TEXTURAL AND COMPOSITIONAL ANALYSIS OF TERRESTRIAL BASALTIC SEDIMENTS: TOWARD A BETTER UNDERSTANDING OF MARTIAN SEDIMENTS AND SEDIMENTARY ROCKS

The composition of martian sediment has been determined by both direct and orbital observational probes, whereas most textural analyses have come from analysis of images taken by an array of cameras on martian rovers. With the high interest in interpreting sedimentary environments on Mars, we have undertaken integrated textural and compositional studies of basaltic, analog sediment under highly controlled natural and laboratory experiments as a means to better describe, interpret, and understand sediments/rocks on Mars. Sediments from three environments were selected as they are common on Mars: impact (simulant using crushed basalt), aeolian (Moses Lake dunes, WA), and fluvial (Death Valley area, CA). One limit on how significant textural properties, such as grain size, could impact sediment compositions is the quality in which textures can be assessed from 2D imagery. As a result, we studied grain size, sorting, sphericity, and rounding in terrestrial aeolian and fluvial sediments by (1) direct 2D v 3D comparisons, (2) comparisons of 2D images at multiple resolutions, and (3) comparisons of 2D images with different shadow lengths. Results show that 2D analysis shows remarkable fidelity with actual 3D measurements, resolution matters only after significant degradation, and shadows have little effect. In all cases rounding is the least reliable textural property. Basalt samples from Kilauea and the Cima Volcanic field were crushed and used as impact simulant. Sediments were sieved in 0.5 phi increments to determine the grain-size distribution, with aliquots removed for petrographic, geochemical, and grain-texture analysis. Part of the pre-sieved sediment was poured onto a flat surface and imaged in bright, even illumination to complete textural analysis in 2D for comparison with 3D grain measurements. From a textural perspective, 2D and 3D analyses strongly resemble each other in terms of grain size and sorting, with 3D sphericity measurements slightly lower than those measured from 2D images. Rounding is the most inconsistent textural property. The sieve fractions have different mineral and geochemical compositions from each other, and from the bedrock. Transport and organization of sediment into grain-size groups would impose compositional trends that resemble those in martian sediments.