Silurian/Devonian Period

Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological Survey and Resource Assessment Division).

Silurian/Devonian Period (441 to 362 million year old rocks)

Silurian and Devonian rocks are lumped together on this map because both are rather scarce in the state. The true cause of this scarcity is still obscure; some contend they were only deposited around the Ozark fringes, and others think that they once existed at least thinly over much of the region, but were removed during a 40 million year long erosional event before the encroachment of the Kaskaskia Sea during the middle Devonian. Some evidence exists that the Ozarks moved upward during this time, as a result regional warping, not volcanism.

Silurian rocks in Missouri occur at the surface near Hannibal, and near Cape Girardeau. Buried Silurian rocks have been drilled into in extreme northwestern Missouri. These layers are thin crystalline to fossiliferous limestones, and shales with a few dolomites.

Silurian strata were deposited during a resurgence of the regressing Tippecanoe Sea--unlike Ordovician age waters, which seemed ubiquitous, these inundations were local in nature, flowing through "seaways", or narrow paths along low lying areas, somewhat like the modern inlets along the Arabian Peninsula. The early Silurian rocks were deposited from the east, which some say suggest the topographic beginnings of the Mississippi Valley, while the ones in the northwest were laid down much later, and probably due to an adjustment of general sea depth.

Devonian rocks have a much wider spread across the state, outcrops being in the the northeast and southeast next to the Silurian rocks, as an outlier in the extreme southwest, and in the subsurface of much of northern Missouri with a major outcrop north of and parallel to the Missouri River. A few early Devonian rocks exist; they are considered the "last gasp" of the Tippecanoe sea deposits. Middle Devonian rocks are mostly limestones and are considered the first stage of Kaskasia flooding, probably from the north; there is no defining base sandstone as in earlier periods. Late Devonian rocks are mixed shales and sandstone with minor limestone. As with the Silurian, Devonian seas are supposed to consist of seaways and bays between higher ground.

Those Devonian strata in Central Missouri (the Cedar Valley, aka Callaway Formation) are very fossil rich, especially in carbonate reef structures containing brachiopods, corals, bryozoans, and trilobites, and have been well studied by geology students at the nearby University of Missouri. An extremely fine grained late Devonian limestone found near Louisana, Missouri, and named for it has been quarried for many years for lithography stones--this same rock is home to Mark Twain and Cameron Caves, whose winding galleries so fascinated a young Sam Clemens nearly two centuries ago he gave us Tom Sawyer.

One final Devonian rock layer of note occurs in the southwest into Arkansas. The Chattanooga Shale is a "black" (deep water) shale which extends, as the name implies into Tennessee. It records a deep sea basin extending all the way to the southern Appalachians.

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