Sills, Dikes, and Pegmatites! EarthCache
Sills, Dikes, and Pegmatites!
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Parking for this EarthCache is at a scenic pull out on the south side of the Woodrow Bean Transmountain Road in the north-trending west tilted fault block Franklin Mountains. You will be crossing 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. Do not park on the north side of the road, there is no place to park.
You will be looking at a section of black microgranite sill which was created when the Red Bluff Granite Complex was formed 1.1 billion years ago by large volumes of molten rock. There are several veins of lighter colored igneous rock that criss-cross the area. These are sills and dikes that were formed as the granite was cooling and other hot liquid materials were injected into the surrounding granite.
A Sill is igneous rock that intrudes between older sedimentary layers of rock in a parallel manner.
Dikes are intrusive rock that is injected through a crack in a pre-existing rock, usually of a vertical angle.
Point 1 - N31° 53.827 W106° 27.388
Look for a horizontal vein of coarsely grained igneous rock with large light and dark crystals. This is a pegmatite and contains crystals of a black mineral known as riebeckite. There are also quartz, feldspar, mica, and biotite crystals.
To log this EarthCache, send me an email with the answers to the following questions:
1. What, if any, differences do you see in the rock of the dikes and sills?
2. Using a tape measure/ruler, what is the width of the widest sill?
3. Describe the shape and size of the riebeckite crystals of the pegmatite.
References:
LeMone, D. V., 1988, Stratigraphy and Structures of the Franklin Mountains, West Texas Geological Society of America Centennial Field Guide-South-Central Section. p. 65-77.
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