Quartz microtextures and microstructures owing to deformation of glaciolacustrine sediments in the northern Venezuelan Andes

W. C. Mahaney, R. W. Dirszowsky, M. W. Milner, J. Menzies, A. Stewart, V. Kalm, M. Bezada

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Glaciolacustrine sediments, ranging in age from Early to Late Mérida (Wisconsinan) in sections in the northern Venezuelan Andes near Pedregal, were studied to assess the effect of smallscale folding and faulting on the surface microtextures of individual grains. At site PED5, faulting of sediments beneath 35 m of Middle to Late Mérida glaciolacustrine, glaciofluvial and probable till deposits, has resulted in a maximum displacement of 1 m and many crushed sand grains are found along the shear zone. These dominantly quartz particles are typically smoothed and/or faceted with surface skins split from parent grains to produce adhering blade or wafer-shaped coarse silts. Faulting with displacement of <40 cm occurs in Late Mérida glaciolacustrine beds at sites PED13 and PED11, beneath ca. 1-5 m of sediment and is thought to result from the removal of lateral support during valley erosion and possibly seismic disturbance. Associated sediments exhibit a greater incidence of smallscale fold structures and convoluted bedding but with similar numbers and types of crushed grains. All of the fault traces examined exhibit microstructural modifications, including microfaulting, folding and multiple domains associated with fluidisation and liquefaction. The microtextures described are distinct from those produced by glacial or glaciofluvial transport processes and potentially may be used to infer glaciotectonic or other post-depositional disturbance of sediments. Rare euhedral quartz grains with distorted crystallographic axes may derive from the shear zone of the regional transform Boconó Fault, although many of these grains appear to be overprinted by glacial and/or recent deformation processes.

Original languageEnglish (US)
Pages (from-to)23-33
Number of pages11
JournalJournal of Quaternary Science
Issue number1
StatePublished - Jan 1 2004


  • Deformation
  • Glaciolacustrine
  • Microstructures
  • Microtextures
  • Venezuela

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