Browsing by Author "Seppelt, Stephan"

A temporary outcrop in southern Lower Saxony permitted the sedimentological, geochemical and palaeontological investigation of a 40.8 m thick Toarcian section, from the top of the Amaltheenton Formation, through the Posidonienschiefer and Jurensismergel Formations, to lower parts of the Opalinuston Formation. Bed by bed collected ammonites and belemnites, bivalve associations, as well as data from neighbouring sections indicate a largely complete sequence of ammonite zones and subzones for the Lower Toarcian. A prominent stratigraphic gap at the Posidonienschiefer/Jurensismergel Formation boundary probably comprises the Semipolitum Subzone as well as the Variabilis and Thouarsense Zones. Above a condensed Dispansum Zone follows the higher Upper Toarcian with a presumably largely complete sequence of zones and subzones, although direct evidence for this is only sporadic. However, a thin condensed bed with stromatolite crusts is recognisable at the boundary Pseudoradiosa to Mactra/Aalensis Subzone. The Toarcian/Aalenian boundary can only be drawn on basis of belemnite finds at another thin condensed bed. Only a few metres above, the Opalinum Zone is evident by ammonite findings. Based on discontinuities, lithofacies, biofacies and correlations with neighbouring sections, a subdivision into alloformations, which largely correspond to formations, is applied. Based on that, a sequence stratigraphic interpretation with respect to third order transgression-regression cycles (T-R sequences) can be inferred: Above the regressive upper parts alloformation 1 (Amaltheenton Formation) with a maximum regression surface (mrs) near its top, the T-R sequence of the alloformation 2 (Posidonienschiefer Formation) is developed, with a maximum flooding surface (mfs) at the transition Falciferum/Commune Subzone and the regressive phase within the later Bifrons Zone. For the Commune Subzone, belemnite alignment indicates a seawater bottom current from SSE. The following maximum regression surface (mrs) lies near the Bifrons/Variabilis Zone boundary. The next sequence is not preserved at the studied location, but is preserved further East as well as further West, represented by the transgressive Dörnten Member (Variabilis and Thouarsense Zone). However, the regressive phase (Fallaciosum Subzone) is also missing there, indicated by a prominent sequence boundary with erosional relief at the base of the Dispansum Zone. The following alloformation 3 (Jurensismergel Formation and lowermost parts Opalinuston Formation) represents another T-R sequence with a maximum transgressive surface (base Mactra/Aalensis subzone) and a slightly thicker regressive Aalensis Subzone. The following maximum regression surface represents the boundary to alloformation 4 (major parts of Opalinuston Formation), followed again by a short transgressive phase (Pseudolotharingicum Subzone), condensation horizon and a longer regressive phase (Opalinum Zone). These sequence stratigraphic interpretations are largely consistent with previous investigations in Northern and Southern Germany. Minor deviations in the timely position of maximum flooding and regression surfaces likely reflect effects of a higher subsidence at variable sedimentation rate in the North German Basin. With respect to the, at the site of investigation, incompletely exposed Opalinuston Formation, further studies on complete drill core sections are required.

Die Exkursion stellt repräsentative Aufschlüsse triassischer und jurassischer Sedimente aus der direkten Umgebung von Göttingen vor. Dabei werden deren stratigraphische Stellung, Ablagerungsmilieus und Fossilgemeinschaften diskutiert. Zunächst werden Aufschlüsse auf der östlichen Schulter des Leinetalgrabens (Muschelkalk- Plateau des Göttinger Waldes), danach auch im Leinetalgraben selbst (Keuper, Schwarzjura) besucht. Hinreichend gute Aufschlußbedingungen sind vor allem in Tongruben (Röt, Mittlerer Keuper, Schwarzjura) und Steinbrüchen (Unterer Muschelkalk) gegeben.

Slope instabilities and reinstatement of the Ludwigskanal ("Ludwig Canal") cutting near Dorlbach permitted a re-investigation of the poorly documented historic Lower Jurassic "Schwarzjura" section formerly exposed in 1840-1841 during the construction of the canal. Hence, for the first time, a detailed description of the exposed strata can be provided for this locality, covering the top 9 m of the Amaltheenton Formation, the 2.1 m thick Posidonienschiefer Formation, the 3.25 m thick Jurensismergel Formation, and the basal 1.3 m of the Opalinuston Formation. About 40 historic fossil findings described and illustrated from this locality by the famous palaeontologist Friedrich August Quenstedt in his monographs can now be assigned to specific beds In addition, new findings of semi-articulated ichthyosaur skeletons in the Bifrons Zone substantiate the conjecture that a 1.6 m sized Temnodontosaurus skull, excavated in 1841 at the Ludwigskanal, comes from bituminous marls of the Bifrons Zone. Apart from the historic fossil findings, the Ludwigskanal section is of special interest with respect to hypothesis on the seawater circulation system of the Lower Jurassic NW-European epicontinental sea. After sedimentation of the Upper Pliensbachian Amaltheenton Formation (with boreal faunal elements), coast-Parallel strong currents from north could have formed a submarine erosional relief including the "Altdorf high", where the condensed, limestone-dominated Posidonienschiefer Formation exposed in the Ludwigskanal section accumulated during Early Toarcian. Later, early during the Late Toarcian, bottom currents from southwest caused slow deposition or even temporary erosion, documented by current-aligned rostra of the "belemnite battlefield".

The morphologically conspicuous bivalve Oxytoma (Palmoxytoma) cygnipes (Young & Bird, 1822), known for its palaeogeographically bipolar distribution, from a limestone bed in the boundary "Belemniten-Schichten"/Amaltheenton formation, Lower Jurassic, in N Germany is described. The occurrence of this palaeoceanographically significant bivalve points to an influx of cool seawater from the Arctic to the North-German Basin at the base of the Upper Pliensbachian, just before the deposition of the Amaltheenton formation. A review of previously reported occurrences on the NW European Shelf indicates two distinct stratigraphic intervals of occurrence of this taxon: the Rhaetian-Hettangian boundary and the Upper Pliensbachian. Whereas the former interval of occurrence may be related to short-term cooling in the course of the end-Triassic extinction event, the latter is interpreted as reflecting the influx of a cool water current to the eastern part of the NW European Shelf, which continued southwards parallel to the coast of the Bohemian-Vindelician High.