormous amounts of carbon dioxide from the environment, and that carbon was buried just as thoroughly as the carbon sequestered in the huge coal measures (and petroleum deposits, for that matter) of the time. Oxygen levels in the atmosphere must have been freakishly high by the end of the Pennsylvanian Period.

In that careless way the Earth has of disregarding all her previous efforts, the deepest Pennsylvanian basin, and the thickest strata in Northern New Mexico and Colorado, were deposited in a trough that subsequent mountain-building activity casually pushed up to form a range of mountains we now call the Sangre de Cristo Mountains. In Colorado these strata were distorted almost out of recognition, but in New Mexico, the rocks rode up in a fairly intact manner. Here is a view looking south over Pecos Baldy, in the heart of the Pecos Wilderness, showing stratified, smoothly-weathered Pennsylvanian rocks in curved fault contact with the Proterozoic quartzite holding up the peak:

Pennsylvania strata on left, rugged quartzite on right
Pennsylvanian strata on left, rugged quartzite on right. Click on the image to enlarge.

A little further south, you can see a beautiful exposure of Pennsylvanian strata at Daltons Bluff, just upstream from the little village of La Posada. There’s even a bit of Mississippian limestone thrown in, visible as the lighter grey rocks at the bottom of the pile, near the river:

Looking down the Pecos River at Dalton's Bluff. Click on the image to enlarge.
Looking down the Pecos River at Daltons Bluff. Click on the image to enlarge.

Geologists have climbed up and down this outcropping like ants.

On the Santa Fe side of the mountains, the Pennsylvanian rocks are not nearly this thick. And what little there is of them is preserved haphazardly in patches and fault blocks low on the western side. It is likely that the block of crystalline basement rock that forms the Santa Fe Range today formed a shallow platform over which only thin layers of sediment were deposited. Subsequent uplift and erosion of the range stripped off most of what did get deposited. The rest went down with the ship, so to speak, when the Espanola Basin floundered into the Rio Grande Rift.

Nevertheless, the beds that are left on the west side of the mountains are easy to access and display such a variability of rock types, sedimentary structures, fossils, and stratigraphy that you could easily illustrate half a textbook on “Sedimentation and Stratigraphy” with them. Most of the rocks are tilted so that you can walk up and down the section by following dry washes:

Dry wash

 

The Pennsylvanian Period was a time in the Earth’s history when sea level fluctuated frequently and with large amplitude. Although New Mexico was near the equator, much of Gondwana, the southern complex of continents slowly assembling into Pangea, was over the South Pole and enduring cycles of continental glaciation. Geologists suspect that, similarly to the Quaternary Period in which we live, sea levels rose and fell with the waxing and waning of continental ice sheets. Near Santa Fe you can walk out at least one cycle of sea level change, starting with beds of limestone deposited in a clear, well aerated, subtidal environment:

Shallow marine limestone
Shallow marine limestone

Higher up the section the limestone struggled with influxes of mud as sea level fell and the sea became murky with the outbuilding of a shallow delta.

Interbedded limestone and mudstone
Interbedded limestone and mudstone

Above these little coarsening-upward cycles of silt and sand appear:

Sand introduced into the basin as the shoreline approaches
Sand introduced into the basin as the shoreline approaches

Many of these beds show beautiful ripple marks on their bedding planes. You can practically feel the shifting tides, reflected in the stone:

Ripple marks on a bed of sandstone
Ripple marks on a bed of sandstone

Higher yet a bed of very well-cemented sandstone containing coarse grains of quartz and feldspar, and showing the lenticular bedding of alluvial channel fill, announces the arrival of the shore. The basin has filled to sea level:

Arkosic channel-fill sandstone forming the floor of the wash
Arkosic channel-fill sandstone forming the floor of the wash

The siltstones that lie on top of this bed contain fragments of plant fossils, like these giant horsetail ferns:

Calamites fossils
Calamites fossils

These deltaic sediments are soon overwhelmed, however, by the return of the sea, and are buried in mud, interbedded with thin beds of limy silt full of marine fossils:

Marine fossil hash
Marine fossil hash

Finally even these beds disappear into thick, organic, featureless shale:

Thick marine shale
Thick marine shale

The sea has returned.

Cycles like these are the story of the Pennsylvanian System all over the planet. The reason we have a record of this kind in Northern New Mexico is the disturbance of the Ancestral Rocky Mountain orogeny, making space for sediment to accumulate, and making highlands to supply the sediment. The mountains that once graced the sounds and bays of tropical New Mexico have long since vanished. The only evidence of their existence the detritus they left behind.

 

 

 

 

The Ancestral Rockies

The first signal – the first hint – that the crust that has supported Northern New Mexico for the last 1 billion years or so might not be as stable as it should be came about 315 million years ago. For all those millennia beforehand, New Mexico sat as flat and dull and stable as Iowa – flatter, actually, and much closer to sea level, and not nearly as green. A section of old rigid continental crust and mantle – the stuff that forms the geologically uneventful interiors of continents – is  sometimes called a craton, derived from the Greek work kratos, or ‘strength’. And the portion of North America, extending from Minnesota to Northern New Mexico, was about as cratonic as they come, forming a continental backbone that stood above the ocean through many cycles of rising and falling sea level that flooded other parts of the continent, stable or otherwise.

Around 345 million years ago, during the Mississippian Period, the sea did creep over the ridgepole to leave a thin veneer of sand and tropical limestone, only to retreat and see much of its work stripped away.

WNA_340_M_PZ_Tect-sm

And then, about 30 million years later, near the beginning of the Pennsylvanian Period, the craton ruptured. The sea began to move in. A set of uplifts now called the Ancestral Rocky Mountains formed, centered in Colorado but reaching southeastward to join similar uplifts in Oklahoma and North Texas:

WNA_300_Late Penn_Tect-sm

A beautiful paleogeographic reconstruction by Ron Blakey dramatizes the change:

WNA_300_PPvir-sm

The above three images were captured from Blakey’s remarkable website which I urge you to visit.

The full set of causes for this failure is disputed, but it does coincide with the assembly of Pangea, the last supercontinent. In the second tectonic map above you can see South America attaching itself to North America along a boundary marked “Ouachita-Marathon orogen”.

Now an orogen is the complement of a craton. It refers to the “mobile” belts of deformed rock that are frequently found on the perimeter of stable cratons. In fact the two terms were introduced by the same German geologist early in the 1920′s. In more familiar language, an orogen is a belt where mountains are formed, with all the folding, faulting, terrane accretion, and volcanism we associate with mountain building. These associated processes are collectively referred to as orogeny. (Oro is the Greek word for ‘mountain’. Something tells me students back then got a better grounding in the classical languages than I did)

As an orogeny the formation of the Ancestral Rocky Mountains is a puzzle, occurring well inboard of the continental margins on old, cold crust. The sea flooded in rather than being driven out. There was no associated volcanism to speak of. Nevertheless, this episode of crustal disturbance completely altered the face of New Mexico and set the course of events here for the next 70 million  years.

 

 

 

 

 

 

 

 

 

 

A Glacial Moraine in the Rio en Medio Watershed

It’s funny how you can walk over something for years without realizing its significance. I’ve headed off for hikes many times from the Ski Santa Fe parking area without giving a second thought about what it might be built on. I’ve picked my way gingerly down the Rio en Medio Trail, which connects with the Winsor Trail just south of this parking area, without wondering just why it is so steep and stony until you get to that first lovely meadow.

The top of the Rio en Medio Trail, at its intersection with Winsor Trail

The top of the Rio en Medio Trail, at its intersection with Winsor Trail

I’ve remarked on the extensive blowdowns near the Winsor-Rio en Medio Trail intersection with the only thought that there must have been a hell of a storm here once upon a time:

Part of a forest blow down along Winsor Trail

Part of a forest blowdown along Winsor Trail

It wasn’t until I hiked up the Winsor Trail from the south that the apple finally fell on my head. I was looking for the first outcroppings of gneiss that I fully expected to find somewhere along the way to the parking area, when I realized I was climbing up a very steep pile of crudely-rounded boulders instead of bedrock. Something that looked just like glacial moraine.

Now it’s not like we don’t have obvious evidence of alpine glaciation in the Santa Fe Range. This image of the north side of the Lake Peak massif, on which Ski Santa Fe is situated, shows a textbook example of a cirque, with a tarn – Nambe Lake – right where it should be, and a steep terminal moraine damming the U-shaped valley just below the lake:

Lake Peak from the north. Click on image to enlarge.

Lake Peak from the north. Click on image to enlarge.

When you swivel your view toward Santa Fe, however, you can see how different the west side of the mountain, where the ski runs are cut, is from the north and east sides:

Lake Peak showing Ski Santa fe

Lake Peak showing Ski Santa Fe. Click on the image to enlarge.

I never expected to see evidence of glaciation in Aspen Basin, which holds Ski Santa Fe, or Big Tesuque, the aspen-covered watershed of Tesuque Creek just beyond. Nevertheless, if you look carefully at this image of the ski area, you’ll see an odd little tongue protruding down from the parking area to the meadow at the bottom:

The parking area of Ski Santa Fe with a tongue of forested moraine below. Click on the image to enlarge.

The parking area of Ski Santa Fe with a tongue of forested moraine below. Click on the image to enlarge.

It’s this area that has all the anomalous features that suddenly fall into place when you realize what you’re hiking on. Groves of trees rooted in loose boulders rather than bedrock. Steep slopes studded with rocks that range from fist to room size:

Looking up from the Rio en Medio Trail

Looking up from the Rio en Medio Trail

A merry little stream that cascades without pause from rock to rock:

Cascades along the Rio en Medio

Cascades along the Rio en Medio

Rocks of a kind that could only have come from the ridges far above:

A migmatite carried down from Ravens Ridge

A migmatite carried down from Ravens Ridge

Even something that looks suspiciously like a glacial erratic:

A boulder perched among blown-down trees

A boulder perched among blown-down trees

Plus – for what it’s worth – the forest here has an attractive, but hard to define quality I associate with glacial country. Raked slopes, sculpturally-placed boulders, the sound of falling water – something. All I know is that this is one of my favorite places to come and find a rock to sit on, under the tall spruce, when I need a retreat from our civilized world. It’s nice to think that the ice did a little creative work on my side of the mountain. And a pleasure to think that it took me quite awhile to discover that fact.

 

 

 

Migmatite in the Santa Fe Range

One of the more unusual kinds of rock, and one of the most striking, can be found near Santa Fe by simply driving up to our ski slopes, Ski Santa Fe, and having a look at the white boulders along the edges of the parking area. Better yet is making the hike up Winsor Trail, which has a well-used trailhead on the northwestern side of the parking area, around to its intersection with the popular Nambe Lake Trail, where you can see intact outcroppings of this rock in a setting of spruce and aspen trees so luminous and peaceful I call it the Zen Forest.

Hiking in the Zen Forest - Winsor Trail near Nambe Creek

The path through the Zen Forest – Winsor Trail near Nambe Creek

And it is a very eye-catching rock. Here is a boulder underfoot in the Zen Forest:

Walking in the Zen Forest

Walking in the Zen Forest

And here is another high up on Raven’s Ridge:

Boulder on Ravens Ridge

Boulder on Ravens Ridge

These rocks are an unusually high grade (you have to visit this link if you like whisky!) of metamorphic gneiss called migmatite. Migmatites are intricately banded interpenetrations of schist and granite which show  evidence of plastic flow. Their name derives from the Greek word migma, or mixture. It’s not the mix of colors you notice so much, however, as the fascinating, almost liquid patterns the dark and light layers make. It almost looks like the rock is in the process of melting:

An outcropping of migmatite on Ravens Ridge

An outcropping of migmatite on Ravens Ridge

Pods of white granite appear to be almost ‘sweated out’ of the original rock, something very likely to have been the case:

An 'eye' of white granite

An ‘eye’ of white granite

and all of the rock seem to be in a state of arrested flow:

Plastic deformation in the migmatite

Plastic deformation in the migmatite

In spite of this appearance the rock itself is very tough and durable, forming sheer crags

Looking down from Ravens Ridge

Looking down from Ravens Ridge

and eroding into a massive bouldery pavement that can be troublesome to hike over:

Rugged ground of weathered migmatite

Rugged ground of weathered migmatite

In fact these rocks hold up some of the highest ground in the Santa Fe Range, underlying the ridge that extends from Aspen Peak, at the bottom of this photograph, all the way up and around to Lake Peak, just above the highest of the ski runs:

Ravens Ridge and Lake Peak. Click on the image to enlarge.

Ravens Ridge to Lake Peak. Click on the image to enlarge.

When the migmatite is blocked out on a map, it seems to form a sort of massive screen or divider between the more abundant granitic plutons around it. Borrowing a term from anatomy, the migmatite forms a septum between the granites, caught up between injections of magma and separating them, as your septum separates your nostrils. This one is called the Aspen Basin septum, named after the drainage basin which cradles Ski Santa Fe.

Migmatites fascinate geologists because they seem to show what granite might look like in the process of formation. They are metamorphic rocks at the extreme limits of metamorphism, caught in the act of passing from metamorphic to igneous. They are a window into conditions deep in the Earth’s crust, where confining pressure and high temperature make even the toughest rock capable of flowing in a plastic state.

This all begs the question, of course, of just what are these rocks doing over two miles above sea level, when they must have formed 15 miles or more below it? But that is a question we’ll have to take up another time.

 

 

 

 

 

A Schist Belt in the Santa Fe Range

One of my favorite hiking trails near Santa Fe is the Chamisa Trail. Its trailhead is just within the border of the Santa Fe National Forest, off of Hyde Park Road, or NM 475, only a few miles from downtown. Because of its relatively low elevation, between 7800′ to 8500′, and the placement of the first mile of the trail along the west-facing slopes of a small open canyon, the trail makes a good walk nearly all year.  The path reaches a saddle after about a mile, and then descends into a shadier drainage for another mile or so, where it intersects with the Winsor Trail along Tesuque Creek.

This is my go-to trail when I need a quick head-clearing walk. It is close by and well maintained. The light is always good. It is particularly luminous in the winter, a time when higher elevation trails require snowshoes. It is practically the poster child for a walk in a mixed conifer forest in the Southern Rockies. If I were designing a diorama for such an ecosystem for a museum, I would copy a few hundred square feet of the forest around the Chamisa Trail.

Ponderosa and White fir along the Chamisa Trail

Ponderosa and White fir along the Chamisa Trail

But for a long time the rocks along this walk puzzled me. The outcrops along the path are not particularly notable; the one in the photograph below is about as good as they get:

A typical outcropping along the Chamisa Trail

A typical outcropping along the Chamisa Trail

Slopes are steep, but rounded, and are covered with a thin soil littered with fragments of rock:

Stony slopes in the forest

Stony slopes in the forest

There is a distinctive mix of colors among the fragments, reflecting the underlying variability of the bedrock:

Rock fragments and pine needles

Rock fragments and pine needles

At a first glance these rocks don’t seem to fit into any straightforward category. They’re clearly metamorphic, with a steeply dipping foliation in outcrop, crystalline, but fine-grained and somewhat blocky close up. Under the hand lens they seem to be made up of either small crystals of quartz and pink feldspar, or small crystals of hornblende and white feldspar. There is very little mica overall. The rocks are moderately platy, but so fractured with finely spaced joints that they weather quickly into angular gravel-sized fragments. (This is not a pleasant trail on which to go barefoot)

Here are some examples:

Chamisa Trail coarse fragments

Chamisa Trail platy fragments

Chamisa Trail variable

Chamisa Trail blocky

 

Metamorphic rocks are classified in the field by their texture. These rocks are certainly not in the coarse-grained gneiss family, nor are they in the directionless-textured fels family. I decided to put them in the schist family by default, in spite of their high feldspar content and low mica content. Schists are typically rich in strongly-aligned micas, causing them to split freely into plates, and giving them a distinctive sheen. Perhaps the protoliths of these rocks – which I suspect are volcaniclastic – were unfavorable for mica to recrystallize. Some of the darker schists break into large angular blocks and are usually called amphibolite in local descriptions of the rocks.

In places along the Chamisa Trail, these metamorphic rocks are intruded by dikes of a distinctive unfoliated pink granite, rich in silvery muscovite mica:

A dike of orange-pink granite intruding the schist

A dike of orange-pink granite intruding the schist

These colorful granites add extra variety to the walk.

There is an entire belt of these schistose rocks in the mountains near Santa Fe, wedged between a mass of monotonous orange gneiss to the south and southwest, and the strongly foliated pink granite, grey tonalite, and high-grade gneiss that make up the core of the Santa Fe Range to the north and east. If you follow the Winsor Trail north from its intersection with the Chamisa Trail at Tesuque Creek, you can walk through the transition from schist to foliated granite within less than a mile.

In this Google Earth view, the Chamisa Trail follows the two end-to-end drainages in the center of the photograph. As far as I can tell, this is just about the center of the schist belt. The higher ridges just to the left of the sharp north turn in the road are made up of foliated granite, underlying Hyde Memorial State Park.

The Chamisa Trail looking north

The Chamisa Trail looking north. Click on photo to enlarge.

Geologic maps of this area show large, but currently inactive, faults cutting through some of the north-south drainages that interrupt the western slopes and streams, including the one the Chamisa Trail follows. These faults were reactivated during past mountain-building activity in the American West, and their movement shattered the brittle schist, gneiss, and granite as they shifted. Shattered rock is more vulnerable to weathering and erosion than neighboring intact rock and as the highlands eroded, the old fault zones helped guide the network of evolving drainages. If you walk down the branch of the trail that follows the dry stream bed back to the Hyde Park Road trailhead, you can see examples of fault breccias:

Fault breccia in amphibolite schist

Fault breccia in amphibolite

and slickensides:

Slickensides ("fault scratches)

Slickensides – the ‘scratched’ pattern

in the outcrops.  Perhaps these strains account for the closely-jointed character of the schist all along the Chamisa Trail.

 

 

 

An Unusual Granite Near Santa Fe

The mountains that make Santa Fe’s beloved eastern backdrop are part of the southernmost range of the Southern Rockies, the Sangre de Cristo Mountains. Named for their spectacular winter alpenglow,

A winter evening in Santa Fe

A winter evening in Santa Fe

the Sangre de Cristo Mountains in New Mexico are made up of a variety of smaller ranges which link together into a broad chain of forested highlands that extend from the Colorado border south to Glorieta Pass. South of Glorieta the uplift plunges beneath the arid high plains of central New Mexico.

Santa Fe basks in warm southwestern light at the foot of the Santa Fe Range. These mountains are held up by a large block of ancient crystalline crust bounded on the east by large faults in the Pecos Wilderness, and bordered to the west by the picturesque badlands of the Espanola Valley. The rocks that make up the peaks and ridges are of a resistant character, gneisses and schists of various types for the most part, well-laced by granite pegmatites, and sturdy ridges of quartzite in the north. The very highest peaks (not quite reaching 4000 meters in elevation) are frost-shattered and bitten by glaciation

The crest of Santa Fe Baldy, elevation 12,631'

The crest of Santa Fe Baldy, elevation 12,631′

but much of the range has a softly rounded appearance in spite of its underpinnings:

Looking back toward Santa Fe from Tesuque Ridge

Looking back toward Santa Fe from Tesuque Ridge

Even the less vegetated, more arid foothills lack a certain ruggedness:

Western foothills north of Santa Fe

Western foothills north of Santa Fe

Which is why the bumpy-looking ridge at the entrance to Pacheco Canyon has always attracted my attention:

Looking down Pacheco Canyon into the Espanola Valley

Looking down Pacheco Canyon into the Espanola Valley

This ridge shows all the characteristic signs of a massive granite outcropping. When you take the Forest Service road into the mouth of the canyon, the signs become unmistakable:

Bouldery granite peak seen near the entrance to Pacheco Canyon

Bouldery granite peak seen just within the entrance to Pacheco Canyon

This is the typical corestone weathering of moderately jointed granitic rock. Outcrops along the road are much bolder than the usual modest rock exposures in the mountains

Jointed outcrop of granite along Pacheco Canyon Road

Jointed outcrop of granite along Pacheco Canyon Road

and a glance at the rock itself confirms your suspicions. It is a beautiful, very coarse-grained granite rich in biotite and muscovite mica:

Coarse-grained granite along Pacheco Canyon

Coarse-grained granite along Pacheco Canyon

So why is this unusual? Geologic maps of the Santa Fe Range show large regions of ‘granite and related rocks’ – plutonic rocks – of Proterozoic age, interspersed with belts of Proterozoic metamorphic rocks. But nearly all the granites you find in the Santa Fe Range are strongly foliated – distorted by regional metamorphism – showing more or less concordant relationships with the surrounding metamorphics. It would probably be more proper to call them granite gneiss. Excellent examples of these ‘metaplutonic’ granites can be seen in Hyde Memorial State Park near Santa Fe. The ages of these rocks are in the range of 1.6 to 1.65 billion years, right in line with the assembly of juvenile crust that built most of the crystalline basement of the American Southwest.

The granite body in Pacheco Canyon is not foliated and it contains a fair amount of muscovite mica, a ‘wet’ mineral which, in granites, hints at the partial melting of thickened continental crust possibly including metasedimentary rocks. The granite displays signs of emplacement under conditions differing from metamorphosed granites. There are clearly associated pegmatites:

Granite pegmatite with spectacular crystals of feldspar

Granite pegmatite with spectacular crystals of feldspar

and aplite dikes:

Fine-grained aplite dike

Fine-grained aplite dike with typical sugary texture

Features such as these are signs that this granite was intruded under lower confining pressure than a truly deep-seated pluton. Any subsequent metamorphism would have strongly blurred these cross-cutting relationships, as well.

Throughout the crystalline basement of the Southern Rockies the framework metavolcanic and metaplutonic rocks that formed 1.8 to 1.6 billion years ago are intruded by massive and crosscutting granitic rocks emplaced around 1.4 billion years ago. These plutonic rocks  comprise nearly half the exposed crystalline rocks in the mountains, and they very likely played an important role in ‘welding’ the continental crust into the rigid basement upon which all the subsequent geological drama of the American West played out.

Although it isn’t mapped as such an intrusion, I strongly suspect the granite at the mouth of Pacheco Canyon is one of these late stage 1.4 billion year old ‘anorogenic’ granites. The only way to confirm this suspicion would be to date the rock by radiometric techniques, something far beyond my capabilities. Someday, however, some graduate student will take up the challenge, and I hope I’m around to hear the results.