Cretaceous Rudist-Reefs of the Mediterranean Realm

Jean M. Philip

Cretaceous rudist reefs are well represented in the Mediterranean region on outcrops, and several subsurface examples are major hydrocarbon reservoirs.

Most rudist reefs are part of large carbonate platforms and show both ecologic and sedimentologic zonation; localized rudist banks are rare and are confined to perideltaic environments. Rudists are the major builders, but rich fauna and flora accompany them. Together, these components produced large amounts of bioclastic debris that accumulated on reef slopes as grain-flow and debris-flow deposits. Much of the primary porosity was occluded by micritic matrix and early cements, but dissolution of the aragonitic rudist skeleton and dolomitization can produce extensive secondary porosity.

Reefs appear in the Aptian, reach their acme in the Cenomanian and Senonian, but are missing in the Turonian. The early Turonian disappearance is probably the result of changes in global ocean circulation, and the Maestrichtian demise correlates with a sea level fall. Three reef provinces are recognizable: (1) western Mediterranean corresponding to the passive margins of Sardinia and Aquitain-Pyrenees-Iberia; (2) African including Maghreb and the Middle East; and (3) central Mediterranean composed of the Italian, Dinaric, Hellenic, and Turkish platforms separated from the western Mediterranean by the Alpine geosyncline.

Rudist reefs occur in a variety of tectonic settings, both extensional and compressional: on tilted blocks, diapiric uplifts, volcanic bumps, and obducted ophiolitic rocks. A most favorable setting for reservoir development is where the reefs interfinger with black shales (scaglia). Future exploration may focus on the transition zone between the outer shelf and proximal basin.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.