Abstract

The Rosso Ammonitico Veronese (Middle--Upper Jurassic) is a thin (25-30 m) lithostratigraphic unit, consisting of red, mostly nodular, pelagic limestones deposited on a current-swept structural high, in the Southern Alps. Sedimentation rates were very low, and omission surfaces and mineralized hardgrounds were widespread. Facies are varied and commonly show evidence of early lithification near the sediment/water interface. Petrographic observations of both mud- and grain-supported facies rich in thin-shelled bivalves enabled reconstruction of the diagenetic evolution. Some layers, or nodules within layers, underwent a phase of early lithification, acquiring a rigid framework that prevented deformation and rearrangement of grains during later burial. In the uncemented beds, and in the matrix of nodular beds, the effects of subsequent mechanical and chemical compaction are clearly recognizable. The response to chemical compaction varied with texture: grain-supported sediments developed fitted fabrics, whereas in mud-supported sediments dissolution seams were generated. After these diagenetic processes, when all the layers had acquired the same degree of coherence, laterally extensive stylolites developed, cutting across older depositional and/or diagenetic fabrics. Total compaction was evaluated quantitatively by different methods. Diverse compaction values, related to the changing interplay of early cementation and burial diagenetic processes, were obtained, but an average of about 60% volume reduction for the whole formation is estimated.

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