CENOMANIAN PHOSPHORITES IN THE FORMER SOVIET-UNION

Cenomanian nodular phosphorites are widespread over vast areas of the Precambrian East European Platform (EEP). They are found in various pa rts of the Cenomanian forming thicker and richer beds close to the Cen omanian/Turonian boundary. They are encountered nearly unbiquitously, but are concentrated on synsedimentary highs in thin and condensed seq uences with numerous hiatuses. All of the economic deposits are confin ed to the palaeohighs. Phosphorite pavements made of amalgamated, rewo rked and concentrated concretions are typical of the condensed sequenc es. More complete successions occur in the adjacent lows, where Upper Cenomanian sediments have been proved. Phosphorite in the lows are fou nd as in-situ concretions either scattered irregularly or aligned in r ows and spaced rather regularly. Calculations show that some 25 Gt of P2O5 was buried during the Cenomanian. A positive deltaC-13 anomaly wa s found to coincide with a thin layer of sandy to silty chalk, contain ing phosphatic intraclasts. The layer lies directly on the youngest an d thickest pavement. The anomaly is believed to represent Oceanic Anox ic Event 2 (OAE-2) on the EEP. Consequently, (1) phosphorite pavement is most probably of Upper Cenomanian age, and (2) phosphogenesis is re lated to OAE-2. There is no black shale amidst the Upper Cenomanian on the EEP and sea-level variation during that time might be responsible for both the spatial and temporal differentiation of phosphorites and black shales. The former were deposited during transgressive phases, whereas the latter correspond to subsequent sea-level highstand. The e xtent of spatial differentiation might be preconditioned by the scale of difference between corresponding sea-levels. An upwelling scenario is hardly probable to explain the origin of the Cenomanian phosphorite of the EEP. A river-input mechanism is more probable, as it was advoc ated by Bushinsky.