PEDOGENIC SPHAEROSIDERITES FROM THE CABALLOS FORMATION (APTIAN-ALBIAN) OF COLOMBIA: A STABLE ISOTOPE PROXY FOR CRETACEOUS PALEOEQUATORIAL PRECIPITATION

The Caballos Formation (Aptian-Albian), Colombia consists primarily of gray mudstones and very fine grained sandstones. Four cores archived at the Instituto Colombiano de Petroleo Core Repository were investigated for sedimentary structures, and approximately 60 hand samples were collected. These rocks contain abundant sphaerosiderites, root traces, burrows, translocated clays and ped structures indicative of paleosols. Sphaerosiderites were microsampled to determine carbon and oxygen isotopic composition. The plotted data generates meteoric sphaerosiderite lines (MSL), linear trends that are defined by variable carbon isotopic compositions and invariant oxygen isotopic compositions. The MSL values from the Caballos Formation range between -6.90 and -3.91‰ VPDB. At a paleolatitudinal zonal mean annual temperature of 30°C, the oxygen isotopic composition of paleo-groundwater was calculated and ranged from -6.95 to -3.96 ‰ VSMOW. Coupled with coeval paleo-groundwater estimates from other latitudes, the data provide insight into the global hydrologic cycle during the mid-Cretaceous suggesting, an intensified hydrologic cycle was an important factor in the Cretaceous greenhouse climate. Our estimated paleo-equatorial groundwater δ¹⁸O values are comparable to results from Albian GCM simulations with embedded precipitation isotope modules.