Capitan Reef Complex

DSCN4446As Brett and I headed east over the weekend on our way to collect mastodon data, we made a few detours to examine geologic features and collect fossils for the museum.At a roadcut just east of El Paso, Texas (above) we spent 30 minutes examining and collecting small fossils:


Notice the oblong dark splotches above the scale bar, that are just a few millimeters long. Here’s a closeup of one of them:


These fossils may be tiny, but they’re actually giants within their group. These are shells (or “tests”) from fusilinids, a type of foraminifer. Foraminifera are protists, so these are shells from single-celled organisms!

Fusilinid foraminifera first show up in the fossil record during the Silurian Period, but they reached their heyday during the Permian Period, when they were present in vast numbers in shallow marine waters; the rocks in this area are Permian rocks. The fusilinid party wouldn’t last, however. After surviving more than 150 million years, the entire fusilinid line went extinct at the end of the Permian Period during the huge Permo-Triassic mass extinction when 90% of the species on Earth were wiped out.

Continuing east, this imposing structure rose up out of the desert in front of us:


This is the southern end of the Guadalupe Mountains, with a huge limestone cliff called “El Capitan” forming the summit:


(See a Gigapan we shot of El Capitan here.)

El Capitan is part of Guadalupe Mountains National Park, so accompanied by Max we made a brief stop at the visitors center:


El Capitan is the namesake of a feature known as the Capitan Reef Complex, a huge Permian barrier reef which is formed in part by the Guadalupe Mountains (Carlsbad Caverns in New Mexico is also part of the reef), and which surrounds a depression called the Delaware Basin. This reef system was made up largely of algal mounds called stromatolites and oncolites, such as the ones below (all the closeup images below are specimens on display at the park visitor center):


Sponges were also major contributors to the reef system:


As with most reefs, there was a vast array of other organisms living on the reef, including brachiopods:






…and bivalves:


…as well as rugose corals, fusilinids, cephalopods, fish, and other animals.

So how did this reef end up in Texas? The origin of the reef is tied in tightly with plate tectonics. In the period immediately before the Permian, the Pennsylvanian Period, a major continental collision began to take place between Gondwana (a continent made up in part of what is today South America, Africa, Australia, Antarctica, and India) and Laurasia (which today forms North America, Europe, and most of Asia). At the end of the Pennsylvanian Period and the beginning of the Permian Period, the Gondwana-Laurasia collision formed the Ouachita and Appalachian Mountains. A large mountain range such as the Ouachitas is so heavy that it pushes down the surrounding continent, forming a low area called a foreland basin. This basin can be hundred or even thousands of meters deep, and if it connects to the ocean it will fill with sea water. The Persian Gulf is a modern example of such a basin. Below is a map from Ron Blakey showing the North American part of Laurasia during the middle Permian (link to the original map):


And below is a version I’ve marked up with the locations of the collision, the foreland basin, and the Capitan Reef Complex:

Permian map

Outside the reef are extensive shallow marine sediments, often with fusilinids and other marine fossils, such as the ones we examined near El Paso. Inside the reef, water flow is restricted and evaporation rates are high. This results in the deposition of salt, gypsum, and other minerals that dissolve in seawater, leaving evaporite deposits:


Since foreland basins always have a mountain range nearby (which is what causes them to form in the first place), they rapidly fill with sediment eroded from the mountains and with limestone produced by marine organisms and evaporites precipitated from seawater. By the end of the Permian Period the Delaware Basin had filled with sediment more or less to the top of the Capitan Reef, and the reef was exposed as dry land. It partially eroded and the remnants were buried, but about 200 million years later another tectonic event, the Laramie Orogeny, formed faults that brought part of the reef, including the Guadalupe Mountains, back to the surface. Subsequent erosion removed many of the softer evaporites and formed caves such as Carlsbad Caverns.


Much of the information in this post was based on this report (pdf):

Standen, A., S. Finch, R. Williams, B. Lee-Brand, and P. Kirby. 2009. Kapitän Reef Complex Structure and Stratigraphy. Texas Water Development Board Contract Number 0804830794, 53 pages plus appendices.



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