Mineralogy and stratigraphy of the Gale crater rim, wall, and floor units

The Curiosity rover has detected diverse lithologies in float rocks and sedimentary units on the Gale crater floor, interpreted to have been transported from the rim. To understand their provenance, we examine the mineralogy and geology of Gale's rim, walls, and floor, using high-resolution imagery and infrared spectra. While no significant differences in bedrock spectral properties were observed within most Thermal Emission Imaging System and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) scenes, some CRISM scenes of rim and wall rocks showed olivine-bearing bedrock accompanied by Fe/Mg phyllosilicates. Hydrated materials with 2.48m absorptions in Gale's eastern walls are spectrally similar to the sulfate unit in Mount Sharp (Aeolis Mons). Sedimentary strata on the Gale floor southwest of the landing site, likely coeval with the Bradbury units explored by Curiosity, also are hydrated and/or have Fe/Mg phyllosilicates. Spectral properties of these phyllosilicates differ from the Al-substituted nontronite detected by CRISM in Mount Sharp, suggesting formation by fluids of different composition. Geologic mapping of the crater floor shows that the hydrated or hydroxylated materials are typically overlain by spectrally undistinctive, erosionally resistant, cliff-forming units. Additionally, a 4km impact crater exposes >250m of the Gale floor, including finely layered units. No basement rocks are exposed, thus indicating sedimentary deposits 250m beneath strata studied by Curiosity. Collectively, the data indicate substantial sedimentary infill of Gale crater, including some materials derived from the crater rim. Lowermost thin layers are consistent with deposition in a lacustrine environment; interbedded hydrated/hydroxylated units may signify changing environmental conditions, perhaps in a drying or episodically dry lake bed. Plain Language Summary The Curiosity rover has detected diverse rocks on the floor of Gale crater; these are interpreted to have been transported there from the crater rim. To better constrain where these rocks came from, we examine the mineralogy and geology of Gale's rim, walls, and floor, using high-resolution images and spectra. While the majority of Gale is dusty and did not reveal any convincing mineral signatures, we did observe some portions of the rim and wall showing iron-rich minerals and probable iron or magnesium bearing clays. Rocks on the Gale floor also have hydration signatures similar to those of clays. These clays and those from the rim and wall are different from those in Mount Sharp because they do not have aluminum; this suggests that the waters they formed in or the rocks they came from were different. Geologic mapping of the crater floor shows that the clay-like materials are typically overlain by bland-appearing, erosionally resistant, cliff-forming units. Additionally, a 4 km impact crater exposes >250 m of material below the Gale floor, including rocks that appear finely layered. None of the original crater rocks are exposed, only rocks that later filled Gale. This indicates later deposits 50 m beneath those traversed by the Curiosity rover. Collectively, the data indicate substantial infill of Gale crater, including materials derived from the crater rim. The lowermost thin layers are consistent with deposition in a lake; alternating levels of hydration in the mapped rock units may imply changing environmental conditions, perhaps in a drying or episodically dry lake bed.