DETERMINING THE CHEMICAL AND PHYSICAL PROPERTIES OF LATE CRETACEOUS SHALE FORMATIONS IN THE EL PASO, TX AREA FOR CORRELATION WITH EOCENE INTRUSIONS

Deformed Cretaceous shale units are present within the margins of Mt. Cristo Rey, an Eocene andesitic laccolith located in southern Doña Ana County, New Mexico. In the El Paso, Texas area, there are multiple Eocene andesite intrusions in contact with Cretaceous sandstone, shale, and limestone. All of these intrusions contact various shale formation deposits that were deformed by thrusting and folding prior to intrusion. Index fossils within interbedded limestones of these shale deposits are currently the only means to identify the shales with their corresponding formation. Multiple contact locations lack fossil exposure for adequate formation correlation. I am investigating the physical and geochemical properties of these shales in situ and in hand samples in an attempt to determine if the shales are related to the Boquillas, Muleros or Mesilla Valley formations of Early to Late Cretaceous ages. I used an Elemental Analyzer and an Isotope Ratio Mass Spectrometer (IRMS) to calculate the carbon, nitrogen, and sulfur content and isotopic ratios of the samples in order to geochemically distinguish one formation from another. Preliminary results indicate that there are variations in carbonate content within each of the shale samples within the different units, some identifiable with index fossils and some presently not identifiable, varying from 1.89% to 77.68%. Carbonate content is not likely to serve as a marker between units because of the wide variances in carbonates within a single unit. It is possible that sulfur content and sulfur isotope ratios could be a marker, but due to processing error by the poor combustion in an Elemental Analyzer, we obtained unreliable calculations for sulfur, thus we were unable to make a thorough comparison. Preliminary conclusions indicate that the use of an IRMS gives reliable data, however a larger sampling pool, combined with a smaller sample to aid combustion in the Elemental Analyzer, may give more substantial results.