SUSPECT MICROBIAL-INDUCED SEDIMENTARY STRUCTURES (MISS) IN FURONGIAN (UPPER CAMBRIAN; JIANGSHANIAN, SUNWAPTAN) STRATA OF THE UPPER MISSISSIPPI VALLEY
The Furongian (upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock and Mazomanie formations), exposed in west-central Wisconsin and southeastern Minnesota, represents a shallow-marine clastic environment during a time of exceptionally high sea level. Lithofacies, from shoreface to transitional-offshore settings, document deposition in a wave- and storm-dominated epicontinental sea. During deposition of a condensed section in the lower part of the Tunnel City Group, eustatic flooding was associated with development and preservation of microbial-mat textures or associated sedimentary structures. Possible fragments of microbial mats similar to “roll-up” structures, wrinkle structures, and syneresis cracks are present in sandstones of the Birkmose and Tomah members of the Lone Rock Formation.
In the Lone Rock Formation, MISS are associated with a trilobite extinction event in the Steptoean–Sunwaptan boundary interval. This conforms to recent predictions that Phanerozoic microbial mats were disaster forms that flourished during extinction intervals. Further investigation of stratigraphic, taphonomic, and other potential biases, however, is needed to fully test this hypothesis.