THE SEDIMENTARY ROCKS OF MARS: A GLOBAL PERSPECTIVE

Mars Global Surveyor Mars Orbiter Camera (MOC) images have scales comparable to aerial photographs. Thousands of the > 205,000 MOC images show outcrops of sedimentary rock. Differing erosional expressions, bedding styles and albedos indicate a variety of rock types. Some materials, such as a fossil delta in Eberswalde Crater (24.3°N, 33.5°W), present evidence for deposition in an aqueous setting. Others, in the craters Gale and Terby, exhibit large ripples preserved in stone. The ripples do not resemble eolian forms, and might reflect currents in deep(?) subaqueous settings. No eolian cross-bedding is seen from orbit. Still other exposures, as in southwest Candor Chasma and in a crater at 8°N, 7°W, exhibit hundreds of layers of repeated thickness and outcrop expression, suggesting episodic deposition of like materials in an environmentally-modulated, subaqueous setting. Not all sedimentary rock exposures are confined to impact craters and chasms. The bedrock of much of northern Sinus Meridiani, including the rover, Opportunity, site, is sedimentary rock. Indeed, the rover has explored < 1% of the > 800 m of stratigraphic section exposed in the region. The plains cut by some of the Valles Marineris chasms exhibit sedimentary rock outcrops, within some of which were preserved ancient stream beds. The layered, yardang-forming rocks of Aeolis, considered for > 2 decades to be tephra deposits, exhibit inverted channels and eroded depositional fans. Some of the stratigraphic sections include erosional unconformities. Impact craters (commonly eroded before burial) and valleys mark locations of erosional unconformities in settings as diverse as Gale Crater and Sinus Meridiani. Impact craters with diameters of 10s of meters to > 100 km have been completely filled, buried, and exhumed or partially exhumed from within sedimentary rock strata. The rocks are usually horizontally-bedded, except where dipping in response to pre-existing topography (e.g., wall of a buried crater). The dip of some rocks may have changed over time, in response to the tectonic forces that created the Tharsis bulge. Still other rocks, as in Oudemans Crater, dip steeply where they have been uplifted in impact crater central peaks and peak rings. The view from orbit shows that the sedimentary rock record of Mars is rich and complex.