Page 63 - Carbonate Facies in Geologic History

50 The Stratigraphy of Carbonate Deposits

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CAUONATf.EvAPo.,n ClASTlC.CAUONA Tf PENN. MIOCONTINENT YOREOAlE
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Fig. 11-25. Common patterns of cyclic sedimentation. These sequences are caused basically by
the timing of terrigenous influx and climatic variations with rising or falling of sea level
relative to land. Since the mechanisms are multiple and somewhat interrelated, results could
be theoretically complex. The observation that in many places patterns such as these are
discernible and repeated through long sequences, means that many times only one or two
mechanisms operate and result in fairly simple cyclic patterns

and closer study of microfacies and sedimentary structure for more accurate
environmental interpretation.
Despite the time and effort required, several advantages exist in the use of
cyclothems in sedimentological analysis of strata. (1) Their recognition may aid in
additional environmental interpretations. Certain rock types are more clearly
interpretable than others. The establishment of an orderly and consistent se-
quence of rock types permits comparison of depositional conditions with beds above
and below any bed in question. For example, a pellet calCarenite lying between
beds of bioclastic micrite more probably represents an organically pelleted lime
mud than an originally current-laid lime sand. (2) Recog9ition of sedimentary
cycles also offers aid to time-stratigraphic correlation within a given depositional
basin. The use of boundaries between cycles or the points of major marine inva-
sion or "deepest deposition" in correlation, has been discussed by Krumbein and
Sloss (1963, p. 383- 386). (See later discussion of correlation.) (3) Discernment of
cyclicity in strata aids in predicting facies distribution. Walther's Law (1893,
p. 979) states that facies vary in analogous sequence both vertically and horizon-
tally. This premise forms the first half of Shaw's TIme and Stratigraphy (1964); see
also Coogan (1972). Cycles will generally change facies in a regular way, some
within a distance of a few miles, some gradually for several hundreds of miles.
Recognition and description of regular vertical repetition in strata is a valuable
first step in construction of lithofacies maps.
Three types of cyclic patterns are typically seen in shelf deposits. These are
diagrammed in Fig. 11-25. It is convenient to organize the vertical sequence into
three main environmental subdivisions: lower terrigenous phase, middle normal
marine, upper shoaling, and final exposure. The categories of cycles are :
(1) Upward shoaling (fill-in) carbonate or carbonate-evaporite hemicycles with
essentially a regressive record (Chapter X); (2) simple terrigenous clastic-carbon-
ate couplets or, more complex and complete clastic-carbonate cycles like those of
the Late Paleozoic shelves of the mid continent (Chapter VII); and (3) cycles with
a transgressive lower carbonate and upper regressive terrigenous phase.
Many other examples of cyclic sedimentation may be found in extensive refer-
ences: The Kansas Geological Survey Bulletin 169 (Merriam, 1964) and Duff et al.