Page 62 - Carbonate Facies in Geologic History

Carbonate Shelf Cycles 49

faunally and lithologically similar over the vast area stretching from Hudson Bay
and Manitoba in the north to Sonora, Mexico in the south, and from Missouri to
Colorado in an east-west direction (Krumbein and Sloss, 1963, Fig. 10-15; see
Fig. 11-24). The lower, carbonate portion of the couplet is almost everywhere a
characteristic brown color, contains large, dolomitized burrow mottling and a
characteristic fauna of large fasciculate corals, Receptaculites, and nautiloids. Its
thickness variation (100--200 m) is slight considering its extremely broad distribu-
tion.
Remnants of the upper portion of the lower Devonian (Camden chert) stretch-
ing from Tennessee through the southwest USA to Chihuahua, constitute an
equally impressive unit of distinctive rock types: light novaculitic and cherty
limestone, everywhere bearing the same brachiopod and trilobite faunas.
The Washita beds of the Texas Cretaceous (Late Albian-Cenomanian) consti-
tute another widespread transgressive carbonate unit of rather uniform, though
argillaceous, character. According to rules of sedimentation in clear-water, epeiric
seas, as set forth by Irwin and Shaw, one would expect such widespread units to
consist of highly restricted carbonates and evaporites, to show gradual facies
differentiation, and be naturally diachronous. Indeed, they do change facies some-
what into the mildly subsiding basins across the vast midcontinent area of North
America, become more laminated and cherty there, but appear always to have the
same faunas. One may conclude that such limestone sheets must have been depos-
ited slowly, with many breaks in sedimentation, perhaps under water a few
hundred feet deep, under hydrologic and climatic condition which encouraged
some degree of circulation. No modern model for such shelf sedimentation is
known.

Carbonate Shelf Cycles

Commonly, shelf sediments consist of depositional cycles in which repetitive
sequences of rock types are spread over vast areas. Individual beds in such cycles
may be traced for many miles. The earliest studied and best known cyclothems of
the geologic record are those in the Carboniferous (Duff et al., 1967, p.4) where
complex combinations of clastics and carbonates make for easy recognition. One
of the most interesting stratigraphic results of carbonate study in the last 20 years
has been the recognition that most thick limestone and dolomite sections consist
of cyclically repeated strata and are not homogeneous units. This is particularly
true of shelf deposits. (See Chapters VII and X.).
One may commonly recognize cyclicity in strata after measuring and describ-
ing a section in some detail; some basic rock types may be repeated in a regular
order. Computer programs are available to aid in this recognition. A further
question is: between which rock types are the stratigraphic contacts most abrupt?
The evaluation of such boundaries should consider evidence of subaerial expo-
sure or depositional still-stand under marine conditions (see outline of sedimen-
tary structures in Chapter III). At this stage interpretations of either truly cyclic
(ABCBA) patterns or rhythmic (hemicyclic-ABCABC) patterns may be possible.
Once cycles are recognized, further work on the section may involve resampling