Origin of a Syndepositional-Early Diagenetic Cavity Network from the Berriasian Carbonate Platform of Eastern Sardinia (Italy): Facies Analysis and Geochemical Characterisation

Alessandro Lanfranchi
University of Milan, Earth Science Departmant “A. Desio” Milan, Italy
alessandro.lanfranchi@unimi.it

The Berriasian, inner platform, carbonates of Eastern Sardinia (Italy), consist of ooidal and bioclastic calcarenites-rudites stacked into meters thick shallowing-upward paresequences, deposited on the Southern European passive margin

These deposits are cross-cut by a pervasive network of mainly sub-horizontal fractures with metric lateral extent and centimeters width. The fractures density increases in correspondence of problematic tepee-like structures, where vertical fractures connecting horizontal veins become more abundant.

The fracture network experienced multiple cycles of wedging. The filling of the fracture is polygenic and is summarized as follow:
     - In the early stage isopachous crusts of blade calcite, often followed by dog tooth cements, began to fill the fractures. During this stage connection with the depositional environment was scarce or even absent. In the youngest deposits fractures show a microbialitic envelope predating the cements.
     - Following stages of fracture filling are characterized by a connection with the depositional surface as the sedimentary filling of the fracture (oolitic grainstones and/or crystal silt containing ostracods and small gastropods) testifies.
     - Equant calcite often close the fracture network.

Fractures developed in subtidal, shallow marine phreatic condition as cathodoluminescence petrography confirms.
Later fracture-wedging events closely follow the previous fractures, nevertheless the driving force of these fracture-wedging events is problematic. The cement crystallization force could be responsible for the early fracturing.