Page 234 - Carbonate Facies in Geologic History

Facies Sequence in the Permian Reef Complex 221

ent facies, must be a response to a different pre-Permian tectonic framework; no
basement faults are known to border the northern edge ofthe basin.
In the Guadalupe Mountains the western rim of the Permian Delaware basin
is a fairly narrow accumulation of massive carbonate fronted by thick foreset beds
which dip 30 degrees into the basin. Vertical depositional relief, that can be seen
by tracing these beds down the front into the basin, increases stratigraphically
upward to almost 700 m in the latest Permian. In the great canyons of the Guad-
alupes, the shelf margin can also be seen to have been built out basinward several
kilometers as it grew upward. The same stratigraphic pattern is visible around the
Delaware basin in the subsurface (Fig. VIll-4). Subsidence in the basin nearly
managed to keep pace with carbonate accretion on the shelves throughout
Permian time. The rate of sinking increased slowly until the carbonate shelf
margin was almost vertical. This is a common occurrence in carbonate bank or
platform history. Behind the rim, well-bedded strata of an evaporitic shelf envi-
ronment developed. Regional facies patterns (Fig. VIII-5) are presented by Galley
(1958), Hills (1972), and Meissner (1972).

Facies Sequence in the Permian Reef Complex

Several major works have described in detail the successive facies belts developed
along the relatively narrow shelf margin (King, 1948; Newell et aI., 1953; Dun-
ham, 1972). Tyrell (1969) also described in detail the facies along the uppermost
stratigraphic unit which can be traced from the shelf across the margin and into
the basin. Earlier, Hickox (in Newell et aI., 1953) traced and described a backreef
bed, Yates A Dolomite. Further details were added by Achauer (1969). The strati-
graphic succession and nomenclature of the shelf and basin deposits is outlined in
Fig. VIII -6. The interpreted facies belts are reproduced as Fig. VIII -7. The facies
belts have been more or less standardized into generally recognized units and the
outcrops of the Permian Reef Complex in the Guadalupe Mountains have been
developed into a classic model for carbonate shelf margin facies. Various un-
solved problems exist, however, as to exactly how some of the facies fit in the
original depositional profile. This is principally because of the great relief, inaces-
sibility of many of the canyons and massive character of the "reef' itself. These
circumstances have to date almost prohibited the detailed section measuring and
sampling necessary to careful tracing of rock types and the clarification of facies
transitions despite the spectacular mountain outcrops. It is a tribute to those
persons who have accomplished part of this arduous work that a more or less
coherent pictures has emerged despite the field difficulties. The facies belts are
generalized in Table VIII-I.
The Permian strata also display a variety of sedimentary structures created
during both deposition and diagenesis. (See comparison with the Triassic in this
Chapter.) These strata illustrate beautifully the depositional facies across a typical
narrow shelf margin (less than 10 km wide) with a steep (25-30°) edge and great
vertical relief (700 m). These marginal belts separate a wide carbonate platform or
ramp from a deeply subsiding basin. Application of the model to other carbonate
facies complexes should be restricted by the consideration that: (1) the sediments