LATE CENOZOIC SOILS OF SOUTH-CENTRAL PENNSYLVANIA: A GLIMPSE INTO LONG-TERM LANDSCAPE EVOLUTION

Surficial geologic mapping shows the northwestern footslope of South Mountain consists of multiple alluvial fans that have coalesced into an alluvial apron of varied thickness and sedimentologic composition. Multiple soil-forming intervals within alluvial fan deposits in South-central Pennsylvania confirm the landscape is the product of episodic geomorphic processes throughout the late Cenozoic, possibly driven by climate perturbations. Soils have developed on the landscape and are buried within the subsurface alluvial fan architecture.

While sparse chronostratigraphic control is available, age estimates are derived by comparison to other deposits in the Mid-Atlantic region. Examination of surface soils suggests they are of at least two ages and represent different soil-forming intervals and can be correlated to Susquehanna River terrace deposits, Coastal Plain deposits and Virginia fan deposits. Brown, sandy and gravelly soils within the area represent a late Quaternary soil forming interval. Relict and rubified soils recognized on the land surface own much of their development to a past soil forming environment and are out of equilibrium today. These soils are located away from the mountain front and escaped the latest episodes of erosion and sedimentation. The relict soils on the distal fan are likely correlative to truncated and buried paleosols in the middle fan region, based on similar weathering characteristics. Thus, these rubified soils and paleosols represent an intermediate age of soil development between the late Quaternary soil and deeper subsurface features.

Within the subsurface, certain features such as clay translocation, buried and stratified soil horizons, soil structure, and re-dox mottling, indicate zones of intense weathering, pedogenesis, and landscape stability within the overall fan architecture. Laboratory data confirm zones of intensely weathered materials that are recognized as relict pedogenic features. These soil forming intervals are only partially preserved and have been progressively altered by both erosional and geochemical processes since initial burial.