Deposition of Interior Layer Deposits within East and West Candor as well as Ophir chasms, Valles Marineris, Mars

Abstract

<p>The Martian Valles Marineris, located on the eastern flank of the Tharsis region, is a 4000 km long linked system of troughs. The formation of the up-to-11 km deep chasmata of Valles Marineris is thought to have taken place during a two-stage process in which ancestral basins collapsed and were later linked by tectonism. Located within most chasmata are enigmatic layered deposits, referred to as interior layered deposits (ILDs), whose origin and mechanism of formation are uncertain. It has been estimated that ILDs cover 17% of the total area, representing 60% by volume of all deposits within Valles Marineris with several deposits nearly reaching the height of the surrounding plateau.</p><p>Here we present the results of a detailed study of the ILD mounds located within three of the presumed ancestral basins in the center of Valles Marineris.  In this study HiRISE and CTX images were used to measure layer attitudes of ILDs within East and West Candor and Ophir chasmata. Because only CTX images cover the entire chasms, the ILDs were grouped into distinct varities of bedding based on their appearance in CTX imagery.  In both East and West Candor and Ophir, the stratigraphically lowest unit is a massive unit which displays no layering in any available imagery.  Layered units with dips between 10° and 20° are deposited on top of this massive unit.  The lowest layered units in all three chasms appear to show multiple prominent benches, indicative of significant competency contrasts. These units can be shown in multiple places to be unconformably overlain by thinner layered units.  It is not possible to correlate or determine the number of thinner layered units because the unconformable contacts cannot be correlated. </p><p>The general similarities of types of units and unit relationships between  the ILDs in these three chasms suggests that they share a similar depositional history. We believe that the ILD morphology is most compatible with an environmental setting in which the ancestral basins were lakes which may have been periodically frozen.  We suggest that the unconformities are the result of multiple erosional events indicating that ILD deposition was not continuous.   A general trend of massive units overlain by thinner layered units may reflect a change in the environment and sediment supply.  Associated with this change is a general observation that polyhydrated sulfate is most commonly found on top of the monohydrated material.  Work to correlate more closely units within these chasms, including their mineralogy, is currently ongoing.</p>