Tectonic and Ecologic Controls on Formation and Drowning of Isolated Carbonate Platforms

David M. Bice, Kevin G. Stewart

During the Early Jurassic, a large carbonate platform--the Calcare Massiccio platform--developed on the Italian continental crust. The subsequent opening of the Ligurian Ocean resulted in crustal thinning and subsidence of the part of the Italian Peninsula that is now the Northern Apennines. A complex network of intersecting normal faults fragmented the large carbonate platform, producing a group of small, isolated platforms separated by interconnected basins up to 1 km deep. The subsequent drowning of these isolated platforms had a profound effect on the later history of the basin and accounts for the important stratigraphic differences between the Northern Apennines and the adjacent Central Apennines.

The sedimentary record indicates two stages of drowning: incipient and complete. After the isolated platforms formed, they entered a state of incipient drowning that lasted until the end of the Jurassic or the beginning of the Cretaceous, at which time they were completely drowned. The stage of incipient drowning was characterized by water depths within the euphotic zone and low rates of sediment production and accumulation. During this stage, a change in the ecology or environment could have returned the rates of carbonate production and accumulation to high enough levels to prevent drowning; but these rates remained unusually low, allowing the isolated platforms to drown completely.

The prolonged state of incipient drowning indicates that crustal subsidence was not the sole cause of the drowning; the important causal factors are those that disabled the normally efficient system of carbonate production and accumulation. These causal factors can be roughly divided into tectonic factors such as the system of extensional faults related to the opening of the Ligurian Ocean, and ecologic factors such as the general scarcity of reef-building organisms during the Lower Jurassic, when these platforms were created. The evolution of these isolated platforms from the Northern Apennines demonstrates the powerful controls that plate tectonics and the history of organisms can have over the development of a sedimentary basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.