Deltas on Mars: A Martian History of Standing Water and Moving Sediments

L. J. Wood
Bureau of Economic Geology, University of Texas, Austin, TX, [email protected]

New satellite imagery from Mars is revolutionizing our knowledge of the geomorphology of this planet. Although Rovers Opportunity and Spirit provide “ground truth” on the planet’s surface, much can be learned from quantitative analysis of the spectacular images pouring from the satellites, Global Surveyor and the Mars Express that orbit the planet. Geomorphic elements associated with flowing water are everywhere on Mars. These elements include debris flows, gullies, debris aprons, all manner of drainage networks, erosional shadows, deltas and deltaic distributaries. One such delta, the Eberswalde Delta is examined in detail in this talk. The delta is composed of six separate phases of lobe development and appears to have prograded some 17 km from the apex. Cross-cutting distributary relationships and compensated deposition of lobes are clearly visible. Sinuosity, radius of channel curvature, meander bend width, and channel width parameters are examined for several lobe systems. Channel sinuosities vary between 1.2 – 1.8, defined by Schumm (1968) as low to moderate sinuosity systems typical of those transporting bed or mixed loads. There appears to be some convergence between age and sinuosity, however some of the younger systems show specific reaches of increased sinuosity associated with abutment against resistant beds of older lobes. This affect of older deposits on the morphology of the channels suggests that these older lobes were fairly well indurated prior to the deposition of the youngest progradational lobes. 86% of distributaries in the Eberswalde Delta are 100-240 meters wide. This number compares with 62% and 44% in the Atchafalya and Wax Lake Deltas of Louisiana, respectively, although the martian delta nearly 14X larger than these two deltas. Such small distributaries may indicate lower average and shorter duration flows in the Mars distributaries than those typical of the wetter Gulf Coast deltas. However, the volume of the material in the Mars deltas suggests long periods of activity and sediment transport. It is probable that this delta formed under rather ephemeral flow conditions, with distributaries that were intermittently active transporting a bed-to-mixed load of relatively coarse-grained sediments. The lobes prograded well out into a standing body of water, possibly in response to fluctuations or regressing water level. Further work should better quantify the flow conditions and history of this delta and numerous other deltaic and alluvial features that have been identified on the martian surface.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009