One of the unexpected pleasures of my recent foray into the Quebradas was spectacular exposures of an unusual layer of gypsum, a member of the Yeso Formation (yeso is Spanish for gypsum). At first I mistook them for marble, though that seemed rather out of place to me even at the time. This isn’t really my area of geologic fascination (I like big layers, faults, anticlines and whatnot, whereas this sort of thing edges perilously close to chemistry, which has never been my thing), so I’ll cop out and simply quote the geologic guide to the Quebradas Backcountry Byway:
….the next several hundred yards of exposure on the south side of the valley consist of massive gypsum (the hydrated form of calcium sulfate, CaSO4 · 6H2O). In most climates, gypsum is removed through dissolution by near-surface ground water. In the warm, dry climate of New Mexico, however, gypsum can reach the surface and survive there for some time. This is an exceptional outcrop, and it is one of only a handful of places in the U.S. that such fresh gypsum outcrops can be seen.
The Cañas Gypsum Member in this area can be as much as 190 feet thick, although at this site only 30–40 feet of gypsum is exposed. The gypsum displays a number of distinctive fabrics, especially laminations of alternating dark and light layers, an eighth to a quarter of an inch thick [saw these, but didn’t get a great shot of them]. The dark layers consist of limestone and organic matter. The light layers contain mainly gypsum. Such repeated cycles are interpreted as representing varves —annual cycles with dry-season evaporation producing gypsum and cooler, wetter seasons yielding organic limestone layers. The laminated deposits are indicative of a large and relatively deep-water saline coastal lake (salina) or restricted marine embayment that extended 110 miles from just north of Mountainair to south of Alamogordo and Truth or Consequences.
The other structures visible in the Cañas Gypsum mainly reflect alteration produced during 270 million years of burial and subsequent uplift and exposure…. Other intervals show small-scale folds within laminated evaporites and more rarely include structures that look like wisps or tongues of flame; both probably result from flow (plastic deformation) within the evaporite masses. That property of plastic flow of evaporites (similar to the behavior of Silly Putty or taffy) can be quite important. For example, evaporite deposits very similar to these are widespread in the Delaware Basin south and east of Carlsbad. Those deposits now host the WIPP site (the Waste Isolation Pilot Plant), the only active underground nuclear waste repository in the U.S. The reason for choosing that unit for waste disposal is because the evaporite deposits, in time, flow in around the wastes and heal any fractures in the rock, forming a largely impermeable seal.
Chemistry aside, they were certainly gorgeous!