The Oxfordian and Lower Kimmeridgian of the Peri-Baltic Syneclise (north-eastern Poland): Stratigraphy, ammonites, microfossils (foraminifers, radiolarians), facies, and palaeogeographical implications

The study of ammonite faunas in five boreholes in the Peri-Baltic Syneclise of north-east Poland (Jastarnia IG 1, Krynica Morska IG 1, Pasłęk IG 1, Bartoszyce IG 1, and Gołdap IG 1) gives a basis for the stratigraphical interpretation and correlation of the Oxfordian and Lower Kimmeridgian deposits. It enables also the correlation of the particular ammonite zonations and makes possible the recognition of the uniform Oxfordian/Kimmeridgian boundary. Because of the large number of ammonites collected, fluctuations in the character of the ammonite faunas (Boreal, Subboreal, Submediterranean) in the succession can be recognized. These changes are correlated with those observed in the foraminiferal assemblages – bearing close affinity to the foraminiferal zones of the Russian Platform – but showing in some intervals a marked admixture of forms of southern origin. The Peri-Baltic Syneclise constituted a basin elongated towards the north-east, and bordered to the north by shallow water sandy and carbonate deposits. The development of silt- and clay-dominated facies took place in its more central part, where also a radiolarian-bearing horizon (recognized herein for the first time) was deposited during the earliest Kimmeridgian. The formation of radiolarian deposits was stimulated by tectonic activity which resulted in the strong subsidence of the central part of the Peri-Baltic Syneclise. The radiolarian horizon may be correlated with similar deposits with radiolarians described from sedimentary basins in Russia whose origin was possibly also related to tectonic activity during latest Oxfordian and earliest Kimmeridgian in the northern part of the East European Platform, although the movements were not everywhere strictly time-correlated. The character of the deposits and faunas described strongly suggests that the Peri-Baltic Syneclise during Oxfordian and Kimmeridgian times constituted the main sea connection between the Boreal and Submediterranean seas of Europe.