Mississippian 0.35 bya (IL only)

Mississippian  (IL only)


354  323 Million years ago

Mississippian Limestones with abundant fossils including brachiopods corals, crinoids and bryozoan occur in the southern two thirds of Illinois. Warm tropical seas teaming with sealife covered this area and much of North American Mississppian  limestones deposited are pure carbonate with little shale and often have caves systems in them. These include:

  •  Redwall Limestone in the Grand Canyon
  • Madison Limestone in Montana, and Wyoming
  • Wind Caves in South Dakota
  • Ste. Genevieve Limestone in Missouri
  • Girkin Formation and St. Louis Limestone including Mammoth Cave in Kentucky

 How did Illinois' fluorite deposits form? Hot water containing fluorine and other dissolved chemicals rose from deep in the earth during the Jurassic Period, about 150 to 200 million years ago. The water flowed through northeast-trending faults and fractures in limestones laid down earlier in the Mississippian Period, about 330 million years ago. When the hot brines reached the calcium-rich Mississippian rocks, the temperature and other conditions were just right for crystallizing fluorite along the walls of the faults and in flat-lying layers parallel to the beds of limestone. These host rocks dissolved and were replaced with the fluorite.