CONCLUSIONS REACHED FROM THE INTEGRATED EXAMINATION OF A LACUSTRINE MUDSTONE AT THE BOTTOM OF GALE CRATER (Invited Presentation)

The first bona fide mudstone known on another planet was examined at the presumed base of the sedimentary succession exposed in Gale Crater on Mars. Analysis of the observations collected by the Curiosity rover allowed us to arrive at a comprehensive interpretation of depositional regime, diagenesis, and burial history. Though lateral relationships remain tenuous, the Sheepbed mudstone likely accumulated in a lake basin that received sediment pulses from alluvial fans that were supplied from the crater periphery. These drainages tapped into as much as 2 km of martian upper crust and provided detritus from a wide range of variably weathered and altered source rocks.

Provenance is reflected in bulk rock chemistry and mineralogy and suggests input of variably weathered rock debris and detrital clay minerals. Clay mineralogy contrasts between closely spaced samples, consistent with at least partial detrital supply of clay minerals. A significant (~30 wt%) amorphous component is consistent with limited post-depositional alteration. Theoretical modeling of diagenetic reactions, as well as kinetic considerations, suggest that the bulk of diagenetic clay mineral formation occurred comparatively late in diagenesis.

Mm-cm scale layering suggests distal pulses of fluvial sediment injections (fine grained hyperpycnites), fall-out from river plumes, and some eolian supply. Diagenetic features include mineralized synaeresis cracks, mm-scale nodules,and stratiform cementation.

Synaeresis cracks indicate shrinkage of water rich-sediment early in depositional history via fabric collapse of flocculated clays. The observed diagenetic features, such as solid nodules, hollow nodules, matrix cement and “raised ridges” (synaeresis cracks) can be explained with progressive alteration of olivine/glass in conjunction with centrifugal and counter diffusion of reactive species.

Anhydrite filled fractures occurred late in diagenesis when fluid pressures built up to exceed lithostatic pressure. Generating fluid overpressure by burial to facilitate hydraulic fracturing suggests a burial depth of at least 1000 m for the underlying strata that supplied these fluids. PT stability ranges of Ca-sulfate minerals and clay minerals suggest 11 degrees per km as a plausible geothermal gradient for the Gale Crater succession.