Sequence stratigraphy and paleo-oceanography of an open-marine mixed carbonate/siliciclastic succession (Late Jurassic,Southern Germany)

Summary The Late Jurassic epicontinental sea of South Germany protruded far to the North forming a wide bay which was rimmed by shallow-water platforms (Swiss and French Jura). This wide shelf is characterized by extensive downslope mud accumulations including siliceous sponge buildups. The bioherms are aligned along the more pericontinental parts of this shelf, which graded to the South into the Helvetic Basin of the Tethys Ocean. Five sedimentary cycles of Oxfordian to Middle Kimmeridgian age (ox. 2–ox. 3, ki.1.–ki.1.2, ki.3, ki.1.3–ki.2.1, ki. 2.1–ki. 2.2) were used for interregional correlation. Each cycle is characterized by a vertical suite from marl to pure limestone. The bases of the marls are characterized by abundant open-marine fossils, glauconite and phosphate (fish teeth and pellets) and interpreted as condensed sections. Deepening is indicated by bioherms changing their growth form before demise from large structures into small isolated buildups, which commonly occur within deeper water. Sequence boundaries, are present at the transition from marl into limestone. Two phases (middle ki. 1.3 and ki. 1/2) of debris-flow deposition, one accompanied by the sudden spreadout of biostromes and basinward shift of bioherms, are interpreted as lowstand phases associated with sequence boundaries. The remaining sedimentary cycles described here lack such lowstand deposits. Instead, corresponding positions in the cycles are characterized by omission features. This problem is still unresolved. It may either be due to sediment trapping on the ramp or to a changing origin of the cycles compared. It is proposed here that highstand sediments, having higher contents of fine-grained siliciclastics than lowstand deposits, formed during a humid and warm climate with high rates of continued runoff and sediment transport. Lowstand deposits consist of pure offbank carbonates, because the associated drier climate reduced fluvial input of terrigenous material. Corresponding patterns of climatic change are also seen in platform sediments from the Swiss Jura Range; however, climatic cycles correlate with eustatic sea level fluctuations in only about 50% of the cases. This misfit may partially result from problems with biostratigraphic correlation (boreal— Tethyan). Small-scale sedimentary cycles with an average duration of 66’000 years (Mutabilis chron, ki. 2) to 95’000 years (Planula chron, ox. 3) provide a tool for detailed stratigraphic correlation in biostromes, small scale lenticular bioherms (1–2 m thickness) and large bedded bioherms (many tens of metres thickness). Interruptions of bioherm growth are due to temporal oxygen deficiency related to plankton blooms. The bathymetrically deepest bioherms-small and lenticular in shape—therefore suffered the highest number of ecological break-downs, whereas massive bioherms continuously remained above the critical level.