MIDCRETACEOUS PHOSPHATE PLATFORMS OF THE RUSSIAN CRATON

The Mid-Cretaceous epicontinental sea covered the central part of the Russian Craton (RC). It was differentiated into a series of syn-sedime ntary highs and lows. The highs are called phosphate platforms. They a re confined to inherited tectonic uplifts where Precambrian basement i s very shallowly buried and earlier Mesozoic and Palaeozoic sediments are thin or wedged out altogether. Cenomanian sediments on the highs r epresent condensed sequences and contain abundant hiatuses. They are c rowned by an extensive phosphorite pavement, 30-50 cm thick, which is a multi-layered phosphorite hardground, built of phosphorite nodules f irmly cemented by a phosphate matrix. The nodules consist of phosphati zed radiolarians, diatoms and hexactinellid sponges. Analyses show no carbonate in excess of what is substituted for PO43- the francolite st ructure, nor is the enclosing sediment carbonate-bearing. Thus the Cen omanian phosphorites of the RC differ from those of the Anglo-Paris ba sin. During the Cenomanian, sea-level rose little because eustatic ris e was compensated far by continued epirogenic uplift of the area. This delicate balance was abruptly broken by a major rise of sea-level whi ch happened around the Cenomanian-Turonian boundary (CTB) and led to t he drowning of the phosphate platforms and deposition of the chalk.