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By the end of the Turonian, a westward-trending flysch trough was well established in what is now the western French Pyrenees. Over the next 22 m.y., to the end of the Maestrichtian, up to 5 km (16,000 ft) of carbonate flysch were deposited, uninterrupted by major silicoclastic sedimentation. Individual carbonate turbidites vary from a few centimeters to occasional massive deposits tens of meters thick. One carbonate megaturbidite can be traced 90 km (56 mi) and is still 10 m (33 ft) thick at its most distal exposure. Carbonate constituents lack shallow water indicators, suggesting that deposition and mass movement initiation occurred on continental slopes. Carbonate sediments were fed into the trough from both north and south.
Are there modern carbonate depositional environments we can turn to that will help us understand how such great thicknesses of carbonate flysch can build up without major clastic input? We believe that the west Florida upper continental slope is one such environment. It is part of a system which has been a carbonate depocenter since Jurassic time. A lime mud slope facies has accumulated at the rate of about 30 cm/1,000 years (12 in./1,000 years) for the last 25,000 years. At that rate (and allowing for a 50% reduction due to compaction) about 3.3 km (11,000 ft) of carbonate could accumulate in 22 m.y. A large variety of structures show that mass movement is continually displacing sediment downslope. If a second carbonate margin were close, accumulation of 5 km (16,000 ft) of carbonate flysch in 22 m.y. in a subsiding trough need not be extraordinary, even in an orogenically quiescent system.
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