Analysis of geomorphic dynamics in ancient quartzite landscape using phytolith and carbon isotopes, Espinhaço Mountain Range, Minas Gerais, Brazil

Although there are important landscapes developed on underlying quartzite in Brazil, there is still very little known about the processes and mechanisms involved in their formation. Some recent research has drawn attention to the presence of karstic features associated with silica dissolution in this lithology. However, there are other relief forms and materials commonly encountered in the domain of quartzite in the Espinhaco Mountain Range in the southeast region of Brazil. The main objective of this research is to analyze the geomorphic processes associated with the material overlying relatively flat surfaces of low inclination that always occur near quartzite outcrops in the study area. This is relevant in order to attempt to understand the development of landforms in quartzite domains in tropical humid areas. A Geomorphic approach for the analysis of the material was used, in which geomorphic sites were identified and two soil profiles were described and sampled for laboratory analysis. This was complemented with laboratory analysis of phytoliths found in the soil, 13C analysis and 14C-AMS dating. The relatively shallow profiles show a sequence of similar horizons, in which the material varies from sandy organic material at the top to stony immediately below. The superficial horizons have predominantly finer sand and also clay, which is expected in humid tropical environments. Both profiles present a stony layer formed of angular to subangular quartz pebbles lying just on the top of the weathered quartzite in the C horizons where the oldest SOM 14C dating values were found. Considering the mean residence time (MRT), it is possible to assume that the first depositions in P2 are more than 10.000 yrs old (10,506-10,230 yr cal BP) and that the first profile in P1, located on the upper slope, presents younger SOM (5,919-5,664 yr cal BP). This suggests that the erosive processes were either more intense or more recent on this portion of the slope and that these deeper horizons may have previously been on the surface, when the material was deposited. Also, the high amount of phytoliths which are unclassified due to their destruction, along with the presence of the most robust types, can be interpreted as evidence of intense erosion and transportation of the material by surface runoff, enabling the assumption that the material is colluvium and that its source is a nearby quartzite outcrop. After deposition along the gently inclined slope, identified as a “colluvial ramp”, the material was relatively undisturbed, which allowed pedogenic processes to take place, forming Quartz-sand Neosols. The ramp itself seems to be the result of very slow rates of weathering and removal of the material, together with strong control from relatively flat Mesoproterozoic depositional macro-structures of the underlying rock. This study is one of the first attempts to use phytoliths combined with carbon isotope analysis for geomorphologic purposes, with very interesting results. Their relevance points to their potential for geomorphic studies.