FORMAL QUATERNARY LITHOSTRATIGRAPHY OF MINNESOTA: A PROGRESS REPORT

The Minnesota Geological Survey, along with interested glacial geologists in the region, is currently developing a formal lithostratigraphy for Quaternary units in Minnesota. Our working group has identified over 100 lithostratigraphic units that have been, or are, in use in the state. However, the formal status of these units varies considerably. Our goal is to review all units ever used in the state and place all units on equal footing. The purpose of this project is to provide clarity, to provide a useful overview of the state's Pleistocene units, to reduce confusion, to help out the applied community, and to identify areas of the state and the stratigraphic column that need work. We have established a framework for naming lithostratigraphic units that follows the North American Stratigraphic Code. Additionally, we have added the following guidelines. (1) We will use the formation as the basic lithostratigraphic unit, and these formations will be based primarily on diamicton units. (2) Most non-diamicton units will be included within the same formation as the diamicton to which it is compositionally and stratigraphically related. (3) Prominent non-diamicton units that cannot be clearly associated with individual diamicton units can be defined as formations. (4) Formations are defined primarily on field criteria. (5) Members of formations may be defined on the basis of lab criteria. (6) Formations should not intertongue. (7) Because it is more difficult to correlate subsurface units than surface units, we have chosen to be cautious in correlating subsurface units extensively. (8) We avoid the use of lobe names for these older units. At the present time, the fate of 113 identified units is as follows: 18 currently formal units are accepted and recognized (for example, the Brenna, Huot, and Peoria Formations), 43 units will be newly defined and formalized (for example, the Browerville and Sauk Centre Formations and the Pierz Member of the Cromwell Formation), 18 units of informal and currently formal status will be discontinued (for example the Granite Falls Till, West Campus Sand, and the Mankato Drift), and 8 currently formal units will be modified (for example, the Cromwell, Hawk Creek and New Ulm Formations). The remaining 26 units will be not acted upon at the present time.