SEDIMENTARY EARLY MARS REVEALED AT THE MICROSCALE AT PAHRUMP HILLS, GALE CRATER

From orbit, Mars is a basaltic planet in composition and spectral character. However, within Gale crater, structures and textures can best be interpreted as having been formed through sedimentary processes. This fact is highlighted in the Pahrump Hills region, an area explored by the Mars Science Laboratory (MSL) Curiosity rover between mission days (sols) 753-948. Here, the MSL team adopted a traditional “walkabout” strategy, where the rover first explored the site with remote sensing instruments, then used this data to downselect the best sites for more detailed, time-consuming interrogation by contact instruments. The MSL Mars Hand Lens Imager (MAHLI) is a contact instrument that acquires sub-mm/pixel scale color images permitting the study of textures down to the distinction between “finer than coarse silt” (< 45 microns), coarse silt (45-63 microns), and very fine sand (63-125 microns). Rock textures in Pahrump Hills were analyzed based on grain size, sorting, and matrix characteristics. Classes include mudstones, siltstones and well-sorted sandstones.

Mudstones contain discernible silt grains 45-60 microns across, and abundant grains unresolvable by MAHLI (16 mm/pixel), silt-sized or smaller. The majority of mudstones contain what appear to be diagenetic nodules and crystal molds, and in places they are cut by Ca-sulfate filled veins. The Pahrump Hills region contains mudstones of at least three different textural types: recessive, recessive parallel-laminated, and resistant laminated-to-massive. Recessive mudstones (e.g., Shoemaker, Confidence Hills, Pink Cliffs) as well as parallel laminated mudstones are slope-forming. The latter display mm-scale parallel (and in some cases rhythmic) lamination that extends laterally for many meters, and are interbedded with recessiv mudstones. Coarse cm- to mm-scale laminae characterize ledge-forming resistant mudstones (Book Cliffs, Alexander Hills, Chinle). Though in some places layering seems entirely absent, all Pahrump Hills rocks appear to be laminated to greater or lesser degrees.

In short, rock textures indicate predominantly aqueous sediments, and the grain sizes seen in Pahrump Hills are difficult to explain without resorting to settling by suspension.