INTERPRETATION ON GENETIC ENVIRONMENT OF FINELY LAMINATED SOLNHOFEN LIMESTONE IN SOUTHERN GERMANY USING CLAY MINERALS AND CYCLOSTRATIGRAPHIC TECHNIQUES

Based on the mineralogical analysis, the fäule and flinz beds contain varying amounts of calcite, quartz and minor amounts of clay minerals which consist mainly of illite, kaolinite and montmorillonite. Among these, montmorillonite has severely undergone illitization through diagenesis. The relative content of kaolinite increased along the increase of the burial depth, but it may be related to the diagenetic process. Also, thin sections show that the deeper the burial depth is, the more strongly the beds have been affected by diagenetic alterations such as dissolution, sparitization, low-Mg calcite precipitation or blocky Fe-calcite cement. The fäule and flinz beds are, on average, 3 cm thick and the thickness ranges from 0.93 to 1.12 m/cycle. Since the most prominent cyclicity in bed-thickness time series corresponds to the Upper Jurassic precessional cycle (ca. 18.2 kyr), the following depositional durations can be calculated: 47.3 years for a fäule bed and 505.2 years for a flinz bed, respectively: i.e. a single couplet of the fäule and flinz beds represent 552.5-years time interval. Thus, on a metric scale the Solnhofen Formation was of long-term cyclic nature related to the orbital cycles, whereas on a centimetric scale the fäule and flinz beds formed under short-term cyclic environment with the period of about half a thousand years. However, the slump units have more terrigenous minerals than the lithographic limestones, and during the time of the slump units, the sedimentation rates are 122-145% higher than in the other lithographic limestones. These reflect changes of the depositional environment between the slump layer and the lithographic limestone. Above all, the slump structure found in the slump layers seems to have been caused by an increased instability of the layer, possibly in connection with a decrease in depth of sea water and a corresponding increase in the rate of sedimentation. This interpretation implies that the top parts of the Lower and Upper Solnhofen Formation may have been deposited during regressive phases.