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Catastrophic Sedimentation in a Palaeofjord During Glacial Retreat
                             
                           

B.C. Kneller1, O. Al-Ja’aidi1, C.M. Buckee1, & J.P. Milana2
                             
                           

1School of Earth Sciences, University of Leeds, LS2 9JT Leeds, United Kingdom
                             
                           

(e-mail ossj99@yahoo.com)
                             
                           

2 Universidad Nacional de San Juan, San Juan, Argentina
                                     
                                     
                                    The Jejenes Formation of the Paganzo Basin in western Argentina records the waning of the Carboniferous ice-sheet of Gondwana. Superb levels of exposure in the arid terrain of the western Precordillera of Argentina allow reconstruction of palaeo-topography. The dissected sedimentary fill of these fjord-like valleys is dominated by deposits of gravity-driven processes, and records the progressive retreat of valley glaciers penetrating seawards into the limestone ranges that bordered the western interior lowlands of Gondwana. We report on the history of infilling of one of these palaeo-fjords at Quebrada Grande in the Zonda range of San Juan Province.
                                     
                                    Basement lithology exerts a strong control both on the form of the valley, and the palaeorelief. Basal in situ and remobilised tillites occur on the floor of the trunk valley and its tributaries, and are overlain by subaqueous gravel fans issuing from the hanging tributaries; these downlap and initially prograde over the valley floor diamictites, but later back-step presumably as consequence of retreat of the parent hanging-valley glaciers. The toes of these fans show abrupt facies transitions related to changes in the dominant flow process, from gravel-dominated debris-flow deposits on the fans to thin-bedded sandy and silty turbidity currents on the fjord floor. Dropstone horizons are common in the turbidite axial fjord fill which onlaps the valley sides and fans. Imbricated armour layers on the fans suggest periodic winnowing of the fan surfaces by meltwater-generated, sediment-laden density underflows.
                                     
                                    The thin-bedded valley-axis turbidite facies is punctuated by several exceptionally thick, normally graded but otherwise structureless beds, that grade up from coarse sand (or locally gravel) at the base into thick, structureless caps of mudstone that form the majority of the bed, locally 5 metres or more in thickness. These beds thicken into the valley axis and onlap the valley walls and gravel fans, and their emplacement thus appears to have been gravity controlled. The mudstone fraction of each is commonly rich in scattered fossil wood fragments, locally decimetres in length. We suggest that these sedimentation events were the result of large surface waves within the fjord, possibly triggered by debris avalanche related to post-glacial unloading of the valley sides, and that these waves stripped the shoreline of vegetation. Such waves have occurred several times in the past century in Lituya Bay, Alaska (Miller, 1960), in a context similar to that which we infer for the Quebrada Grande; they have attained heights in excess of 100 metres, and produced conspicuous trim-lines in the forest on the valley sides.
                                     
                                    Miller. D.J., 1960. Giant waves in Lituya Bay, Alaska. US Geological Survey Professional Paper 354-C.