EARTH SEDIMENTS AS AN ANALOGUE FOR MARTIAN DEPOSITS: A NEOPROTEROZOIC MODEL

A fundamental question concerning sedimentary deposition on the surface of Mars is how the planet’s atmosphere evolved over time and whether earlier climates supported widespread surface water. If we wish to employ Earth analogs, it is difficult to utilize Phanerozoic models due to the ubiquitous effects of plant and animal life in modifying the fabric and chemistry of sediments. On the other hand, Precambrian sediments - particularly shallow water clastic deposits - lack most of these modifying features; as such, they should provide a reasonable assemblage of potentially analogous, readily recognizable surface and near-surface depositional features.

The Mesoprotrozoic Grinnell Formation, which is cropping out in and around Glacier National Park (Montana), appears to be an excellent example for sedimentary deposits having formed in shallow seas in the absence of fabric-destroying bioturbation. As such, the Grinnell can be used as an analog for Martian surface conditions during periods of moderately warm climatic conditions. It provides both wave-rippled and desiccated bedding surfaces, raindrop imprints, halite surface impressions, displacive and cementing halite and other minerals formed below desiccated surfaces, and sporadic layers and channel fills containing cross-bedded sands, the grains of which were wind-abraded and rounded. A reference-collection of such features on the Martian surface should prove useful in the interpretation of sedimentary structures. Recognition of these sedimentary morphologies, along with those from exposed, arid surfaces known in other areas on Earth, should permit clear, visual morphological recognition of mineral deposits formed in Martian sedimentary settings.