Middle Devonian Unconformity in Michigan
Introduction An unconformity or disconformity is usually discovered by carefully observing a zone of supposed contact and discriminating lithologically and faunally the beds above and below that zone. Nearly all the accepted disconformities have been discovered in this way. In western and south‐western Michigan, however, evidence of unconformity can be obtained only with considerable difficulty. The entire area is covered with glacial drift, and at no place do the Devonian beds crop out. Drilling has not furnished sufficient core to permit the differentiation of faunal zones and the problem must, therefore, be attacked by other means. Twenhofel 3 has tersely defined an unconformity as a surface of erosion or nondeposition separating two groups of strata. The purpose of this paper will be to show that an unconformity exists in western Michigan, both because of the erosion of the Dundee beds (and perhaps of the Monroe beds) and because of the nondeposition . . .