258
6 DEPOSITIONAL SYSTEMS
extensive diagenesis. This is due to a variety of factors, including high primary porosity,
unstable mineralogy, and invasion by hypersaline fluids when sea levels drop and lagoons evaporate (see Section 9.2.5.2). Because of these factors, reefs and banks are
frequently recrystallized and dolomitized, so that their primary fabric is undetectable.
Many of the Devonian "reefs" of Canada are of this type. Because of this diagenesis, it
may be impossible to prove whether a lenticular carbonate buildup was actually once a
reef in the ecologic sense.
There is a second important point related to the diagenesis of carbonate buildups.
It has already been mentioned that a drop in sea level exposes the crests, not only of
orthodox reefs, but also of shell banks and carbonate sand shoals. These may all be cemented by early diagenesis. When sea level rises again, all of these features, regardless of their genesis, now form wave-resistant masses complete with talus slopes and
lagoons. Repetition of this process can build up wide shelves with margins formed of
early cemented carbonates of diverse facies with aprons of slope deposits that grade
down into a basin. The Capitan Formation (Permian) of west Texas rims the Delaware
basin, separating shallow water evaporites and continental red beds on the northwestern shelf from deep water shales and turbidites (Fig. 6.62). This was once interpreted as
a classic example of a barrier reef (e.g., Newell et al., 1953). The tendency now is to
interpret this as a feature that had a wave-resistant scarp, but that this was composed
of organic bioclastic shoals and banks which gained their rigidity, not from the syndepositional activity of encrusting algae, but from cementation during marine low stands
(Achauer, 1969; Dunham, 1969; Kendall, 1969).
The story of the Capitan "reef" suggests that it may be helpful to recognize and disFig. 6.62. Photograph of the jebel E1Capitan, type locality of the Permian Capitan "reef," Guadalupe Mountains, west Texas. The massive reef carbonates overlie basinal black shales and turbidites of the Delaware
Mountain Group that crop out in the foreground. (Courtesy of E. Purdy).
6 DEPOSITIONAL SYSTEMS
extensive diagenesis. This is due to a variety of factors, including high primary porosity,
unstable mineralogy, and invasion by hypersaline fluids when sea levels drop and lagoons evaporate (see Section 9.2.5.2). Because of these factors, reefs and banks are
frequently recrystallized and dolomitized, so that their primary fabric is undetectable.
Many of the Devonian "reefs" of Canada are of this type. Because of this diagenesis, it
may be impossible to prove whether a lenticular carbonate buildup was actually once a
reef in the ecologic sense.
There is a second important point related to the diagenesis of carbonate buildups.
It has already been mentioned that a drop in sea level exposes the crests, not only of
orthodox reefs, but also of shell banks and carbonate sand shoals. These may all be cemented by early diagenesis. When sea level rises again, all of these features, regardless of their genesis, now form wave-resistant masses complete with talus slopes and
lagoons. Repetition of this process can build up wide shelves with margins formed of
early cemented carbonates of diverse facies with aprons of slope deposits that grade
down into a basin. The Capitan Formation (Permian) of west Texas rims the Delaware
basin, separating shallow water evaporites and continental red beds on the northwestern shelf from deep water shales and turbidites (Fig. 6.62). This was once interpreted as
a classic example of a barrier reef (e.g., Newell et al., 1953). The tendency now is to
interpret this as a feature that had a wave-resistant scarp, but that this was composed
of organic bioclastic shoals and banks which gained their rigidity, not from the syndepositional activity of encrusting algae, but from cementation during marine low stands
(Achauer, 1969; Dunham, 1969; Kendall, 1969).
The story of the Capitan "reef" suggests that it may be helpful to recognize and disFig. 6.62. Photograph of the jebel E1Capitan, type locality of the Permian Capitan "reef," Guadalupe Mountains, west Texas. The massive reef carbonates overlie basinal black shales and turbidites of the Delaware
Mountain Group that crop out in the foreground. (Courtesy of E. Purdy).