DIAGENETIC ALTERATION AND CONCRETION FORMATION IN THE JURASSIC BRUSHY BASIN MEMBER OF THE MORRISON FORMATION, COLORADO PLATEAU, USA: AN ANALOG STUDY FOR THE YELLOWKNIFE BAY FORMATION AT GALE CRATER, MARS

Lithology, mineralogy, and geochemistry of sediments and concretions in the terrestrial Brushy Basin Member are compared to the Yellowknife Bay formation at Gale Crater, Mars – rocks that have recently been analyzed by the Mars Science Laboratory mission – to examine patterns of diagenetic alteration and determine the potential influence of biota on fluid/rock interactions. The upper section of the Brushy Basin Member was deposited in an alkaline, saline, groundwater-fed, fluvio-lacustrine system during the late Jurassic. The ephemeral lake system was centered on the Four Corners region of the western US and a back-arc basin to the west produced an abundant supply of volcanic ash to the lake sediments. Biotic influence is recorded in the sediments in the form of trace fossils (burrows, reduction halos, algal molds) as well as vertebrate fossils. The Yellowknife Bay formation is also interpreted as a fluvio-lacustrine depositional environment and exhibits similar lithofacies such as horizontally laminated to massive mudstone and cross-stratified sandstone.

Concretions are present in both the terrestrial and martian rocks and are important because they record fluid/rock (and biota, on Earth) interactions and can preserve organics and micro- and macro-fossils. Yellowknife Bay formation concretions exhibit uniform morphology and are enriched in Fe, Mg and Cl relative to the host rock. By comparison, Brushy Basin Member concretions exhibit much more diverse mineralogies, sizes, and morphologies within areas of tens of square centimeters despite similar lithology in both the terrestrial and martian rocks. The geochemistry of the Brushy Basin Member concretions is quite similar to the host rock with only slight enrichment of certain elements which implies limited transport of reactants. The diversity of concretion mineralogy and morphology with restricted reactant transport suggests the concretions form in diagenetic microenvironments and are likely influenced by biotic interaction. In contrast, uniform concretion morphology and mineralogy in the Yellowknife Bay formation indicate abiotic precipitation mechanisms. Although the Yellowknife Bay formation represents a potentially habitable paleoenvironment, the record of biotic interaction in diagenetic reactions and concretion formation is lacking.