Monthly Archives: August 2013

Krystallos

I was on a mountain excursion earlier this week with some 14 year-old hikers, high in the ancient rocks that outcrop east of Santa Fe, when their attention was drawn to this luminous rubble along the trail:

Milky quartz float along the trail

Milky quartz float along the trail

I picked up a piece and asked them what they thought it was. “Crystal?” asked one. “Quartz!” asserted the other.

They were both right. The coarse-grained granite that splits the gneiss in the foothills of the Sangre de Cristo Mountains above Santa Fe spills its crystalline contents all along the mountain trails, mixing with the pines, penstemons, and desert plants there. It covers the ground with pink feldspar and adds icy accents of quartz and mica. Everyone notices the quartz.

The components of granite: feldspar, quartz, and mica

The components of granite: feldspar, quartz, and mica

Each of these minerals is composed of chemical elements common in the Earth’s crust. Among these elements, oxygen is by far the most abundant, making up 47 percent of the rocks by weight, and a whopping 96 percent by volume! Oxygen is a big atom. When you look at a granite mountain like Pikes Peak you are basically looking at a big pile of oxygen with some impurities in it. This fact never fails to impress me.

In second place is the element silicon, making up around 27 percent of the crust by weight. Since oxygen and silicon can link together chemically, it follow that their compounds utterly dominate the composition of the Earth’s crust. There is so much oxygen and silicon around that their simplest combination, two atoms of oxygen sharing one atom of silicon, or silica, is exceptionally common. We know it as the mineral quartz.

Under ideal conditions, such as in the cavities of mineral veins, quartz can be found as transparent, six-sided prisms, each terminated by a pyramid with six shining faces. These noble light-forms are perennially fascinating – visit any mineral shop or New Age bookstore – and they seems to captivate everyone who sees them. The ancient Greeks gave the name krystallos (clear ice) to these forms, considering them a kind of ice that had been eternally frozen. Transparent quartz is still called rock crystal, and from that simple beginning, any mineral or chemical substance that develops symmetrical forms bound by planar faces is now known as a crystal.

A crystal of quartz

A crystal of quartz

With the development of the atomic theory of matter, we now realize that the beautiful symmetry of a crystal is simply the reflection of its internal structure, a lattice-work of linked atoms repeating themselves in endlessly in three dimensions. Even when a mineral is confined and unable to develop its ideal outer expression of crystal faces, its internal order may still reveal itself when it is broken. Cleavage planes in feldspar scattered along the forest trail flash back at you like mirrors when the light is right, literally reflecting the mineral’s crystalline structure:

A cleavage plane in feldspar shining along the trail

A cleavage plane in feldspar shining along the trail

So both of my young hikers were right. In two words they accidentally captured an ancient linkage that gave a permanent name to the crystalline nature of minerals.

 

 

 

 

 

 

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.