Morphology, Mineralogy, and Hydrology of Soils in the Triassic Culpeper Basin of Maryland

Abstract

Soils derived from red parent materials are problematic in terms of morphologically assessing their hydrology because these parent materials are known to retard the development of redoximorphic features within their sola. This retardation phenomena may lead to misinterpretations of these soils for both agricultural and engineering purposes. Such soils occur within the Triassic Culpeper Basin of Maryland. Eleven pedons along two topohydrosequences within this basin were therefore investigated to determine the interrelationships between the soil morphology, mineralogy, and hydrology, particularly as these pertain to the development of redoximorphic features in these soils. These soils are derived from dusky red (SYR 4/4) parent materials which retard the development of redoximorphic features. Morphological descriptions of each pedon were made and the water table was monitored in both open boreholes and slotted PVC pipes at staggered depths. Bulk samples of each horizon were collected for mineralogical and chemical analyses. Results indicate that hematite is the only iron oxide in the underlying red shales and sandstones of this basin. The hematite in the soils is then inherited during pedogenesis and is responsible for the red hues exhibited throughout the profiles of the better-drained pedons and in the lower sola of the more poorly-drained pedons. Results of parent material uniformity indicate the presence of either loess, colluvium, alluvium, or mixtures of these materials with residuum within the upper sola of these same pedons. Residual, or principally residually-derived, soils remain predominantly red throughout their profile, independent of hydrology. Soils formed from non-uniform parent materials exhibit yellower hues, particularly within their upper sola. Thus, the pedogenic yellowing of these pedons may be due to either the incorporation of more easily altered foreign material than that derived from weathering of the red residual parent materials or to the presence of a seasonally-high water table in the upper sola of these pedons. Even though redoximorphic features are created through redox processes, the nature of the parent materials appears to mediate their degree of development. This study suggests that the degree of parent material uniformity be assessed prior to any judgment on the drainage classes of similar soils.