4 November 2014

Joints highlighted with hematite, Anapra Sandstone, Cristo Rey

Posted by Callan Bentley

Good morning. Here are two images from last March’s “Border to Beltway” field trip to West Texas, on the north flanks of the Cristo Rey laccolith. Specifically, these are Cretaceous strata of the Anapra Sandstone, looking at the bedding plane of the rocks. Cutting across bedding are a series of fractures (joints) that have been highlighted by the oxidation of iron (rusting) along their edges.

In the first photo, the highlighted joints are all sub-parallel, and are cross-cut by later joints that formed after the rusting fluids had already passed through the rock.


In the second photo, there is a more complicated picture. On the left, there is are two joint sets that are more or less orthogonal to each other, and both sets of fractures existed at the time the orange-staining fluids moved through this body of rock. In the middle, a radial pattern emerges from a depression in the sediment –


This depression is actually a dinosaur footprint, and it contorted the strata sufficiently that it changed the stress field when these rocks broke – and the joints formed in different orientations than they would have if the sandstone was un-stepped-on.

Just for fun, here’s another dinosaur trace fossil from this same site:


Your perspective has changed in this photo – rather than looking down on the top of a bed of sandstone, you’re now looking “up” at the bottom of a bed, and these sinuous lines are casts of the hollow furrows that must have once existed in the underlying bed (now removed). These were interpreted by our guide as toes dragging in the wet sand, but since so little weight was imparted to the foot which left them, perhaps the dinosaur that made this track was swimming, and its weight was partially supported by the water column.

There’s no structural information to be gleaned from this trace, but who doesn’t love the idea of a dinosaur swimming a little out of its depth, toes scrabbling for a hard purchase on a bottom that’s almost out of reach?