26 March 2014

The Great Unconformity in the Franklin Mountains

Posted by Callan Bentley

Good morning! Let’s take a walk up the east side of the Franklin Mountains, north of El Paso, Texas, to walk across the Great Unconformity.

The basement rock exposed here is the Red Bluff Granite, a 1.1 Ga felsic magma that intruded the columnar basalts of the Mundy “Breccia” and the Castner Marble. (It is unknown what substrate the Castner Marble was deposited upon.) This is what the Red Bluff looks like from the parking lot of the Wyler Aerial Tramway, which we rode to the crest of the Franklins on a very cold and windy morning.


Typical high-potassium granite. Now let’s climb up the trail…

It’s hard to spot where you leave the Red Bluff and enter the overlying conglomerate, which is made of Red Bluff cobbles and pebbles. It’s called the Coronado Hills Formation, and this is what it looks like:


This isn’t a granite. It’s a conglomerate. Yes, I know: the cobbles are really hard to spot. Let me outline a few that I spotted, and then you can go back to the unannotated original photo and see if you can spot any others.


Atop this is the Bliss Formation, a Cambrian quartz arenite (quartz sandstone). It has Skolithos trace fossils in it, just like Virginia’s Antietam Formation (which is of the same age):


Though these aren’t the most impressive Skolithos I’ve ever seen, they are at least diagnostic of vascularized body plans, and therefore of the Cambrian period of geologic time.

Better yet, or at least more impressively expressed, are the cross-beds:


Notice how in one bed, the cross beds tilt to the left, and in the next, they tilt to the right. This is indicative of currents that shift direction. Sometimes, it’s dubbed “herringbone” cross-stratification.

Here’s another lovely exposure showing these primary sedimentary structures:


Beds highlighted in black, cross-bedding in white, and the interpreted current flow direction shown with red arrows:


A great little hike – gaining 500 million years in far less than 500 vertical feet.