Compositional Trends of Cretaceous Conglomerate Provenance: Tracing The Evolving Nature of Tectonic Environments in the Northwestern Colombian Andes

Abstract:

The composition and provenance of the sedimentary record is a sensible marker of the evolving nature of source , basin paleogeography and tectonic assemblage.

The Cretaceous geological evolution of the northern Andes is characterized by the succession of different tectonic environments that include: An early Cretaceous magmatic quiescence that follow former Jurassic arc magmatism, Albian-Aptian subduction resume and associated arc - back-arc formation and the late Cretaceous collision with an allocthonous oceanic arc that marks the beginning of the Andean orogeny.

Such tectonic evolution had been mostly reconstructed from the magmatic record or the stratigraphic analysis of inland basin far from the arcs and suture zones. Along the western flank of the central cordillera outcrops two different stratigraphic units with notable differences in the provenance and timing of accumulation.

The Abejorral Formation is the oldest sedimentary sequence (Albian-Aptian) that discordantly overlies the Triassic continental margin. this unit include two lithofacies clearly distinguishable, a lithofacies consist mostly of conglomerate, characterized by abundant quartz content , low compaction, rounded clasts and moderate sorting ; and the other is a interbedded of fine size sandstone, mudstone and chert; also with abundant quartz content further muscovite, containing basement and volcanic material .

To the west, sedimentary rocks including within the Quebradagrande Formation conform a turbidite sequence with a well defined Bouma type succession that concordantly overlied a Campanian marine volcanic arc succession. The conglomerates associated to this unit are characterized by containing mainly sedimentary and volcanic rock fragments ,high compaction, subrounded clast, and low sorting. This sequence is overlying by the volcanic component in a concord contact.

Whereas the Albian-Aptian record of the Abejorral Formation exhibit the unroofing of the continental basement and deepening of an extensional basin that end up with effusive volcanism, the turbidite record with mixed continental and arc provenance associated to the Quebradagrande Formation is interpreted as the sedimentary marker of the arc-continent collision, marking the initiation of the Andean orogeny in the Northern Andes.