LATE JURASSIC ROCKS OF THE COLORADO PLATEAU AS DEPOSITIONAL AND DIAGENETIC ANALOGS TO GALE CRATER, MARS

The Morrison Formation (the upper Brushy Basin and the Tidwell Members) and Wanakah Formation are proposed as terrestrial analogs to sedimentary rocks exposed in Gale Crater, Mars, due abundant sedimentological and diagenetic similarities. These late Jurassic terrestrial units are well exposed in western Colorado and eastern Utah (Colorado Plateau) and the depositional environments are interpreted as hypersaline lake systems with abundant volcanic ash input. The terrestrial volcaniclastic, clay- and iron-rich shales are similar to the putative lacustrine sedimentary rocks at Gale Crater in terms of lithology, mineralogy and chemistry and this terrestrial study allows inference of the depositional environment and diagenetic history of Gale Crater. Preservation potential for biosignatures in martian sediments is evaluated by documenting diagnostic biogenic features present in the terrestrial units.

Five primary lithofacies are present in both the Colorado Plateau and the Mars rocks: silt-/claystone, sandstone, and conglomerate, sulfates and carbonates. Both terrestrial and martian silt-/claystone lithofacies are interpreted as lacustrine depositional environments due to parallel laminated, massive, and vuggy sedimentary structures. Fluvial features are also present in the analog units and the Gale Crater rocks such as cross-bedded sandstones and imbricated conglomerates. Both the Tidwell Member and the Wanakah Formation contain meters-thick horizons of gypsum analogous to sulfates in the rocks at Gale Crater. Additionally, gypsum veins are present in the Tidwell Member and at Gale Crater. Thin, laminated limestones are common in the Tidwell Member and the Wanakah Formation and the Brushy Basin Member hosts a 3-4m thick limestone sabkha deposit. Carbonates are also found at Gale Crater. Concretions are present in all the Colorado Plateau units and at Gale Crater.

Research on the Brushy Basin Member, the Tidwell Member and the Wanakah Formation will inform the Mars Science Laboratory mission on important scientific questions: 1. What is the depositional environment?, 2. What is the history of fluid-rock-(biota?) interactions, 3. What past chemical environments (i.e., fluid composition) existed during deposition and diagenesis?, and 4. What types of biomarkers can we expect to find?