The carbonate-rich Espanola Formation forms part of the Huronian Supergroup, deposited in Early Proterozoic time between about 2.5 and 2.1 Ga ago. The Espanola Formation overlies glacigenic diamictites of the Bruce Formation and is gradationally overlain by fluvial sandstones of the Serpent Formation. In the southern part of the outcrop belt, the Espanola Formation comprises a lower limestone member, a middle siltstone member, and an upper heterolithic member. These rocks record what may be the first marine incursion in the early Huronian and perhaps indicate a pre-rift phase of sedimentation. The limestone and siltstone members reflect low-energy conditions with sporadic influxes of fine-grained siliciclastics and carbonate debris from turbidity or storm-derived currents. Deposition took place subtidally, either in a shallow-marine setting or in a large lake, following the end of Bruce glaciation. Deposition of the coarser grained heterolithic member took place in higher energy environments, dominated by shallow-marine tide and storm processes.In the study area, most carbonate is detrital in origin. Paleocurrents suggest a northerly source. The restricted nature of the basin, postglacial warming, and shallower marine conditions could have been factors in carbonate precipitation.