Barbara Curry
Roadside Stop 1
31.49.766 N 105.59.750 W
Elevation: 1470m
Leaving El Paso, our entourage headed east toward the Guadalupe Mountains on Highway 62. After about 45 minutes we made our first stop to discuss the geology of the structures we were observing. To the north of the highway we noticed tan colored mountains with observable layers. The layers, though, did not consistently fit the basic (parallel to the Earth’s surface) layer cake structure that one thinks of with sedimentary rocks. In some of the mountains the layers appeared to be at odd angles to other layers. It was obvious that events had taken place that caused changes in the position and angle of the layers.
We learned that what we were observing was the Heuco Mountains. These mountains were formed from deposits layed down at two very different times in geologic history. Everything that we could observe had been deposited under water. Most of what we were observing was the heuco limestone, deposited during Permian times (286-245 mya) under shallow seas. The puzzle, though, was the odd angles that we were observing. Apparently, we were looking at layers that were much older than we originally thought.
A sea covered this part of Texas 500 million years ago during Ordovician times. Deepwater sandstones and shales were deposited with the limestone over millions of years. There was a major event that caused uplift of these layers at a steep angle. With time (150 million years) the deposits were eroded.
Original Deposition: Uplift: Erosion:
During Permian times (286-245 mya) shallow waters once again covered this region of Texas. As a result, the rocks of the Heuco Formation were deposited, and, as with the Ordovician deposits, consisting of limestone, shale and sandstone. These deposits rest on the early Paleozoic deposits, but there is a huge gap in time (150 million years) between these deposits that has eroded away. The result is what is called an unconformity. Much time passed, and during Tertiary times (58-24mya) a second major episode of uplift exposed the Ordovician deposits. As a result, the unconformity was exposed and erosion has taken place, explaining the structure we see today.
Permian Deposits: Uplift Occurs:
Stop 2
31.49.659 W
104.49.726 N
Elev: 1373m
After leaving the valley, we started the climb up into the Guadalupe Mountains. Our second stop brought us to a terrific view of El Capitan, the remnants of a huge reef formed during Permian times. The portion we see is composed of the Capitan Limestone and is approximately 1300 thick. This is the largest, and best preserved ancient reef complex in the world. It contains examples of structures and characteristics typical of reef complexes throughout the world. This reef was comprised of algae rather than the corals we think about today.
We are standing in the forereef area, comprised of thinly bedded limestones and sandstones. The basin is below us. All of the structures we are looking at show us how this area looked during Permian times. The only thing missing is the 1500 or water that covered the area.
Above and behind the reef was a lagoon where sediments consisting of a mix of sands and lime deposits gradually filling in the lagoon.
After the sea retreated, uplift during Tertiary time exposed the reef, basin and lagoon structures, subjecting them to erosion. This is what has allowed us to see the mountains as they are today.
I think the most amazing thing about this is to think about all of the water that used to be here and how, what we see today in the reef, forereef and basin looks very similar to what it did as it was being formed. The only thing missing is the water!
Saturday, June 3, 2006
