Mass-transport as a fundamental control on deglacial and postglacial sedimentation; the Carboniferous Jejenes Fm., San Juan, Argentina

M. Dykstra, Benjamin Charles Kneller, J. P. Milana

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Abstract

Quebrada de las Lajas, near San Juan, Argentina, preserves an early Pennsylvanian deglacial-postglacial succession in a highly confined paleofjord setting. The sedimentary succession records four distinct stages in the evolution of the valley fill. Stage 1 is represented by the deposits of subglacial diamictites, ice-contact deltas, and related deepwater lacustrine environments, including several subaqueous channels. Stage 2 records a glacio-eustatic marine transgression, and a slow-down of the clastic supply into the paleofjord. Stage 3 records a sandy, confined turbidite environment. Stage 4 consists of a coarse-grained delta, which represents a significant rejuvenation of the sedimentary system in the paleofjord. The transition from stage 1 to stage 2 was abrupt and basinwide, and has proven to be a good regional correlation marker. In this paleovalley, the glacioeustatic rise probably caused floating and consequent rapid melting of the valley glacier. This resulted in a paleovalley-wide turbidite event that is up to 5 m thick and indicates an overall waning character. The rapid sediment emplacement in this event and resulting loading of the paleovalley sediments may have helped cause widespread mass-transport events at the stage 1-2 boundary. All stages show evidence of mass-transport-related deposits, but stage 1 records the most widespread mass transport, with a large spectrum of processes represented, including coherently slumped material, rafted blocks, and completely mixed debris-flow deposits. These mass-transport deposits range from a few meters to over 50 m thick, and up to hundreds of meters wide and long. Most of the large mass-transport deposits in the lower part of the fjord fill occurred at the transition to stage 2, implicating a rise in relative sea level as a possible trigger for the slope failures. Additionally, stage 3 deposits were affected by several thrust-sense dislocations that have large offsets relative to the scale of the sedimentary succession (tens to over 50 m vertically and hundreds of meters laterally); strata associated with the dislocations exhibit growth, indicating very early movement, and they are interpreted as the frontal ramp zones of mass-transport deposits. The abundance and size of mass-transport deposits in stages 1 and 3 helped control sediment pathways throughout the paleofjord.

Original languageEnglish
Pages (from-to)913-937
Number of pages24
JournalGeological Society of America Bulletin
Volume118
DOIs
Publication statusPublished - 2006

Keywords

  • mass transport
  • deglaciation
  • fjord
  • climate change
  • Carboniferous
  • Argentina
  • SEA-LEVEL CHANGE
  • WESTERN ARGENTINA
  • BRITISH-COLUMBIA
  • FACIES
  • PALEOSEISMICITY
  • FJORD
  • BASIN
  • SEISMOTECTONICS
  • SEISMICITY
  • PAGANZO

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