FIRST REMARKS ON LATE PLEISTOCENE LACUSTRINE DEPOSIT IN THE BERCETO AREA (NORTHERN APENNINES, ITALY)

Abstract

Bertoldi R. et al., First remarks on Late Pleistocene lacustrine deposit in the Berceto area (Northern Apennines, Italy). (IT
ISSN 0394-3356, 2004).
During a geognostic survey performed by the Servizio Difesa del Suolo of the Emilia-Romagna District Administration in correspondence to the Berceto village (Northern Apennines, Parma Province), a lacustrine sequence of about 25 metres was found in the borehole Berceto SPz2.
This paper shows the preliminary results coming from the analysis of the Berceto lacustrine deposit and suggests the morphological
setting that may have contributed to the origin and development of the lake.
The geological and geomorphological surveys highlight that the landslides are the prevalent processes affecting the Berceto slope. The lacustrine sequence is a part of the filling of a trench that lies behind the dormant rotational rock slide involving the upper part of the slope.
The entire core is 50 metres long. Sedimentologic, pollen and microscopic analyses, the latters carried out on thin polished sections
by both optical and scanning electron microscope (SEM), together with radiocardon datings, allowed its division into 7 units and permitted us to obtain their features, ages and some inferences on their formation.
From the top to the bottom of the core, a first unit (0-4.30 m) represents material reworked by the strong anthropic use of the place.
Below it, a level of peat (unit 2; 4.30-6.10 m), represents the ending of the lacustrine basin. It records a floristic phase characteristic of
the middle-lower Postglacial of the Northern Apennines.
The unit 3 (6.10-9.20 m), together with the subsequent units 4 (9.20-17.00 m) and 5 (17.00-31.00 m), represent the real lacustrine
deposit. The presence of laminated structures is the main feature of this sequence. Both clastic graded millimetric laminae and couplets of dark and pale clayey ones are present. They are evident in unit 5 and begin vanishing proceeding towards the top of the
sequence (unit 4), being almost completely destroyed in correspondence to unit 3. The units are affected by deformations of different
types (penecontemporaneous and/or post-depositional) and extent, occurred either very close or far from the water-sediment interface. The lacustrine sequence developed during the Upper Pleistocene.
Below the lacustrine deposit, a breccia (unit 6; 31.00-48.00 m) is present. It represents a weathered cover derived from the rocks outcropping in the area, subsequently involved in mass wasting processes of the slope. It is probably related to the rotational rock slide affecting the western side of the Berceto area, that may have represented the natural dam of the ancient Berceto lake. The uppermost portion of the breccia contains a clayey matrix that may have waterproofed the bottom of the lake. The last unit 7 (48.00-50.00 m) may represent the substratum of the deposit and the Berceto landslide.
On the basis of the results of the whole study, it is possible to refer the formation of the lake starting from a geomorphologic context
of the Berceto slope very similar to the present one. In such a situation, the movements of the Berceto landslide determined the presence of a trench, in which the lake progressively developed. The landslide movements started in the Upper Pleistocene and continued in the Holocene up to the development of the present hollow of Berceto.