HYPOTHESES FOR SEDIMENTARY MAGNESIUM SULFATE AT GALE CRATER, MARS

The clay-sulfate transition region of Gale crater as observed by the Curiosity rover is marked by texturally and chemically distinctive diagenetic fabrics—color variations, pits, nodules, and veins—that vary in abundance and diversity throughout the stratigraphic section. Heterogeneities in the distribution of nodules in particular are consistent with sedimentological variations in the Carolyn Shoemaker and Mirador formations, suggesting grain size and compaction controls on fluid pathways influenced their formation. The primary strata in the Pontours member are entirely overprinted by nodules, while the base of the clay-sulfate transition region, just above the less permeable clay-bearing Glen Torridon region, sees the highest diversity in diagenetic fabrics of this study.

Enrichments in Mg sulfate have been linked to specific nodules as well as bulk rock measurements of densely nodular bedrock, suggesting that an increase in Mg sulfate upsection correlates with precipitation of enriched pore-filling diagenetic cements. While there are a few examples of nodules associated with polygonal features interpreted to be desiccation cracks, the lack of widespread evidence for primary evaporative concentration of salts leads us to additional mechanisms for the migration and precipitation of the sulfates in the subsurface, in addition to the primary evaporite mechanism.

Three proposed hypotheses include a “top-down” model, in which bedded evaporites deposited in a yet-unknown stratigraphically higher unit (potentially the boxwork formation) are remobilized as dense brines that sink through the section via gravitational drive; “bottom-up” precipitation from circulation of warmer fluids up through an early, deeply buried salt deposit; and “top-down” precipitation due to downward groundwater retreat during the global aridification of Mars.