The controls on the major- and trace-element evolution of shales, siltstones and sandstones of Ordovician to tertiary age in the Wet Mountains region, Colorado, U.S.A.

Abstract

A vertical section of shales, siltstones and sandstones from Ordovician to Eocene age in a limited geographic area east of the Wet Mountains and Sangre de Cristo Mountains in Colorado have been analyzed for major elements and a variety of trace-element concentrations, including the REE. In addition, the petrography of the sandstones has been determined. The quartz-rich arenites contain considerably lower concentrations of most elements relative to coexisting shales or siltstones at the same outcrop. The arkosic sandstones contain similar to lower concentrations of most elements compared to coexisting shales or siltstones in the same area. Accordingly, the average concentrations of Al 2O 3, Fe 2O 3, MgO, TiO 2, LOI, Rb, Th, Co, Sc, Cr, Cs, Nb, Y and REE are significantly lower in all sandstones relative to all shales and siltstones. The exceptions are SiO 2, Na 2O and Ba concentrations, and the Eu/Eu ★, La Sc and Th Sc ratios as they are higher in the sandstones than the shales and siltstones. There is no significant difference in the MnO, CaO, K 2O and Sr concentrations or the sol La Co , Th Co , La Ni , Cr Th and ( La Lu cn ratios of the average sandstones relative to the shales and siltstones. Many of these differences may be related to the higher quartz and feldspar and lower clay mineral amounts in the sandstones than in the shales and siltstones. For example, average La Sc , Th Sc , Eu Eu ★ and ( La Lu cn ratios of th arkosic sandstones tend to be higher than the coexisting shales and siltstones due to enrichment of the arkoses in feldspar relative to the other minerals (e.g., Eu Eu ★ = 0.82 ± 0.19 in average arkoses in the Trinidad, Vermejo, Raton, Poison Canyon and Cuchara Formations; Eu Eu ★ = 0.63 ± 0.19 in average shales and siltstones in the same formations). In addition, the elemental concentrations and ratios are more variable for most elements in the sandstones than the shales and siltstones. This suggests that the shales are rapidly homogenized near the source. The elemental fractionation in the arkosic sandstones due to enrichment of feldspar relative to other minerals suggests that they are poorer indicators of provenance that the associated shales, whereas, the more homogeneous elemental distributions in the shales and siltstones suggest that they may be better indicators of provenance than the sandstones. Nevertheless, the elemental ratios that are most similar in the sandstones and in the shales and siltstones are consistent with their derivation from similar average intermediate to silicic source rocks.