SEQUENCE STRATIGRAPHY IN A MIXED CARBONATE-SILICILASTIC DEPOSITIONALSYSTEM (MIDDLE MIOCENE, STYRIAN BASIN, AUSTRIA)

The mixed carbonate-siliciclastic Weissenegg (Allo-) Formation records three depositional sequences corresponding approximately to the TB 2. 3, TB 2.4 and TB 2.5 global cycles. Sea-level fluctuations were of the order of at least 30 m. Siliciclastic lowstand systems tracts compris e lignite deposits, reworked basement and tidal siltstones (above a te ctonically enhanced sequence boundary) as well as coastal sand bars. C oastal sands of the transgressive systems tract contain distinct layer s of well cemented nodules. They are interpreted as the first stage in hardground formation and record superimposed minor sea-level fluctuat ions. Coral patch reefs and rhodolith platforms developed during trans gressive phases and were subsequently drowned and/or suffocated by sil iciclastics during early highstand. Shallowing upwards siliciclastic p arasequences, each terminated by a bank of rhodolith limestone, form t he (late) highstand systems tract. The limestone beds record superimpo sed fourth-order transgressive pulses. Occasionally a carbonate highst and wedge developed. Lowstand carbonate shedding occurred where the to p of a platform which suffered incipient drowning during highstand was near sealevel again during the following lowstand. Late highstand del ta progradation is common.