Day 5 : Stop 1

 DAY 3 (9/28/2025) - STOP 2

"Sitting Bull Falls - Second Visit"

COORDINATES: 32.24519° N, 104.69691° W

*Also Stop 1 of Day 3*

    While we already visited Sitting Bull Falls on day 3, we missed an important feature of this stop due to rainfall. Now, we returned to discover one of this site’s most unique features, the presence of Pecos diamonds (fig. 37).

    The Pecos diamonds found at Sitting Bull Falls have the same geologic and geomorphic context described for quartz in the Seven Rivers Formation. While not actual diamonds, these quartz crystals are made of SiO2 and are given their name due to their diamond-like shape. These crystals originate in a Permian back-reef sabkha system dominated by gypsum, with interbedded dolomitic limestone and thin shales that created salinas or shallow salt-pan conditions. Their distribution is patchy because these subenvironments were isolated pockets within a broad evaporite plain. Within these settings, early dolomitization and shifts in groundwater chemistry mobilized silica and allowed quartz to precipitate in open spaces or replace earlier dolomite. This process produced the characteristic doubly terminated crystals and the pseudocubic or trigonal forms, along with inclusions of gypsum or anhydrite and the common red to honey-brown coloration from oxidized iron in the host beds (Albright, Lueth, 2003).

    We were able to find many of these crystals because when gypsum-rich strata weather, these quartz crystals are released from the matrix. The crystals at Sitting Bull Falls are therefore exposed not where they formed, but where erosion exposed the Seven Rivers evaporites to the surface.

Fig. 37 Pecos Diamond Hand Specimen on Paper Towel

External References:

Albright, J. L., & Lueth, V. W. (2003). Pecos diamonds-quartz and dolomite crystals from the Seven Rivers Formation outcrops of southeastern New Mexico. New Mexico Geology, 25(3), 63–74. https://doi.org/10.58799/nmg-v25n3.63

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