Detrital Geochronology and Provenance Analyses of Andean Molasses as Markers of Northward Tectonic Transport

The inter-Andean Magdalena Valley in the northern Andes is flanked by coarse-grained deposits folded and faulted at the toe of valley-verging fold-and-thrust belts. The southern part of these deposits have been collectively called the Gualanday Group (Figure 1), a succession of pebble and cobble conglomerate and interbedded mudstones, some of them with pedogenic origin. It is also known as the San Juan de Rio Seco, and Carmen de Apicalá formations in other areas. We performed clast-countings at 10 sites (400 clasts each) over a strike distance of about 330 km along the western edge of the Magdalena Valley, where the Gualanday Group crops out. At the same localities of clast-countings, we made U/Pb detrital zircon geochronology analyses for the first time in this sequence. Additionally, we report zircon trace element concentration and zircon typology for the samples analyzed. Detrital zircon geochronology defined four main populations: Precambrian, Jurassic, Early-Cretaceous and Late-Cretaceous. Trace element concentration and typology in zircons shows that most of them are magmatic, have crustal affinity and that only the older ones have been redeposited. The Upper Cretaceous population is present in the northernmost samples, while the Jurassic population is absent. Conversely, the southernmost samples contains Jurassic but no Cretaceous populations. We obtained clast compositions of 66.6%, 16%, 4.3%, 3.6%, and 4.3% of chert, sedimentary, quartz, andesitic volcanics and metamorphic clasts respectively for the uppermost conglomerate of the sequence (Doima Fm.) and 76.3%, 7%, 11%, 0.3%, and 5.5% respectively for the lower conglomerate (Chicoral Fm.). These results suggest that the source for the Gualanday Group conglomerate must have been located in the Eastern Cordillera and in the western flank of a perhaps discontinuous Central Cordillera, at a more southerly latitude. The absence a Paleocene age population, expected as a Paleocene arc that is present in the Central Cordillera, suggests northward transport along the NW margin of the South American Plate.