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Weimer Distinguished Lecture: John Grotzinger, California Institute of Technology
December 5, 2019 @ 3:30 pm - 5:30 pm
John Grotzinger, California Institute of Technology
Student Center Grand Ballroom, 4-5PM
Stratigraphy, Sedimentology, and Diagenesis of a Martian Lacustrine Deposit, Murray Formation, Gale Crater, Mars
Abstract: The Mars Science Laboratory Mission’s Curiosity rover landed at Gale crater, Mars, on August 6, 2012. For the past ~6 years Curiosity has been exploring a lacustrine deposit exposed in dissected terrain of crater-interior central mountain. The lacustrine deposit (Murray fm., >300m thick, early Hesperian age) overlies and is laterally equivalent to fluvial-deltaic deposits of the Bradbury group that Curiosity explored earlier in the mission. These rocks are unconformably overlain by the Stimson formation, an eolian sandstone that was deposited above a surface representing significant denudation of crater-filling strata.
The Murray contains one major depositional facies – laminated mudstone – deposited in a lake; and minor additional facies – ripple cross laminated and trough cross bedded sandstones – representing subaqueous delta foreslope, fluvial, or eolian environments. The persistence of fine lamination, locally with scour and drape truncation surfaces, and general absence of desiccation cracks, prism cracks, intraclasts, displacive evaporite crystals and nodules, or bedded evaporites, all suggest a perennial lake with depths great enough to avoid seasonal desiccation. Intercalated thin sandstones, of potentially eolian or fluvial origin, might indicate base-level lowering during longer-term lake level oscillations or a period of normal regression. Two compositional facies are observed: a hematite-sulfate-clay (HSC) facies and magnetite-silica (MS) facies. The HSC facies comprises the lower few meters of stratigraphy of the Murray, and transitions upward into the MS facies which persists through the subsequent ~10-15 meters of the Murray. This is followed by a return to HSC facies, which dominates the upper 250 meters of the presently measured Murray. These data could be explained by variations in the composition of fine clastic detritus delivered to the lake via marginal sediment plumes, coupled with redox oscillations in the composition of authigenic minerals precipitated from the lake. In a second model, originally reduced sediments of either detrital or authigenic origin are oxidized by groundwaters, converting reduced species, e.g. magnetite and sulfides, to hematite and sulfates. Stratigraphically higher members of the Murray, including those leading up to and comprising Vera Rubin Ridge, record elemental mobility during later diagenesis due to chemical weathering in an increasingly temperate paleoclimate as shown by feldspar alteration and phyllosilicate mineralogies. Uncommon Ca-sulfate crystal molds and pseudomorphs postdate compaction and suggest oxidative weathering of trace sulfides. Furthermore, localized Mg-sulfates also crosscut bedding and may be related to fluids that percolated down through the overlying Stimson formation, which contains Mg-sulfate concretions. This suggests that salts associated with extreme desiccation postdated deposition of the Murray formation, perhaps associated with localized and small concentrations of Fe-sulfates which have been shown to be late (ca. ~2 Ga) based on K-Ar dating.
Considered collectively, the early Hesperian environment at Gale crater is inferred to have recorded a persistently aqueous environment characterized by mild salinity and acidity. This contrasts sharply with the extremely saline and acidic environment recorded by eolian sandstones observed by the Opportunity rover at Meridiani planum. In that regard, Mars increasingly seems similar to Earth, where spatial variability in regional/global environments is as important to understand as temporal variability.