Uwagi o stratygrafii senonu niecki północno sudeckiej

Abstract

Przedstawiono dyskusję z poglądem J. Milewicza na stratygrafię senonu niecki północnosudeckiej oraz próbę sformułowania nowych wariantów podziału stratygraficznego tego regionu. Wykazano kruchość przesłanek palinologicznych, na których oparto precyzyjne ustalenia wieku serii lądowo-brakicznej senonu. Przeprowadzono weryfikację oznaczeń inoceramów .senońskich z otworu Węgliniec IG 1, powodującą zmianę położenia granicy koniak - santon, a także zaproponowano własną interpretację znalezisk przewodniej fauny w otworze Spremberg 101 (NRD). Pozwoliło to na nowe ustalenie stratygrafii morskich osadów senonu niecki p6łnocnosudeckiej i górnołużyckiej.

SOME REMARKS ON STRATIGRAPHV OF THE SENONIAN

IN THE NORTH-SUDETIC BASIN

In the North-Sudetic Basin Senonian deposits are developed in two facies: marine and brackish-limnic-potamogenic. The views on stratigraphy of the two sedimentary sequences and their correlation, presented in works of J. Milewicz (1956 - 1979; syntheses: 1973 and 1979), cannot be treated as well established. It appears impossible to accept a single stratigraphic scheme as paleontological evidence for continental series remains unconvincing. The value of palynocomplexes described from that series by W. Krutsch (1957, 1966) to show its Lower Santonian age is debatable, In the best case the assemblages ,may indicate the age of samples ranging from the Upper Coniacian to Middle Santonian as no index taxa (sensu Synopsis, 1979) for individual substages or stages were found in the deposits. Thus it may be stated that the premises for b10stratigraphic subdivision of the series up to 550 m thick are insufficient (T. Leśniak et aI., 1978).

Several errors have been made in zonation of marine Senonian rocks penetrated by the drilling Węgliniec IG 1 (J. Milewicz et al., 1968). The same may be the case of stratigraphic boundaries drawn in the borehole column Spremberg 101 (R. Musstow, 1968). The revision of inoceramids from the borehole Węgliniec IG 1 showed that the Coniacian/Santonian boundary should be drawn at the depth of 325 m which means decrease in thickness of marine deposits younger than the Coniacian to 165 m in that borehole column (Fig. 1). Assuming that identifications of ammonites and inoceramids from the borehole Spremberg 101 are valid (W. Hailer, 1963) and that the microfloristic method is imprecise, faunal succession in that borehole was reinterpreted and the stage and substage boundaries redrawn. The repeated occurrence of younger and older faunas and the lack of evidence for the Lower Santonian and Upper Coniacian in that borehole column may be explained by tectonic style (Fig. 2) noted in structure of that area, characterized by the presence of compressional half-grabens and half-horsts (J. Oberc, 1972). The occurrence of Coniacian fauna in Santonian deposits at the depths of 445 and 750 m may be explained by redeposition.

The reinterpretation shows that it is not possible to exclude interfingering of lower part of continental-brackish deposits and the marine Upper Coniacian as well as upper part of continental deposits and the marine Upper Santonian (or Lower Campanian). presumably eroded in the borehole column Węgliniec IG 1.

The dating of continental deposits is rather imprecise so several variants of their correlation with different stages developed in marine facies may be proposed (Fig. 3). In the variant I, it. is assumed that SE part of the basin was subjected to uplifting movements and erosion leading to removal of the Upper Coniacian, followed by sedimentation , of deposits of the two facies till the end of the Middle or Late Santonian. In the variant II, neither sedimentary gap nor erosion of Upper Coniacian rocks are assumed but rather an uplift of SE part of the basin, followed by deposition of continental facies deposits from the beginning of the Santonian. The deposits, occurring there in the same succession, are gradually passing to NW into those of the marine facies. The variant III involves gradual uplift and migration of continental facies to NW to comprise eastern part of the basin in the Coniacian and the Lusatia in the Late Santonian. The variant IV adiminits development of marine facies throughout the basin in the Late Coniacian and Early Santonian. SE part of the basin became uplifted at the end of Early Santonian which resulted in erosion of sediments down to Middle Coniacian sandstones. The subsequent continental sedimentation covered various members of marine Coniacian or Lower Santonian. In this interpretation, marine equivalents of continental facies would be confined to the Lusatia only.

The above presented four possibilities of interpretation of geologicalal and paleontological data do , not exhaust all the possibilities.·The variants may be also modified by some other factors, including the actual age of the youngest Senonian deposits (Upper Senonian or Lower Campanian) or the extent of post-Senonian erosion.