Variety and complexity in the mound of sedimentary rock in Gale Crater, Mars

NASA's Mars Science Laboratory rover, Curiosity, will be used to explore a portion of the lower stratigraphic record of the northwest side of a mound of layered rock ∼5 km thick in the 155 km-diameter Gale Crater. The rock materials are of a sedimentary origin, though the proportions of clastic sediment, tephra, and chemical precipitates are presently unknown. The mound is usually described as having lower and upper units separated by an erosional unconformity. However, some investigators recognize that it is considerably more complex. The stratigraphy displays vertical and lateral complexity; multiple erosional unconformities; filled, buried, interbedded, and exhumed or partly exhumed impact craters; evidence for deposition along the base of the mound followed by retreat of less-resistant rocks and abandonment of erosion-resistant materials shed from the mound; lithified sediments deposited at the mouths of streams that cut mound rock; inversion of intra-canyon stream channel sediment; and widening of canyons. On the northeast side of the mound there are landslide deposits, shed from the mound, that contain large blocks (10s to 100s of m) of layered rock in various orientations. The mound's highest feature does not exhibit layering and has been interpreted by some as being Gale's impact-generated central peak. However, its highest elevation exceeds that of most of the crater rim, an observation inconsistent with central peaks (where they occur at all) in martian craters of diameters similar to Gale. The layered materials that occur highest in the mound are also at elevations that exceed most of the crater rim; these exhibit repeated stratal packages that drape previously-eroded mound topography; they produce boulders as they erode, attesting to their lithified nature and requiring that a lithification process occurred in materials located ≥ 5 km above the deepest part of Gale. The lower mound strata, including the Curiosity field site, are diverse materials; they include strata of differing thickness, erosional expression, and tone. Resistant rocks form cliffs that shed boulders, less resistant rocks form shallow slopes. One relatively thin, dark unit, interpreted to be a marker bed that outcrops at various places across the lower mound (doi:10.1029/2009GL041870), is more resistant to erosion than sub- and superjacent beds and retains many small impact craters. Some of the lower mound rocks are cross-cut by channel or cavern fills; others are cut by reticulated patterns of filled cracks or ridges formed by inversion of these cracks. These reticulated features might be evidence of interaction between the lower mound rocks and groundwater; we anticipate the Curiosity team will find abundant evidence for dissolution and precipitation of minerals in these rock outcrops.