August 24. Capitan Reef

El Capitan. Photo by Richard Gibson (1980)

That huge reef that developed around the margins of the Delaware Basin portion of the Permian Basin is called the Capitan Reef. It’s named that for outcrops in the Guadalupe Mountains, specifically for El Capitan, a prominent mountain there which is the core of the Permian reef. El Capitan stands about 2,000 feet above the adjacent valley floor today – just about the same elevation that the ancient reef stood above the adjacent deep sea basin. The overall geometry of the reef was a long, narrow ribbon encircling the oval Delaware Basin. There were some breaks in it, but its total length was about 450 miles.

Source: National Park Service

The reef was constructed in the standard way, by the skeletons of tiny calcite-secreting organisms including algae, sponges, bryozoans, and some corals, but not mostly corals. Other life, including brachiopods and fusulinids, contributed their shells to the reef complex as well, so that it became a huge edifice nearly a half-mile high. The top would have been just about at the water line, the warm, aerated shallow zone where life lived, as in the modern Great Barrier Reef of Australia or the reefs that make atolls around tropical islands.

The critters were sustained by an influx of nutrients from all directions. The back reef, the shallow lagoonal area on the shelf, was much like the shallow seas we heard so much about throughout the Paleozoic. Lots of life – crinoids, even more fusulinids, the large single-celled organisms we talked about a few days ago, plus gastropods (snails) and more. The forereef area, the deep ocean in front of the high reef itself, also contained abundant nutrients that would flow up the slope to feed the reef organisms. Organic matter that settled into the deep forereef basin also helped create some of the rich hydrocarbon source rocks that have matured and migrated into the oil and gas fields we exploit in this area today.

Source: Texas Water Development Board

The area was tropical to subtropical, with the Permian equator running approximately from what is now northern California to Newfoundland, making the region similar to the Great Barrier Reef in terms of latitude.

The modern El Capitan and Guadalupe Mountains are exposed and high-standing because of much later tectonics and erosion that exposed the reef core. In arid country like West Texas, carbonates tend to be resistant. They dissolve better in rainy country because of the weak carbonic acid produced by the interaction of rainwater and carbon dioxide in the atmosphere. The shales and sands in the forereef area, a broad plain today, are less resistant and end up eroding away more easily. This is not to say that there was no dissolution of these limestones. There was, in spectacular fashion – and we’ll talk more about that tomorrow. 

* * *

The eruption of Mt. Vesuvius in Italy on this date, August 24, 79 A.D., destroyed the cities of Pompeii and Herculaneum. Most of the estimated death toll of 16,000 were buried under ash falls or pyroclastic flows. Vesuvius has erupted many times, including 17 since 1700. The last major eruption was in 1944. The volcanism in southern Italy and Sicily is ultimately related to subduction caused by the collision of Africa with the Italian Peninsula, but it’s pretty complicated because there are multiple small blocks involved in the collision. Italy itself is pushing like a finger into Europe, raising up the Alps. Oceanic crust beneath the Tyrrhenian Sea north of Sicily and west of southern Italy is probably subducting or being overridden by small continental blocks, resulting in the volcanism.

—Richard I. Gibson

Links and references
Guadalupe Mountains 
Permian Reef 
Capitan Reef  (TX Water Development Board – source of cross-section)