Page 226 - Carbonate Facies in Geologic History

Interpretation of Midcontinent Pennsylvanian Cyclothems 213

sand flats, and beach-bar units. The traditional lower clastic unit of the Kansas
megacyclothem is the most variable and correlation errors are best eliminated by
careful delineation of its units. Channels downcutting more than 30 m are known
in such strata. Multiple sources of terrigenous influx are recognized. In an east-
ward direction less regular sequences occur (Wanless, 1972).
2. The black shale, its underlying thin (number 2) limestone, and its eastward
equivalent coal facies represent the maximum transgression across a remarkably
flat terrain during a time of tectonic stability when there was no ingress of clastics
from the east. These beds then offer a key for tracing the cyclic sequences up or
down the paleoslope. This triplet changes facies less rapidly than other members.
It constitutes the most reliable time stratigraphic marker. Sediments of the other
parts of the cycles were formed during a substantial migration of depositional
environments during deltaic progradation and retreat. Each of these rock types
has facies equivalents in time in both shelf and basinward directions.
3. The downdip limestone buildups (largely buried in the subsurface of Kan-
sas and north central Texas) represent low-lying but extensive organic banks
which must have strongly restricted circulation in the Kansas and north Texas
eastern shelf by sheltering the lagoons from the open sea to the west. This offers
an explanation for the thin black shale sheets.
4. Petrographic distinction of the various limestones in the megacycles of
Kansas is important to their interpretation; they differ greatly in depositional
environment. The lower limestones, where argillaceous, contain a marine mollus-
can fauna; the middle limestones (numbers 2 and 3) record clear, warm water with
open circulation; and the higher limestones are indicative of shallower, locally
more restricted seas, being commonly oolitic, onkoidal, or algal.
In the midcontinent United States there exist between 50 and 60 records of
what must have been extensive transgressions and regressions across the central
part of the North American craton during the 25-30 million years of the later
Pennsylvanian and Wolfcampian. Many explanations for the cyclicity have been
advanced (Chapter II and references above). Basic agreement exists that, whatever
the causes, considerable sea level fluctuations occurred over the midcontinent
shelf during this time, that the cyclicity is unusually apparent because of terrige-
nous influx, and that probably both eustatic sea level change and tectonic activity
contributed to the repetitive pattern. The sequence of a half dozen or so compli-
cated megacyclothems in Kansas (Fig. VII-6) indicates a surprising order to this
complex and argues for a simple and systematic mechanism. To date, however, no
general theory as to the basic cause for the megacyclothem patterns is completely
satisfactory.
One may compare the midcontinent cyclothems with those of the same age
described by Van Siclen (1972) and Galloway and Brown (1973) in north central
Texas (Fig. VII-7). Where terrigenous influx is important, purely sedimentologic
processes may be responsible for cyclicity and are superimposed on eustatic or
other extrabasinal causes; in some places the sedimentological controls are domi-
nant. Shifting of deltaic distributaries on a steadily subsiding shelf, restriction of
sedimentation by plant growth, and differential compaction of coals, shales and
limestones are all mechanisms which control changes in rates of sedimentation
relative to sea level and which may be cyclically repeated.