Geochemical and Petrographic Characterization of Ash in the Cretaceous Eagle Ford Formation

Abstract:

The Eagle Ford Formation is composed of highly laminated, organic rich shales and marls interbedded with volcanic ash. Discrete ash beds are easy to identify in outcrop as recessed layers between more resistant rock. In the finely laminated shales, the ash cannot be identified visually, which fosters the questions of whether ash is present in these shales and how that can be determined. The ash is thought to come from volcanic activity in western North America during the Cenomanian and Turonian, depositing in the Western Interior Seaway in what is now South Texas. Samples of known ash-rich beds from the Eagle Ford were analyzed using micro-XRF and thin section petrography in conjunction with ICP-MS laser ablation to determine the geochemical composition of the samples. The high CaCO₃ content of the marls diluted the ash in each sample so elemental data were used to separate the two components. The amount of Ca in the ash from the total measured Ca was unknown. Carbonate takes Sr but not Al, therefore the y-intercept of a Ca/Al vs. Sr/Al graph gave the concentration of Ca in the non-carbonate components. This method was used for every cation to gather a generalized overall composition of the present day ash. The ash was found to have been altered to clays, resulting in a substantial loss of Si and thereby making the original composition of the ash indeterminable. However, certain elements like Ti and Zr are not as significantly affected by weathering. Using an empirical relationship between Ti/Zr and SiO₂ in magmatic rocks from the Cretaceous Peninsular Ranges batholith, the likely source of ash, our measured Ti/Zr was used to determine the original SiO₂ percentage in the ash, giving a range of 60-75 wt%. This was also checked by a Ti/Al regression analysis from the same Peninsular Ranges data, which gave a range of 67-72 wt% SiO₂. These results suggest that the ash came from andesitic to rhyolitic eruptions. The discrepancy in Ti/Al and Ti/Zr calculated SiO₂ suggests that there may be an excess in Al associated with a detrital component. The detrital component can be separated from the ash by calculating the amount of Al in a rock with the SiO₂ given from our Ti /Zr data and then subtracting that from the Al of the present day ash. This allows us to evaluate whether ash and indicators of biological productivity are correlated.