Abstract: Anatomy of a synrift Jurassic pelagic carbonate platform and basin system, Rossa Mountains, Central Apennines, Italy

Fabrizio Galluzzo, Massimo Santantonio

Synrift evolution and anatomy of Jurassic pelagic carbonate platform (PCP) and basin systems are documented from the Rossa Gorge Mountains, Umbria-Marche Apennines, from structural and sedimentological relationships and ammonite biostratigraphy. Liassic extension resulted in the break-up of shoal-water carbonate platforms. These platforms formed the foundation of younger pelagic platforms. Sediment productivity on the pelagic carbonate platforms was typically low, as evidenced by the abundance of ammonite-rich condensed limestones. Hermatypic corals were an important component to PCP sediments in the lower Tithonian. Coral-bearing strata formed localized progradational wedges which extended into adjacent deeper basins. Water depth of the PCP was no deeper 150 m and may have been as sh llow as 30 meters.

Fault scarps bounded the margins of the pelagic platforms and, due to their steep slope, became the sites of prolonged exposure, subjected to only minimal sedimentation. The longevity of the paleoscarps is documented by the wide range in the age of condensed pelagic carbonates covering the scarps. The upper scarp typically developed convex-upward profiles due to submarine slope erosion. For example, an inferred Liassic fault separates a PCP from a down-dropped, rotated block of peritidal limestone. The tilted block of peritidal limestone never went through a PCP stage. Instead, the peritidal block was rapidly drowned and received minimal sediments for some 40 m.y., and eventually was onlapped by basinal cherts and pelagic carbonate turbidites derived from distant platforms. Rapid bloc tilting of more than 4-5° and removal from the carbonate productive portion of the photic zone is envisaged as the cause for the apparent sediment starvation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994