Przekątne uwarstwienie niektórych skał fliszowych

Abstract

Current bedding in Carphatian Flysch The Carpathian Flysch formation is composed of alternating sandstones and Shales with subordinate beds of conglomerates and marls. This formation shows graded bedding in most beds. A bed of sandstone commences with coarse, often conglomeratic material, which passes upwards into fine grades. Besides this „simple“ graded bedding a „composite“ graded bedding may be distinguished in some parts of the Flysch series; it is characterized by alternating bands of graded bedding2) inside one bed of sandstone. Graded bedding according to E. B. Bailey1) has been formed on the bottom on which no appreciable currents were operative. According to this writer, graded bedding characterizes géosynclinal sedimentation. Usually graded bedding occurs in deposits in which current bedding is absent. Therefore it is interesting to note that some of Flysch beds possess also well marked current bedding, although most of Flysch beds exhibits only graded bedding. There are two series, both of the Lower Cretaceous age in which current bedding appears: the Cieszyn (Teschen) shales (Valanginian) and the Lgota .beds (Aptian-Lowest Albian). Current bedding is very frequent in the Eocene Beloveza beds,, in sandstones of the Menilite series, it also occurs in Magura sandstones. The Upper Cieszyn shales consist of dark thin-bedded, well laminated and often corrugated calcareous sandstones alternating with black shales. The Lgota beds are composed of banded, often siliceous sandstones and black shales. In both cases sandstones are fine-grained. Some of sandstone beds of these two series show current bedding. Lamination bands, slightly curved, run obliquely through the bed and approach the lower surface of the bed tangentially, at a very small angle, while the upper surface truncates lamination bands at a larger angle. This type of current bedding may be termed „normal“ as opposite to the „torrential“ current bedding.1 The upper surface is evidently due to submarine erosion and the contact with overlying shale is sharp and discontinuous, while the upper surface of graded sandstone usually exhibits a transition contact toward the covering shale In some beds several sets of current bedding may be observed. Lamination bands at a distance from the lower surface are truncated and another set of laminae is surimposed. This may be repeated in one bed several times (fig. 1, comp. Polish text). In a few instances current bedding is marked only in the lower portion of a sandstone bed; it dissappears upward and bedding becomes more or less regular. There are also cases in which normal bedding is marked in the lowest and uppermost portions of the bed, while the intervening part shows current bedding. In all examined instances the direction of inclination of foreset laminae is fairly constant in one bed what points to a constant direction of currents. The foreset angle is always small what according to W. H. Twenhofel3) indicates strong currents and small supply of material. At present no sufficient observations are available for more precise determination of current direction. The presence of current bedding in some beds of the Cretaceous series indicates that these series must have occasionally been deposited under the direct action of currents. As these series are of marine origin, one must assume that bottom currents have been responsible for this type of bedding. Very likely one should adscribe this current bedding to un dertow regularly working on the sea bottom. Conditions under which sandbars are deposited cannot be accepted because the sediments of this kind show great irregularities in crossbedding 4). It is most probable that beds with current bedding have been deposited at a distance from the shore. Principally the Cieszyn and Lgota beds must have been laid down on a bottom without currents as they mostly possess graded bedding, but if sedimentation had built up the bottom to the base level, sediments with current bedding could have been formed. Flysch sediments have been deposited in a basin slowly but continuously subsiding5), therefore the bottom was after a certain time sufficiently lowered and again graded bedding could prevail. Anyway one may assume that the Cieszyn and Lgota beds have been deposited on a bottom lying below but on the whole in the proximity of the wave base. This may serve to explain banded bedding of the Lgota beds. Sandstones of these beds are characterized by a very well marked lamination. They are composed of light and dark bands, usually very thin. Generally light bands are less finegrained than dark laminae. Shales alternating with sandstone beds are always dark what points to bad aeration of bottom waters. The banded bedding of sandstones can be interpreted, partially after J. Barrel6), that the sandy sediment was stirred up from time to time by occasional storms and finer particles could be washed out and subsequently deposited again with a certain lag. This interpretation can be adopted if the Lgota beds may be considered as being formed near the wave base. The rhytmical lamination of the Lgota beds may thus be referred to some seasonal changes. If this rhytm is due to annual changes, each pair of light and dark bands may correspond to one year. One pair of lamirae in the Lgota beds is in average 0,80—1,2, sometimes several mm thick; this value is greater than values usually quoted, but obviously is due to abnormal rate of Flysch sedimentation which in any case seems to have been greater than the rate of deposition in other areas. Recently F. J. Pettijohn7) has given an estimation of deposition rates in different areas. Compared with his data the rate of sedimentation of the Lgota beds occupies an intermediate position between the geosynclinal and „platform“ facies. The fine-grained character of the Lgota beds indicates that their sedimentation was slower than of other Carpathian Flysch beds. Especially rapid rate of sedimentation should be expected in the Middle and Upper Cretaceous beds (Godula and Istebna beds). The presence of currents in the Flysch sea is also pointed out in another way. It seems that most of the fossilbearing layers in the Flysch series owe its origin to violent currents which have burried shells together with coarse material. It may generally be obserwed that fossils occur in conglomeratic layers and are very rare in fine-grained sediments. If in a series large foraminifera occur (Nummulites , Discocyclina) , conglomeratic layers contain as a rule larger and abundant foraminifera, on the contrary fine-grained layers contain smaller and less abundant foraminifera. It appears that the distribution of large foraminifera is governed to some extent by the same principle as of clastic particles, which if coarser, accumulate on the bottom of greater agitation of water, while the finer material is deposited in parts of stiller water8). 1) New light on sedimentation and tectonics. Geol. Mag., 67, 1930, p. 88. 14 . Sedimentation in relation to tectonics. Dull. Geol. Soc. Amer. 47, 1936. 2) M. P. Billings. Structural Geology N. York 1946. 3) Treatise on sedimentation, 2nd ed, 1932, p. 618. 4) W. H. Twenhofel, 1. c. p. 620. 5) R. Zuber. Flysch and Petroleum. Lwow 1918, p. 306 (in Polish) 6) Rhytms and the measurement of geologic time. Dull. Soc. Geol-Amer. 28, 1917. 7) F. J. Pettijohn. Archaean sedimentation. Dull. Soc. Geol. Amer. 54, 1943, p. 967. 2 P. D. Trask. Sedimentation in the Channel Islands region, Calif. Econ. Geol. 26, 1931, p. 32-33.