Lacustrine Sedimentary Rocks from An Alkaline Environment in Becquerel Crater, Arabia Terra, Mars

Becquerel Crater is a 170 km diameter impact feature (22^oN, 353^oE) in the Arabia Terra region of Mars that contains a well-exposed sequence of layered sedimentary rocks.

High Resolution Imaging Science Experiment (HiRISE) images reveal gently-dipping meter-to-decameter thick layers in Becquerel Crater that we infer to be lacustrine sedimentary rocks. The images also show rotated and faulted strata, bounded by disrupted bedding, which we interpret as subaqueous slump blocks. Thin (1-2 m) high-albedo layers cap darker 2-10 m layers, and such dark-light layer pairs occur in cyclical sets. The high-albedo layers likely represent chemical sedimentation, either from standing water, or by evaporation at the sediment-air interface. Cyclical layer sets are conformably underlain by massive units (~40 m thickness) that may represent deeper water sedimentation. The absence of fluvial features indicates air-fall dust as a primary sediment source.

Thermal Emission Spectrometer (TES) data show an emissivity minimum centered near 9.27 µm that could be produced by phyllosilicate and/or tectosilicate minerals, such as clays, feldspars, and zeolites. The shape of the spectral feature is best modeled as mixture of plagioclase feldspar, pyroxene, and zeolite.

Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra do not show metal-OH absorption bands in the short-wave infrared, thereby ruling out phyllosilicates. However, hydration bands at 1.4 and 1.9 µm are also absent in CRISM spectra. Thus, zeolites, if present, must be dehydrated and/or spectrally diluted by anhydrous minerals, such as feldspars. Zeolite sensitivity to dehydration and/or UV degradation under Martian conditions require additional experimental study.

Assuming only episodic availability of liquid water, early Martian lakes must have been important loci for chemical alteration of primary volcanic materials. Zeolites are common in terrestrial alkaline lake deposits, and zeolites in Becquerel Crater would signify that alkaline conditions also existed in some Martian lacustrine settings.