Abstract

Certain salient points regarding organic limestone reefs are as follows:

1. “Coral” reefs is a name generally applied to present day reefs built up by organisms, even though corals play only a minor part in their building.

2. Except where reefs rise from great oceanic depths, their submerged slopes are generally low.

3. Modern reef masses are cavernous and structureless; clastic rocks along the flanks generally have a steep inclination.

4. Alteration of reef rock commonly destroys much or all of the remains of organic life.

5. Barrier reefs are developed only on subsiding foundations.

The Capitan reef is described as a “type” of the reefs of the Permian basin. As exposed in the Guadalupe Mountains its chief characteristics are (1) its thickness of 1800 to 2700 feet and (2) it is a true barrier reef and separates a normal marine basin from a supersaline lagoon.

An attempt is made to harmonize conflicting reports regarding the geology of the Guadalupe Mountains. Most of the differences are due to differences in nomenclature, though there are some differences in geological interpretation.

The primary features by which subsurface reefs are recognized and projected are:

1. The reef zone is generally represented by a continuous section of limestone or dolomitic limestone. By present methods of work, however, it is not possible to recognize reef limestone from well cuttings.

2. The “seaward” side of the reef is characterized by a steep “dip” of the “top of the lime.” At the outcrop this steep front is characterized by steeply dipping forereef beds, but these are not recognized in well cuttings.

3. Lagoonal beds close to the reef are dolomites with interbedded clastics. With increasing distance from the reef the dolomites grade laterally into anhydrite and then into salt. Clastic sandstone and, in some cases, shale are interbedded with the dolomites, anhydrites, and salt. In some cases the lagoonal beds are almost entirely limestone or dolomite.

The theory is advanced that porosity in the limestones is mostly primary porosity developed in the reefs and in oolites and pisolites adjacent to the reefs on the lagoonal side. This primary porosity was probably modified by solution and-or cementation during periods of uplift. Conversely, high porosity in limestone is evidence of the presence of a reef.

The theory is advanced that the origin of the oil is closely associated with the building of the reefs. Three possible sources are (1) the deep in front of (seaward from) the reef (Adam’s theory), (2) the growing top of the reef, (3) the lagoon adjacent to the reef. The first and third are probable, the second improbable.

The converse of the theory that the origin of the oil is closely associated with the growth of the reef is that the presence of oil in large quantities in limestone or dolomite is suggestive of the presence of a reef.

The theory is advanced that the reefs formed barriers separating supersaline lagoons, depositing anhydrite and salt, from basins in which open marine conditions prevailed. The precipitation of the anhydrite and salt is made possible by the growth of the reef barriers. This is a modification of the bar theory of Ochsenius to account for thick deposits of salt and gypsum (or anhydrite). A corollary of this is the theory that the reefs whose lagoons carry anhydrite and salt must have been continuous. This principle is important in the projection of reefs in subsurface studies, especially where information is meagre. The converse of the reef barrier theory is that where thick deposits of anhydrite and salt are found we may confidently predict the presence of a reef separating this area from one in which open marine conditions prevailed.

The presence of anhydrite instead of gypsum and the building of the reefs are convincing evidence of a warm climate without marked seasonal variations. The average temperature was about 80 degrees Fahrenheit, or higher. Evaporation over the water surface varied from 70 to 180 inches per year, depending on the aridity. Precipitation was probably not the same in different parts of the basin. The average precipitation was probably not far different from that of west Texas today, 20 to 30 inches per year. It could have been 40 inches per year or higher. It is not necessary to postulate arid conditions to account for the origin of the saline residues.

The growth of the reef and the precipitation of the saline residues were governed by a number of variable factors. Chief of these were alternating uplift and subsidence of the basin and variations in rainfall and runoff from adjacent land areas. The reefs grew and the lagoons were developed during periods of gradual submergence. Periods of minor uplift interrupted the reef growth and furnished conditions favorable for the establishment of new reefs on the seaward flanks of the old. In this way complex reef systems could be built up.

With a period of pronounced uplift followed by renewed subsidence, reefs would become established farther seaward and a new stage of reef development would by inaugurated. At least three such stages of pronounced uplift separating stages of reef development can be recognized in the Permian of the Permian basin.