UPPER CAMBRIAN SAND STROMATOLITES OF CENTRAL WISCONSIN

Insight into microbial communities in ancient siliciclastic settings comes from macroscopic textural features inferred to represent growth, trapping, and/or binding activities of microbes. “Sand stromatolites” are a relatively unstudied example of these microbial structures, and are common in Upper Cambrian siliciclastic strata of Wisconsin, Missouri, and Ontario. Sand stromatolites are domal buildups of quartz sand that are circular in plan view and are preserved in convex epirelief. They may be clustered or isolated and commonly exhibit evidence for synoptic relief and sediment cohesion during transport and deposition of adjacent sediments.

Intertidal - supratidal facies of the Elk Mound Group of Wisconsin contain abundant sand stromatolite heads ranging from 0.2 – 7 cm in diameter, and up to 2 cm in height. Heads occur in very fine-coarse quartz arenites. To assess the range of paleoecologic and paleoenvironmental conditions under which sand stromatolites occur, six stromatolite horizons were examined. The first horizon contains clusters of stromatolite heads in association with Protichnites trackways; the clusters have larger heads concentrated near the center and heads with smaller radii located further from their center. The second horizon contains relatively large individual heads, which become more concentrated downslope – possibly along the margin of a shallow tide-dominated embayment. The third horizon contains Climactichnites trackways and unclustered stromatolites, including heads that grew in trackways after trackways were produced. The fourth horizon contains individual stromatolites in association with stranded scyphomedusae. This surface shows a diffuse boundary across which the stromatolite population exhibits a drastic reduction in size. The fifth surface is characterized by patchy ripples; the non-rippled areas are dominated by small non-clustered heads. The sixth surface is characterized by evenly distributed small heads exhibiting accumulations of sand on their downcurrent side – causing heads to appear teardrop-shaped in plan view.