ORIGIN OF THE CALCAREOUS, PRE-WISCONSINAN PIERCE AND MARATHON FORMATIONS, WISCONSIN

Till of the Pierce Fm. in western WI and till of the Medford Member (Marathon Fm.) in central WI are very dark gray to yellowish brown, silty, and calcareous.  Baker and others (1987) proposed that a pre-Illinoian Des Moines Lobe from the Winnipeg Lowland deposited the Pierce and Medford tills during one glaciation based on stratigraphic position, paleomagnetic work on the Pierce till, lithologic similarities, and boulder trains/till fabrics suggesting ice flow from the NW/NNW. 

However, clay mineralogy work by Thornburg and others (2000) and compilation of regional till data lead us to question this correlation.  Mean values are reported below (carb. ratio for weight % coarse silt fraction; K=kaolinite & V=vermiculite; (number of samples)).

Till Unit	Snd:Slt:Cl %	Calc:Dolo	%K	V/K
Pierce Fm.	39:37:24 (41)	4.2:1 (5)	23.3 (13)	0.5 (13)
Edgar Mb, Mar.Fm.	40:40:20 (283)	0.7:1 (105)	6.8 (3)	2.4 (3)
Medford Mb, Mar.Fm.	35:46:19 (26)	0.3:1 (8)	5.7 (2)	3.2 (2)

Pierce till has much higher K and lower V/K values than either the Medford Mb or the slightly younger Edgar Mb.  Carbonate in Pierce till is calcite-dominated; Marathon till is dolomite-dominated.  We suggest four hypotheses to explain the relationship of the two units. (1) Baker and others’ (1987) correlation is correct, and K derived from the Cretaceous weathering surface in MN was diluted downflow and the transition zone in Barron/Chippewa Co. removed by erosion. (2) Both units are from the Winnipeg Lowland, but Marathon tills are older and either completely removed from western WI, deeply buried/undiscovered, or mistakenly identified as Pierce till. (3) Both units are the same age but deposited by different lobes from the Winnipeg Lowland. (4) Carbonate/clays in Marathon tills have a Hudson Bay provenance.