The Biwabik Iron Formation is a â¼1.9 billion year-old sequence of iron-rich sedimentary rocks that was metamorphosed at its eastern-most extent by â¼1.1 billion year-old intrusions of the Duluth Complex.he nature of the intrusive contact between the gabbroic rocks of the Duluth Complex and the Biwabik Iron Formation is fairly well known from exposures in the Dunka Pit and from hundreds of drill holes that penetrated the footwall of the Duluth Complex.