Formations

You’re not around geologists very long before you hear the word “formation” mentioned. For that matter, almost anybody out enjoying a landscape with rocks in it is apt to use the word themselves, as in “there were the coolest rock formations out by Diablo Canyon!” Geologists cringe just a little when they hear that – not that you’d notice – because to them the word “formation” has a very definite meaning.

This is not a formation. It is, however, "Camel Rock"

This is not a formation. It is, however, “Camel Rock”

A formation is a group of sedimentary strata, volcanic beds, or igneous intrusions with upper and lower boundaries that can be easily traced and mapped across the countryside.

The word ‘mapped’ is critical in this definition. The first step geologists make in their attempt to understand the natural history of a region is to construct a geologic map, showing the relations between the different rocks there. In order to do this, he or she has to make some distinctions among the various kinds of rocks, subdividing the outcrops into “meaningful units” that are large enough to plot on the map and distinctive enough that other geologists can agree on their selection.

Typically the distribution of formations is shown by using different colors or patterns:

A simple geological map of New Mexico

A simple geological map of New Mexico

Because of the way they are chosen, formations have lithologic significance, consisting of a single rock type, or a cluster of closely associated rock types. Formations are the basic Rock Unit of stratigraphy.

This is reflected in the formal names given to formations, based on their definition at a type section, at a specific geographical location, where other geologists can inspect the choice. For example, the Mancos Shale, named after a town in Colorado, or the Redwall Limestone, named after the famous cliff in the Grand Canyon, are cases where the formation is basically one rock type. In cases where the formation is chosen to be a mix of associated rock types (still distinctive enough to trace and map!) the geographic name prefixes the word “Formation”. An example is the Galisteo Formation, named after a village in New Mexico.

In sedimentary formations, the strata within a formation tend to be more or less parallel, or conformable, with one another.

The Galisteo Formation, showing a mix of strata

The Galisteo Formation, showing a distinctive mix of conformable strata

Because of the way they are chosen, formations also have a genetic significance. Each one records a time of fairly uniform environmental or depositional conditions, different from adjoining formations. Understanding this is vital in our attempt to work out how conditions changed with time.

Finally, formations have a built-in time significance. By virtue of the principle of superposition – younger layers must rest on older layers – formations can be put into a relative time order, from oldest to youngest. In the case of igneous intrusions, the principle of cross-cutting relationships serves the same purpose. If an intrusion cuts through another body of rock, it must be younger in age than the rock it intrudes.

That’s a lot of significance packed into one common word! “Formation” truly is a useful concept in geology.

And yet, as geologists – especially petroleum geologists – soon discover, the formation is not the perfect stratigraphic bookmark we tend to think it is.

Among sediments, depositional environments can exist simultaneously, side by side, in a given area. Think of shrimp boats dragging their nets through the mud while swimmers frolic on clear sandy beaches at the shore. These environments – one accumulating mud, the other sand – migrate with time. A barrier beach may slowly build out over a muddy marine shelf, which, millennia later, will show up in the geologic record as sandstone over shale. We’d very likely define two formations in our mapping – say, the Point Lookout Sandstone over the Mancos Shale – and consider that the sandstone is everywhere older than the shale. But we would be wrong. The two formations actually interfinger and there are places where the sandstone here is the same age as the shale there.

Such formations are called diachronous – “passing through time”. Most sedimentary formations are diachronous to some extent. Now this may sound like the sort of hair splitting only a stratigrapher could enjoy – but understanding these sorts of relationships can be critical in defining potential petroleum traps, sources, and seals in an oil-bearing basin. An entire branch of stratigraphy called sequence stratigraphy has developed among oil companies (rooted in the insight of perceptive geologists long before, I must emphasize) in order to establish accurate time lines within and across formations, repackaging the strata into a different kind of “meaningful unit” called a sequence.

But these are subtleties we can let rest for now. You have to start somewhere, in every science, and the notion of a formation – properly used –  is one of the first stepping stones in geology. So bite your tongue next time you hike in Zion National Park or wander through Carlsbad Caverns. A geologist might be listening.

 

If you are a hunter of fossils

Brachiopod

 

“Up here,

what’s real

is the shallow warm sea that all these seashells knew.

On this mountain,

every rock still holds the memory of that time.

When you are here, you hold it too.

The ocean’s salt is in your blood.

Its lime is in your bones.

Its waves rise slow and green around you

and you feel the pull of tides.

It never seems to be now.

Here, time flows back and forth so easily that any day

can be wrapped up inside some other day

that came and went a hundred million years ago.

Here, when I find a brachiopod

or mollusk

or a round sea urchin,

I don’t just see it as it is . . .

on a mountain locked in a rock.

I see it in that ancient lapping water.

I see the tiny clams plowing through mud.

I see sea lilies sway.

I see all the creatures with shell and plates and spines.

Slowly moving, glimmering, they hide in the crevices,

creep into holes in the rocks.

Up here,

you are never surprised by things like that.

Sometimes you even feel

the long slow terror in that world

when water turned to mud.

Now that sea is a mountain of rock

that I climb with a shell in my hand.

If you are a hunter of fossils

you know how the day always ends.

You know how it is to go home

feeling glad that you walk in the sun . . .

breathing air.

You always walk home kind of proud.

You always hold on to that long chain of life as you go.”

Limestone

Excerpted from “If You Are a Hunter of Fossils” by Byrd Baylor and Peter Parnell.

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.

 

 

 

 

 

 

Red beds

My recent scouting adventures here in Northern New Mexico have reminded me just how important “red beds” are to our colorful landscape. Red beds are sedimentary rocks, usually sandstones, siltstones, or shales, that are stained various shades of red and orange. As you make the drive up to Ghost Ranch from Santa Fe, on your way to visit the Georgia O’Keeffe country, you abruptly pass from a pastoral river valley lined with small farms and green cottonwoods to this lurid scene:

On the road to Ghost Ranch

On the road to Ghost Ranch

Or you take a drive up from Albuquerque to Jemez Springs and watch the sandy, juniper-studded badlands give way to flaming cliffs:

Entering San Diego Canyon on the way to Jemez Spings

Entering San Diego Canyon on the way to Jemez Springs

Rocks like these are so ubiquitous here that the first great summary of the State’s geology, published by N. H. Darton in 1928, was called “Red Beds” and Associated Formations in New Mexico.

The reddish colors in red beds are due to ferric oxides – oxidized iron (rust, basically) – that coat the tiny mineral grains that make up sandstone, siltstone, and shale. It doesn’t take much: as my artist friends tell me, a little red goes a long way.

Except for the obvious fact that these stained rocks must have contained a few iron minerals and have been exposed to free oxygen sometime during their genesis, understanding the conditions under which red beds form has been surprisingly elusive. For a long time it was assumed that they formed in ancient deserts and always recorded the existence of a hot arid climate. This was based on analogy with modern red deserts like those found in the American Southwest and Australia.

Many modern deserts are grey, however, and most of the soils and sand of the red deserts are reworked from the red rocks that already outcrop there. Permian rocks, like those around Jemez Springs, and Triassic sediments, like those you see at Ghost Ranch, are famous for their red beds and these rocks crop out all over the American Southwest, contributing a vast share of modern sediment.

More recently red beds have been taken to give evidence of seasonally dry conditions – monsoon climates – in the past. Modern areas with monsoons are generally considered semi-arid, hot and dry much of the time, then soaked in rain. Such alternative drying, then wetting with oxygenated water, seems to agree with a chemistry that would stain sediment with iron. During the Permian and Triassic Periods the Earth’s continents were assembled into a vast supercontinent named Pangea, whose climate must have varied dramatically from the modern dispersed continents. Parts of Pangea may have experienced mega-monsoon conditions (to go with its “supercontinent” status, I guess) and this has been used to explain the prominence of red bed from that time.

But lately doubts have arisen. The safest thing to say is that the red color indicates former good drainage in the sediments. Terrestrial conditions. Other clues hidden in the red sediments must be sought and added to understand the ancient climate in which they formed. Here is another set of New Mexico red beds seen along the “Turquoise Trail” that links Santa Fe with Albuquerque:

The Galisteo Formation near Cerrillos

The Galisteo Formation near Cerrillos, New Mexico

These rocks are Eocene in age and record the weathering and erosion of the first ranges of the Rocky Mountains as they were born. This was a very lush and wet time here, almost tropical compared to the modern climate. Perhaps red beds are forming in the Amazon basin, today.

No matter what ultimately created them, red beds make a big contribution to the scenic beauty of New Mexico and indeed, all of the American Southwest. And their elusive origin reminds us that geologic investigation, like all the sciences, is never static.

 

The Colorful Folds of the Sierra Nacimiento Mountains

Sedimentary rocks hold many charms for geologists. They contain the Earth’s archives, recording the distribution of ancient environments, outlining areas of subsidence and uplift, and tracking changes in climate and fluctuations in sea level over the ages. They preserve the history of life on Earth in the fossil record. They contain mineral fuels like coal and petroleum.

But there is an additional, almost incidental aspect of their record-keeping abilities which never fails to fascinate even casual observers. It’s tucked into the formidable phrase “original horizontality”.

Because sediments at the Earth’s surface are originally laid down in approximately horizontal layers, dispersed by moving water or blowing wind and settling under gravity, we can infer subsequent deformation of the outer crust by their displacements from the horizontal. Deformation by flexure – or folding – is one of the most striking manifestations, and some truly spectacular examples can be found here in New Mexico along the southern flanks of the Sierra Nacimiento Mountains, only a short drive west of Albuquerque.

Here is a Google Earth Image of the San Ysidro anticline, an up-arched buckle in the Earth’s crust beautifully outlined by a layer of white gypsum encircling a core of red shale:

The San Ysidro Anticline

The San Ysidro Anticline

Erosion has scraped out the soft center of this fold, exaggerating its appearance. Other sedimentary beds tilt away from the elongated center of the fold – its axis – in all directions. The axis of this anticline dips below the surface – or plunges – toward the lower left.

This part of New Mexico is just on the edge of the Colorado Plateau, which stretches from here far to the north and west. The Colorado Plateau is famous for its colorful strata, laid out in buttes and mesas or exposed in deep, sheer-walled canyons, and these wild color contrasts make this area of folding more spectacular than most.

Here is a view of the fold taken from the opposite direction which includes its companion fold, a plunging syncline:

Complementary plunging syncline and anticline

Complementary plunging syncline and anticline

Synclines are down-buckles in the Earth’s crust, U-shaped in cross-section. Rumple your napkin up by pushing it across the table and you’ll get the idea of why these folds come in pairs.

Sedimentary rocks make a cover – a sort of thin blanket – at the top of the Earth’s crust, resting on older and unrelated crystalline rocks like granite or schist. Geologists often refer to this ancient foundation as the crystalline basement. Just to the north of these folds, an upthrust of the crystalline basement rocks in the Sierra Nacimiento has flexed these same strata sharply upward, almost to the vertical:

Along the western front of the Sierra Nacimiento

Along the western front of the Sierra Nacimiento

Scenes like this are common throughout the Southern Rocky Mountains and the Colorado Plateau. Some famous examples include the Flatirons near Boulder, Colorado and the Garden of the Gods, near Colorado Springs.

One pleasant byproduct of folding sedimentary strata, at least for geologists, is the fact that you can go up and down the geologic record by walking among the tilted strata, rather than scaling a cliff or drilling a well. Here is the transition from the colorful shales and sandstones of the Morrison Formation, to the right, into the duller grey and yellow shales of the Dakota Formation, off toward the left:

Tilted Mesozoic strata in the east limb of the San Ysidro Anticline

Tilted Mesozoic strata in the east limb of the San Ysidro Anticline

A scramble along one of the dry washes that cuts the flank of the anticline will let you examine a huge thickness of rock.

Of course, you may decide that today is not the day for scrambling down a gulch of slippery shale, and are content to simply enjoy the view:

Enjoying New Mexico

Enjoying New Mexico

It’s all good.