Pedogenesis and Stability of Two Late Cenozoic Paleosols Developed on Alluvium in East-Central Pennsylvania

The ridge and valley province of central Pennsylvania is named for the resistant ridges of sandstone and quartzite adjacent to more easily eroded valleys of limestone and shale. Colluvium and alluvium have accumulated against the base of the ridges, enabling pedogenic processes to develop and preserve a rich history of paleoenvironmental conditions in buried soils. Our group builds on the legacy of existing scholarship on these paleosols by deciphering the timing of the initiation of pedogenesis, and characterizing the most likely paleoenvironmental conditions of soil formation.

In this work, we report on our first efforts to study two paleosols developed through alluvium deposited on the north-facing slope of South Mountain adjacent to the Cumberland and Lehigh Valleys. The first is a periglacial kame deposit near Emmaus, Pennsylvania that hosts a 4 meter thick soil, and the second deposit is a series of alluvial fan sequences near Mainsville, Pennsylvania that host a 10 meter thick soil. Common features observed in both these soils include a deeply rubified color and horizons with abundant iron and manganese oxides. In some localities, these iron oxides were abundant enough to be a source of iron ore during in the late 1800s. The relative proportion of amorphous iron oxides, as calculated from the ratio of oxalate extractable iron to dithionite extractable iron can be used to coarsely estimate an age for these soils. Ratios in the shallow modern brown-colored soils range from 0.21 to 0.35, and then evolve towards values of 0.088 to 0.143 in the rubified paleosols as the amorphous iron is converted to crystalline iron minerals. Evolution of iron crystallinity from comparable profiles reported in the literature takes between 1e5 and 1e6 years, suggesting a fairly long period of stability in these hillslopes.