LATE CAMBRIAN ENIGMATIC RING STRUCTURES FROM SOUTHWESTERN WISCONSIN, USA

Ring-like structures from upper Cambrian (492 Ma) heterolithic strata are exposed near Muscoda, Wisconsin (USA) and presumably were formed on the seafloor before burial. These approximately circular rings (ellipticity rarely exceeds 1.2) are preserved on at least two specific bed surfaces within a quarry at the contacts between sandstone and mudstone beds and are preserved discretely as sand infilling mud or mud infilling sand. No noticeable differences in grain size, texture, or sedimentary structures occur within the area enclosed by each ring, which extends no more than 5 mm into the substrate. Ring diameters range from 2 to 10 cm, and ring wall width is related to casting medium, not to ring diameter, being narrower in rings cast by mud than by sand. Ring walls are orthogonal to bedding or beveled slightly, expanding upward. Although rings may overlap, their distribution is dispersed, suggesting a tendency to avoid intersection. Polished cross sections suggest the rings were infilled from above.

The origin of these rings is enigmatic. Their dispersed distribution and restriction to specific horizons suggests formation over a short time interval. No non-biological source for their origin appears consistent with their regular form and distribution, yet it also remains hard to conceive of what organismal activity might have produced them. No entry or exit point exists in the enclosed ring-like structures, and nor is there evidence of activity, such as rotation about a central tethered stalk. Rather it appears that either the circles were formed by imprinting into the seafloor from above, or by the movement of an organism through the sediment in a circular path. The latter explanation is supported by the preservation of an incompletely developed ring from another locality in the same rock unit, which may represent preservation of a structure in the process of being formed.