Carbonate turbidite sequences deposited in rift‐basins of the Jurassic Tethys Ocean (eastern Alps, Switzerland)

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Abstract
Syn‐rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault‐bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates.The most abundant redeposits are bio‐ and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high‐ and low‐density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow‐water carbonates and skeletal debris from nearby submarine highs.After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge‐shaped with a horizontal upper boundary and consists of a thinning‐ and fining‐upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin‐bedded turbidites and hemipelagic limestones. The thick‐bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick‐bedded turbidites, thinning‐ and fining‐upward cycles are common. The overlying thin‐bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine‐grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin‐plain association developed comprising fine‐grained and thin‐bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.