These areas include Gale Crater, Mawrth Vallis, Oxia Planum, and Nili Fossae, among others, and date to 4.0-3.7 Ga.[12][13] There are two hypotheses to explain the formation and distribution of phyllosilicate clays on Mars: (1) subsurface and hydrothermal activity and/or diagenesis[14][15][16] which yield trioctahedral phyllosilicates, and (2) surface / subaerial chemical weathering, e.g., pedogenesis[10][11][12][17][18][19][20][21][22] which yield dioctahedral phyllosilicates. Importantly, some of these areas (Mawrth Vallis and Oxia Planum) have weathering profiles of Al-smectites overlain by Fe/ Mg smectites (all of which appear to be dioctahedral), and then poorly crystalline / amorphous phases like allophane and imogolite. These weathering profiles are capped by an igneous deposit of an estimated 3.7-3.6 Ga which may be a pyroclastic deposit or a mafic sandstone,[23] similar to paleosols buried beneath igneous deposits on Earth.[24][25][26] These stratigraphic profiles appear to be up to 200 m in thickness, with individual layers of 10 m in thickness or less.[23] This stratigraphy reflects the possible cooling and drying of Noachean Mars, and may preserve organic matter or other biosignatures because of the exceptionally high clay content (~50 wt %) and clay mineralogy (2:1 smectites)[27] of these buried rocks.[13]