Litho- and cyclostratigraphy of the Aalenian Opalinusclay Formation in the Swabian Alb deduced from downhole logging data

From 2019 to 2021, three cores were drilled at different locations in the southern German Swabian Alb as part of the SEPIA project (Sequence Stratigraphy of the Aalenian in Southern Germany). They comprise sediments of 200 to 250 m length and are penetrating Lower and Middle Jurassic strata from the Pliensbachian to Bathonian stages (~ 190-166 Ma). The aim of this project is the development of a sequence stratigraphic model of the South German Basin at the transition from the Lower to Middle Jurassic time. Conclusions should be drawn towards the source area of the sediments as well as on the influence of sea level fluctuations on sedimentation.

Today, the Swabian Alb is a SW-NE trending mountain chain consisting of mainly carbonate rocks, and is one of the most distinctive regions in Germany where Jurassic strata is cropping out. During the middle Jurassic, Europe was almost completely covered by a shallow epi-continental sea including several small emerging areas or islands, located at latitudes about 15° lower than today. Southern Germany experienced predominant deposition of fine clastic sediments in a tropical climate. The most common sediments of this period are dark clays and oolithic ironstones, whereas condensation and discontinuity surfaces occur in many instances. Accommodation space for these sediments was not only generated by changes in sea level, but also by continuing subsidence of the area, explaining the inhomogeneous thickness and changes in facies of the sediments.

The geophysical downhole logging data of the stratigraphic record is used to develop a lithological classification and correlation of the boreholes sediments by the application of a cluster analysis to the data. Furthermore, the downhole logging data is used to perform cyclostratigraphy in selected intervals. The focus of the intervals chosen for cyclostratigraphy lies on the Aalenian stage, as this stage holds the most continuous and extended record in all three boreholes. Predicted timespans of these intervals yield similar results of ~800-1100 ka for all three boreholes and might provide a new benchmark for progressive improvement, especially for cyclostratigraphic analyses of the Lower Aalenian Opalinusclay Formation.