ALTERNATIVE INTERPRETATION FOR THE EBERSWALDE DELTA, HOLDEN NE CRATER, MARS

Perhaps the best example of a deltaic system filling a crater on Mars is the Eberswalde Delta, Holden NE crater. Since it's original identification, a number of studies have detailed the geometry of the fluvio-deltaic distribution system and concluded it is consistent with a long-lived, Noachian-aged crater lake requiring persistent liquid water on the Martian surface. Our examination of black and white images and anaglyphs taken with the High Resolution Imaging Science Experiment (HiRISE) camera reveals that critical components of a delta are not present and that alternative non-deltaic explanations also must be considered as viable. One of the critical observations in the delta interpretation is the presence of a kilometer-scale meander bend (with others less well defined) feeding the delta plain. Our examination of the anaglyphs reveals that the inferred cut-bank side of the loop is topographically higher than the inner point-bar, which demands uphill migration of the channel. This renders the idea of scroll-bar topography unlikely. Furthermore, image analysis of this bend shows that the lithologic color alternations represent a flat-lying stratigraphy and not the lateral accretion typical of a fluvial meander. Inspection of the terminus of the inferred delta front shows that all layers are essentially flat lying, unlike the expected inclined foresets typical of a prograding delta sequence. Topographic relief generated from erosion exposes much of the vertical stratigraphy of the succession, which does not show any of the expected features of a delta, including distributary channels, progradational or aggradational stratigraphic packaging, or a general upward coarsening from the prodelta to the delta plain. Instead, all that can be identified is fundamentally horizontal layers of unknown composition or grain size whose stacking does not favorably compare with known deltaic deposits on Earth. If our interpretation is true, then the deposits here permit, but do not require, persistent liquid water to have formed them. Any surficial expression of a delta is more a consequence of fortuitous erosion than primary depositional environment.