Lake Omeo is a relic of a more extensive lake formed when a basalt flow, dated as Upper Pliocene in age, dammed the flow of Morass Creek. This larger lake is referred to as Lake Morass. Downfaulting of part of the Lake Omeo basin and upthrow
of a narrow horst between the Morass Creek and Minute Creek catchments probably occurred at about the same time or later. Sedimentation of the natural overflow gap of the Lake Omeo basin during the existence of Lake Morass resulted in the basin becoming
isolated from Morass Creek as the basalt flow was incised. Clay sediments over 40 m thick were deposited in the downfaulted part of the basin. Two clay lunettes to the southeast of the present lake, and material of fluvial and lacustrine origin provide