Implications of elemental concentrations for provenance, redox conditions, and metamorphic studies of shales and limestones near Pueblo, CO, USA

Abstract

Major element and some trace element compositions (including the REE) of shales, carbonate-rich shales, and limestones of Late Cretaceous age have been analyzed at two outcrops near Pueblo, CO. Elemental ratios that are characteristic of the provenance of terrigenous debris that are the least variable with changing percent acid insoluble residue vs. percent calcite are Th/Cr, La/Co, (La/Lu) cn, and Eu/Eu*. The Ce/Ce*, La/Sc, and La/Cr ratios, however, are only constant when greater than 30% of a sample is composed of acid insoluble residue. At less than 30% acid insoluble residue, these elemental ratios increase markedly. The Th/Co and Th/Sc ratios are fairly constant from nearly 0% to about 60% acid-insoluble residue. Above 60% residue, these ratios increase due to the high concentration of Th in the Graneros to Hartland shales at Everhart Ranch. The average of the Th/Co, Th/Sc, Th/Cr, La/Co, La/Sc, and La/Cr ratios are similar to those of the MCS (mid-continent shales) and PAAS (Post-Archean Australian shales). Thus, these carbonate-rich to carbonate poor rocks analyzed in this study contain terrigenous debris that has been derived from granitoids similar to those that supplied debris to the MCS and PAAS. The Ce/Ce* ratios are lower and the Mn* (Mn*=log[(Mn sample/Mn shales)/(Fe sample/Fe shales)]) values are more positive in the more calcite-rich Bridge Creek and Ft. Hays limestones than in the other units, suggesting that they formed in an oxidizing environment. The Ce/Ce* are the highest and the Mn* values are the most negative in the Graneros to Hartland shales, suggesting that they formed under more reducing conditions. The elemental concentration of one sample relative to that of another sample over a few meters distance usually vary in small amounts (e.g., medians of the ratios of the same elements between adjacent samples ranges from 1.06 to 1.70). However, the ratio of elemental concentrations between adjacent samples can sometimes be quite large. The least variation of elemental ratios between adjacent samples often occurs within the more carbonate-poor shales; the greatest variation occurs within the limestones. Thus, trying to decipher local movement of elements due to metamorphism in similar carbonate shale-limestone sequences should be done cautiously since local elemental variation due to sedimentary processes can be large.