DIRECT PRECIPITATION OF ONEOTA DOLOMITE OF THE UPPER SAUK MEGASEQUENCE

As part of the Sauk Megasequence, the first of six megasequences across North America, the Oneota Formation with oscillatory depositions of dolomite oolites and stromatolites has been extensively studied. Historically, much of the Oneota dolomite has been described as either syndepositional dolomite or dolomitized during syndepositional diagenesis (Smith and Simo, 1997). Oneota dolomite origin has been recently challenged by studies indicating that spherulite textures within this formation are the result of primary dolomite precipitation. Evaporite dissolution brecciation and paleo mud cracks are present, implying an evaporative setting capable of producing these features (Smith and Simo, 2016). Our work further supports these claims with additional observations of the Shorewood Hills Quarry in Madison, Wisconsin, providing evidence of primary dolomite precipitation using well-preserved spherulite and micritic textures. The spherulites display cross extinction, which are similar to those in synthetic disordered dolomite in the presence of dissolved silica. Recent laboratory work has shown that disordered and proto-dolomite can precipitate directly from solutions with dissolved silica (Fang and Xu, 2021). These observations support a primary dolomite origin for the Oneota instead of earlier theories. Within the Oneota, there are multiple stromatolite-rich layers that contain micritic dolomite. Previous work has shown that modern stromatolites precipitate disordered and proto-dolomite (Fang and Xu, 2019). Phanerozoic stromatolites precipitate dolomite (and carbonates) at a fast rate to avoid predation. The local paleoenvironment can be inferred as shallow and evaporative with a high concentration of dissolved Mg, Ca, and silica, promoting fast precipitation of dolomite. Solutions with similar concentrations of silica only occur in modern settings such as playa lakes (i.e., Deep Springs Lake or Coorong region lakes) compared to silica depletion in modern seawater. These settings produce modern primary dolomite and the evaporites, which might have dissolved and brecciated from the Oneota formation through diagenesis. During the Oneota dolomite formation, fast precipitated dolomite may indicate intense chemical weathering and very high atmospheric CO₂ released from volcanism.