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Article 6 - Volume 17/1 - 2008

Paper title:

Preliminary data concern the ravenation processes genesis in some sedimentation structures of The Moldavian Tableland

Pages:

67 - 78

Abstract:

The goal of the study is to establish the contribution of geochemical systems, from sedimentary structures of southern Moldavian Plateau, in triggering and evolution of gully processes. In this purpose, a case study has been carried out on Ivesti gully, in which, the textural and geochemical properties of original subsurface structures had been analyzed. The site is 650 meters long and approximately 35 meters wide located east to Ivesti village along the DN11A road. The analyses where based on 32 samples taken from along the gully and from four control section in order to ensure a better cover. The contribution of geochemical processes in gully formation it refers to alteration of hydrophisycal and mechanical properties of subsurface deposits, when a salinization process is taking place and those deposits contain over 10% sodium ions in cationic exchange capacity. These characteristics lead to a water holding pressure in clay minerals over 140 atm., which determine the beginning of deposits mobility. In this case, the salinization process is not present; the soluble salts concentration is between 17 and 116 mg/100g soil and 205 – 217 salts mg/100g soil along median line of right bluff, downstream from active gully processes. None of the control section contains deposits in saline class. The general ionic formula of soluble salts contained in surface deposits is Ca-Mg-Na-K-NH4, respectively SO4-HCO3-Cl-NO3 alternating with HCO3-SO4-Cl-NO3. The preliminary data regarding the chemistry of sodium, potassium, chlorine, sulfates and hydrogencarbonates shows that in actual deposits, the salts of these ions do not reach the values to start the chemical suffusion processes. In this gully, the most important factor is the nature of the bedrock - a weak cohesive material which allows breaking of the kinetic equilibrium of the slopes (7,5-8°). The process is amplified by the hydrological characteristics of the region. In these conditions, the viscosity of the bedrock decreases and the sand is transported exclusively because of gravitational attraction. In the main section, but especially in the secondary gullies located on the right side of the main section, chemical suffusion processes might be present in the early stages of the gully development, because of the primary salinization of the deposits, now levigated. Also, between other factors that contribute on gully development, its worth to be mentioned the climatic particularities of the entire area, especially precipitation regime, the land use represented by vineyard plantations and the existence of pluvial flow rills from slopes, which can contribute to evolution of actual active processes.

Keywords:

gully, sediments chemistry; soluble salts; exchangeable cations, sediments mobility