Category Archives: Colorado Plateau

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.

 

 

 

 

 

 

 

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.