As with the Cambrian-Ordovician, surface distribution of Silurian
(from 439-409 million years ago) and Devonian (from 409-363 million years
ago) age rocks is limited to the structurally uplifted areas of the Arbuckle
and Ouachita Mountains, as well as in the Ozark Uplift. Limestones and dolomites
continued to form across the shelf, while sandstones and shales were formed
in the deeper Ouachita Basin during the Silurian and Early Devonian. The limestones
are represented by the Hunton Group, which formed under shallow marine water
conditions, and host a variable array of fossil trilobites, brachiopods, crinoids,
and bryozoans. Thicknesses of these limestones vary from 100-500 feet (Johnson,
1996).
Sometime during the Devonian Period (possibly during the Middle Devonian),
most of Oklahoma experienced a major withdraw of the epicontinental sea, and
which exposed these, and older rocks, to erosion. It is estimated that some
500-1000 feet of sediment was removed during this time, and produced what
is now called a major unconformity in the rock record. An unconformity is
essentially a gap in the rock record caused by an extensive period of erosion
or non-deposition. As a consequence, there is no recorded history for these
times in Oklahoma. Withdraw of the sea was not total, however, as deposition
of the Arkansas Novaculite continued uninterrupted in the Ouachita basin during
the Middle and Late Devonian.
Epicontinental seas flooded all of Oklahoma once again by the Late Devonian,
and deposited thick sequences of black marine shale (the Woodford and Chattanooga
Shale) across most of the state.
To see what Oklahoma was like during this period go to the Paleogeography of the Silurian-Devonian.
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.