The Coronation Supergroup, a 1.9 Ga west-facing, continental- margin prism in the Wopmay orogen, represents the opening and closing of a basin; but was this an open ocean or a back-arc basin? The ≈10 m.y. life span of the basin suggests a Korean-type margin, but geological arguments have been raised against a back-arc interpretation. The most serious argument is the lack of ash beds in lagoonal facies of the shelf despite prevailing onshore winds. On the basis of combined paleomagnetic data, stromatolite elongation, starved fore- slope, and other sedimentological evidence, the margin faced east and deposition occurred in the belt of trade winds at fat 10°-20°N. However, ash is generally not distributed by low-level winds, such as trades, but by winds in the troposphere; ash erupted from an offshore arc located near lat 15°N would probably have been transported away from the margin, not toward it. This nullifies the strongest geological argument against a back-arc origin for the Coronation Supergroup, which is supported by many independent lines of evidence. The method described has potential for determining whether other ancient basins represent back-arc basin fill or continental-margin deposits of an open ocean.