An application of sequence stratigraphy to Mars: the Eberswalde fan delta

Kurzfassung

We present a review of our work on an intriguing feature on Mars, the Eberswalde delta-like feature, based on detailed geological analyses aimed to recognize and map the depositional environments and to infer their evolution through time. The topset-foreset-bottomset geometry, typical of delta progradation, has been observed in the Eberswalde outcrops allowing to interpret this feature as a fan delta. We distinguished delta plain, delta front and prodelta depositional sub-environments in the exposed stratigraphic succession. The delta plain is made of distributary channels and interdistributary areas, in which crevasse splays delivered coarser sediments from the channels. Polygonal shrinkage cracks formed probably after exposure in the interdistributary areas. At places the polygons have been reworked by more recent distributary channels causing the formation of intrabasinal breccia. In the delta front inertia and friction-related processes appear to be dominant in different phases of the fan delta evolution, suggesting fluctuations of the controls on sedimentation through time. The dynamics of the recognized depositional environments has been evaluated in terms of sequence stratigraphy, recognizing geometries such as offlap and downlap, erosional truncations and stacking patterns, entering them in a stratigraphic framework. The sedimentary architecture of the fan delta appears to be organized in three cycles, which we tentatively interpreted as depositional sequences. The evolution of these sequences would have been controlled by relative water level fluctuations showing a longer term trend toward decreasing water content inside the basin. The evolution of the Eberswalde fan delta appears to be controlled by autogenic processes within the single discharge lobes. Allogenic processes, such as tectonic activity and/or climatic forcing, would have instead caused fluctuations of the level of the water table in the lake, consequently driving the switching among the deltaic lobes.