Cyclic phosphate-rich successions in the upper Cretaceous of Colombia
Abstract Upper Cretaceous neritic to hemipelagic successions from the eastern Colombian Cordillera display frequent and rhythmic intercalations of phosphate-rich sediment. Their accumulation is attributed to a back-arc setting between the Andean arctrench system and the Guayana cratonic shield. In three examined sections near Tausa, Tunja, and Iza (all north of Bogota), respectively, the phosphate-rich sediments occur in 1–15 m thick coarsening-upward series ideally consisting — from the base to the top — of porcelanite, organic-rich claystone, siltstone, sandstone, and a condensed and thoroughly burrowed top bed. Phosphatic particles appear either in thin gravity-flow deposits or in pristine, in-situ occurrences near the base of these successions, intercalated in fine-grained biosiliceous or clay-rich sediment, or in the condensed top bed. The major portion of this coarsening-upward series (porcelanite to sandstone) is considered a shallowing-upward succession and the thin condensed phosphatic top bed a deepening-upward succession. These rhythmic successions are interpreted as parasequences resulting from fourth-order relative sea-level changes. Based upon biostratigraphic age estimates, the time span of formation of these parasequences range between approximately 100,000 and 200,00 yr. The allochthonous phosphate intercalations near the base of the parasequences are derived from condensed phosphatic top beds, which may have been exposed at the sediment-water interface in proximal directions. This suggests that the parasequence boundaries, i.e., marine flooding surfaces, are diachronous and become younger in onshore directions. using the vertical stacking patterns of these parasequences, we distinguish between transgressive and highstand-systems tracts (TST and HST). TST's are characterized by the dominance of phosphatic sediment, laminated and organic-rich claystone, and laminated porcelanite. This suite of sediments documents high nutrient fluxes and the presence of an oxygen-minimum zone, both probably induced by coastal upwelling. HST's include laminated to well-bioturbated siliciclastic successions, which may reflect a weakening or basinward shift of upwelling cells and higher levles of bottom-water oxygenation. The dominance of siliciclastics in HST's is indicative of high detrital fluxes, which outpaced sediment-accomodation rates on the shelf. Upper Campanian ammonoids have been found in three levels of the Lower Plaeners Member of the Guadalupe Formation in the section near Tausa — Nostoceras (Nostoceras) liratum sp.n., Exiteloceras jenneyi (Whitfield, 1887), and Libycoceras sp. E. jenneyi is an important zonal marker in the U.S. Western Interior that is also known from the basal Mount Laurel Sand of Delaware, USA. Its occurrence at Tausa is the first record outside the USA and provides an important datum for intercontinental correlation. The type of Libycoceras sp. encountered in Tausa is also known from the upper Campanian of Peru and Angola. Together with the presence of Andalusiella polymorphia (Malloy, 1972), a dinoflagellate cyst, an age range is given for the formation of the Lower Plaeners Member at Tausa (late Campanian to early Maastrichtian).