Paper No. 94-8
Presentation Time: 10:10 AM
TRACING BASALTIC SEDIMENTATION FROM DEPOSITION TO LITHIFICATION
The surface of Mars is dominated by basalt, with a sedimentary record strongly influenced by fluvial processes that have resulted in the generation of clastic sedimentary rocks with basaltic compositions. A fundamental question that remains to be answered for Mars is: what was the nature of the early climate that was to produce clastic sedimentary rocks with mafic compositions? Complicating matters, basaltic sedimentation is an understudied process on Earth, and as a result, we lack an adequate reference frame to interpret the sedimentary rock record of Mars. Exploring terrestrial analogs is one way to develop a better understanding of the sedimentary processes of early Mars. In this work we trace the processes of sedimentation and sedimentary rock genesis within the Columbia River Flood Basalts. We assess the geochemical and mineralogical changes that occur as basaltic sediments transform during burial lithification and subsequent diagenesis. A field-based investigation is coupled with laboratory sample analyses, including X-ray diffraction and fluorescence, scanning electron microscopy with energy dispersive spectroscopy, inductively coupled plasma optical emission and mass spectroscopy, and thermal emission spectroscopy. The combination of these techniques provide high-resolution results for mineralogy and geochemistry, in addition to serving as a direct links to Mars orbiter and rover instrumental capabilities. Field investigation of an ancient stream deposit has produced samples with intact secondary cementing products preserved. Preliminary results from SEM-EDX analysis of the sharp contact between the basalt pebble and cement display a host rock composition with elemental concentrations typical of a tholeiitic basalt. However, the constituent cementing material loses more soluble elements as a result of chemical mobility. The result is a cement that is rich in Si, O, Al, Fe, and Ca, which suggest a smectite clay family member. Furthermore, mineralogical results confirm a montmorrillonite smectite but also display distinct changes in clay mineral phases in the loose unconsolidated clay fraction, potentially highlighting the transition from an immature smectite phases to an evolved illite.