TECTOGENIC SEDIMENT DISPERSAL FROM RETROARC FORELAND OROGENIC WEDGES: EXAMPLES FROM THE CORDILLERA OF NORTH AND SOUTH AMERICA

Tectogenic sediment, derived from eroding orogenic wedges comprising retroarc foreland fold-thrust belts, is delivered to wedge-top and foredeep depocenters of foreland basins through a variety of alluvial depositional systems. Major loci of sustained fluvial gravel deposition along thrust-belt margins are located at transverse structural zones where: 1) abrupt along-strike differences in orogenic wedge geometry focus large, long-lived antecedent drainage basins that tap multiple thrust sheets in the orogen interior, and 2) gradual along-strike differences in wedge geometry give rise to fluvial systems flowing around anticlinal ridge margins, along thrust ramps, and through multiple piggyback basins before emerging in the foredeep. Fluvial megafan systems in the Andean foreland are likely modern analogues for thick (>3000m) conglomerate complexes in the Cordilleran foreland basin of western North America.

Piggyback basins develop where portions of the foredeep are transferred to the wedge top under supercritical-taper conditions. Piggyback basin margins often comprise anticlinal ridges that nucleate in foredeep settings characterized by axial to transverse gravelly braided fluvial systems tapping well-integrated drainage basins in the orogen interior. Tectogenic sediment is sourced locally from small, poorly-integrated drainage basins of the anticlinal ridges and deposited synthetically and antithetically in flanking alluvial fans. Texturally immature fold-derived fan gravels possess labile petrofacies, unroofing sequences, evidence of sediment recycling, and growth strata development. Braided fluvial gravels are compositionally and texturally mature; clast unroofing sequences are absent. Anticline uplift diverts the braided channels through surface deformation and alluvial fan growth leading to interaction of distinct coarse-grained depositional systems.

In the Cordilleran orogen of western North America, Upper Cretaceous strata in southwest Montana and southern Nevada document alluvial fan/fluvial interaction resulting from frontal fold growth. Comparable modern analogs for fan/fluvial interactions are present in the Argentine Precordillera where the Las Peñas thrust and nascent Montecito anticline record active thrust belt propagation into the foredeep.