Sequence stratigraphy of the Upper Jurassic mixed siliciclastic-carbonate deposits in the North German Basin (Lower Saxony, Hildesheimer Wald)

The Wendhausen-6 core, drilled in the Hildesheimer Wald, 30 km SE of Hannover, provides an insight into the Oxfordian deposits of the North German Basin consisting of the Ornatenton, Heersum, and Korallenoolith formations. Eight facies are recognized based on the different fossil combinations, sedimentary structures, and the carbonate content. A facies model of a mixture of a deltaic system and a carbonate ramp is presented based on the interpretation of these facies, which helps the understanding of the facies and sequence stratigraphic evolution in the mixed system in the North German Basin and could be applied to other shallow-water mixed depositional systems. Sequences of three hierarchies are delimited. Small-scale sequences are defined by the changes in components, grain size, and the vertical facies stacking patterns. Medium-scale sequences are bounded by distinctive exposure surfaces. These surfaces are characterized either by the enrichment of iron minerals or by irregular base of the lowstand channel-fill sandstones overlying on them. Large-scale sequences are derived from the facies proportion statistics of each medium-scale sequence. The similar trend between the regional sea-level curve and the facies statistic curve implies that sea-level fluctuations have a great control on the large-scale sequence evolution during the Oxfordian age. Sequence stratigraphic correlation was achieved at the medium-scale sequence level, and four surfaces of Oxfordian age were correlated across the North German Basin.