FORAMINIFERAL REEFS OF THE URGONIAN FROM HAUTE-SAVOIE (FRANCE)

Massive biostromes built by the arenaceaous foraminifer Bdelloidina CA RTER (Coscinophragmatidae) atone have been encountered in the Urgonian limestones of Barremian age from the massif du Haut-Giffre. Resemblin g stromatolites in the field they can reach up to one metre in height and extend over a few kilometres. They display four morphological type s depending on the hydrodynamical conditions and sedimentation rate : stratiform, columnar, coated ball or occurring as small dispersed colo nies encrusting bioclasts. This Barremian species represents the only agglutinated foraminifer able to erect such large monospecific buildup s. The Bdelloidina buildups are encountered in foraminifera- and algae -rich bioclastic grainstones and wackestones, deposited on a shallow w ater carbonate platform influenced by waves and tides. This facies bei ng rather common in the Urgonian limestones, one might expect that suc h reefs have a larger geographical distribution.