Biostratigraphy and sedimentary sequences of the Toarcian Hainberg section (Northwestern Harz foreland, Northern Germany)

Abstract

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.