STRATIGRAPHIC CONTEXT OF A LATE CAMBRIAN EXTINCTION EVENT, TUNNEL CITY GROUP, UPPER MISSISSIPPI VALLEY

Although a primary focus for research in Late Cambrian paleontology and stratigraphy in the first half of the 20^th Century, the siliciclastic succession of the Upper Mississippi Valley has received comparatively little recent attention.  New data from detailed measured sections permit a comprehensive revision of the stratigraphy, facies and trilobite faunas of the Sunwaptan Tunnel City Group (Lone Rock and Mazomanie formations) in Wisconsin and Minnesota. Heterogeneous sandstones, comprising at least six lithofacies along a depositional transect from shoreface to transitional-offshore environments, document sedimentation in a shallow marine epicontinental sea.

Our data provide new insight into the relationship between the trilobite extinction at the base of the Sunwaptan Stage and regional lithofacies shifts.  Pre-extinction trilobite faunas at the top of the Wonewoc Formation are dominated by Camaraspis.  In south-central Wisconsin, onset of extinctions is marked by a sharp shift from shoreface sandstones of the Wonewoc to thinly-bedded sandstones of the Tomah Member (Lone Rock) that were deposited in an transition-offshore setting.  An Irvingella-Comanchia fauna of the Irvingella major Zone (basal Sunwaptan) appears abruptly at the base of the Tomah.  To the west, in west-central Wisconsin and southeastern Minnesota, the Tomah Member does not directly overlie the Wonewoc.  Rather, it succeeds a strongly glauconitic unit, the Birkmose Member (Lone Rock), which is not present in south-central Wisconsin.  Data from this and previous studies indicate that the I. major Zone is not recorded in the Birkmose.  Lowermost Sunwaptan strata of the overlying Taenicephalus Zone form a condensed, conglomeratic interval. Synthesis of successions in the study area indicates that onset of extinctions is related to a major flooding surface, possibly amalgamated with a Type II sequence boundary, that passes offshore (westward) into a sediment-starved, condensed interval.  The apparent abruptness of the faunal change at the base of the I. major Zone in south-central Wisconsin is at least partly related to a pause in deposition at this flooding event. As such, stratigraphic context exerts an important control on the apparent tempo of the terminal Steptoean extinction in the Upper Mississippi Valley.