Deep karst system research, Michigan, USA

At least three major evaporite dissolution events have developed deep karst in two zones of the northern Michigan basin. Dissolution of outer margin Silurian evaporites resulted in the Mackinac Breccia during the early Devonian. This rebound and exposure or a later, brief basin rebound also caused the development of surficial evaporate epikarst in the middle Detroit River Group before the next subsidence and new deposition. Pleistocene glaciation pumped fresh water into shallow Silurian and Devonian carbonate aquifers and stressed the formations by repeated loading and unloading. Buried early Devonian paleokarst in the Detroit River Group was affected by hydrostatic pressure from the glacial waters and exposure along hydraulically open faults and fractures which developed the ancient karst. The current karsted zone varies from totally removed evaporites and collapsed formations along the northern margin of the Detroit River Group subcrop, to an irregular active evaporite dissolution front 29-39 km further basinward. The last glacial retreat from the Lake Huron basin approximately 10,000 years before present (ybp), opened and flooded the basin, The level dropped approximately 150 m in the Lake Huron basin to Lake Stanley stage at 10,000-7,500 ybp to approximately 58 m compared to the present 177 m above sea level of Lake Huron. This is likely the time when most of the visible collapse sinkholes in Northern Michigan formed. Some of these sinkholes have been identified in Lake Huron recently by subbottom profiling. Water venting from them into the lake has conductivity values ranging from 123 to 1,821 mu S/cm, attributed to high levels of chloride and sulfate. A benthic ecosystem of anoxic chemosynthetic bacteria is established in mat and bacteria filament forms at the vents. There are no land springs from this system which has hundreds of square kilometers of active swallow area.