Page 224 - Carbonate Facies in Geologic History

Interpretation of Midcontinent Pennsylvanian Cyclothems 211

crusting calcitornellid foraminifera. Some of these masses encrust and are interwoven
with spirorbid tubes (Toomey, 1962). Girvanella tubules usually abound in the laminae.
Environment: Matrix may be of coarser texture but is commonly micrite, showing
quiet-water deposition. Probably accumulated in areas of slow deposition as loose balls
on the sea floor and later were deposited with lime mud or silt. The specific gravity of
these balls may have been low because of the large amount of plant matter. Such beds
occur commonly in cyclic shelf deposits of all ages, are known also at disconformities,
in tidal channels, and in low places behind bioherms. Modern onkoids, formed as lime-
mud accretions by blue-green algae, are known in Florida and the Bahamas, scattered
in tidal channels and shallow bays or straits, but true beds of the structures such as are
seen in the geologic record have not yet been observed in the Holocene. These consti-
tute standard microfacies type 22.
P 11. Basinal, somewhat argillaceous lime mudstones-wackestones. Dark, well-bedded strata
with dominantly normal marine fauna. Indigenous fusulinids are very rare; apparently
fusulinids did not normally drift so far downslope. Brachiopods are represented mainly
by productids and their spines, and spicules. Trilobite and ostracod tests and mollusk
shells are common. Echinoids are more common than crinoids. Detrital algal plates,
when present, are well-preserved and may show cortical structure. Such shallow ba-
sinal limestones, when farther away from the shelf, may consist mainly of spicules,
presumably from sponges.
Environment: Quiet, normal marine water, below shallow wave base and at or below
common O2 level. Turbid water induced slightly reducing conditions by inhibiting the
normal in-fauna. Some burrowers were present. A somewhat euxinic variety of stan-
dard microfacies 9 (Plate II).

Interpretation of Midcontinent Pennsylvanian Cyclothems

Numerous authors in the Kansas Geological Survey Bulletin (1969) and the West
Texas Geological Society volume on cyclic sedimentation in the Permian Basin
(1969, 1972) described these Late Paleozoic shelf cycles and discussed their possi-
ble causes. These papers derive in large part from life-long investigation by
R.C.Moore, J.M.Weller, H.R.Wanless as well as more recent studies of
D.C.Van Sic1en,J.Imbrie,J.W.Harbaugh, L.F.Laporte, E.G.Purdy, D.F.Toom-
ey, A.R.Troell, L.F.Brown, and H.C.Wagner. FigureVII-6 is based mainly on
work of Wanless (1972) and describes facies within an ideal shelf cycle across
Illinois and Kansas. Figure VII-7 similarly describes a cycle from north central
Texas (Van Siclen, 1972). Here the shelflay closer to the source of clastics and had
a steeper gradient.
Most of the above studies have been made following the long outcrop of these
cyclothems which stretches from Iowa into north Texas. Any reasonable explana-
tion of the strikingly repetitive vertical sequence must in addition consider the
total east-west facies sequence, much of which is downdip in the subsurface of
western Kansas and central Texas. Only in the last few years have explanatory
models for cyclothems adequately considered downdip relations. See particularly
the papers ofH.R.Wanless and L. F. Brown. The above figures have attempted to
show the complete facies pattern.
Modern studies of midcontinent cyclothems have accepted the following keys
to interpretations:
1. Careful tracing of eastward-derived terrigenous beds shows that these thin
deltaic units may be environmentally subdivisible into channels, distributaries,