Page 391 - Carbonate Facies in Geologic History

378 Summary

Table XII-3. Use offacies models
Holocene model Well known
with similar ancient
geography facies model
,,/
.....
.....
..... "
..... " " plus imagination!
.....
..... "
......... /
Holocene process Re-evaluated
model 1-----1 ancient model
' ..... I
.....
..... I projection toward unknown
..... I
.....
..... ...... '"
..... Incompletely
'~ known ancient
model
preferably of
same age as
original model

shelves, and Sunda shelf which is flooded with terrigenous clastics and has only mar-
ginal carbonate buildups.
c) Shelf seas across major cratonic blocks do not exist at the present time. Comparisons
with ancient areas must be made with relatively shallow bays and lagoons along the
present coasts. These commonly contain banks or mounds of bioclastic lime mud. E.g.,
the Florida Bay; lagoons on northeastern Yucatan coast; Shark Bay, Western Aus-
tralia.
The above outline relates the geographic settings of modern depositional
models to tectonic framework so that models will be more applicable (see Malek-
Aslani, 1973). As pointed out above this does not work entirely satisfactorily. It is
possible in many places to use parts of a geographic model regardless of the exact
match of the whole. The Persian Gulf Trucial Coast model comes closest to a
complete display of the standard carbonate facies progression but only when
applied to an arid climate. Similarly, Florida Bay is a useful place to study the
products of restricted circulation but is part of a larger geographic setting which is
complicated by Pleistocene history. The outer barrier to the Bay is triple, consist-
ing of the Florida Reef tract, the shallow Florida strait, and the Pleistocene rim of
the Keys.
Perhaps the most valuable use of Recent carbonate deposition is to study the
processes and to use this information to pique our imagination when transferring
attention to an ancient geological example. This is usually more productive than
merely matching facies patterns on models of different geologic age and perhaps
of different geographic settings. This procedure is diagrammed in Table XII-3. A
particularly thorough application of a Recent model to geological interpretations
is found in the work of Griffith et al. (1969) on the Cretaceous of Mexico and
Texas.
The exploration geologist normally begins with stratigraphic knowledge of a
few wells or sections and a general understanding of the basin tectonics. As