Carbonates of late Llandoverian to Ludlovian age, divided into the Severn River, Ekwan River, and Kenogami River formations, crop out in the drainage basin of the Harricana River. The Severn River Formation consists of a succession of alternating wackestone and packstone beds 17.5 m thick. The packstones are ascribed to storm events and carry a deeper water fauna than the wackestones. The Ekwan River Formation is divided into four units: a barren dolostone at the base, sandy wackestones and packstones with a coral fauna, another barren dolomudstone, and at the top a unit of argillaceous, nodular limestone rich in leperditid ostracods. The units are ascribed to shallow subtidal, intertidal, and supratidal environments. Three benthic assemblages, characterized by ostracods, corals, rhynchonellids, and the brachiopods Brachyprion and Dalejina, are distinguished in order of increasing depth. Two transgressive events in Llandoverian C4 and C6 times are marked by coral-rich units in the Severn River and Ekwan River formations.