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HiRISE Reveals the Stratigraphy in Holden Crater

Wednesday, January 24, 2007
John
Grant (Smithsonian Institute)

HiRISE images of Holden Crater in southwest Margaritifer Terra, Mars, reveal outcroppings of mega-breccia of likely impact origin that are overlain by deposits associated with two Noachian-era periods of lacustrine activity. The first lacustrine episode was likely sustained in part by groundwater and was initiated when the crater formed into sedimentary materials filling the pre-existing Holden Basin. By contrast, the second, shorter lacustrine phase was the result of discharge of ponded water from Uzboi Vallis. Exposed stratigraphy reveals the mega-breccia is poorly sorted and incorporates variably rounded blocks up to 50 by 25 meters across. The approximately 100 meters of sediments deposited during the first lacustrine episode are continuously bedded and generally in thinner towards the top. By contrast, up to tens of meters of poorly bedded sediments deposited during the second lacustrine episode unconformably drape the older deposits. Lacustrine and other deposits around Holden Crater record the end of a wet late-Noachian period on Mars. Subsequent gradation has been limited and accomplished by mostly eolian and perhaps limited fluvial activity.

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