Page 392 - Carbonate Facies in Geologic History

Review of Tectonic Settings for Carbonate Buildups and Cycles 379

diagrammed on Table XII-3, a specific example existing in the stratigraphic re-
cord is observed and an attempt is made to find a geographically comparable
Holocene model to be studied. From the Holocene study comes an understanding
of sedimentary processes which can be measured and evaluated from marine
investigation. The process model may then be used, with a little imagination, for a
reevaluation of the well-known geological example and ultimately for projection
to an incompletely known carbonate facies complex and for more accurate pre-
diction of trends and facies in it.
As an example of the valuable interplay of Holocene models with ancient
examples, consider the evolution of thought about the Permian Reef Complex.
Newell et al. (1953) after extensive study of the Bahama Banks and following post
World War II studies of the Pacific reefs, used the only known Holocene sedimen-
tary models to help interpret the Guadalupian strata of New Mexico and West
Texas as a major barrier reef trend. Size and geography of the Recent and Ancient
models agreed quite well. But subsequent petrographic study of the Permian
"reef' showed no framework capable of organic stabilization of the abrupt shelf
edge and much discussion ensued in the literature about its origin. Beginning in
1960, processes of diagenesis in a strongly arid environment and in a landlocked
sea were studied in the Persian Gulf. This was an area whose geographic setting
did not serve as a very apt model for comparison with the Permian Reef Complex.
Nonetheless, many diagenetic features along this arid coastline were recognized
as identical to those of the Permian: evaporites on sabkha, splash zone drusy
cementation, rapid hardening of peloidal sands on tidal flats, extensive develop-
ment of bulbous algal stromatolites, desiccation polygons, and tidal flat dolomiti-
zation. Dunham (1972), recognizing the similarity of processes based on similarity
of diagenetic features, proposed that lithification during a time of extreme climate
and periodic drops of sea level was responsible for stabilization of the sharp shelf
margin. At the present time, investigation of the classic Triassic banks of The
Dolomites is progressing rapidly and these structures are being reinterpreted,
aided both by an understanding of Recent processes of vadose zone diagenesis
and by an improved interpretation of the Permian Reef Complex, a model not far
removed in time from the Ladinian strata of the Alps (Bosellini and Rossi, 1974).
Further investigation on the interiors of modern reefs has revealed the impor-
tance of submarine cementation in maintaining them. The role of this process
now needs to be evaluated in the Permian buildups of the Guadalupe Mountains.
Thus geologic thought directed toward interpretation of carbonate buildups
evolves as continuously as do the organisms which generate such masses. We
have still much to learn. The didactic method employed in this volume, the
numerous outlines, and classifications should not obscure this fact. Concepts will
almost certainly alter as more data appear and are analyzed by experienced
stratigraphers and petrographers. The author hopes that this volume has laid a
beginning foundation for the description of carbonate depositional facies-a base
from which more accurate interpretations of environments and prediction of
vertical sequences and geographic patterns is possible. The study of present-day
processes and development of Holocene models is vital to our interpretations of
ancient strata, but the recognition and classification of what actually exists in the
geologic record must form the primary basis for our concepts.