Original paper

Das Niedersächsische Oberjura-Becken -- Ergebnisse interdisziplinärer Zusammenarbeit

[The Upper Jurassic of the Lower Saxony Basin -- results of interdisciplinary team-work.]

Gramann, Franz; Heunisch, Carmen; Klassen, Horst; Kockel, Franz; Dulce, Gesine; Harms, Franz-Jürgen; Katschorek, Tanja; Mönnig, Eckhard; Schudack, Michael; Schudack, Ulla; Thies, Detlev; Weiss, Michael; Hinze, Carsten

Kurzfassung

Für die Entwicklung des Niedersächsischen Beckens ist der Oberjura ein sehr wichtiger Zeitabschnitt, da das bislang relativ einheitliche Becken durch differenzierte Rift-Prozesse intensiv in lokale Strukturen gegliedert wurde, die sich durch große Mächtigkeitsunterschiede und engräumige Faziesvariationen unterscheiden. Die sich aus dieser geologischen Situation ergebenden Probleme der Stratigraphie, Fazies und Tektonik sollten im Rahmen einer interdisziplinären Arbeitsgruppe gelöst werden. So konnten Fragen der Paläogeographie und synsedimentären Tektonik durch die lithostratigraphische Neugliederung von 1340 Bohrungen und die Auswertung reflexionsseismischer Daten geklärt werden. Im Rahmen der paläontologischen Untersuchungen wurden eine Ammoniten-Zonierung für das Oxfordium, eine Zonierung mithilfe von Ostracoden in 23 Ostracodenzonen, Zonierungen durch Charophyten, Dinoflagellaten-Zysten, Sporen und Pollen für die nordwestdeutsche Malm-Gruppe (Oberjura plus Oberer Münder Mergel und Serpulit) erarbeitet sowie der stratigraphische Wert von Mikrovertebratenresten im "Kimmeridge" festgestellt. Unter Einbeziehung der palökologischen Faktoren (Fossilien, Sedimentstrukturen) und Hinweise auf Meeresspiegelschwankungen konnte ein umfassendes Bild der paläogeographischen Entwicklung des entworfen werden.

Abstract

The Late Jurassic times are very important for the development of the Lower Saxony Basin of NW Germany. During Late Jurassic times, the Lower Saxony Basin, in which previously rather uniform conditions pertained, became subdivided by synsedimentary rifting into several smaller units. These differ considerably in the thickness and facies development of the individual stratigraphic units. Coordinated interdisciplinary studies have been carried out to tackle the resulting stratigraphic, sedimentological and tectonic problems. Palaeogeography and the influence of synsedimentary tectonics were studied by correlating lithostratigraphic data from 1340 well logs and reflection-seismic data. The ammonoid-based zonal scheme for the local stratigraphic units of the Oxfordian has been revised on the basis of outcrops. Correlation of ammonite zones younger than the Oxfordian are problematic because of the extreme scarcity of identifiable species. Only the Eudoxus Zone of the Kimmeridgian is well documented. The latest unit to be dated by ammonoids is the Gigas-Schichten (Gigas Formation) of Tithonian age. The whole of the Upper Jurassic to lowest Cretaceous sedimentary succession, known as the "Malm" (Malm Group) in NW Germany, can now be subdivided by means of 23 formally defined ostracod zones. Zonal schemes based on charophyte remains, dinoflagellate cysts, spores and pollen grains have been developed and tested. Dinoflagellate cysts have been found to be very useful for subdividing the Oxfordian and earliest Kimmeridgian, whereas spores and pollen grains are more useful for the late Kimmeridgian and Tithonian to Berriasian. This also reflects the departure from normal marine conditions in the Lower Saxony Basin. The distribution of important foraminifera is shown in a range table. Alvaeosepta and Everticyclammina have a similiar distribution as in Oxfordian-Kimmeridgian shallow-water sediments of the Swiss Jura Mountains. The stratigraphic potential of vertebrate fossils, mainly isolated teeth of microscopic size, was also studied. A comprehensive geological and palaeogeographical history of the Upper Jurassic has been worked out on the basis of palaeontological data, sedimentary structures, characteristic lithologies, and unconformities. In the Lower Saxony Basin, the Upper Jurassic lithostratigraphic units are usually separated from the underlying rocks by a Late Callovian to Early Oxfordian hiatus. In restricted areas, the Callovian "Ornatenton", a shaly mudstone sequence, continues up into the Early Oxfordian Mariae Zone. The "Heersumer Schichten" (Heersum Formation) of Early to Middle Oxfordian age is developed as a spotted marine mudstone sequence, which is strongly bioturbate and increasingly rich in sponge spicules towards the top. The next unit, the Middle to Upper Oxfordian, the "Korallenoolith" (Korallenoolith Formation or Group), is restricted to the eastern part of the Lower Saxony Basin. Oolitic shelf limestones interfinger with minor ferruginous oolites and bioclastic limestones locally including thin patchy reefs of scleractinian corals. In the western part of the basin, this limestone facies is missing, either due to replacement by deltaic sands or because of non-deposition. The Oxfordian/Kimmeridgian boundary is difficult to locate accurately, as ammonoids have become extremely rare. It may be traced within the "Humeralis-Schichten", the uppermost unit of the "Korallenoolith", at least with the help of ostracods and palymorphs. In NW Germany, the so-called "Kimmeridge" comprises three formations. Typical for the "Unterer Kimmeridge" are alternations of marly shales with sublithographic or detrital limestones. Salinity variations are documented by the local occurrence of anhydrite in drill cores and mine galleries. Reduced salinity or freshwater input is indicated by the appearance of charophyte remains, as well as certain genera of ostracods, fishes and gastropods in some layers. A return to more stable marine salinities, probably as a result of sea-level rise, has a biotic response and is also documented by transgressive onlaps of the "Mittlerer Kimmeridge". The main part of the "Mittlerer Kimmeridge" belongs to the Eudoxus Zone of ammonoids. The "Mittlerer Kimmeridge" extends far to the west of the basin to the outcrop area, where the "Korallenoolith" is missing. The "Oberer Kimmeridge" has been found to contain Aulacostephanus contejeani (Thurmann) the index fossil of the latest Subzone within the Eudoxus Zone. The next coastal onlap is at the base of the "Gigas-Schichten", which are Tithonian in age. The highly variable fossil Gravesia gigas has been collected within this formation. The main lithology is bioclastic to oolitic limestones. They are overlain by a formation of dark fissile shaly limestones called the "Eimbeckhäuser Plattenkalk", probably of lagoonal origin. Land-locked conditions become prominent in the "Münder Mergel" (Münder Formation). Red-bed clays and evaporites including rock salt are the main lithologies. They are interrupted only in the middle part of the formation by oolites, or serpulite or stromatolitic limestones. The first occurrence of certain Cypridea (ostracods) was recorded in the upper part of the Münder Formation, in the Katzberg Member. Finally, a coastal onlap and calcareous sediments are known from the outcrop area.