Stratigraphy of North Polar Deposits on Mars: Major New Findings

Introduction: We have begun detailed photo-geologic analyses of parts of the north polar region on Mars that expose key geologic elements and relationships. Our goal is to better understand the interplay of local stratigraphy, H 2 O ice and CO 2 frost, climate, and insolation and eolian processes in the development and modification of north polar stratigraphy and geomor-phology. At present, extensive data sets for the north polar region include a MOLA DEM at 115 m/pixel resolution and thousands of THEMIS VIS and IR, HRSC, and MOC wide-and narrow-angle images acquired at various times. TES and THEMIS data provide useful albedo and thermophysical information. The Rupes Tenuis unit: Much of the basal part of Planum Boreum within and west of Chasma Boreale consists of neary horizontally bedded strata of possibly Early Amazonian age [1] exposed on the floor of Chasma Boreale and along and above parts of Rupes Tenuis [1]. This prominent scarp runs from the southwest margin of Chasma Boreale westward to ~240º E. and rises hundreds to ~1000 m above Vastitas Borealis and ~600 m above the floor of Chasma Boreale. We name the basal strata of Planum Boreum the Rupes Tenuis (RT) unit. Along Rupes Tenuis, the RT unit forms most of the thickness of Planum Boreum but is partly buried by younger layered deposits and mantle material that obscures its morphology (THEMIS V05209008). RT is crowned at 287ºE. by two massive layers having a combined thickness of ~100 m and whose eroded margins include circular to elongate knobs mostly <1 km across. The massive layers appear to dip gently westward. Below these, about 20 layers (and/or layer sequences) totaling <850 m can be discriminated in better exposures, but they generally cannot be traced con-tinously for more than several kilometers. These beds appear to have somewhat uneven thicknesses. Locally, scarps cut across the beds and may result from faulting. The unit thins westward and cannot be traced with certainty west of ~242ºE. For the Chasma Boreale floor platform, the RT unit is reduced to ~10 layers totalling 200 to 350 m in thickness [2]. MOC images (e.g., R2300500 and S0100879) indicate minor faulting, tilting, and fracturing and local burial by thin layered material. None of these better exposures of the RT unit display dunes emanating from them and thus the unit does not make up part of the dark platy deposit described as