CHRONOSTRATIGRAPHIC REVISION OF THE GIVETIAN (MIDDLE DEVONIAN) MILWAUKEE FORMATION

The late Givetian Milwaukee Formation of western Michigan Basin was deposited at and below storm-wave base and contains a diverse marine fauna. Initial study of these rocks was on quarry exposures of the lower part of the formation (Berthelet and Lindwurm Members) that are now mostly covered, and subsequent subsurface studies of more complete successions focused primarily on lithostratigraphic subdivision of the unit. To better understand the depositional history and identify units of the Milwaukee Formation that can be correlated at regional to basinal scales, we use integrated lithostratigraphy, elemental and δC¹³_carb chemostratigraphy, and conodont biostratigraphy. With this methodology, we (1) identify a previously unrecognized depositional sequence boundary and unit in the uppermost Milwaukee Formation, (2) constrain the age of the Milwaukee to the hermanni through norrisi conodont biozones, and (3) quantify lithologies and confirm positions of unconformities using chemostratigraphy. The contact of the basal Berthelet Member (lowermost Milwaukee) with the underlying microbialite and evaporite-dominated Middle Devonian Thiensville Formation is a pyritic and phosphatic encrusted disconformity surface. Berthelet conodonts are Lower hermanni Zone in age. The contact of the Berthelet and overlying Lindwurm Member is a disconformity marked by an abrupt lithic change from muddy dolostone to dolomitic mudstone, phosphate pebbles, and a shift in the δC¹³_carb profile to slightly lighter values. Lindwurm conodonts are Lower hermanni to Lower disparilis Zone age. The Lindwurm and overlying North Point Member contact is transitional and our data indicates they are chemostratigraphically similar, with Upper disparilis Zone faunas in the North Point. At the top of the Milwaukee Formation, a previously unrecognized unconformity-bound unit is observed underlying the Upper Devonian Antrim Formation; this may have been considered part of the North Point by past researchers. This interval consists of argillaceous dolomitic pack-to-grainstone, with abundant pyrite and chert, and norrisi Zone conodonts. Given the relative lack of lithologic difference between the Lindwurm and North Point, and the unique lithology and younger age of the uppermost Milwaukee Formation, stratigraphic revision is warranted.