The Tertiary Fusagasugá Succession; A Record Of The Complex Latest Cretaceous-Pre-Miocene Deformation In An Area Between The Magdalena Valley And Sabana De Bogotá

The Tertiary succession in the Fusagasugá syncline includes unconformities, thick marginal to alluvial deposits, and an upsection change in sandstone composition that constrain pre-Miocene deformation eastward of the present western flank of the Cordillera Oriental. Five informal units were mapped along the western flank of the syncline. Lower Paleocene finegrained coastal plain deposits of Unit I (499 m) rest paraconformably upon upper Campanian shallow-marine sandstones and calcareous shales of the Guadalupe Group; palynological data document the absence of middle and upper Maastrichtian strata. The disconformity between paleosols at the top of Unit I and estuary sandstone and siltstones of Unit II (153 m) marks a second stratigraphic surface with a biostratigraphic gap that probably includes the Paleocene-Eocene boundary. The remaining succession (Units III to V; >1400 m) is not dated, and the lithofacies association of sandy siltstones and sandstones indicates that depositional environments prograded from coastal to alluvial plains. Mappeable sandstone intervals in Units II to V show lateral extent of several hundred of meters, intraformational gentle angular unconformities, and gentler dip of beds toward the axis of the syncline; these map patterns are similar to those predicted in growth strata in continental settings, suggesting accumulation during deformation. The upsection increase in population of lithic fragments and feldspars between the Guadalupe Group and Unit I, of metamorphic lithic fragments between Units I and II, and of volcanic lithic fragments between Units III and IV indicate the unroofing of source area(s) and volcanism during deposition of Units IV and V.