Geology and Geochemistry of Noachian Bedrock and Alteration Events,
Meridiani Planum, Mars: MER Opportunity Observations
Abstract
We have used Mars Exploration Rover Opportunity data to investigate the
origin and alteration of lithic types along the western rim of
Noachian-aged Endeavour crater on Meridiani Planum. Two geologic units
are identified along the rim. The Shoemaker formation consists of two
types of polymict impact breccia: clast-rich with coarser clasts in
upper units; clast-poor with smaller clasts in lower units. Comparison
with observations at terrestrial craters show that the lower units
represent more distal ejecta from one or more earlier impacts, and the
upper units are ejecta from Endeavour crater. Both are mixtures of
target rocks of basaltic composition. Subtle compositional differences
are caused by differences in post-impact alteration along the crater
rim. The lower Shoemaker units and the Matijevic formation represent
pre-Endeavour geology, which we equate with the regionally mapped
Noachian subdued cratered unit. An alteration style unique to these
rocks is formation of Si- and Al-rich vein-like structures crosscutting
outcrops, and formation of smectite. Post-Endeavour alteration is
dominated by sulfate formation. Rim-crossing fracture zones include
regions of alteration that produced Mg-sulfates as a dominant phase,
plausibly closely associated in time with the Endeavour impact.
Calcium-sulfate vein formation occurred over an extended time period,
including pre-Endeavour impact and after the Endeavour rim had been
substantially degraded, likely after deposition of the Burns formation
that surrounds and embays the rim. Differences in Mg, Ca and Cl
concentrations on rock surfaces and interiors indicate mobilization of
salts by transient water that has occurred recently and may be ongoing.