Two distinctive assemblages of biogenic structures are present in the Mt. Simon Formation, basal Upper Cambrian quartzarenite in western Wisconsin. A Skolithos trace assemblage is dominated by specimens of Skolithos and Arenicolites, which commonly occur in medium to very coarse-grained, cross-bedded sandstones. Skolithos traces are typically 10 to 100 mm in length, 1 to 5 mm in width, and are oriented normal or slightly inclined to bedding. Specimens are present throughout the formation, even in basal conglomeratic beds, and thus point to a marine origin for the entire unit. Skolithos density increases upward to concentrations of 5 to 7 burrows/sq cm. A Cruziana trace assemblage is dominated by specimens of Cruziana, Rusophycus, and Planolites, which commonly occur in th n beds of very fine to medium-grained, horizontal to ripple cross-laminated sandstones. Rusophycus specimens are typically 10 to 100 mm in length and 6 to 60 mm in width, and are preserved in convex hyporelief on the bases of sandstone beds.

Distribution of the two trace assemblages and the associated physical sedimentary structures points to two different environmental regimes present within a shallow subtidal to intertidal setting: (1) a higher-energy tidal channel environment in which coarser-grained, cross-bedded sandstones containing a Skolithos assemblage were deposited; (2) a lower-energy tidal flat environment in which finer-grained, horizontal and ripple cross-laminated sandstones containing a Cruziana assemblage were deposited. The two subenvironments coexisted within a complex environmental mosaic, because the two trace assemblages are observed to intergrade both laterally and vertically, probably the result of lateral migration of each subenvironment. The pronounced upward increase in trace fossil density indi ates a significant upward decrease in energy conditions and a reduced sedimentation rate, probably due to tidal flat progradation.

End_of_Article - Last_Page 701------------