The Pennsylvanian (from 323-290 million years ago) rocks are
well represented in Oklahoma, and cover nearly 25% of the surface area of
our state. Two events were occurring simultaneously that give the Pennsylvanian
rocks of Oklahoma a unique character.
First, the Pennsylvanian Period was a time of extensive mountain building
activity, also called orogenesis. Uplift and erosion of the Ancestral Rocky
Mountains contributed large volumes of sediment from the west, while the Wichita
orogeny in southwest Oklahoma uplifted older (Precambrian through Mississippian)
rocks, exposing them to erosion during the Early Pennsylvanian. This event
was followed by the Ouachita orogeny during the Middle Pennsylvanian, which
caused strong deformation of the sediments in the Ouachita Basin, and formed
the Ouachita Mountains. Once exposed above sea-level, rocks of the Ouachita
Mountains started to erode, and produced a large volume of sediment that was
deposited to the north (in the newly formed Arkoma Basin) and to the south
in Texas. By the Late Pennsylvanian, the Arbuckle orogeny was occurring, and
reactivated mountain building and uplift of all regions in the southern part
of the state. With all of this tectonic activity, clastic sediments (i.e.,
sands, silts, and clays) were being shed into the deepening Anadarko, Ardmore,
and Arkoma basins situated north and south of the highlands, as well as onto
the shallower marine shelf areas in northern Oklahoma (Johnson, 1996).
Along with these Mountain building events, extensive continental glaciation
was occurring in the southern part of the world (South America, Africa, and
Australia), which caused drastic fluctuations in sea level. The Pennsylvanian
rocks of Oklahoma reflect these fluctuating conditions as most of the rock
sequences exhibit alternating beds sandstone, shale, and sometimes limestone
that formed under marine and non-marine conditions. This almost uniform, and
rhythmic alternating between sandstone, shale and limestone lithologies is
called cyclothems). The rocks formed under non-marine conditions typically
contain abundant plant fossils of petrified wood, carbonized leaves, and extensive
layers of coal. The rocks formed under marine conditions contain a host of
invertebrate fossils, including: brachiopods, clams, bryozoans, and snails
to name a few.
Click either the Early, Middle, or Late Pennsylvanian to view the paleogeography of Oklahoma at those times.
Reference: Johnson, K.S. 1996. Geology of Oklahoma, p. 1-9. In, K.S. Johnson and N.H. Suneson (eds.), Rockhounding and Earth-Science Activities in Oklahoma. Oklahoma Geological Survey Special Publication, 96-5.