Problemy formalnej litostratygrafii czwartorzędu w Polsce

Abstract

Omówiono różne aspekty formalizacji jednostek litostratygraficznych czwartorzędu. podkreślając zagadnienia, któłre muszą być potraktowane odmiennie niż w odniesieniu do utworów przedczwartorzędowych. Dyskusję zilustrowano przykładem kreowania formalnej jednostki litostratygraficznej, tj. formacji z Ferdynandowa (fm).THE QUESTIONS OF FORMAL LITHOSTRATIGRAPHY OF THE QUATERNARY IN POLAND The principles of formal lithostratigraphy of the Quaternary are highly consistent with those proposed for pre-Quaternary strata (Zasady polskiej klasyfikacji ... , 1975) in basic stratigraphic categories. The differences are merely limited to conceptional range of the applied terms and criteria for defining formal lithostratigraphic units. The principles for cretation of formal lithostratigraphic units of the Quaternary are illustrated with the proposal of the Ferdynandów Formation. By definition this unit comprises lacustrine sediments of the Mesopleistocene age and the sedimentary sequence is built of sands, muds, bitumen-bearging clays, diatomous earth, gyttja, and peats accumulated in the Ferdynandów Lake. The maximum thickness of these sediments exceeds 27 m but they are usually thinner. Their top surface is situated at about 135 m a.s.l. in central part of the reservoir and about 142 - 148 m a.s.l. in marginal parts. The stratotype (holostratotype) section is situated in the vicinities of Ferdynandów village, being based on the borehole column B and those of the boreholes A and C (Figs. 1. 2), situated about 8 m north of the Wieprz River near Jeziorzany village and about 15 km north-west of Kock town, Lublin voivodeship. The Ferdynandów Formation comprises sediments from the depth interval 35.00-53.75 m in the section B (holostratotype), that is the complex no. VI (Fig. 2). The above example clearly shows that differentiation of a lithostratigraphic unit is mainly based on genetic (lithogenetic) criterion whereas purely lithological determinations of the type of sediment (e.g. sand or clay) are treated as of subordinate importance. This is the first major conceptional difference in relation to the principles of formal lithostratigraphy of pre-Quaternary rocks. In the orthodox approach it is foreseen that the whole geological sections will be formalized as stratigraphic units. This may be accomplished in the case of marine Quaternary sediments but appears inexecutable and sometimes even impractical in that of continental sediments predominating in Poland. The latter are characterized by very high genetic-facies variability in all the directions and their numerous varieties usually differ in lithostratigraphic value. Moraine tills, loesses, and lacustrine and marine sediments may be treated as good bearers of lithostratigraphic information whereas fluvial and fluvioglacial sediments are rather of local value and deluvial and others may be of negligible importance for lithostratigraphic correlations. Moreover, Quaternary sequences are usually characterized by numerous sedimentary gaps (breaks in sedimentation and erosional boundaries) or even angular unconformities related to glacitectonic processes. Therefore, the time span of sedimentary gaps usually exceeds greatly that of sedimentation of layers preserved in a section. This creates a major obstacle in formalization of whole geological sections of the Quaternary as well as the recently proposed treatment of the whole complex of sediments of a given glaciation as the basic lithostratigraphic unit, that is formation. Genetic features of a given sediment are characterized by its whole inventory of structural, textural, petrographic, mineralogical and other properties. It is also known that the whole assemblage of features primarily reflects variability of the quantitative type and only exceptionally that of the qualitative type. This makes it necessary to characterize lithostratigraphic units quantitatively and therefore, to carry out both field and laboratory tests. Practically this means that formalization of lithostratigraphic units may be performed not before the relevant sediments are covered by analytical studies. The discussion of criteria for differentiation of lithostratigraphic units shows that there is no need to introduce a new category of stratigraphic units for the Quaternary, that is that of allostratigraphic units.