A depositional record of deglaciation in a paleofjord (Late Carboniferous [Pennsylvanian] of San Juan Province,Argentina):The role of catastrophic sedimentation

摘要

The combination of high seidment supply frates and ample accommodation within fjords provides high-resolution records of deglaciation.Ancient fjord fills provide the potential for three-dimensional views of the evolution of depositional environments in response to changing sediment supply and base level through the deglacial process.we describe the fill (Jejenes Formation) of a well -exposed Late Carboniferous (Pennsylvanian)glacial valley and its tributaries;these deposits record the progressive marine floding and infilling of topography by glacially derived sediments.the geometry fo the valley and its tributaries is controlled by the underlying lower Paleozoic lithologies:a deep valley with steep sides exists where the bedrock is massive limestones,and a broader,shallower valleyexists where the bedrock is generallya finegrained olistostrome.The valleys are locally floored iwth diamictites,including both in site tillites and remobilzed diamictites.In the trunk valley these are locally overlain by a small,shallow-water delta.The major part of the valley fill consists of a <=150-m-thick mudstone-dominated succession (probably generated by plumes of glacial outwash)containing numerous dropstones that decrease in abundance down thefjord.The mudstones contain numerous thin sandstones contain numerous thin sandstone and conglomerate turbidites that were supplied latrally via subaqueous gravel fans feeding in from the tributary valleys,each of which has a distinctive clast suite related to the local subcrop.The entire succession is overlain by >300 m of sandy turbidites,of which the upper part includes large mass-transport complexes.Intercalatd within the succession in the trunk valley are structureless,graded,silty mudstons lacking dropstones but with abundant large wood fragmetns.Close to the steep western magin of the trunk valley,each of these massive mudstones is underlain by a slump or debris flow,locally containing meter-scale blocks of Ordovician limestone from the valley side.We interpret these as a consequence of rockfalls from the stepper valley sides,triggering debris flows on the subaqueous fjord slopes.We suggest that large solitary waves were generatedas the rockfallsentered the water,traveling along the fjord and stripping amounts of mud and silt were thrown into suspension during these events and subsequently settledfrom suspension to form the and other mass-flow deposits within the succession,emphasize the potential importance of catastrophic sedimentation within deglacial successions.