THICK SAPROLITES IN THE NORTHERN CENTRAL CORDILLERA, COLOMBIA

Recent geophysical determinations show that saprolites with thicknesses reaching 100 meters underlie several plateaus located in the northern Central Cordillera, Colombia. These saprolites are derived from a Cretaceous quartzdiorite batholith possessing a remarkable petrographic homogeneity. The slightly undulated surface of the plateaus is partially mantled by weathered andesitic volcanic ashes, underlain by a stone line which, on basis of OSL dating corresponds to Last Glacial Maximum. For the two better defined erosion surfaces (2600 m and 2000 m respectively) stratigraphic correlation indicates ages of Miocene and Lower Pleistocene. On the other hand, preliminary calculations based on present rates of chemical weathering show an approximate age of 5 M years for these saprolites. If these surfaces were formed near sea level, chemical weathering started as soon as the Central Cordillera uplift began. It may be assumed that the exceptional development and preservation of these thick saprolites were possible due to predominantly humid climates, which permitted a permanent forest vegetation cover. Stable isotope analyses of undisturbed “isovolumetric” kaolinic clays derived from plagioclase could possibly offer a sequence of altitude-related rain temperatures and thus of uplift rates. On the other hand the presence of well defined inselbergs reaching 140 m above erosion surfaces could eventually permit further datings of denudation events through cosmogenic isotope methods.

Our group is presently carrying out detailed mineralogical analyses of saprolite and planning detailed paleomagnetic work; we would welcome association with laboratories interested in performing stable and cosmogenic isotopic analyses in order to unravel the evolution of these apparently anomalous landscapes.