Day 4 : Stop 3

DAY 4 (9/29/2025) - STOP 3

"Walnut Canyon"

COORDINATES: 32.18731° N, 104.42121° W

Fig. 31 Pisoid Size Gradient - Larger Pisoids (Top), Smaller Pisoids (Bottom)

   Our next stop was a back-reef de

Fig. 32 Semi-Elliptical Shape Overhang

posit in the Capitan system, within the upper Permian back-lagoonal facies of the Guadalupe region. This part of the Capitan Limestone does not represent a high-energy reef system, but rather a lagoonal environment. Circulation is restricted, water depth remains shallow, and carbonate precipitation occurs under distinct conditions. These physical and chemical conditions explained the formation of pisoids at this site. Pisoids typically nucleate around a small shell fragment or quartz grain and are layered with carbonate coats as they roll back and forth in shallow water. We observed a size gradient with larger pisoids concentrated toward the base and smaller ones near the top (fig. 31). This reflects changes in water agitation and growth time, with the larger grains having remained suspended for longer intervals before burial, while the smaller grains represent later, less favorable formation conditions.    

Fig. 33 Chert Nodule on Cave Floor

The modern topography at this stop depicts alcoves and overhangs in the cliff face. Over geologic time, they’ve visibly eroded into semi-elliptical shapes (fig. 32). The alcove morphology itself reflects ongoing weathering and water-driven erosion similar to features we observed at Sitting Bull Falls, where semi-circular openings form along planes of weakness in carbonate rock.

    Along the cliff floor, we also examined several rounded, silica-rich nodules that precipitated within the carbonate host rock. Their presence relates to the sponge-dominated composition of the Permian reef complex. Under lagoonal conditions, this silica precipitated as non-iron-rich, distinct from goethite or other iron oxides, and now appears as chert-like masses dispersed through the carbonate beds (fig. 33).

Fig. 34 Lithology Changes - Sandstone (Bottom), Dolomite (Top)

   

 There are distinct rock layers at this site as well. The lithology changes from carbonate-dominated pisoidal to sandstones and dolomite beds belonging to the Yeates Formation. The lower beds consist of sandstones comparable in composition and texture to the siliciclastic layers exposed farther downslope. Above those layers is highly porous dolomite (fig. 34). This porosity is not a primary structure, but is instead secondary, as it was caused by the dissolution of gypsum that was once interbedded within the dolomite units. Because dolomite in this region forms where freshwater mixes with marine water, the chemical conditions are favorable for gypsum precipitation. However, the later dissolution of this gypsum left behind a distinct secondary porosity in the upper exposure.