Resumen

KROHLING, Daniela  y  ORFEO, Oscar. Sedimentology of loessic units (Late Pleistocene - Holocene) of the center-south of Santa Fe. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.2, pp.135-154. ISSN 1853-6360.

An integral characterization of the outcropping loess-paleosols sequence in a type area of North Pampa is presented, based on the stratigraphic analyses and the grain size and mineralogic variability. Two loess profiles representative of interfluve areas of southern Santa Fe province were selected for detailed sedimentological analyses (Tortugas and Carcarañá Profiles). The loessic cover is part of the quaternary aeolian sedimentary body of the Argentine plain, defined as "Pampean Aeolian System" by Iriondo (1990a) and Iriondo and Kröhling (1995). Both geomorphological and sedimentological systems compose the Pampean Aeolian System: a Sand Sea and a Peripheral Loess Belt. The Loess Belt, about 2,000 Km long and 250-300 Km wide in North Pampa, is located at the northeast of the Sand Sea. The sediments of the Pampean Aeolian System accumulated during the first stages of the last Pleistocene glaciation; later on they were partially reworked during successive dry phases and underwent pedogenesis in humid periods (Iriondo and Kröhling, 1995). The outcropping sedimentary sequence of the area is represented by fine aeolian units interbedded with buried soils. The Tezanos Pinto Fm (Iriondo, 1987) is the typical loessic unit of the North Pampa. In most places (Carcarañá Profile) it lays in erosive contact on a dissipation deposit of aeolian sand (Carcarañá Fm; Kröhling, 1999a). The loess is a loose deposit, silt with subordinated clay and very fine sand, light brown in colour. The loess thickness generally ranges from 4 to 10 m in the interfluves. The loess is a homogeneous, massive and permeable deposit. The sedimentary mass is calcareous; it contains powdery concentrations and hard concretions of CaCO₃. The loess body is crossed by fine rhizoid ramified canalicula and very fine micropores of tubular forms. It is stable in steep walls, in parts altered by subcutaneous subfussion and shaped by columnar disjunction. Two members of the loess unit are differentiated in both profiles. The lower member, 2-3 m thick, has a moderate degree of consolidation and is very calcareous. There are abundant molds of CaCO₃ concretions. Sedimentary structures filling regular hollows are interpreted as originated by aeolian erosion and subsequent alluvial action. Locally there are discontinuous table-like or lense-shaped intercalations of clayey silt, dark brown in colour, strongly structured and including abundant coats of Fe- and Mn-sesquioxides. The lower member was accumulated in subaerial environment and in sectors with imperfect drainage occupied by non-permanent swamps with oxidation periods. The upper member, is a 2-4 m thick, friable, massive, and slightly calcareous loess. Krotovinas appear in the lower part. The upper part includes numerous fossil nests. The upper member characterises a subaerial environment and represents the typical loess of North Pampa. The members are separated by an intraformational unconformity. Locally, a level with abundant CaCO₃ rhizoconcretions was preserved along this unconformity, as an indicator of pedogenesis. Preserved relicts of a well developed buried soil appear in the Tortugas Profile, at the level of discordance. The top of the Tezanos Pinto Fm is marked by a partially eroded soil, 0.40 to 1.60 m thick, typically represented by the Bt- and BC-horizons. It is a moderatelly developed Argillisol; locally the buried soil forms a pedocomplex. The Bt-horizon is formed by a dark brown clayey silt. The common illuvial clay features are represented by clay coatings of root channels. The Bt-horizon has a moderately to well developed structure in fine to medium angular prisms, strong consistence, with very fine and fine cracks between peds. There are abundant root molds and fine tubular micropores. A younger loessic formation lays in erosive discordance on the paleosol above the Tezanos Pinto Fm. It was defined as the San Guillermo Fm by Iriondo (1987). It is composed of a grey silt with scarce proportions of very fine sand and clay. The typical thickness is 0.30 m, with 0.55 m maximum thickness. The unit is massive, non-calcareous, friable, porous and moderately structured in very coarse resistant prisms limited by very fine fissures. It contains numerous root molds, abundant macropores, very fine and fine canalicula and tubes generated by bioturbation. This unit is partially the product of deflation of the A-horizon of the underlying soil and the subsequent deposition of dust. The San Guillermo Fm covers as a continuous mantle the Tezanos Pinto loess; in general culminating the sedimentary sequence of the area. According to thermoluminiscence datings, the lower member of the Tezanos Pinto Fm was deposited during the Last Glacial Maximum (between 36,000 and 16,000 years BP). The upper member was accumulated during the late Pleistocene / early Holocene period equivalent, in broad sense, to the "Younger Dryas" (between 14,000 and 8,000 years BP). The Hypsithermal period -Optimum Climaticumof the Holocene (between 8,000 and 3,500 years BP) is represented by the buried soil developed at the top of the loess. A semiarid pulse occurred during the late Holocene (between 3,500 and 1,400 years BP) generated the deflation of the A-horizon of the soil and the subsequent deposition of the San Guillermo Fm. The average textural composition of the Tezanos Pinto Fm in both profiles is 70% silt with subordinated clay and very fine sand. In general, the median ranges between 4 and 6 Φ; (coarse silt to medium silt, 63-15 µm). According to the values of the mean size of the loessic members, a progressive increase in grain size to the top can be noted. The mean size are bigger than median, indicating abundance of fine materials (positive skewness in the distribution). Typical loess is very poorly to poorly sorted. On the Bt-horizon on top of the loess, the silt fraction diminishes (on average: 55%) and the clay fraction increases (on average 40%) in comparison with the parent material. The mean grain size is located between 7 and 8 Φ;. The San Guillermo Fm is composed of 70% silt, with clay and colloids (25%) and scarce very fine sand (<5%). The mean grain size ranges between 6 and 7 Φ;. Cumulative frequency percent-probability curves of both loessic units normally exhibits three straight-line segments, with truncation points at 4 and 5 Φ;. These include a poorly sorted saltation population, a well sorted suspension population and a poorly sorted suspension population. The sediment of the Tezanos Pinto Fm was principally originated by aeolian suspension; it represents around 70% to 90% of the identified transport mechanisms. Saltation and surface creep processes were scarcely represented. The dominance of materials of volcaniclastic nature from Andean Cordillera is revealed in the modal sand fraction of the Tezanos Pinto Fm in both profiles: polycrystalline quartz, volcanic glass shards derived from acidic rocks, microaglomerates of silt particles composed of quartz and volcanic silica, plagioclases and lithic fragments. The plutonic-metamorphic minerals derived from the Pampean Ranges are minor components: quartz and K-feldspars. The silt skeleton of the typical loess is predominantly quartz, with feldspars as subsidiary minerals. Illite, kaolinite, calcite and allophane are minor constituents. The mineralogy of the clay fraction of the loess is dominated by quartz, with important percentages of feldspars and illite and subordinated smectites and kaolinite. The colloidal fraction is composed of a mixture of poorly crystallised minerals. Illite is the dominant mineral, with scarce smectites, feldspars, quartz and amorphous silica-allophane.

Palabras clave : Loess; Pleistocene; Holocene; Grain size analyses; Mineralogy; North Pampa.

        · resumen en Español     · texto en Español     · Español ( pdf )