An integrated environment analysis—lithofacies, chemofacies, biofacies—of the Oligocene calcareous-siliciclastic shelf deposits in northern Germany

During the Oligocene monotonous fine-grained siliciclastic and calcareous shelf sediments of the Septaria Clay Fm. were deposited in the North German Basin. The uniform grain size spectrum and the homogeneous outward appearance of these sediments forces environmental analysis to be based on a joint investigation of the inorganic and organic facies. The transgression of the Rupelian sea caused tidal sand sheets to form off the mouth of estuaries. Upon deepening of the basin the benthonic oxygen content of the bottom water diminished and inner shelf muds were deposited. Following this early transgression, a conspicuous change may be recognized when a siliciclastic shelf was replaced by a calcareous-siliciclastic shelf upon shoaling of the basin and warming of the seawater. These outer shelf conditions persisted until fine-grained clastic sediments became more and more dominant during the early Late Oligocene. Their depositional environment may be described as shallow lagoon. In the succeeding formation of the Oligocene tidal bars and estuarine sands are characteristic. The uppermost formation penetrated by the drill hole in the Gartow area contains estuarine channel sands. In conclusion, the Oligocene sedimentary record reflects a symmetrical facies pattern showing calcareous-siliciclastic cyclothems in the central section and siliciclastic shelf deposits below and above, reflecting the cyclical nature of the Oligocene sea in a dominantly tidally-influenced depositional environment. The use of clay minerals, heavy minerals, carbonates, phosphates and sulphides together with dinocysts, nannofossils and foraminifera has proved to be a successful tool in palaeoecological and palaeoenvironmental analyses.