CONIACIAN RUDIST BUILDUPS IN WEST CENTRAL TUNISIA - SEDIMENTOLOGY, PALEOGEOGRAPHIC CHARACTERISTICS AND SEQUENCE INTERPRETATION

Coniacian rudist-buildups have been discovered in the Dernaia-Tamesmid a area (west central Tunisia). They are sedimentologically studied in detail in this paper. Reefs sensu stricto are made up of rudists (Hipp uritidae and Radiolitidae) and they show a height:lateral extension ra tio of about 1:50. The fore-reef is composed of southwest prograding p elletoid sandwaves, whilst hemipelagic micrite has been deposited in t he distal talus. The back-reef lagoons (NE side) include green algae a nd annelid, miliolid, ostracod and gastropod biotopes. Perilagoonal oo litic bars may border these environments. These reefs evolved in a sha llow, relatively open environment. They correspond to barrier-reef com plexes which developed at the margin of a generally shallow carbonate ramp. In terms of the sequence stratigraphy, they characterise late lo w sea levels and especially the high sea levels of the uppermost Turon ian-Coniacian depositional sequence. (C) 1997 Elsevier Science Limited .