Earth Geology Geologic Time Scale Ordovician Marine Deposition

Ordovician Period

Marine Deposition

505 to 440 million years ago

The end of the Cambrian and the beginning of the Ordovician brought a temporary change in conditions. The area was still part of a tropical sea, but the first layer of the Ordovician is a sandstone layer on top of the Cambrian carbonates. This sandstone, the Gunter member of the Gasconade Formation, indicates different conditions and, in places, erosion of the top of the Cambrian. The Gunter is continuous throughout the Ozarks and is present below the surface in the Bryant area. After the Gunter sandstone was deposited, the sea level conditions changed and the remainder of the Gasconade formation records marine carbonate formation. Sea level and sediment source conditions changed and the interbedded sandstones and cherty dolomites of the Roubidoux formation were deposited. The rocks of the Roubidoux record changing or fluctuating conditions of sea level. The next sequence of rocks, the Jefferson City/Cotter formations, represent somewhat more stable conditions with mainly carbonate sedimentation.

After the Jefferson City/Cotter formations were formed, the central Ozarks apparently uplifted during the formation of the Ozarks dome. Younger rocks still of Ordovician age are present around the edges of the dome, but they were either not deposited or eroded away from the central part of the dome, including the Bryant area.

The northern and central Appalachian Mountains were forming at this time, and the uplift of the Ozarks dome may be related to the mountain building and continental collisons in the east.

The Ozarks region was still in the tropics during the Ordovician Period and the few fossils preserved in the rocks of the area are representative of tropical marine organisms, the snails and crustaceans that crawled over and burrowed in the lime muds on the seafloor.

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