ASSESSING THE PRESERVATION POTENTIAL OF BIOGENIC FEATURES IN PRE-QUATERNARY TUFAS AND TRAVERTINES- APPLICATIONS TO EXOBIOLOGY

If evidence for life on Mars is ever found, it may come from Martian paleo-groundwater deposits (tufas and travertines). Tufas and travertines uniquely preserve evidence of life from both surface and subsurface environments. What is known about the sedimentology, geochemistry, and paleontology of tufa and travertine is biased towards Holocene examples. Geologically older examples remain poorly studied, with a few exceptions, possibly due to poor preservation potential in erosional terrestrial environments, or because they have been overlooked or misclassified. Documented examples exist of Eocene, Jurassic, Triassic, and Devonian age, but most of these studies focused only on the mineralogy and/or textures and structures such as pisolites and stromatolites. This study describes possible biogenic microstructures detected using SEM and petrography. We have compared tufas and travertines from the Eocene Chadron Formation (Badlands of South Dakota), the Jurassic Shuttle Meadow Formation (Hartford Basin), and Quaternary samples from San Ysidro Quadrangle, New Mexico. Our goal has been to discern trends in the preservation potential of microstructures through geologic time. The dominant microstructures we have observed are of probable bacterial origin, such as clotted micrite and pseudo-stromatolitic features (eg. shrub-and-ray dendrites). Quaternary samples include pisolites, coated grains, lithoclasts, encapsulated “fresh” algae, and aragonite shrub-and-rod dendrites. Eocene samples include pisolites, oncoids, coated grains, lithoclasts, shrub-and-ray dendrites, macrofossils (molluscs, ostracodes, and vertebrate bones), charophyte stems and gyrogonites, microcolonial fungal sacs, coccoliths, and unidentified microfossils. Jurassic samples include dendrites, calcite microspherules that have been associated with algal and/or bacterial growth, and yet unidentified microfossils and microstructures. Our results indicate that identifiably biogenic microstructures can be well preserved in tufas and travertine deposits as early as the Early Jurassic.