The cored Konrad 101 borehole (Cenomanian–Lower Coniacian, Lower Saxony): calibration of surface and subsurface log data for the lower Upper Cretaceous of northern Germany

Abstract

The lower Upper Cretaceous succession of the cored and geophysically logged Konrad 101 borehole (Salzgitter area, Lower Saxony, northern Germany) is 239m thick and can be subdivided into 12 lithological units. The base of the Cenomanian is taken at a facies change from the Upper Albian Flammenmergel towards softer, greenish, marly claystones at a depth of 250m. Sedimentation commenced in the early, but not earliest, Early Cenomanian (lower Mantelliceras mantelli Zone), correlating with the ultimus/Aucellina transgression (decrease to SP peak 0). Important marker beds are the Sponge Beds at SP peak 3, the Pycnodonte Event above SP peak 7, the Facies Change above SP peak 9, and theplenus Bed (SP peak 10). The base of the Turonian is inferred to lie at c.162.4m, whereMytiloides debris becomes common, giving a thickness of c.88m for the Cenomanian Stage. The Rotpläner facies of Early and Middle Turonian age (SP peaks 10 to base of 16) shows a threefold subdivision, and the thickness exceeds that of all known sections. Otherwise unknown in this interval, the Lower Limestone Unit (Upper Turonian) is characterized by slumps and other allochthonites indicating synsedimentary tectonism. Important marker beds are the Weisse Grenzbank (SP peak 13), marls MEand MEchen(base of SP peak 18), as well as tuff TFand the Micraster Event (around SP peak 22). The base of the Coniacian, taken at the first flood of Cremnoceramus ex gr. deformis (29.5m; below SP peak 23), falls within the upper portion of the Grau-Weisse Wechselfolge (marl dominated marl-limestone alternation), and gives a thickness of c.133m for the Turonian Stage. Eleven third-order depositional sequences are recognized. Sequential and event stratigraphical comparison with exposures in northern Germany and the Anglo-Paris Basin shows, especially in the Cenomanian, remarkable correlation. Time-series analysis of geophysical log data indicates stable sedimentation during late transgressive and highstand conditions within individual third-order depositional sequences. For these intervals, periodic cyclicities of the Milankovitch frequency band are recognized and can be used to calculate accumulation rates. This case study permits a detailed stratigraphic subdivision of boreholes, and their integration into existing correlation frameworks of surface data as individual peaks of geophysical logs can be referred to macrofaunal zonations, stratigraphic events and sedimentary sequences. As a consequence, the reconstruction of the basin history and its tectono-sedimentary development can now be considerably improved for the lower Upper Cretaceous of northern Germany.