The Sedimentary Rock Record of Mars

Abstract

ABSTRACT The last decade of Mars exploration produced a series of discoveries that provide compelling evidence for the existence of sedimentary rocks on Mars. Previously, Mars was regarded principally as a volcanic planet, the dominant surface processes of which were eruption of lavas and pyroclastic deposits, although early studies did recognize valley networks, enormous outflow channels, and the required transport of sedimentary materials to the northern plains of Mars. In contrast, our new view of Mars shows a rich history of interactions between water and the surface, with weathering, transport, and deposition of sediments by water as well as eolian processes. Surprisingly thick accumulations of stratified rocks extend back into the Noachian Era—the oldest of which were likely formed over 4 billion years ago, making these rocks much older than any sedimentary rocks preserved on Earth. Some sedimentary rocks were formed and deposited locally, whereas others accumulated as vast sheets that can be correlated for hundreds of kilometers or farther. Local deposits were formed in alluvial fan, deltaic, sublacustrine fan, and lacustrine environments in addition to deposits that fill canyons and valleys possibly carved during catastrophic floods. These former deposits indicate more gradual erosion and sedimentation, perhaps even involving meteoric precipitation, and they provide support for the notion of clement conditions on early Mars. In contrast, rapid erosion and sedimentation may have occurred within large, regional outflow channels thought to have resulted from outbursts of groundwater. Regionally extensive sedimentary deposits have less obvious origins, but the presence of hydrated sulfate minerals indicates that some of these deposits may have formed as lacustrine evaporites, particularly in the Valles Marineris network of open and closed basins. Others may have involved eolian reworking of previously deposited sulfates, or perhaps aqueous (groundwater) alteration of previously deposited basaltic sediments. Another major type of regionally extensive sedimentary deposit occurs as meter-scale stratification with highly rhythmic organization. These deposits occur in several places in the Arabia Terra region of Mars and are also observed at the top of a 5-km-thick stratigraphic section in Gale Crater. The significant lateral continuity of relatively thin beds, their distribution over broadly defined highs as well as lows, and the lack of strong spectral absorption features indicate that these rocks may be duststones, formed by weak lithification of fine particles that settled from the Martian atmosphere. The most ancient sedimentary deposits on Mars may be dominated by stacked, impact-generated debris sheets, similar to those seen on the Moon, and may include impact melts. In the absence of plate tectonics, it appears that the flux of sediment on Mars has declined over time. Early on, primary sediments may have consisted mainly of impact- and volcanic-generated particles that would have been transported by fluvial and eolian processes. Chemical weathering of fragmented bedrock in the presence of circum-neutral pH fluids would have generated clay minerals and carbonates, though the latter are surprisingly rare; weathering under more acidic conditions generated dissolved salts that precipitated as sulfates, halides, and oxides. With time, Mars is regarded to have evolved from a rather wet planet, in which chemical weathering by circum-neutral pH fluids was common, to a regime in which more acidic chemical weathering took place and, eventually, to a cold, dry environment dominated by physical weathering. As the flux of impactors and volcanism declined, and as the planet’s hydrologic cycle decreased in vigor, the formation of sedimentary rocks also declined. Today the Martian highlands appear to be in a net state of erosion, and outcrops of sedimentary rocks are exposed as a result of wind-driven denudation. This erosion is likely balanced by deposition of sediments in the Martian lowlands. Orbiter observations of depositional framework, bed-scale textural/morphologic attributes, and mineralogy provide the basis for an “orbital facies” classification scheme. Orbital facies include Massive Breccia (MBR); Complexly Stratified Clay (CSC); Laterally Continuous Sulfate (LCS); Laterally Continuous Heterolithic (LCH); Distributary Network (DNW); and Rhythmite (RHY). These orbital facies are observed in several key reference sections, and their succession allows for correlation between widely separated regions of Mars, leading to a more refined understanding of environmental history. The oldest terrains on Mars are dominated by MBR and CSC facies, whereas younger terrains are characterized by LCS, DNW, and RHY facies. However, some occurrences of clay-bearing DNW and LCH facies may be contemporaneous with large sulfate deposits of the LCS facies, which are typically regarded as Hesperian in age. This indicates that the climatic evolution of Mars may be more complex than a simple global alkaline–acidic transition and that important regional variations in aqueous geochemistry and the relative roles of surface waters and groundwaters may be preserved in the Martian sedimentary record.