Geology 4

The Salem Plateau is composed of layers 4-9 (Figures 5 and 6) and will be covered in a summary discussion in the next section. For those that desire a complete detailed description of entire geologic column formation please refer to Imes and Emmett (1993) or Hays et al. (2016). The various sandstones of the St. Francois Aquifer (layers 8-9) crop out in the St Francois Mountains and surrounding countryside. In the Cambrian, the Mountains were a large island in the shallow sea. The LaMotte Sandstone was the sands of that time and the sandstone and Bonneterre Dolomite were formed as seas receded. Note that the word formation here means deposition, lithification, and when appropriate dolomitization. According to Imes and Emmett (1993) there was a reef encircling the land mass and metals were transported there through “hot sodium-calcium-chloride brines precipitated (metals) as sulfides by biogenically reduced sulfur” (Imes and Emmett citing German and Meyers, 1993). Hence metal ore and mining now in the Viburnum Trend. As the seas disappeared and re intruded, dolomite formation continued. A cyclical increase in shale and calcareous mud content in the depositional environments formed confining layer 7 (dolomites plugged with calcareous clays and interbedded with shale). Other dolomites in the late Cambrian and early Ordovician times were formed in subsidence basins around the future mountains. These basins acted as depositional basins for the formation of layers 5 and 6 aquifer dolomites and sandstones (remember that formation is a simplification). An important event in this early Ordovician time was uplift and fracturing of many layers followed by rains and water erosion in what Imes and Emmet term “development of extensive paleo karst”. There were many episodes to follow through the late Ordovician through the early Devonian. These events were repeated through many episodic depositions and receding and advancing seas. When seas were low and the land mass was above water, little sedimentary material is found in the area of the present Ozark Aquifer. In the late Devonian the predominant load of clay and silt sediment lithified and formed the Chattanooga Shale. This is the confining layer of Salem Plateau, sandstones and dolomites under the areal extent of the Springfield Plateau. In the Salem Plateau the Ozark Aquifer is largely unconfined.