Page 230 - Carbonate Facies in Geologic History

Chapter VIII

Permo-Triassic Buildups
and Late Triassic Ecologic Reefs

The Permian Reef Complex, surrounding the Delaware basin of West Texas and
New Mexico, and the Middle Triassic (Ladinian) of the Dolomites of the South
Alps are two of the worlds greatest and best known ancient carbonate buildups.
They are closely related in age and organic content. Surprisingly enough, they still
present unsolved problems of origin and subsequent diagenetic history. This
Chapter briefly reviews and compares these famous models of carbonate facies
focusing on both the deposition and diagenesis of the sediments.

Permian Reef Complex

The almost featureless plains of West Texas and southeastern New Mexico con-
ceal rocks of Late Paleozoic age with a complex structural framework. The Cen-
tral Basin Platform, a north-south horst, separates two structural depressions, the
Midland basin on the East and the Delaware basin on the West. During Pennsyl-
vanian time these basins, now covered by younger strata, were deep, euxinic and
starved. During Permian time the Midland basin subsided slowly and was rapidly
filled; in contrast, the Delaware basin sank more quickly, was filled partly with
fine sand and silt, but remained relatively deep until the end of the Permian. A
carbonate rim almost completely encircled this basin, separating it from the
surrounding shelves on which accumulated a shallow water lagoonal and evapo-
ritic tidal flat facies. The general facies and structural relations have been known
since the 1930's and are reviewed by King (1942, 1948). An interesting chapter on
the development of ideas on facies changes and "reef' recognition in the complex
Permian strata of the southwestern U.S.A. is found in King's textbook, The Evolu-
tion of North America (1959). Lloyd (1929) is generally given credit for recognizing
that the Guadalupe Mountains on the western side of the basin contained expo-
sures of a kind of fossil "reef' which rimmed the entire subsurface Delaware basin.
Lloyd's idea was an important one, for the northern and eastern sides of the basin
lie below the almost featureless surface of the plains and furnish no hint of the
underlying structure. The eastern border of the Delaware basin was discovered to
form the western edge of the Central Basin Platform, one of the world's great oil
producing structures. Permian strata produce most of this oil (Galley, 1958; Hills,
1972) from beds just behind the carbonate shelf margin. It is a rare opportunity
when strata of the subsurface can be studied in nearby outcrops, in the same
facies, and on the opposite side of the same basin.
Rising west ofthe Pecos River in New Mexico, the Guadalupe Mountains, which
contain the Permian reef outcrops, form the front of the Rockies. They are a great
triangular block, tilted gently northward and bounded on the west by a large