Karst Geology and Hydrology
Tyrone J. Black, CPG
Senior Geologist
DEQ Cadillac District
Gaylord Field Office Office, Office of Geological Survey
August 2008
Tyrone Black is a Senior Geologist with the MI DEQ, Office of Geological Survey. He has worked for the Geological Survey for over 31 years in oil and gas regulation. During his free time he has studied, observed, and written several papers on Michigan's karst.
Sink holes, swallows, and caves are “karst” features. Karst is the term used for the many expressions left on or in bedrock from of water action. Such features can have substantial impact on farming, construction, ground water, and surface waters. Karst allows unfiltered water, soils, and surface debris entry into the subsurface. The solids settle in the cavities and caverns while water and soluble materials flow deep underground to the Great Lakes.
Pleistocene glaciers scraped across the bedrock of Michigan removing most surficial karst features. The last glacier began its retreat from Southern Michigan 12,000 years ago and 9,000 years ago finally left Northern Michigan. Most karst exposed features have developed since then.
The karst development is in a group of rocks called the Detroit River Group (named for an exposure near Detroit). It is over 900 feet below the surface at Alpena, and nearly 100 feet below surface at Rogers City. It composed of a sequence of limestones, shales, and evaporite minerals (gypsum and salt). Surface and ground waters invaded the formation through open faults. As the evaporites were removed by solution the formations above collapsed and brecciated (broke up). Sinkholes developed from uneven collapse and additional solution at swallow areas. Many sinks are visible along a fault line as the Shoepac Lake-Rainy Lake and Sunken Lake to Misery Bay line of sink holes and sink hole lakes of Presque Isle and Alpena counties. Northern Michigan sink holes also exist in the north-east corner of Otsego County and west of Wolverine, with a few features on the west side in the Charlevoix-Grand Traverse-Leelanau-Manistee area.
Outflow of the system is toward the nearest of either lakes Huron or Michigan. Currently discharge is known only at El Cajon Bay, Middle Island, and a remote sink hole 15 miles from Alpena into Lake Huron. With a karst path that zigzags along caverns and fractures deep underground, contamination would be difficult to nearly impossible to trace or capture.
The system began forming millions of years ago but only sinkholes "active" since the retreat of the glacier approximately 10,000 years ago, have become visible at the surface. The last glacial advances plugged or covered over older features. However, the older features may be actively swallowing ground water as it filters through the cover and collapsed formations.
To better understand the natural impact to lakes Michigan and Huron, and man’s potential impact, we must study the geological setting, origin, cause, hydrology, and remediation of sinkholes. It is important to understand the recent geological and hydrological history (probably the past 2 my in Michigan) to more completely understand the current karst geology and hydrology.
