SHALLOW STRATIGRAPHIC WELLS SOLVING DEEP GEOLOGICAL ISSUES: A CASE IN THE NORTHERN RANCHERIA BASIN OF COLOMBIA

Major questions in Caribbean geology include the nature and timing of inboard deformation as Caribbean-South American interactions took place during Cenozoic times. The Rancheria Basin in northern Colombia contains the record of such activity in a thick Paleogene clastic wedge overridden by the large, northwest-verging Cerrejon thrust sheet. A cored, shallow borehole located near the tip of the thrust sheet was key to document the angle of the fault, timing of deformation, and to identify units in both, hanging wall and footwall blocks. In the northern Ranchería basin, the shallow-dipping Cerrejon thrust places volcaniclastic rocks of Triassic-Jurassic age over lower Eocene conglomerate and mudstone, and a 1-km-thick coal-bearing, reptile fossil-rich, clastic wedge of late Paleocene age. A 350-m-deep stratigraphic cored-borehole, drilled in the hanging wall of the Cerrejon thrust and 500 m inboard of the fault trace, crossed the Cerrejon thrust as predicted by surface mapping. Our analyses from the core indicate that Middle Jurassic volcaniclastic rocks (U/Pb volcanic zircon ages) are thrust with a shallow dip of ~10º over lower Eocene coal-bearing strata (palynological ages). These findings constrain NW-SE shortening values to be at least 20 kms along the Cerrejon thrust. Sandstone petrography and detrital zircon geochronology in the Middle Jurassic volcaniclastic succession constrain the history of the nearly 7-km thick Jurassic sequence and Paleozoic strata in this area. Sedimentological and provenance analysis in the lower Eocene succession crossed by the borehole indicate that coal-bearing strata become younger eastward as predicted by tilting of the blocks in response to coeval arc-continent collision along the Caribbean margin. Provenance data on Eocene strata point to combined supply from both the collisional margin and the onset of inversion of the Jurassic graben.