PALEOPEDOLOGY OF THE MIDDLE PASSAIC FORMATION, LATE TRIASSIC, POTTSTOWN, PA

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 subaerially exposed. Milankovitch orbital forcing is proposed to be 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 exposed for over 30 meters along a railroad cut that follows Manatawny Creek near Pottstown, PA. A detailed measured section of a portion of this exposure was constructed, and oriented samples were taken from each lithologic unit for petrographic analysis. Root traces can be seen in the exposure and in thin section and define six paleosol surfaces. Microfabric analysis of paleosols suggests only weakly developed soils in this section. At least three of these paleosols are overlain by lake beds of differing depth and lithology; representing very shallow siliciclastic-dominated lakes lower in the section to deeper (laminated) carbonate-dominated lakes up section. Lake sediments contain millimeter-scale fining upward sequences. Algal laminations, suggestive of laterally linked hemispheroidal stromatolites, represent scenarios where deep lakes did not form over soils. This is a manifestation of variation in orbital forcing affecting lake depth over longer scale cycles. Sediment came from at least two sources: sedimentary terrane(s) and metamorphic terrane(s). Detrital sedimentary grains are ubiquitous throughout the section and vary from well-rounded to angular. Detrital metamorphic minerals are well-rounded, but are not common. This may be related to increased precipitation or increased gradient within the basin. Magnetic susceptibility (MS) was measured at 2 cm increments over select intervals. Preliminary results suggest a correlation between relatively high MS and deep lake facies, possibly associated with a shift in sediment sourcing. Soil development is greatest at the top of each VHC, especially soils underlying deep lake beds.