THE FIRST FIELD GEOLOGIC MAPS ON ANOTHER PLANET: COMPLEX CRATER STRUCTURE AND DRAPE FOLDS (Invited Presentation)

Geologic maps derived from in situ ("field") observations during the traverse of Mars Exploration Rovers Spirit and Op­portunity are the first tests of field geologic mapping methods at the human scale on an­other planet. Opportunity's current mapping campaign on the western rim of the 22-km-diameter Noachian Endeavour im­pact crater is also the first in situ geologic trav­erse, including Earth, on the preserved topographic rim of large (complex morphology) impact crater. Prior to this traverse the most basic outcrop-scale geologic, structural, lithologic, and petrologic characteristics of the landforms on large impact craters were unknown. From outcrop scale correlations, along-azimuth discontinuities in rim structure and variable relief define distinctive structural and relief segmentation of the crater rim. Enhanced concentrations of some mobile elements in outcrop at these discontinuities are consistent with alteration of bedrock from fluid flow along the abundant fractures associated with these structures. Unexpected exposures of both inward- and outward-dipping impact breccias and elevation differences in stratigraphic sections challenge current simple models for complex crater geology.

Outcrops examined by Spirit on the Columbia Hills generally dip non-conformally and more steeply than the hill slopes. Differences in apparent degree of alteration have resulted in complex and variable elemental analyses from outcrop to outcrop within a single lithology, but petrographic characteristics between local stratigraphic successions are more easily correlated. Based on multiple stratigraphic sections along the traverse, we define a hill structure that is a simple draping of clastic materials over a pre-existing relief. Erosion of materials on summits exposed stratigraphically lower units on the upper slopes as with any truncated antiformal drape fold. Similar truncated conformal stratigraphy and draping structure may be pervasive on Mars.