Areally restricted beds of dolomite-rich carbonate occur in rocks transitional between the Upper Ordovician Juniata and Sequatchie Formations in the southern Appalachian Valley and Ridge. The gray biomicrites and biomicrudites of the Sequatchie facies represent offshore-subtidal deposition on a shallow, open shelf; Sequatchie rocks show evidence of periodic scouring, and are not visibly burrowed or desiccated. Rocks of the Juniata facies are red, silty and clayey, pelletal dolomicrites and dolomitic shales, and contain abundant evidence of desiccation and intermittent scouring. An infaunal element produced isolated, vertical burrow structures in these sediments. Juniata-facies rocks represent deposition on high tidal flats, which were flooded only occasionally. Rocks intermediate between subtidal Sequatchie and supratidal Juniata facies contain both vertical and horizontal burrow structures, and are extensively bioturbated and mottled. They represent deposition on low tidal flats to very shallow-subtidal, nearshore zones. Dolomite is much more abundant in the supratidal rocks than in rocks of other facies, and is interpreted to have formed on the high tidal flats by precipitation from evaporating pore waters trapped in the sediment after periodic floodings. This model is similar to that proposed for the modern Bahama dolomites.