Geomorphology of paleosurfaces in the sierras de comechingones, central pampean ranges, Argentina

Abstract

Remnants of pre-Andean erosion paleosurfaces have been already described by several authors in different sectors of the cratonic area of the Sierras Pampeanas of Argentina. Their origin and age are still under discussion, but in general, it is assumed that they correspond to etchplains and pediplains developed between the Middle-Late Jurassic and the Paleogene. The objective of this chapter is to present a morphological-morphometric characterization of the planation surfaces that are preserved in the summit portions of the Sierra de Comechingones (Sierras Grandes, Córdoba), between 32° 22'-32° 52' S and 64° 45'-64° 57' W, and propose genetic-evolutionary models that would consider the incidence of lithology and fracturing degree on the configuration and preservation of these paleolandforms. These features are located between 2,150 and 1,500 m a.s.l. and they are developed upon the Precambrian gneissic-migmatitic rocks (the Monte Guazú Complex), mylonites and ultramylonites (the Guacha Corral shear zone), and Devonian granites (the Cerro ¡spero Batholith). The latter are accompanied by fluorite epithermal mineralizations (Early Cretaceous). The paleosurfaces were affected by the Andean orogeny and modeled by the Neogene erosion cycle, being partially covered by Quaternary loess units. To complete this analysis, petrographic, structural, stratigraphic, sedimentological, metalogenetic, and geochronological studies were performed both at the local and regional scales, thus generating our own information in the field and in the lab. Based upon topographic sheets, satellite images, and digital elevation models (DEM), the paleosurfaces were identified and mapped, with special attention to the different lithology on which they have been carved, the drainage network and the depth of fluvial incision were defined, the morphometric parameters were obtained (such as slope, area, elevation, drainage density, and mean length of first-order stream channels [Lm1]), and six topographic-geological sections, considered as representative of the regional conditions, were prepared. In the field, the existing landforms and morphology aspects, active processes, and lithology sections, among other parameters, were described. In the metamorphic environment, the erosion surfaces show a homogeneous relief (generally crest-like ridges) with low topographic and morphological variability (slope, 2-4 %; drainage density, Dd < 4; Lm1 = 150-250 m; incision depth, 10-20 m), and, in general, they are better preserved than those corresponding to the granite environment. The latter are more heterogeneous (slope varying between 4.5 and 7.5 %); they exhibit landforms of varied size and type (crests, rounded boulders, tafoni, tors, domes, among other features) and show greater fluvial dissection (Dd, 6.5-7.5; Lm1, 70-110 m; incision depth, 100-110 m). The geomorphological analysis particularly integrated with metalogenetic and geochronological studies of the fluorite deposits and the stratigraphic and sedimentological investigation of the adjacent Cretaceous sedimentary basins allow the following considerations: (a) the studied cratonic area was a positive element of the landscape at least since the Carboniferous-Permian, and since then, it has been the subject of different denudation cycles; (b) previous to the Andean orogeny, there was a very long period of stability in which the tectonic and erosion processes, generally of low activity, favored the development of these erosion surfaces, which began to be denudated much later than these movements, with their remnants located today in the summit or water-divide areas; (c) at least since the Late Cretaceous, the Cerro Áspero Batholith was not being exhumed and, thus, the studied paleosurfaces may be assigned to a Late Cretaceous-Paleogene age; (d) the studied erosion surfaces belong to one single paleosurface and its altitudinal variations are due to the tilting during the Andean events; (e) the paleosurfaces are polygenetic, a result of the oscillation between wetter periods (dominant chemical weathering, channeled fluvial action) and more arid ones (pedimentation), thus conforming spatial arrangement of the palimpsest type; and (e) a marked lithological control on the morphology and degree of preservation of coeval paleosurfaces do exist.