A Depositional Record of Deglaciation in a Paleofjord (Late Carboniferous of San Juan Province, Argentina): the Role of Catastrophic Sedimentation

Benjamin Charles Kneller, J. P. Milana, C. M. Buckee, O. S. Al Ja'aidi

Research output: Contribution to journalArticle

47 Citations (Scopus)

Abstract

The combination of high sediment supply rates 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 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 exists where the bedrock is massive limestones, and a broader, shallower valley exists where the bedrock is generally a finegrained olistostrome. The valleys are locally floored with diamictites, including both in situ tillites and remobilized 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 less than or equal to15-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 supplied laterally 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. Intercalated within the succession in the trunk valley are structureless, graded, silty mudstones lacking dropstones but with abundant large wood fragments. Close to the 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 fjord slopes. We suggest that large solitary waves were generated as the rockfalls entered the water, traveling along the fjord and stripping vegetation from the shoreline. Large amounts of mud and silt were thrown into suspension during these events and subsequently settled 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.

Original languageEnglish
Pages (from-to)348-367
Number of pages19
JournalGeological Society of America Bulletin
Volume116
DOIs
Publication statusPublished - Mar 2004

Keywords

  • deglaciation
  • fjord
  • mass transport
  • catastrophic
  • Carboniferous
  • Pennsylvanian
  • Argentina
  • BRITISH-COLUMBIA
  • WESTERN ARGENTINA
  • EAST GREENLAND
  • SAANICH INLET
  • GRAVITY FLOWS
  • GLACIER BAY
  • STRATIGRAPHY
  • ALASKA
  • FJORD
  • PLEISTOCENE

Cite this

A Depositional Record of Deglaciation in a Paleofjord (Late Carboniferous of San Juan Province, Argentina): the Role of Catastrophic Sedimentation. / Kneller, Benjamin Charles; Milana, J. P.; Buckee, C. M.; Al Ja'aidi, O. S.

In: Geological Society of America Bulletin, Vol. 116, 03.2004, p. 348-367.

Research output: Contribution to journalArticle

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AB - The combination of high sediment supply rates 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 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 exists where the bedrock is massive limestones, and a broader, shallower valley exists where the bedrock is generally a finegrained olistostrome. The valleys are locally floored with diamictites, including both in situ tillites and remobilized 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 less than or equal to15-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 supplied laterally 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. Intercalated within the succession in the trunk valley are structureless, graded, silty mudstones lacking dropstones but with abundant large wood fragments. Close to the 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 fjord slopes. We suggest that large solitary waves were generated as the rockfalls entered the water, traveling along the fjord and stripping vegetation from the shoreline. Large amounts of mud and silt were thrown into suspension during these events and subsequently settled 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.

KW - deglaciation

KW - fjord

KW - mass transport

KW - catastrophic

KW - Carboniferous

KW - Pennsylvanian

KW - Argentina

KW - BRITISH-COLUMBIA

KW - WESTERN ARGENTINA

KW - EAST GREENLAND

KW - SAANICH INLET

KW - GRAVITY FLOWS

KW - GLACIER BAY

KW - STRATIGRAPHY

KW - ALASKA

KW - FJORD

KW - PLEISTOCENE

U2 - 10.1130/B25242.1

DO - 10.1130/B25242.1

M3 - Article

VL - 116

SP - 348

EP - 367

JO - Geological Society of America Bulletin

JF - Geological Society of America Bulletin

SN - 0016-7606

ER -