Category Archives: tectonics

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.