WHEN IS A S0-CALLED ‘FOSSIL REEF’ TRULY A REEF?

The answer to that question is: when field evidence shows unequivocally that any “so-called reef” can be proven to have been constructed by wave-resistant/frame-building organisms, and these remains are in situ at their site of origin.

In the published literature, however, there are some so-called “reefs,” when in reality, they are actually pseudoreefs. Their sedimentary bedding planes by been masked by weathering, thereby creating massively looking structures that can resemble non-bedded true reefs. Even more critically important, a reef fauna is not present.

A case in point is discussed here. It was the topic of my master’s degree in geology (Squires, 1968), which focused on Permian marine rocks in Last Chance Canyon, southeastern New Mexico. Based on spending a summer of studying several so-called “reefs”, I found no evidence that could be used to paleontologically or lithologically differentiate these rocks from the surrounding, laterally equivalent strata. The only fossil material present consisted of broken fusulinids, fragments of brachiopods, and occasional crinoid columnals. These fossils together usually comprised no more than 10 percent of the rock in question, which consisted of dolomitized mudstones and muddy wackestones.

Pseudoreef (area "2," outlined in dashed lines) in upper part of upper member of the San Andreas Formation (from Squires, 1968).

Pseudoreef (outline in dashed lines = area between 1 and 2) in upper part of upper member of the San Andreas Formation (plate 10 from Squires, 1968).

Photomicrograph of a typical example of the dolomite rock that comprises the pseudoreefs (plate 3 from Squires, 1968). 

Although the pseudoreefs do have massive-looking outcrops that resemble true reefs, this resemblance is only superficial and is related to weathering associated with steep-walled, south or south-facing outcrops. Another complicating factor is that eastwardly, within the studied area, the San Andres Formation interfingers with the sandstone tongue of the underlying Cherry Canyon Formation, which locally can have moderately large channel-and-fill structures. This interfingering relationship makes it seem like the eastward sides of the pseudoreefs transition into “reef-talus debris,” but this is not the actual case because these supposedly “reef-talus beds” consist of sandy dolomite with some fossils, but with no reef-forming fossils.

My pseudoreef interpretations are also supported by the work (unknown to me at the time) of Harrison and Jacka (1967), who reported that in the area of Last Chance Canyon, the San Andres Formation and the Cherry Canyon sandstone tongue accumulated in a deep-water submarine canyon. The fossils in these two units are death assemblages of shallow-marine species (e.g., fusulines, bryozoans, brachiopods, crinoid columnals, and echinoid spines) that were transported by submarine currents into deeper waters (approximately 600 to 1,000 feet deep).

References Cited: 

Harrison, S.C. and A.D. Jacka. 1967. Depositional environment of Cherry Canyon Sandstone Tongue, Last Chance Canyon, New Mexico. American Association of Petroleum Geologists Bulletin 51(3):p. 268 (abstract). Only the abstract is free.

Squires, R.L. 1968. Origin of reeflike masses in the upper member of the San Andres Formation, central Guadalupe Mountains, Eddy County, New Mexico. Master of Science Thesis, University of New Mexico, 124 pp.