Big Bend – Ernst Tinaja

Ernst Tinaja is a narrow slot canyon in the eastern part of Big Bend National Park.  In Figure 1 you can see the tinaja or erosional pothole in the thick whitish-gray limestone beds of the Buda Formation. 

Figure 1. Entrance to Ernst Tinaja slot canyon.

The Cretaceous Buda limestone was formed at a time when dinosaurs were walking the Earth and this part of Texas was likely a carbonate shelf.  Above the Buda lies the more thinly bedded Boquillas Formation.  You can see the wavy bedding of limestones, shales, and less common volcanic ash beds. The Boquillas Formation contains abundant marine fossils including the extinct bivalve fossil seen below in Figure 2.  Also present with the fossil is liesegange banding, where iron-rich fluids (possibly from local volcanic rocks) have caused secondary alteration of the rock.

Figure 2. Large bivalve fossil and “liesegange” (ringed) banding in the Boquillas Formation. 

There is significant folding and faulting seen in the Boquillas Formation (Figures 3 and 4).  The folding is not seen in the Buda Formation below it and may be due to slumps and slides that occur due to differential compaction and the contrasts between the more ductile shale layers and the less ductile limestone layers.  Notice the thickness of layers remains much the same throughout the folded strata indicating that these are likely flexural-slip folds (Figure 3).

Figure 3. Small scale synclinal and anticlinal folds in the Boquillas Formation with the author for scale..

Figure 4. Image of faulting in the Boquillas Formation above a small tinaja.

Erika Locke, Nature Lover & Geology Professor – Del Mar College, Corpus Christi, Texas