The Origin and Pedogenic History of Quaternary Silts on the Delmarva Peninsula in Maryland

Abstract

Soils formed in Quaternary age silts are widespread on the Delmarva Peninsula in Maryland. The origin, mode of transportation and deposition, and age of the sediments in which these soils formed have long been debated and are important to understanding climate change and to investigations of the prehistory of the Delmarva. This study was undertaken in an effort to resolve the issue of the origin of parent sediments, to examine the pedogenic history of the soils, and to gain insight into the paleoclimate of the region. Thirty nine profiles were described and sampled in two north-south transects on the upland and the broad terrace along the Chesapeake Bay on Maryland's Eastern Shore. Laboratory analyses included determination of particle size distribution, determination of Zr, Ti, Ca, and K contents of coarse silts, mineralogical analysis, and the examination of biogenic opal. The silty mantle overlying sands ranged in thickness from 150 cm to less than 50 cm, with considerable variation across the study area. Textures of this mantle were silt loam and silty clay loam with 53 to 94 percent clay-free silt and a mean clay-free particle size of 41 mm. The Zr content of the silts was uniform within profiles and across the study area while that of Ti, Ca, and K varied. Mineralogy of the silts was homogeneous across the study area. There were no features diagnostic of either fluvial/estuarine or eolian processes in the silt deposit. Minimal coarse fragments and no stratification were observed. Low chroma matrix colors of soils reflected modern drainage conditions rather than a reducing depositional environment. Pedological development argued for relatively young soils (< 30,000 years) and archaeological materials from surface horizons buried by the silts dated the onset of deposition to the end of the Pleistocene (approximately 10,500 14C years BP). The youthfulness of the silts precluded them from having been deposited during the Sangamon transgression, which occurred no more recently than 82,000 years BP, and proved unequivocally that the silts are loess. Buried paleosols were indicative of the landscape stability prior to loess deposition while phytoliths reflected a climate shift.