Marble in the Mountains EarthCache
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The posted coordinates will take you to a dirt pull out on the east end of a road cut along the Woodrow Bean Transmountain Road. You will be walking along a major road with traffic, please be careful and stay as close to the road cut as possible and be aware of falling rocks. Sturdy shoes would also be advised.
As you walk along this road cut, you will be looking at the upper section of the Proterozoic Castner Marble, the oldest rock in the north-trending west tilted fault block Franklin Mountains. It was also a roof pendant of a magma chamber.
About 1.25 billion years ago, sediments of coarse calcareous (calcium carbonate) limestone and sandy mud was deposited horizontally in shallow tropical marine waters that were exposed to different tidal and wave activity.
During a period 1.1 billion years ago, contact metamorphism (baking by high temperature and pressure) by the overlaying Red Bluff Granite Complex caused re-crystallization of these sedimentary layers into marble. This layering appears as white or pink coarse grained marble and dark green or black hornfels.
Point 1 - N 31° 53.650 W 106° 28.219
The reddish-brown looking crystals embedded in this section of the marble are metamorphic garnets. They are small and not of gem quality. Small orange crystals exposed on the sides of and as veins in the marble are potassium feldspar that can be seen in this area as well.
Point 2 -N 31° 53.647 W 106° 28.234
All along the road cut you will be seeing the Castner Marble. At this point you are away from the road and can spend time looking at the marble. The marble has repetitive layering; referred to as rhythmite. Is there a specific pattern in the layering? Are all layers the same thickness?
Point 3 - N 31° 53.649 W 106° 28.250
This is an intrusive breccia dike in a weakened area of the marble. Breccia usually does not travel far, therefore rock fragments of other rocks with somewhat sharp edges are embedded in a basalt (volcanic) matrix.
Point 4 - N 31° 53.656 W 106° 28.318
Here is a layer which is different than that of the rhythmites. This layer was created when high currents of water, possibly by earthquake activity, caused some of the sedimentary layers to be ripped apart before it hardened and combined into a single layer. This is considered to be flat-pebble conglomerate. Describe the differences you see between the rhythmite layers, the conglomerate, and the breccia dike at Point 3.
Also notice in this area the bending and folding of the layers. This happened when Mundy Breccia was being laid down over the Castner Marble, and the marble underwent soft sediment deformation before it had completely hardened.
To log this EarthCache, send me an email with the answers to the following questions:
1. At Point 2: Is there a specific pattern in the layering? Are all layers the same thickness?
2. At Point 4: Describe the differences you see between the rhythmite layers, the conglomerate, and the breccia dike at Point 3.
References:
Hoffler, R. L.,1976, Contact Metamorphism of the Precambrian Castner Marble, Franklin Mountains, El Paso County, Texas [M.S. Thesis]: El Paso, Texas, University of Texas, 76 p.
Thomann, W. F., and Hoffer, J. M., 1989, Stratigraphy, sedimentary structures, and petrology of the Middle Proterozic Castner Marble, Franklin Mountains, far West Texas: Contributions to Geology, University of Wyoming, v27, p. 33-39.
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