[1] We utilized images and stereo-derived topographic data acquired by the High Resolution Stereo Camera (HRSC) and Thermal Emission Imaging System (THEMIS) images together with other data in order to study the geology of ''layered ejecta structures'' associated with relatively pristine Martian impact craters. The geomorphology and morphometric properties indicate their origin as complex combinations of a variety of impact processes. The studied (inner) layered ejecta structures often exhibit ground-hugging characteristics, and many of them do not have topographic profiles expected from simple ballistic emplacement. Such profiles include ones that are plateau-shaped or thickening outward. We think that water-rich fluidized flows driven by the momentum due to the impact and by gravity, together with ballistic emplacement and vortex produced by the atmosphere-ejecta curtain interaction, were essential to the (inner) layered ejecta structure formation. We hypothesize that the thinner outer layered ejecta structures were formed by various combinations of shockwave-induced liquefaction of water-rich near-surface sediments, ballistic emplacement of ejecta-entraining water, and strong winds (expanding vapor, vortex, base surge) related to the impact. The contribution of each proposed layered ejecta structure formation mechanism should have been variable depending on the condition of the impact.
The ∼150 km wide Holden crater lies in an area characterized by high density of valley networks implying conditions conducive to forming of water‐related environments. We undertook geological mapping and a stratigraphic survey in order to probe the evolution of water‐related landforms and their paleoenvironmental implications. Our investigations lead us to propose that the Holden area was subjected to a “wet” lacustrine phase of Hesperian age and an “icy” phase during the Amazonian. Deltaic, coastal, and lacustrine environments occurred during the “wet” phase, some displaying a cyclic depositional pattern presumably related to autogenic processes. Water was delivered to the basin by the Uzboi Vallis and by surface runoff channels from a series of drainage basins along the crater walls. Fan delta geometries and coastal onlap enabled estimation of major water levels. Two levels of major stand of the water have been recognized, possibly reflecting allogenic controls. Geologic units related to this “wet” lacustrine phase were subsequently eroded by glacial abrasion and plucking and were disconformably overlain by glacial deposits of Amazonian age, defining an “icy” phase. These features are consistent with a warm‐based glacier entering the Holden crater through the wide Uzboi Vallis to form a proglacial lake in the central part of the crater. Changes in sedimentary units reflect changes of depositional environments probably connected with climatic variation.
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