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

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.