Grain coatings: Diagenesis of Jurassic sandstones in south-central Utah and implications for targeting fossil microbes on Mars

Abstract

Clusters of large-scale (decameters to > 100 m dia) fluidization pipes have been identified at several locations in Jurassic Navajo and Entrada sandstones in south-central Utah. These typically cut the cross-bedded aeolian-sabhka host rock at right angles. Similar pipe-like structures have been tentatively identified on Mars, and the question of the diagenesis of host rock has important implications for the genesis of pipe structures. Current thinking regarding the genesis of host rock structures is that the grains were derived from wet aeolian-sabkha environments. EDS of all samples indicates a Si:Al ratio of 2:1 suggesting smectite clay minerals are to be expected in a dry mid-latitude climatic zone, and unlike the pipe structures which contain hyphopodium-like structures (fungal filaments), similar in morphology to modern tropical leaf parasites, the host rock is relatively bereft of microbes entombed in grain coatings. Moreover, microtextural data from host rock grains in these wet aeolian (wet-sabkha) environments suggest a lack of volatility judging from infrequent observation of percussion crack evidence of grain collision presumably during the time of formation, a microfeature commonly found within the pipe structures. The diagenesis process can be reconstructed from the composition of the coatings on recovered grains of quartz, feldspar, mica, and occasional calcite as imaged with the SEM and studied by EDS. XRD of the coatings provides confirmation of the clay mineralogy, and INAA is used to cross check the chemistry obtained by EDS. If similar microfeatures exist on grains from both Earth and Mars, materials transported by similar aeolian processes, one would not expect to find fossil evidence of life on grains in host rocks. While martian pipe structures may be the optimum target for the astrobiological exploration of the red planet, host rock might be relatively devoid of fossil microbes if diagenesis of sandstones on Mars is anything like Earth.