CONSIDERAZIONI SUI CRITERI DI CORRELAZIONE TRA UNITA’ STRATIGRAFICHE CONTINENTALI QUATERNARIE

Abstract

Carraro F. & Giardino M., A few remarks on the use of correlation criteria among Quaternary continental stratigraphic
units. (IT ISSN 0394-3356, 2004).
Starting from a general definition of the term “correlation” (“demonstration of the lithological or chronological equivalence of two or more geological phenomena in different areas, by means of paleontological or physical evidence”) a discussion will be developed on the topic of stratigraphic correlations, which can be considered a “blind area” in the field of modern Quaternary Geology. In the Italian geological literature concerning continental Quaternary, correlation are frequently presented between stratigraphic units l.s. (meant as sedimentary bodies or erosional surfaces) of different areas, or between stratigraphic units s.l. and climatic or tectonic “episodes” (events meant as time intervals of various length, during which certain conditions started and persisted). Correlations can in turn be based on lithofacies, stratigraphic position or chronological elements of reference. By analysing the logical development of scientific papers presenting correlation hypothesis, it can be drawn that correlations are often based on one or more than one of some arguments:

- a stratigraphic succession is an almost continuous record of geological evolution of an area, in the sense of a series of sedimentary
bodies and erosional surfaces representing the whole group of sedimentary and erosional events taken place through time in that area;
- the time interval of formation of a stratigraphic unit can be compared by duration to the time interval of a climatic or tectonic event;
- the diacronicity (i.e. the difference between the minimum and maximum age) of the whole lenght of bottoms and tops of a stratigraphic unit s.l. is much shorter than time interval of a climatic or tectonic event;
- the stratigraphical units trace back to a single distinct climatic or tectonic event, in the sense of an event excluding the other;
- the available chronological elements are always characterized by absolute values and good resolution.
In this paper arguments are critically analysed, and some consideration are derived. In the majority of cases, continental successions are quite full of lacunae, in the sense they do not represent a complete collection of sedimentary or erosional events describing the whole geological history of the area; stratigraphic units of different rank can correspond to quite different time intervals. Also the duration of different climatic events controlling depositional or erosional processes can be quite different. Up to date studies on recent and active tectonics are now showing the validity of the above considerations also for interpreting syn-sedimentary deformations. In geodynamic environments characterized by a mobility of factors controlling the continental sedimentary system, in addition to the dynamic role of exogenetic and endogenetic factors, a “static” control has to be considered, due to different erodibility of the through-time outcropping rock and sediment units. This latter is responsible for the strong inertia of the system response to the inputs from the first groups of conditioning factors. It derives a strong diacronicity along the whole lenght of bottoms and tops of sedimentary units and erosional surfaces; it also derives that stratigraphic units of the same rank can correspond to very different time intervals. In the paper field data and interpretations are presented and graphically illustrated to show how very different climatic or tectonic events can be testified by stratigraphic units of the same rank.Then the real significance of numerical datings is analysed: these can be affected by three types of conditionings, the most important being the effective associations of dating elements to the events to be dated. The second is represented by the possible openings of the geochemical system. The third conditioning is the unavoidable errors of laboratory measures.
From the above presented arguments it can be sufficiently documented that correlations based on a low number of dating elements
are weak and open to challenge. In the tentative correlations is therefore suitable to go further only to the correlation of single sedimentary bodies containing dating elements, not to the whole sedimentary units, less than ever to the whole succession that comprises it. Any other effort will remain an attempt and it is opportune to be presented as such in synthesis papers.