THE DROWNING OF A CARBONATE PLATFORM - AN EXAMPLE FROM THE DEVONIAN-CARBONIFEROUS OF THE SOUTHWESTERN HOLY CROSS MOUNTAINS, POLAND

The broad Middle Devonian carbonate platform of the Holy Cross Mountai ns was fragmented during the Frasnian and afterwards submerged as isol ated blocks and transformed into a topographic depression. The section exposed in the Ostrowka quarry illustrates a well-documented drowning of the last small fragment (seamount) of this carbonate platform whic h submerged during Late Devonian and Early Carboniferous times. The re cognized facies succession reflects a progressive, stepwise lowering o f the sediment surface through time from a peritidal, lagoonal environ ment to a deep-water, anoxic basin. The Frasnian shallow-water carbona te platform is represented by lagoonal facies with peritidal sediments . Small-scale deepening-upward cycles punctuated with emersion surface s are a feature of this stage. They resulted from a combination and co ntinuous balance between carbonate production, subsidence and low-orde r; eustatic sea-level fluctuations. The carbonate platform aggraded at rates around 100-125 m/Ma. The shallow-water peritidal carbonates are separated from the post-platform deposits by an unconformity surface developed during subaerial exposure. The flooding of the faulted block of Galezice in the pre-late marginifera time and the subsequent initi al drowning resulted primarily from a rapid sea-level rise. Small dime nsion of the seamount and its cemented upper surface were the most imp ortant factors that facilitated the sediment removal and thus suppress ed the sediment accumulation. A condensed section was deposited on the top of the seamount. Its stratigraphic succession and rare cephalopod storm beds account for a model of redistribution of sediment over the swell, disintegrated by small-scale faulting. During drowning, the se amount traversed down through the photic zone at an average rate not h igher than 20-25 m/Ma. Transformation of the swell into a basin accele rated in the late Toumaisian anchoralis Zone. It is manifested by an i ncrease of deposition rate and by a lithology of alternating limestone s and clays, with participation of fine-grained calciturbidites. The a ssociated tephra deposits and syndepositional ferruginous hydrothermal mineralization were the result of extensional tectonics on the facies evolution. The drowning was completed in the Visean with deposition o f black siliceous shales in the starved basin. The 'death' of the Devo nian carbonate platform in the Galezice area illustrates well the fame d 'paradox of drowned platforms' as the subsidence rate was much slowe r during the drowning than during the phase of the platform aggradatio n.