THE ENIGMATIC LATE CRETACEOUS (CENOMANIAN) WINDROW FORMATION IN MINNESOTA

The transgression eastward of the Cretaceous Inland Sea during the Cenomanian produced the non-marine Windrow Formation in Minnesota, Wisconsin and Iowa. First mentioned in 1874 near Devil’s Lake, Wisconsin it was described as a stream gravel conglomerate of probable Cretaceous origin. The Windrow Formation has been interpreted to be of deltaic flows and lacustrine deposits as the seaway transitioned from marine sources in the west to non-marine deposits to the east. Further work during 1895 showed fossil leaves and pollens that were assigned to the mid to late Cenomanian in age. The Windrow Formation has been reported in western Illinois, north western Missouri and central Iowa, however many of these localities have been lost due to urban development over the last one hundred years.

The type locality for the Windrow Formation was described in 1919 from East Bluff near Tomah, Wisconsin. This description refers to the East Bluff (upper) and the Iron Hill (lower) Members of the Windrow formation and is typically found on higher elevations in the Driftless areas of western Wisconsin and in south-central Minnesota. It can be found in several quarries of Ordovician age in southern Minnesota in deep fissures or presumably stream cuts that were present during its original deposition. An isolated outcrop in Goodhue County Minnesota was described in 1958 and was visited in 2016 and documented. In 2018 a site along the Cottonwood River near New Ulm, Minnesota, directly opposite from the locality from 1895 where fossil plants were recovered, was visited and a 10 meter section of the Windrow Formation was documented in-situ well within the Cretaceous Dakota aged sandstones. Associated with this are cross bedded sandstones and varved clay deposits showing the deltaic and lacustrine depositional environments.

Recent revisions of the Windrow Formation attribute the East Bluff member to reworked Pleistocene pebbles and gravels (1990). However with the Windrow Formation locality in-situ near New Ulm, Minnesota it can be correlated to the original description of the type locality. Further investigation of the Cretaceous deposits of southern and central Minnesota, although sparse, will help to interpret the eastern most margins of the Cretaceous Inland Sea.