A depositional record of deglaciation in a paleofjord (Late CArboniferous -Pennsylvanian- of San Juan Province, Argentina): The role of catastrophic sedimentation.

KNELLER, B., MILANA, J.P., BUCKEE, C. AND JA`AIDI, O

GEOLOGICAL SOCIETY OF AMERICA BULLETIN

GEOLOGICAL SOC AMER, INC

Lugar: Boulder; Año: 2004 vol. 116 p. 348 - 367

0016-7606

The combination of high sediment supply rates and ample accommodation within fjords provides high resolution records of deglaciation. Ancient fjords 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 flooding and infilling of topography by glacially derived sediments. The geometry of the valley and its tributaries is controlled by the underlying lower Paleozoic lithologies: a deep valley with steep sides exits where the bedrock is massive limestones,  and a broader, shallower valley exists where the bedrock is generally a fine-grained olistostrome. The valley are locally floored with diamictites, including both in situ tillities and remobilized diamictites. N the trunk valley these are locally overlain by a small, shallow-water delta. The major part of the valley fill consists of a ¡Ü 150-m- thick mudstone-dominated succession (probably generated by plumes of glacial outwash) containing numerous dropstones that decrease in abundance down the fjord. The mudstones contain numerous thin sandstone and conglomerate turbidites that were supply laterally via subaqueous gravel fans feeding in from the tributaryvalleys, each of which has a distinctive cast 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. Intercalated within the succession in the trunk valley are structureless, graded, silty mudstones lacking dropstones but with  abundant large wood fragments. Close to thr steep western margin 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 steeper valley sides,triggering debris flows on the subaqueous flord slopes. We suggest that large solitary waves were generated as the rockfalls entered the water, travelling along the fjord and stripping vegetation from the shoreline. Large amounts of mud and silt were thrown into suspension during these events and subsequently sttled from suspension to form the structureless graded beds. These deposits, and other mass-flow deposits within the succession, emphasize the potential importance of catastrophic sedimentation within deglacial successions..