Page 241 - Carbonate Facies in Geologic History

228 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

Problems of the Permian Reef Complex

The profile across the shelf: What was the original topographic profile across the
shelf and its margin? Did the ramp-like shelf slope steadily seaward owing its
restricted circulation to sluggish currents and lack of wave action across its vast
expanse? Or did a barrier island complex lie at the outer margin of a platform-a
barrier which was exposed at low sea level stands, subjected to diagenesis by
meteoric water, and which offered a protection to a broad shelf lagoon? Both
interpretations of the shelf environments are possible and both conditions may
have prevailed at different times. Detailed mapping is needed to ascertain the
relative amounts and distribution of lagoonal, tidal flat, and sabkha environments
in the Chalk Bluff facies. More careful and stratigraphically controlled mapping
in the Guadalupe Mountains could outline the configurations of old barrier
islands by using signs of vadose diagenesis. Quite possibly, positions of mean sea
level can also be ascertained from mapping distribution of algal stromatolites.
Dunham's marginal mound hypothesis (Fig. VIII-9) suggests that a belt of these
stromatolites existed on the back reef, lagoonal side, and marked the inner shore
of a barrier island.
A further profile problem concerns the position of the massive limestone
interpreted as "organic reef' by Newell et al. (1953) who conceived the shelf
margin as forming a barrier reef in the breaker zone analogous to a modern coral
rim. Further petrographic studies (Achauer, 1969; Dunham, 1972) have demon-
strated a large amount of lime mudstone, fine broken debris, low-lying encrusting
organisms, loose sediment dwellers, and in many places, almost complete absence
of relatively large framebuilding biota (Fig. VIII-8). Sedimentologic interpretation
would favor a downslope, quiet-water environment for such an accumulation.
The position of the micrite on the original depositional profile is made unclear by
the difficulty of tracing bedding planes and individual bodies of sediment through
the facies and the gigantic size of the outcrops involved. A start on this was made
by Smith (1973) who traced an organic framework of bryozoans on some canyon
bed exposures and interpreted the shelf margin as an ecologic reef. Three interpre-
tations of profile as epitomized by Dunham are given in Fig. VIII-9. The hy-
potheses are termed Barrier Reef, Uninterrupted Slope, and Marginal Mound.
The author believes (following Dunham) that at some places careful observation
of slopes and bedding planes indicates that the original high point on the profile is
in a lime sand-barrier island facies and that the "reefy" wackestone, rich in
sponges, Tubiphytes, and lined cavities ("organic reef' of Newell et al.) is a down-
slope deposit. The Permian Reef Complex is thus considered to represent a Type I
shelf margin-a downslope mud accumulation rich in organisms (see Chapter XII
and Wilson, 1974). Dunham's interpretation, that the mass developed rigidity
more because of early diagenetic cementation than through organic binding, is
discussed below.
Problems of sea-level fluctuation: Silver and Todd (1969) and Meissner (1972)
demonstrated how cyclic and reciprocal sedimentation may be responsible for the
strikingly abrupt Permian facies changes from carbonate-evaporite of shelf and
shelf margin, to predominantly sandstone-siltstone strata in the basin. Dunham's
work (1972) helps to document key facies relations in the Guadalupe Mountains.
That the thin dark limestones of the basin may be traced up the slope of the shelf