Middle Turonian (Upper Cretaceous) ammonites from a gravel pit in Wisconsin

Abstract

W.A. (Bill) Cobban and E.A. Merewether’s 1983 classic publication on the molluscan record and strata from the eastern side of the Cretaceous Western Interior Seaway gave us our first indication of the immense size of the North American epicontinental seaway during the Upper Cretaceous Turonian. Pleistocene glaciation and erosion have obliterated most evidence of the Western Interior Cretaceous Sea that once covered portions of present-day Minnesota, eastern South Dakota, Iowa, Illinois Wisconsin, northern Michigan, Manitoba and Ontario. As a result, ammonites from the eastern margins of the Western Interior Seaway are extremely rare. Prior to this study, the eastern most known occurrences for Cretaceous ammonites within the Western Interior were from the basal conglomerate Coleraine Formation (upper Cenomanian) deposited in the Mesabi Iron District near Calumet, MN, located about 160 miles north of Minneapolis in northeast Minnesota (Bergquist, 1944). The specimens were found in-situ within depressions on the erosional surface of the Precambrian ironstone. Cobban (1983, p. 19) also reported on a single ammonite specimen found in glacial drift south of Minneapolis. Two Turonian age belemnites, Actinocamax manitobensis (Whiteaves, 1989), were also reported in glacial drift from eastern Iowa (Cobban 1983 p. 20-21). Two middle Turonian (Upper Cretaceous) ammonites, a Placenticeras pseudoplacenta Hyatt, 1903 and Scaphites carlilensis Morrow, 1935 along with several septarianized, carbonate concretions were discovered in 2004 in a glacial-till/gravel quarry near River Falls, WI, located about 35 miles east of Minneapolis. The glaciers that deposited the glacial till, and these ammonites reportedly originated north-northeast of this site, possibly from the Keweenaw/Superior region (Syverson and Colgan, 2004). Because these soft carbonate rocks are susceptible to destruction during long distance transportation in glacial till, we believe that the fossils were deposited not far from where they were ultimately recovered. This discovery extends the eastern margin of the seaway further east than was previously known into central and possibly north-central Wisconsin. The ammonites are from the middle Turonian Stage of the Late Cretaceous, and thus about 10 MY younger than those specimens previously reported from eastern Minnesota by Bergquist (1944) and are comparable in age to specimens reported in western Minnesota and northwestern Iowa by Cobban (1983).

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019