Geology
The Saint Francois Mountain Range rises above the Ozark Plateau and is the geological core of the highland dome. The igneous and volcanic rocks of the Saint Francois Mountains are the exposed remains of a Proterozoic mountain range. The mountains are the exposed portion of an extensive terrane (the Spavinaw terrane in part) of granitic and rhyolitic rocks dating from 1485 to 1350 mya that stretches from Ohio to western Oklahoma. The core of the range existed as an island in the Paleozoic seas. Reef complexes occur in the sedimentary layers surrounding this ancient island. These flanking reefs were points of concentration for later ore-bearing fluids which formed the rich lead-zinc ores that have been and continue to be mined in the area. The igneous and volcanic rocks extend at depth under the relatively thin veneer of Paleozoic sedimentary rocks and form the basal crust of the entire region.
A major unconformity in the region attests that the Ozarks was above sea level for several hundred million years from the time of the volcanism in the Precambrian until the mid-Cambrian with an erosionally produced relief of up to 1500 feet. The seas encroached during the late Cambrian producing the LaMotte sandstone, 200 to 300 feet (61 to 91 m) thick, followed by carbonate sedimentation. Coral reefs formed around the granite and rhyolite islands in this Cambrian sea. This carbonate formation, the Bonneterre now mostly dolomite, is exposed around the St. Francis mountains, but extends in the subsurface throughout the Ozarks and reaches a thickness of 400 to 1,500 feet (120 to 460 m). The Bonneterre is overlain by 500 to 600 feet (150 to 180 m) of dolomite, often sandy, silty or cherty, forming the Elvins Group and the Potosi and Eminence Formations. Withdrawal of the seas resulted in another unconformity during the latest Cambrian and early Ordovician periods. Hydrothermal mineralizing fluids formed the rich lead ore deposits of the Lead Belt during this time.
Sedimentation resumed in the Ordovician with the deposition of the Gunter sandstone, the Gasconade dolomite and the prominent Roubidoux sandstone and dolomite. The sandstone of the Roubidoux forms prominent bluffs along the streams eroding into the southern part of the Salem Plateau. The Roubidoux and Gunter sandstones serve as significant aquifers when present in the subsurface. The source of the sands is considered to be the emerging Wisconsin Dome to the northeast. The Ozark region remained as a subsiding shallow carbonate shelf environment with a significant thickness of cherty dolomites as the Jefferson City, Cotter and Powell formations.
Portions of the Ozark Plateau, the Springfield plateau of southwest Missouri and northern Arkansas, are underlain by Mississippian cherty limestones locally referred to as Boone chert consisting of limestone and chert layers. These are eroded and form steep hills, valleys and bluffs.
During the Pennsylvanian Period the Ozark Plateau was uplifted as a result of the Ouachita orogeny. During the early Paleozoic a deep ocean basin existed in central and southern Arkansas. Then South America collided with North America creating the folded Ouachita Mountains and uplifting the Ozark plateau to the north.
Read more about this topic: The Ozarks