Page 222 - Carbonate Facies in Geologic History

Petrography of Cyclothem Beds 209

following is an attempt to define and interpret 11 microfacies types common in
Pennsylvanian-Wolfcampian strata. It is based principally on petrographic stud-
ies in the midcontinent region of the U.S.A. (e.g., Toomey, 1969a and b, 1972;
Troell, 1965) and on the author's experience in West Texas, New Mexico, and
Utah. Excellent illustrations of the microfossils and textures are found in papers
by Toomey. Despite obvious limitations of study confined to certain provinces,
these rock types are believed basic and useful for a general environmental inter-
pretation of Pennsylvanian shelf sediments. They are widely repeated in many
sections throughout the western and midcontinent United States. The rock types
are numbered P I-P 11 to distinguish them from the standard microfacies types.
P 1. Molluscan argillaceous, ferruginous lime mudstone-M yalina beds. Outcrop expression:
thin, yellow, soft-weathering limestone, a shell hash oflarge Myalina shells.
Environment: shallow, muddy water; significance of iron concentration is not known.
P2. Very fossiliferous, marly shale. Grey nodular-weathering shale with well-preserved fos-
sils and a varied marine fauna. Chonetids, productids, Neospirifer, Composita, Derbya,
Enteletes are all common brachiopods; horn corals; cyclostomate bryozoans forming
large lumps; disarticulated pieces of crinoids and echinoids are common; some mol-
lusks, fusulinids, and smaller foraminifera. Much of the fauna is vagrant benthos. These
shales provide most of the well-known Pennsylvanian-Lower Permian fauna from the
Midcontinent. Environment: soft substrate, but at times moderately clear water, open
circulation and normal marine salinity but very little current or wave action; moderate
depth-a few meters to tens of meters.
P 3. Normal marine wackestone with 20-25% bioclasts.
Same fauna as contained in the above shale, plus algal plates, a few dasycladacean algae
and many foraminiferal-onkoidal structures. Generally contains a highly varied biota.
Syringopora occurs. Echinoids are more abundant than crinoids. Matrix is micrite or
microspar, generally the former. On outcrop, bedding is wavy-nodular to planar with
shale partings. Texture homogenized by extensive burrowing. Organic debris com-
monly somewhat macerated but with well-preserved internal structure.
Environment: Clear, calm water of open circulation and of moderate depth (a few
meters to tens of meters determined from facies gradations downslope from bioherms);
substrate soft. Standard microfacies type 9 (Plate VI B).
P4. Foraminiferal limestones. Poorly sorted debris with many sorts of foraminifera and of
packstone or wackestone texture. Calcitornellids most commonly occur as flat-sided
coiled tubules, which encrusted soft-bodied forms, and also are found as tiny masses or
irregular balls. Endothyrid concentrations with packstone texture are known in earlier
Pennsylvanian beds. Many mobile foraminifera are present, the same genera occurring
with normal marine limestones: Bradyina, H emigordius, Tuberatina, Climacammina, Pa-
leotextularia, Globivalvulina. Outcrops of such limestone are wavy bedded and are
indistinguishable from the above normal marine limestone.
Environment: Believed to be almost the same as for the above normal marine lime-
stone; perhaps the foraminifera are concentrated by gentle winnowing of currents or
waves; they appear to be more or less ubiquitous. Perhaps slight restriction or higher
water temperature inhibits other biota. A variety of standard microfacies type 9.
P 5. Fusulinid coquina. Fusulinids concentrated into coquinas but also commonly found in
limestone with other members of the varied normal marine biota. They occur in well-
bedded strata. Such coquinas are generally packstones with some intergranular solu-
tion and microstylolitization, particularly when the matrix is slightly argillaceous. Envi-
ronment: Apparently normal marine, clear water, on shelves which ranged from depths
of a few meters to a few tens of meters. Fusulinids must have been able to live high on
biohermal slopes and hence in quite shallow water, for major detrital accumulations of
them occur down slopes between bioherms standing as much as 25 m above the sea
floor (Plate XXIII A).
Fusulinids formed resistant particles, which were easily moved, and are found in many
types of sediment but most commonly in carbonates. Standard microfacies type 12.