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