The origin of the Massive Member of the Capitan Formation in the Guadalupe Mountains of West Texas and New Mexico is controversial. It has been inteipreted in many different ways, including as a barrier reef, a deep water skeletal wackestone mound, a cement boundstone mound, and a linear complex of buildups. The reality is that it is not simply one or the other, but a complex with characteristics of all of the above — a reefal complex that changes both in time and space.

The Massive Member of the Capitan Formation contains the components of a framework reef: frame-building organisms — bryozoans, platy sponges, and Tubiphytes', binding organisms — Archaeolithoporella and microbialite; infilling internal sediment; and marine cement. Much of the Capitan was probably deposited below wave base as a diverse framework reef. The profile of the Capitan reef varied throughout its deposition. At the mid-point of Capitan development, the slope of the reef was very steep (80° or more) and the living reef extended to a depth of approximately 140 m. The organisms that lived on the reef varied with depth. The shallowest portions of the reef were inhabited by Collenella and a diverse assemblage of other organisms. In slightly deeper water, large platy sponges formed meter-scale over-hangs and cavities, which were inhabited by a cryptic community dominated by sponges. In the deepest parts of the reef, frondose bryozoa created decimeter-scale framework and internal cavities inhabited by cryptic pendant sponges. These internal cavities were filled by a succession of encrustations and cements, beginning with thick layers (1-3 cm) of microbialite followed by profuse botryoidal aragonite (up to tens of centimeters thick), thin (1—4 mm) layers of radiaxial fibrous cal- cite, and much later, large crystals of meteoric spar.

The Capitan reef influenced deposition in surrounding environments. Throughout its growth the Capitan was a “sediment factory” that generated prolific amounts of sediment that accumulated in the fore-reef. Study of the upper Capitan at Walnut Canyon suggests that the living Capitan reef probably created a depositional high that modified sedimentation in the lee of the reef on the outer shelf. Study of the middle Capitan and equivalents at McKittrick Canyon suggest that at other times circulation on the outer shelf was unrestricted by the reef escarpment and its living surfaces.

Many questions about the Capitan remain unresolved, for example, what was the response of the reef to sea-level change? How did the reef profile change through time? Is there consistent evidence of exposure in the reef? Does diagenesis vary throughout the reef? How diverse is the biota in the lower and middle Capitan, and does biotic diversity change through time?