Page 221 - Carbonate Facies in Geologic History

208 Late Paleozoic Terrigenous-Carbonate Shelf Cycles

usually lie in the upper half of these cycles. The cycles were formed across what
Krumbein and Sloss have termed "unstable shelves" and are remarkable for
widespread and uniform distribution of thin beds of varying rock types. Much
shale, as well as limestone and sandstone, occurs in these strata-almost as much
as in the basinal strata off the shelves. This imparts a characteristic erosional
pattern and the rapid vertical alternation of thin beds of sandstone, shale, and
limestone is a hallmark of Pennsylvanian sedimentation in North America (Ma-
thews, 1974, Fig. 16-7) and also see Figs. VII-6, VII-7 for typical cyclic patterns of
the Midcontinent. These diagrams have great vertical exaggeration and may not
adequately convey the impressive repetitive alternations of the thin, and very
different, strata.

Petrography of Cyclothem Beds

Two types of sandstones are commonly seen: (1) On the shelves thin deltaic se-
quences occur with channel conglomerates, shallow marine bars and beaches, etc.
These may occupy channels of distributaries or occur as thick, localized pods of
clastics in areas of great sediment influx. Petrographically the sands are variable,
mostly slightly feldspathic, lithic arenites. Detailed studies on lithologic features
of these have been published by Wanless (1964, 1972); Wanless et aI., (1963);
Brown (1972) and Galloway and Brown (1973). (2) Fine-grained, dark, and more
uniform sands are known in the subsurface basins in contrast to such variable
shelf sandstones. These sands represent sediment by-passed from the shelves dur-
ing low sea-level stands (cyclic and reciprocal sedimentation; see Galloway and
Brown, 1973).
Thin black shales: Studies of the very thin (1 m) and widely distributed Upper
Pennsylvanian black shales of Kansas, such as the Heebner (Evans, 1966) and the
Excello (James, 1970) have added much to a logical understanding of the cy-
clothem sequences. Such fissile, platy, organic-rich shales contain mainly a pelagic
nektonic fauna of conodonts, fish teeth, spores, and limited benthos, including
even a few brachiopods and mollusks. The shales contain phosphatic concretions
and significant trace amounts of uranium. Their environment is clearly euxinic
marine. They occur most commonly in the middle of the megacyclothem sequences
of the Kansas Upper Pennsylvanian, lying east of the dominant limestone
buildup in the subsurface. Similar black shale formed in coastal marine ponds and
swamps in Illinois, has been extensively described by Zangrl and Richardson
(1963).
Carbonate microfacies types: Special organic constituents and textures in
Pennsylvanian-Early Permian shelf and shelf-margin limestones have already
been described in Chapter VI. Environmental determination ofthe limestone mem-
bers of shelf cyclothems is also of critical importance to interpretation of their
geologic history and much attention has been given to this in the last 15 years.
The organic composition of these Late Paleozoic limestones is particularly varied
and despite much study, not all of the fossil organisms have been properly identi-
fied biologically. The abundance of organic debris creates packstone texture
which is commonly associated with a microspar matrix of problematic origin. The