Arkell.1956.Jurassic..

CHAPTER 19
THE BALTIC REGION AND POLAND
THE SOUTHERN BALTIC REGION
Since the Caledonian orogeny the depression partly occupied now by the
southern Baltic Sea has undergone relatively little geological change.
It has remained essentially a broad, shallow depression fringing the
southern part of the ancient Baltic or Fenno-Scandian shield and collecting
sediments (and, in the Pleistocene, glacial debris) resulting from almost
continuous erosion of the crystalline rocks of the shield. At times of high
sea-level, during Jurassic and Cretaceous transgressions, the sea flooded
in from the west. At its periods of maximum extension sea covered all
the stable shelf of north Poland and the Russian platform and connected
NW. Europe with the central Tethys by way of the Caspian basin. At
times of relative elevation of the shield the marginal depression became
a collecting ground of river debris. Deltaic sandstones and shales with
plant-remains and even coal seams were laid down at these times. The
deltaic deposits resemble those which in the Middle Jurassic covered
Yorkshire, but in the Baltic area they have a much greater stratigraphical
range, reflecting the greater proximity of the shield.
The only place where Mesozoic rocks are seen at the surface in direct
contact with the shield is Scania, the southern peninsula of Sweden
(Nathorst, 1910; Troedsson, 1951). Here the Trias with Rhaetic Beds
is followed by Lower Lias, both containing coal measures and marine
intercalations. Most of the coal is Rhaetic, but there are also Pteria
contorta beds. The Hettangian is of mixed marine, brackish and freshwater
facies. During the Lower Sinemurian and Lower Pliensbachian
there were major marine periods with ammonite faunas, but the Upper
Sinemurian is missing. A similar succession occurs on the Island of
Bornholm, but here both the Rhaetic and the marine Lower Sinemurian
are absent. The Pliensbachian marine phase is well developed, however,
and ends with ammonites of the Spinatum Zone. The Lias of Bornholm
contains coal seams and insects. In both Scania and Bornholm it is
overlain by sandstones believed by some to be Wealden.
In Pomerania the earliest marine Lias is Lower Pliensbachian (Ibex
Zone), but borings indicate a south-easterly and easterly extension of the
non-marine sandstone and shale facies with plants under most of the basin
of the Vistula, through northern and central Poland, to the borders of the
Carpathians near Cracow (fig. 74). At Jastrow these beds, underlying
marine Middle Jurassic, are 402 m. thick, and as far east as Heilsberg
the thickness is still 94 m. (Brinkmann, 1927, p. 73 ; Hohne, 1933, p. 73).
The facies ('Baltic') is reminiscent of the Gresten facies of the Alps and
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