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ABSTRACT

Trapezoidal-type fan deltas, lacking bottomset deposits, were studied in two different tectonic settings: extensional (rifted), and compressional (piggy-back) basins. In both cases studied fan deltas were characterized by: (i) an absence of bottomsets; (ii) development in protected or narrow basins and sub-basins confined by intrabasinal basement highs or by topographic highs, respectively; (iii) coarse-grained sediment fluxes, dominated by mass-flows forming fan deltas that prograded from steep nearshore slopes basinwards; and (iv) a high-energy environment, with powerful underflows that probably bypassed the basins and transported fine-grained sediments outside the basins. The location of channels cut by such underflows is influenced by local tectonic style. When the supplied sediments in the rifted basins overstepped the intrabasinal basement highs, trapezoidal fan deltas were replaced upwards by Gilbert-type deltas, with bottomsets.