Over the past 10 years, the Mars Science Laboratory (MSL) rover Curiosity has been investigating the plains of Aeolis Palus and the lower reaches of Aeolis Mons (informally known as Mount Sharp), a 5 km tall mound of sedimentary rocks in Gale crater (Figure 1). After traversing 27 km and nearly 600 m of vertical stratigraphy, three lithostratigraphic groups have been identified: Bradbury, Mount Sharp, and Siccar Point (SP). The Bradbury group consists of fluvial, deltaic, and lacustrine sedimentary rocks [1-2]. The Mt. Sharp group mainly consists of laminated mudstones with minor fluvial sandstones, interpreted as evidence of a long-lived lacustrine environment [1]. Locally, exposures of the Mt. Sharp group are unconformably overlain by aeolian cross-bedded sandstones of the SP group, interpreted to have deposited on an aeolian deflation surface [3]. While these three groups show evidence of deposition in specific environmental and climatic conditions, knowledge of their stratigraphic relationships is a key information to understand the evolution of environmental conditions in Gale. Yet, no clear stratigraphic contact has been observed at the boundary between the Bradbury and the Mt. Sharp groups. Because the mean dip of the Bradbury group is approximately horizontal, the MSL team suggested that the Bradbury group might be stratigraphically lower than the Mt. Sharp group, and therefore lower than the SP group [1]. Nonetheless, orbital analyses of the region suggested that capping strata of the Bradbury group could be part of the SP group [4]. Chemical data from the ChemCam and APXS instrument suites of Bradbury and SP group rocks have recently shown that both groups have similar compositions and possibly similar sediment sources [5-8]. In this study, we aim to reappraise the stratigraphic and chemical relationships between the Bradbury and SP groups using Mastcam [9-10] and ChemCam data [11-12] to characterize the evolution of Gale’s ancient environment.