The Ninmaroo Formation (Upper Cambrian-Lower Ordovician) is one of several epeiric carbonate sequences in the Georgina Basin. It comprises ooid, peloid, flat-pebble conglomerate, skeletal and mixed carbonate lithofacies, terrigenous sandstones, and a late diagenetic crystalline dolostone lithofacies. Ninmaroo sedimentation took place in a broad, shallow epicontinental sea under normal marine, increasingly saline, and evaporitic conditions. A barrier complex of ooid, peloid and skeletal sands formed the seaward limit of these environments. Channels through this barrier extended weak tidal effects to the periphery of an extensive, non-tidal complex of semi-emergent shoals in a patchwork pattern, Here, algal and nonskeletal carbonate sedimentation predominated because higher salinities restricted the fauna. Sedimentation was cyclic, probably due to repeated shoaling from storms. Sabkhas developed locally on emergent areas of shoals and landward where the complex was transitional with an emergent pavement. Where emergence was prolonged, microkarst resulted with both carbonate and sulphate dissolution. Lithofacies patterns indicate two regressive depositional sequences. An initial southeastern progradation of the barrier and epeiric environments was over a stable and extensive flat shelf. Subsequent instability increased shelf slope locally, and depositional environments along margins were condensed. With increased tidal circulation, skeletal carbonate production became dominant, producing a second offlap to the southeast.