Page 228 - Carbonate Facies in Geologic History

Interpretation of Midcontinent Pennsylvanian Cyclothems 215

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Fig. VII-8

sylvanian except that perhaps the single basal limestones are more persistent. In
contrast, the traditional explanation of midcontinent cyclothems has emphasized
extensive east-west regression and transgression caused by extrabasinal sea-level
fluctuation or by tectonic "tilting" or "bobbing" of the shelf. The reason for this is
the much wider cyclic shelf in the Midcontinent, the extensive downcutting of
certain channels and the complexity of the section interpreted to represent initia-
tion of transgression.
In the Midcontinent, conventionally the base of the cycle is marked at the
most striking lithologic boundary; this is commonly the base of a channel con-
glomerate. This disconformity has been interpreted to represent a significant and
sudden sea-level lowering. A few channels in the Midcontinent are large and
widespread, filled with coarse conglomerate, and cut down through tens of meters
of section. The beds above the channel are a mixed clastic and carbonate section
interpreted to have formed when the following marine transgression was barely
successful in overcoming clastic influx. Many channels, however, may represent
merely meandering distributaries during periods in which regressive and progra-
dational terrigenous sedimentation occurred. Mathews (1974, Figs. 16.1 and 16.7)
compares the two interpretations.
The north-central Texas cycles and those of the Yoredales are conventionally
portrayed to begin with a widespread marine limestone whereas those of the
midcontinent United States are conceived to commence with a variable section of
limestones and terrigenous clastics.
If the lower part of the Kansas megacyclothems actually represents a part of a
prograding regressive sequence, the Midcontinent, Y oredale, and north central
Texas patterns are very similar. In any event, the triplet of coal, thin limestone,
and black shale appearing in the basal Y oredale and in the middle or basal
Kansas megacyclothem represents the most inundative and stable phase of sedi-
mentation and hence the best time-stratigraphic marker in these complex sec-
tions.
Other Pennsylvanian cyclothems occur farther west and south in the Rocky
Mountain areas. They continue far out into the desert southwest and up the