Four samples of cuttings from two bores provide the first record of marine deposition at the northern margin of the Murray Basin near Mutooroo. At depths between 94.5 m and 127.5 m two marine units of glauconitic clay sand and minor limestone are separated by a regressive non-marine carbonaceous sand. The upper marine unit carries Lepidocyclina howchini which, as along the River Murray, is probably lower Balcombian, Early to Middle Miocene. On less certain foraminiferal evidence the lower marine unit is probably Batesfordian, late Early Miocene. Two of the marine samples yield similar microfloras which suggest Balcombian to possibly Batesfordian age. There is a microfloral link with the Etadunna Formation. The regressive episode was relatively brief. The lowest sample also contains clay with abundant siderite pellets, and fragments of weathered phyllitic rock, both suggesting proximity to bedrock at this depth.