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TEXTURES OF GALE CRATER ROCKS AS VIEWED BY THE MARS HAND LENS IMAGER (MAHLI): MECHANICAL AQUEOUS ALTERATION DOMINATES
Mudstones. These rocks contain framework grains smaller than the highest resolution MAHLI images (16 mm/pixel), and thus are interpreted to consist of grains that are silt-sized or smaller. Some rocks contain nodules, sulfate veins, and Mg-enriched erosionally-resistant ridges.
Well-sorted sandstones. Rocks in this class are made of gray, fine-to-medium sand and exhibit little to no porosity. Two examples of this class show fine lineations with sub-mm spacing. Aillik, a target in the Shaler outcrop, shows abundant cross-lamination.
Poorly-sorted sandstones. This class is subdivided into two sub-classes: rounded, coarse-to-very coarse sand grains of variable colors and lusters, set in gray, fine sand; and dark gray, well-cemented, and fine grained, with rare pebble-sized clasts. The latter also exhibits pores or vugs that may have resulted from removal of these larger clasts.
Pebbly sandstones. This texture is characterized by a poorly-cemented, poorly sorted matrix of coarse sand to granules with a variety of colors and lusters. Whereas two endmembers in this class (Bardin Bluffs and Altar Mountain) have a similar fine-grained matrix, they exhibit different populations and proportions of granules to cm-sized pebbles. Bardin Bluffs displays a fining upwards texture and grain-to-grain contact; the stratigraphically lower Altar Mountain does not.
Massive and Vuggy rocks. These two classes are gray and fine-grained. Many of these rocks occur in a sedimentary context, suggesting that differences in grain characteristics and relationships stem from variations in cementation and/or weathering history.
In general, most rock textures indicate fluvial or possibly lacustrine sediments; MAHLI has not unambiguously identified eolian or igneous rock textures, although some pebble-sized clasts may have an igneous provenance and some might derive from impact melt.