Asociaciones mineralógicas de las arcillas en la Cuenca de Ischigualsto-Ischichuca: Parte II: Perfiles de la Hoyada de Ischigualasto

• Gerardo E. Bossi ^[1]
  1. [1] Fundación e Instituto Miguel Lillo
• Localización: Acta geológica lilloana, ISSN-e 0567-7513, Vol. 11, Nº 4 (Texto completo no disponible), 1970, págs. 73-100
• Idioma: español
• Títulos paralelos:
  • Mineralogic associations of the clays in the Ischigualsto-Ischichuca basin.: Part II: Hoyada of Ischigualsto-Ischichuca
• Texto completo no disponible (Saber más ...)
• Resumen

  • The mineralogic and paleoenvironmental characters of the clay fraction of Triassic continental sediments outcropping in Hoyada de Ischigualasto are described. This paper is the second part of a series devoted to the clay mineralogy and sedimentology of the Ischigualasto- Ischichuca Basin of La Rioja-San Juan provinces of Argentina Ther stratigraphic sequence is composed of five sedimentary formations: Tarjados, Chañares, Los Rastros, Ischigualasto and Los Colorados, from older to younger.

    The clay fraction or Tarjados Fomation (that lays over Caballo Anca Granite) is mainly montmorillonitic. Beidellite, 57-43 quasi-regular illite-beidellite mixed layers, illite and kaolinite are subordinate. The greenish color of the formation (normally red) could be associated to diagenenis link with deposition and compactation on Chañares Formation.

    Chañares Formation is a 40 m shale-tuff grayish horizon with pillow and ball sandstone structures intercalated. The clay fraction is typically beidellitic with minor amounts of montmorillonite and illite.

    Los Rastros Formation is a 400 m bituminous sequence arranged in fluvial cyclo thems. Kaolinite and illite are the main minerals bue the clay fraction of all the tuffaceous horizons is a 57-43 quasiregular illite-beidellite, 70-30 irreguar illite montmorillonite illite, assemblage with no kaolinite. Most evidences (clay and sand fractions petrology) point toward illitization as the principal diagenetic process.

    Ischigualasto Formation consists in gray calcareous and dolomitic sandstones and brownish to greenish shales and shale-tuffs, arranged in fluvial cyclotthems. It could be cosidered a transitional facies between the dark bituminous facies of Los Rastros and the red gypsiferous facies of Los Colorados. The clay mineralogy is quite complex. Up to fice mineral assemblages have been identified. Montmorilonite (high and low charge varieties) is the dominant mineral but kaolinite, chlorite and illite are quite common in the sandstone channel lithopes.

    Los Colorados Formation is a red termnal facies. The sequence is well organized in fluvial cyclothems composed by dark red siltstones and medium to light red sandstones. The clay mineral composition is remarkably uniform. Beidellite-montmorillonite-illite-kaolinite assemblage is the mos common but few samples also bear illite and kaolinite as dominant minerals (channels sandstones).

    All climate implications of these formations indicate a hot dry-humid alternate climate wich is consistent with other evidences (paleontological and sedimentological) already published.

    A megacyclic sequence: red initial facies (Tarjados)-bituminous dark facies (Los Rastros)-red terminall facies (Los Colorados) may be related to cyclic tectonic changes and minor climatic adjustments in the source and basin. Intensity of vulcanism associated (represented by tuff and volcanic sandstones is correlated with this sedimentary-tetonic megacycle.