Ruben D. Uribe1, Luis A. Gonzalez2, Scott J. Carpenter3
(1) The University of Iowa, Iowa City, IA
(2) University of Iowa, Iowa City, IA
(3) Univ. of Iowa, Iowa City, IA
ABSTRACT: Stable Isotope Geochemistry of Upper Cretaceous Carbonate Concretions and Veins in the Cariblanco Formation of South-central Puerto Rico
Matrix and sediment infills in carbonate concretions within tuffaceous mudstones in the Upper Cretaceous Cariblanco Formation of south-central Puerto Rico have d13C values ranging from -30 to -15 o/oo PDB. These values, together with a brightly luminescent character, and the presence of pyrite indicate formation in sulfate-reducing environment with influx of diffusing methane.
Carbonate cements are enriched in13C (d13C values from -18 to -8 o/oo) and depleted in 18O (d18O values from -4 to -12 o/oo PDB). The isotopic composition of carbonate cements, together with fluid inclusion studies, indicates maximum burial temperatures between 150 and 200°C. Calcite cements and microspars were formed by the circulation of progressively warmer fluids, with warming induced by the gradual emplacement of the nearby Los Panes intrusion. This intrusion probably caused intense normal faulting, induced extensive warm fluid circulation, and resulted in high geothermal gradient responsible for early hydrocarbon generation.
Calcite veins present in the Cariblanco Formation (calcareous siltstone/mudstones and limestones) have d18O values between -17 and -5 o/oo and d13C values between -59 and 2 o/oo. These extremely low d13C values of vein calcites indicate that the source of carbon for these cements is oxidized methane.
The lack of detailed diagenetic or source rock potential studies of the sedimentary rocks of south-central Puerto Rico make it difficult to evaluate whether significant amounts of hydrocarbons were generated within the Cretaceous Cariblanco Basin. Additional research is needed to evaluate the fate of the hydrocarbons during the intense post-Campanian fracturing, plutonism, and volcanism that affected all of the Cretaceous rocks in this area.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado