Chemical and mineralogical comparison of the Pennsylvanian Fountain Formation, Colorado, U.S.A. (an uplifted continental block) to sedimentary rocks from other tectonic environments

Abstract

The mineralogic and chemical composition of composites of cobble and pebble-sized rock clasts, clay-stones, siltstones, fine sandstones and size fractions of two sandstones of the Fountain Formation near Beulah, Colorado were determined. The sandstones are arkosic and most plot in the basement uplift or transitional continental fields of QFL diagrams. The mineralogic composition of the sandstones is similar to that of the composite of rock clasts at a given location. Al, Mg, Fe, Ti, Mn, Sc, Th, REE and Cs are concentrated in the clay minerals or heavy minerals and are diluted by quartz or feldspar. Consequently they are inversely correlated with Si. Cr, Hf, K, Rb, Ba and Sr are poorly correlated with Si. Cr is concentrated in clay and in heavy minerals in the silts. K, Rb, Ba and Sr are correlated with K-feldspar, mica, or clay mineral abundances. Sedimentary rocks from the Fountain formed from an uplifted continental block. The element concentrations plot in the fields of active or passive continental margins in many tectonic discrimination diagrams probably as a result of derivation from granitoid sources of similar composition. The REE patterns, Eu anomaly size, K and Rb concentrations of the fine sandstones are mostly similar to those of the corresponding composite of rock clasts at a given location, suggesting that these parameters might be the most reliable in distinguishing sources. The most reliable discrimination diagram for uplifted tectonic blocks like the Fountain may be a plot of K 2O vs. Eu anomaly size since both of these parameters were very similar in concentration in both the sedimentary rocks and presumed source of the Fountain. Most sedimentary rocks in this study have larger negative Eu anomalies than those from the NASC (North American Shale composite) or MCPS (midcontinent platform sediment). This suggests a more evolved source for the Fountain than that for average platform sediment.