Organic Matter Enrichment and Anoxic Conditions in Cretaceous Black Shales from the Upper Magdalena Valley (Colombia): Latest Results of an Ongoing Sedimentological and Organic Geochemical Study

Ute Mann Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

The extended black shale deposits in Colombia are records of a Cretaceous epicontinental sea in northwestern South America. During lower and mid-Cretaceous times, black shale deposition was favoured by suboxic and anoxic conditions triggered by the regional tectonic evolution of the basin (Fabre, 1985; Mann, 1995). Since the mid-Cretaceous, there are numerous indications for global mechanisms (e.g. eustatic sealevel variations) controlling the sedimentation of the organc carbon rich sediments (Mann, 1995). Because the depositional environment and the processes controlling the Upper Cretaceous sequences are nearly unknown so far, detailed organic geochemical investigations were performed on three Upper Cretaceous sequences within the Upper Magdalena Valley (UMV), Colombia.

Focus of interest are the mechanisms controlling the accumulation of organic matter in the Upper Cretaceous sequences such as eustatic sealevel oscillations, upwelling conditions causing enhanced primary productivity of marine organic matter, or climatic variations and therefore amplified fluviatile or aeolian supply of terrigenous organic matter. Are the variations in the organic fractions (marine/terrigenous) randomly appearing events, do they correlate with global anoxic events (OAE), or/and do the variations display cyclicity/short term variations corresponding to Milankovitch cycles. How is the relation of the controlling mechanisms and how do their changes vary through time (Lower vs. Upper Cretaceous). The results of these investigations will be compared to data from the world ocean and help to understand the enigma of the global anoxic events.

The investigated sections range stratigraphically from the late Albian to Maastrichtian and comprise a thickness of more than 800 m. The strata is built of dark grey to black, laminated marly claystones alternating with dark grey fossiliferous limestones, interbedded with small, calcerous nodules (Hondita Fm.) and intercalations of cherty claystones (Lomagorda Fm.). The upper part of the sections consist of two chert layers intercalated with dark grey fine grained sandstones and claystones (Olini Group). These chert layers are of great regional extend and can also be observed in the La Luna Formation of Venezuela (Macellari, 1988). The uppermost part of the sections consist of dark laminated claystones and dark grey, silty sandstones (Nivel de Lutitas y Arenas Fm.). Latest results of the investigtions will be presented here.


Fabre, A., In Proyecto Cretacico, Contribuciones (Etayo Serna, F. & Laverde-Montano, F., eds.) XIX 1-20 (Bogotá, 1985).

Macellari, C. E., J. South Am. Earth Sci. 1, 373-418 (1988).

Mann, U., In Berichte 63 (ed. Fachbereich Geowissen-schaften, Universität Bremen) 153 pp. (1995).