Abstract

BLASI, Adriana M; ZARATE, Marcelo A  and  KEMP, Rob M. Quaternary sedimentation and pedogenesis in the northeast of the pampa bonaerense: a case study from Gorina locality. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.1, pp.77-92. ISSN 1853-6360.

The Gorina locality is representative of the Quaternary sedimentary record of the northeastern Pampas of Buenos Aires province. The main goals of this contribution are to analyze the composition (grain-size and mineralogy) of the deposits and discuss their nature and provenance. The general balance and interaction of the sedimentation process with pedogenesis and erosion are also evaluated. The studied section (middle to late Pleistocene and Holocene) is a quarry exposure about 11 meters thick. It was sampled at approximately 25 cm intervals for grain size, mineralogical and micromorphological analyses. The micromorphological results are fully discussed in a separate contribution (Zárate et al., 2000). Pedological features are common throughout the succession. Thus, a combined macromorphological and micromorphological approach was used to identify and differentiate paleosols. Five pedological units (from top to bottom Pu1, Pu2, Pu3, Pu4, Pu5), apparently representing four discrete paleosols plus the surface soil, were initially differentiated on the basis of field-scale morphological properties (i.e., color, texture, structure, consistency, boundaries) following the approach applied by other authors in the region. Micromorphologically, the presence of excremental fabrics throughout the vertical column suggests that bioturbation processes were active as the sediment accreted with buildup of the surface resulting in conversion of A into B and C horizons (Zárate et al., 2000). The micromorphological data do not support the field differentiation of Pu4, Pu3, and Pu2. Pedological features (e.g. excrements, secondary carbonate coatings, illuvial clay coatings) occur throughout without any obvious breaks or patterns that might warrant the separation into discrete paleosols. This ca. 8 m sequence is interpreted as an accretionary and/or welded pedocomplex with superimposition of secondary carbonate on illuvial clay features and illuvial clay coatings on excrements as the surface built up and horizons became transformed (Zárate et al., submitted). According to their grain size composition, the lower two units (Pu5, Pu4) are composed of sandy silt textural facies, whilst the upper three units (Pu3, Pu2, Pu1) have mud and silt textural facies (Folk, 1954). The coarse/fine ratios (g/f 31μm and g/f 15μm) indicate a progressive grain size decrease from the lower part of Pu4 upwards with dominance of finetextured sediments in the upper three units (Pu3, Pu2, Pu1). The muds are unimodal (modes at coarse and medium silt) consisting of up to 30-50% fine clay and colloidal clay. The silty textural facies are characterized by a non-prominent mode (coarse silt fraction) including up to 20% of fine clay and colloidal clay. The sandy silt facies are unimodal and can be divided into coarse sandy silts (mode at the very fine sand fraction) and fine sandy silts (mode at the coarse silt fraction). The mineralogical composition of the very fine to coarse sand fractions consists of both deformed quartz and feldspar, microcrystalline quartz and garnet derived from metamorphic rocks; microcline, micas and quartz from granitic rocks along with volcanic lithics and pyroxenes from basaltic extrusive and intrusive rocks. Rare fresh volcanic shards occur throughout the succession, apart from in the lower parts of Pu4 and Pu1 where they are dominant and are probably related to volcanic ash falls. The clay fraction is composed of illite, smectite, kaolinite and interstratified illite-smectite. The illite, dominant at Pu1, Pu4 and Pu5; smectite-interstratified illite-smectite, predominant in Pu2; kaolinite very rare, though relatively constant in every unit. In Pu3, the abundance of illite and smectite-interstratified illite-smectite are comparable. Both illite and smectite display low crystallinity, particularly in the lower units (Pu3, Pu4, Pu5). The sedimentation of the parent material of Pu5 was related to a supply of eolian dust, characteristic of a typical loess (mean grain size: medium silt). Sandy loess (mean grain size: coarse silt) accumulated during the deposition of the lower part of Pu4, changing gradually to finer textured loess upwards (typical loess-clayey loess). The upper horizon (Ab) of Pu4 coincides with the transition between the lower coarser and the upper finer textures of the succession. The Pu2 and Pu3 sedimentary interval appears to represent a relatively continuous sedimentation process with no significant changes in the sedimentary supply corresponding to a fine eolian dust (mean grain size: fine to very fine silt). The accumulation of this fine eolian dust (clayey loess) was also dominant during the time interval recorded by Pu1. At some levels of the succession, aqueous transport (hillslope wash?) played a secondary role in the reworking of the sediments. The dominant granitic and metamorphic mineralogical composition of the very fine sand fraction suggests a source area related to crystalline basement rocks which might correspond to the Precambrian bedrock of Uruguay, the Paraná Basin and/or Sierras Pampeanas of Córdoba and San Luis. Volcanic ash falls from distant Andean sources played a secondary role except at two stratigraphic levels. The origin of the clay minerals could not be established, though other authors have related them to either neoformation or direct derivation from source areas. The Quaternary sedimentary succession of Gorina resulted from the accretion of coarse to fine eolian dust. Pedogenesis was simultaneous with the sedimentary process throughout the succession with two significant depositional hiatus recorded by Pu5 and Pu1 and two erosional episodes that stripped away the surfaces of both Pu5 and Pu2. The interval represented by Pu2, Pu3 and Pu4 might reflect relatively continuous sedimentation. Alternatively, welding of pedological profiles may have masked any evidence for major depositional hiatuses.

Keywords : Sedimentation; Pedogenesis; Paleosols; Quaternary; Buenos Aires.