Paleopedology of the Late Triassic Middle Passaic Formation, Newark Supergroup, Pottstown, PA

Abstract

Cyclic stratigraphy has been recognized in the Newark Basin for many years. Each package, referred to as a Van Houten Cycle (VHC), generally has three divisions: shallow lake, deep lake, and subaerial exposure. Van Houten (1964) first proposed that Milankovitch orbital forcing was responsible for the manifestation of these ~21 kyr cycles. Although root traces have been observed in VHCs by others, no detailed paleopedological analysis has been performed that examines the relationship between individual VHCs, orbital forcing, and paleosol development. The Middle Passaic Formation of Late Triassic age is continuously exposed for over 30 meters along a railroad cut that follows Manatawny Creek near Pottstown, PA. Six VHCs were identified at this location and the upper most three were selected for detailed study due to their strong development. Three Van Houten Groups (VHGs), consisting of VHC Division 3, Division 1, and Division 2 respectively, were formed in order to group paleosol profiles (Division 3) with stratigraphically adjacent lacustrine units (Divisions 1 and 2) since the lakes directly affect the paleosurface through inundation and erosion. Petrographic analysis suggests that soils in this section only developed to the degree of Entisols or Protosols. Voids are lined with chalcedony and cored with calcite indicating diagenetic alteration. Molecular weathering ratio calculations proved unreliable due to diagenetic alteration of the strata. Magnetic susceptibility was measured on two intervals of the section, but is not well-suited to fractured, massive rock due to signal attenuation. Paleosol development is greater in instances where the overlying lake is poorly developed. Paleosols that are associated with a shallow lake or no lake likely have more time to develop than paleosols associated with deep lakes as the precipitation filling the lake would saturate the soil, hindering pedogenesis. The VHCs' ~21 kyr interval forces time to be the limiting factor for pedogenesis in this section, ending in either sedimentation or inundation. However, time is also tied to climate as it modulates from relatively moist to relatively drier within a VHC. Orbital forcing is the ultimate controlling factor in soil formation since time, climate, insolation, and precipitation are all interrelated and influenced by it. Relief is independent of orbital forcing and a possible control on soil formation within the Basin. Soils that formed distal from the bounding fault may not have been subject to inundation due to their higher elevation. Further research is needed to establish paleocatenary relationships of soil within the Newark Basin.