The Astoria Formation (formerly known as the Astoria shales) is a geologic formation in Washington state & Oregon. It preserves fossils dating back to the early to middle Miocene (but was formerly thought to date to the Oligocene).
The Astoria Formation is a thick marine formation representing a near shore, relatively shallow-water shelf deposit. The formation spans a considerable amount of time, with its base considered to be lower boundary of Newportian Stage (late Early Miocene) & its top to be upper boundary of Newportian Stage (middle Middle Miocene).