DEVELOPMENT OF A STROMATOLITE REEF DURING EARLY ORDOVICIAN TRANSGRESSION, NE WISCONSIN

Cambro-Ordovician siliciclastic-carbonate cycles have received little study in NE Wisconsin due to extremely rare outcrop exposure. A new quarry reveals unusual three-dimensional exposures of a stromatolite reef, tentatively interpreted as part of the Lower Ordovician Oneota Fm of the Prairie du Chien Grp. Vertical facies variations in both stromatolite type and sedimentary features suggest the reef developed in response to a marine transgressive cycle.

The lower Oneota reveals an upward progression from cryptalgal structures of unknown origin to laminated mats, layers of laterally-linked hemispheroids, cabbage-head stromatolites, concentric domes, and mounds of elliptical, compound columar structures. A corresponding variety of sedimentary features indicate change from subaerial to shallow marine conditions. Near the base of the dolostone (DS) a lag of angular intraformational breccia clasts and rounded silicified sandstone pebbles records an erosional event, and desiccation cracks in the overlying fine grained beds indicate subaerial exposure. Interbedded sandy DS, exhibiting planar- and cross-lamination, wave ripple marks, flaser- and wavy-bedding, suggests deposition by waves and currents. Closely associated with laterally-linked hemispheroids, these features likely indicate an intertidal or shallow subtidal environment. The presence of nodular anhydrite implies evaporation in a restricted lagoon, also evidence of intertidal conditions. Ooids occur as carbonate-coated quartz grains low in the section, and as concentrically laminated grains higher up-section. Silicified ooids are associated with cabbage-head stromatolites. A few DS beds contain glauconite.

Up-section, the DS beds gradually thicken, contain less quartz sand, and enclose more abundant, larger stromatolites. Bedding surfaces show algal domes concentrated in mounds, with relief of 20-30 cm and increasing to >1 m at the top of the quarry. Large (>1 m long, ~30 cm thick) elliptical domes are aligned SE-NW parallel to the paleocurrent direction of surrounding cross-bedded, oolitic grainstones. This association suggests that stromatolite form and growth were strongly influenced by subtidal currents, and that a mounded reef-like structure evolved as marine transgression increased water depths.