Origin of phosphatic stromatolites in the Upper Cretaceous condensed sequence of the Polish Jura Chain

The Turonian stromatolite-bearing condensed sequence in the Polish Jura Cha in (the European epicontinental basin) provides good insight into the envir onment of formation of Cretaceous phosphatic stromatolites, owing to their purely phosphatic development and negligible post-depositional alteration. The sequence developed as a result of slow pelagic sedimentation and microb ial mat phosphatization on a submarine swell surrounded by local basins wit h non-condensed carbonate deposition. Diagenesis of organic matter and diss olution of biogenic apatite were the major sources of reactive phosphorus f or the microbial mat phosphatization. Stromatolite growth occurred due to p ulses of amorphous or poorly ordered calcium phosphate precipitation follow ed by crystallization of carbonate fluorapatite (CFA). The phosphogenic env ironment left an imprint on the isotopic composition of limestone carbon an d lattice-bound carbon and sulphur in CFA, and on the light rare-earth elem ent (LREE) distribution in CFA. The delta(13)C of the stromatolite-bearing sequence shows a negative excursion (-1 to -3 parts per thousand), standing in marked contrast to positive carbon values of the surrounding basinal ca rbonate. Most of the delta(34)S values of CFA (+20 to +21 parts per thousan d) fit the value range of the coeval seawater sulphate, and the LREE distri bution shows a well-defined seawater pattern. This geochemical signature is indicative of intense diagenesis of organic matter at the seafloor, pelagi c carbonate dissolution, and prolonged exposure of the deposited phosphate towards the water column. The enhanced deposition and diagenesis of organic phosphorus in the stromatolitic environment reflects elevated levels of th e epicontinental basin nutrification related to sea-level rises and the ass ociated oceanographic and geochemical changes. (C) 2000 Elsevier Science B. V. All rights reserved.