Here we report detections of Fe/Mg phyllosilicates and hydrated silica in discrete stratigraphic units within the knobby terrains of Acidalia Planitia made using data acquired by Compact Reconnaissance Imaging Spectrometer for Mars. Fe/Mg phyllosilicates are detected in knobs that were eroded during southward retreat of the dichotomy boundary. A second later unit, now eroded to steep-sided platforms embaying the knobs, contains hydrated silica, which may have formed via localized vapor weathering, thin-film leaching, or transient water that resulted in surface alteration. These are then overlain by smooth plains with small cones, hypothesized to be mud volcanoes which previous studies have shown to have no hydrated minerals. In spite of Acidalia's location within the putative northern ocean, collectively, the data record a history of aqueous processes much like that in the southern highlands with progressively less intensive aqueous chemical alteration from the Noachian to Amazonian.