U-PB DETRITAL ZIRCON AND HEAVY MINERAL SIGNATURES OF CENOZOIC CLASTIC SEDIMENTS IN THE TACHIRA SADDLE, WESTERN VENEZUELA

The Merida Andes is a NE-SW oriented mountain range in western Venezuela. Formed by an active convergent boundary interaction among the southern margin of the Caribbean plate, the Maracaibo Block, and northern South American plate.

Previous geological evidence suggests that a multiphase uplift of the Merida Andes diachronically diverted a Pre-Miocene northward drainage system to the more recent fluvial system that drains northeast. This created a mayor change in the sedimentation patterns and therefore a shift in the potential sediment sources in the surrounding basins.

To test this hypothesis, a series of 19 outcrops samples were collected from 5 sections across the southern flank of the Merida Andes. These outcrops expose a Tertiary sequence from the Tachira saddle and the Barinas basin. A complete provenance analysis was made with heavy mineral analysis including U-Pb detrital zircon geochronology in 6 selected samples from the most complete sections.

Preliminary data has shown a shift in the relative abundance of heavy mineral suites. Stables mineral suites in Cretaceous to Eocene samples change to a more unstable heavy mineral association in Oligocene to Pliocene samples. Results from the geochronologic signatures in pre-Miocene sediments show a main concentration in Precambrian ages associated with Guyana Shield in contrast with Post-Miocene samples that presents an age signature dominated by Greenville to Mesozoic Andean sources and reduced signal from Guyana shield sediments.

Results from this work may generate new insights about the depositional history of the Táchira Saddle and Barinas basin and the dynamic of the ancient sedimentation patterns in this area. This should allow proper integration with the Llanos Basin in eastern Colombia and will generate a new temporal constraining to the uplift of the Merida Andes.