Textural Distribution of the Sediments from Some Gully Structures in the Moldavian Platform and Its Role in Gully Formation Processes

Abstract

This paper is part of an ample study about the genesis of gullies, especially about the influence of geochemical and structural
factors which describe the sedimentary deposits. In this stage, it will be discussed the
influence of a gully development on detrital deposits distribution and the main factors
responsible for sorting of primary material. It was taken in the study a gully developed in
geological formations of the Tutova Hills, located nearby Iveşti village. The samples represent
9 points: five of these are active heads, and four are located on the bottom of the gully,
distance of approximately 150 meters. Textural study of sedimentary deposits was done by
collecting a total of 24 samples. The projection of all samples in Shepard diagram (1954) is in
the sand field, the content exceeding 75% in all cases. Granulometric and distributive curves
shows the homogeneity of the deposits and the participation of 0,250-0,056 mm fraction in
cumulative proportions of 70 up to 95%. In transverse profile, it is obvious that the deposits
are sorted with the decreasing of sand content (from 85 to 81%) and constant increasing of silt
fraction (from 15 to 19%). The most likely cause of these variations is the physical and
chemical transport of particles under the action of water infiltration. The action of the latter is
placed in the record transport solution, which is manifested by the appearance at depths of
over a meter of limestone formation, which appears as either impregnation or as concretions
size centimeters). In longitudinal profile, the content variations of different granulometric
fractions are much lower. Researches carried out showed a slight fall of sand contents (from
85 to 81%), in detriment of silt fraction (from 15 to 19%). At clay fraction level it is not a
constant variation, the clay fraction being feeble. These variations are caused mainly by the
meteoric waters that are leaking out in the gully; smaller fractions (silt and clay) remains in
suspension longer than sand fraction and thus, it will be transported along a greater distance.
Other major factors that contribute to the spatial distribution of sediments are the insular
development of vegetation and the distribution of obstacles represented by wall caving or
hydrotechnical developments. The emergence and development of this gully are caused by
several major factors: a fusiform relief covered with heavily eroded soils, faulty farming
practices and the existence of permanent groundwater.
Seen as a whole, textural distribution of sediments in the gully shows a drop in sand fraction
and an increase of the siltic fraction, both in longitudinal and transversal profile. Changes in
the content of clay particles are irregular; these are driven primarily by the vegetation growth
and the existence of physical obstacles with anthropic origin.