Integrated stratigraphy of an offshore succession influenced by intense siliciclastic supply: Implications for Coniacian tectono-sedimentary evolution of the West Sudetic area (NW Bohemian Cretaceous Basin, Czech Republic)
Abstrakt
We present the interpretation of tectono-sedimentary evolution of the West Sudetic area (central Europe) during the latest Turonian-middle Coniacian as recorded by deposits of the NW part of the Bohemian Cretaceous Basin. This paper provides the first strontium isotope curve from the Upper Cretaceous of the Bohemian Massif.
The exact stratigraphic framework was provided by combining of macrofossils (inoceramids) and calcareous nannofossils. Six inoceramid zones were distinguished, from Cremnoceramus deformis erectus Zone to Volviceramus koeneni Zone.
Biostratigraphic data were combined with XRF curves and geophysical logs which allowed for correlation of several key sections within the study area. Using the genetic stratigraphy, contrasting parts of the depositional system interpreted as nearshore to deltaic were successfully correlated.
Six elementary sequences were defined within the studied succession. These are TUR 7 (latest Turonian), backstepping, aggradation-dominated, with a short-term progradational episode and CON 1 to 5 (early-middle Coniacian), deposited during a period of increasing depth through time.
The progradational pattern is most typical for the CON 4 sequence. Intensified sediment supply resulting in pronounced progradation is also evidenced by increased siliciclastic influx to the offshore zone and resulting changes in calcareous nannofossil assemblages.
Three major transgressive events are interpreted at the base of sequences: (1) CON 1 (close to the Turonian-Coniacian boundary); (2) CON 2 (near FO of Cremnoceramus crassus crassus); (3) CON 5. The transgressions were predominantly driven by basin-floor subsidence, although the transgression at the Turonian-Coniacian boundary and at the base of sequence CON 2 likely carried a component of eustatic sea-level rise.
The accelerated basin-floor tectonic subsidence and source uplift in the NW part of BCB falls within the early Ilsede phase of the Late Cretaceous ('Subhercynian') deformation of the Alpine foreland.