The controls on the major and trace element variation of shales, siltstones, and sandstones of Pennsylvanian-Permian age from uplifted continental blocks in Colorado to platform sediment in Kansas, USA

Abstract

Shales, siltstones, and sandstones of Pennsylvanian-Permian age from near the source in Colorado to those in the platform in eastern Colorado and Kansas have been analyzed for major elements and a number of trace elements, including the REEs. The near-source sandstones are significantly more enriched (Student t-test at better than the 99% confidence level) in SiO 2 and Na 2O concentrations and more depleted in Al 2O 3, Fe 2O 3 (total), TiO 2, Th, Hf, Sc, Cr, Cs, REEs, Y, and Ni concentrations and La Co and La Ni ratios than the near-source shales and siltstones, most likely due to more plagioclase and quartz and less clay minerals in the sandstones than in the shales and siltstones. There are no significant differences in K 2O and Sr concentrations and Eu Eu ∗ , La Lu , La Se , Th Sc , Th Co , and Cr Th ratios between the near-source sandstones and the near-source shales and siltstones. Samples of the Molas, Hermosa, and Cutler formations near the source that were formed in different environments in the same area contain no significant difference in Eu Eu ∗ , La Lu , La Sc , Th Sc , Th Co , and Cr Th ratios, so a generally silicic source and not the environment of deposition was most important in producing these elemental ratios. For example, Cr Th ratios of near-source shales, siltstones, and sandstones range from 2.5 to 17.5 and Eu Eu ∗ range from 0.48 to 0.78, which are in the range of sources of sediments derived from mainly silicic and not basic sources. Near-source shales and siltstones contain significantly higher (Student t-test) and more varied concentrations of most elements (Al 2O 3, Fe 2O 3, MnO, TiO 2, Ba, Th, Hf, Ta, Co, Sc, REEs, Nb, Y) but significantly lower concentrations of Na 2O and Eu Eu ∗ than platform shales and siltstones in Kansas (e.g., La = 65.7 ± 40 and Eu Eu ∗ = 0.55 ± 0.07 in near-source shales and siltstones and La = 23.7 ± 8.7 and Eu Eu ∗ = 0.64 ± 0.08 in platform shales and siltstones). The SiO 2 and CaO concentrations are not significantly different in platform shales and siltstones compared to the near-source shales and siltstones, so dilution of other minerals by quartz and calcite is not the main reason for the lower concentration of most elements in the platform relative to the near-source shales and siltstones. Rather the lesser concentrations of most elements in clay minerals of the platform shales and siltstones can account for the lower concentration of most elements compared to corresponding near-source shales and siltstones. The lower concentrations of many elements in clay minerals in the platform shales and siltstones may be a result of having been derived from recycling of clay minerals from older rocks. The greater homogeneity of elemental concentrations of the platform shales and siltstones compared to those in the source is also consistent with homogeneous mixing of such recycled material. Also there is no significant difference in Th Sc , La Co , Th Co , La Ni , and Cr Th ratios of the near-source sedimentary rocks in Colorado to the platform shales and siltstones in Kansas, and the latter are also consistent with derivation from mostly silicic source rocks.