The enigmatic Middle Range (MR) unit that unconformably overlies the Joana Limestone in the Middle Range, Utah, was deposited on the eroded surface of the Joana bank of the Antler forebulge in the Lower Mississippian in warm, tropical sea water, and fits the classification of a Waulsortian or Waulsortian-like mudmound due to fossil types and presence of algal remains. Conodont fossils found in the MR unit date it as having been deposited during the middle Osagean (Upper typicus conodont Zone), while the Joana Limestone had previously been recorded as being deposited only from Kinderhookian to earliest Osagean. Unconformities bound the MR unit above and below, due to competing sea level changes caused by tectonic uplift of the Antler forebulge combined with world-wide eustatic rise and fall. Although sea level continued to rise into the middle Osagean (Upper typicus to anchoralis-latus conodont Zones), tectonic uplift was sufficient to outpace eustasy and the region was again exposed to the air, causing the MR unit to erode. Once eustatic sea level approached its peak in the middle Osagean (anchoralis-latus conodont Zone), the Delle Phosphatic Member of the Chainman Shale was deposited unconformably on the resubmerged MR unit. The enigmatic MR unit may therefore be classified as a regionally isolated uppermost member of the Joana Limestone.