GEOCHEMICAL EVIDENCE FOR THE ORIGIN OF TRIASSIC FLUVIAL SEDIMENTS AND THEIR REMOBILIZATION DURING INTRUSION OF THE PALISADES SILL

When exposure of a rock unit is limited, as is often the case in built up areas, it is important to use multiple approaches to identify and interpret the lithologies. Here, we focus on one such outcrop beneath the Palisades Sill in the Newark Basin in North Bergen, NJ. It is composed primarily of psammites and metapelites; i.e. sandstones and shales that show some low-grade metamorphic alteration of clay minerals to biotite and cordierite due to the intrusion of the Palisades Sill. Our study included field observations, followed by mineralogical and bulk geochemical analysis.

Previously, this location was mapped as the Lockatong Formation, a predominantly lacustrine unit, but initial field observations indicated a fluvial origin more typical of the Stockton Formation, which is stratigraphically lower. We compared the major and trace element geochemistry of the psammites and minor metapelites with previously published data for both the Lockatong and Stockton Formation. Specifically, in plots of Co, Cr and Ni against MgO, which differentiate between the two formations, data from this location agree with data collected elsewhere from the Stockton Formation, supporting our initial field observations. Despite the low-grade metamorphism observed mineralogically, there is no evidence of any overall movement or redistribution of major or trace elements related to the intrusion.

In addition to the sequence of sand beds that are typical of the Stockton formation, there is a single large channel deposit in this outcrop that cuts across earlier beds. This channel unit has very similar major and trace element geochemistry to the beds, however, rare earth elements are much more fractionated and do not have the negative Eu anomaly typical of the regular beds. This suggests a different sediment source for this single large channel. Additionally, in the area of the channel, there is evidence of sediment remobilization after deposition, including a clastic dike. This most likely occurred during the intrusion of the Palisades Sill, and suggests that the channel deposit was wet and not fully lithified at the time of the intrusion. Again, geochemical analysis supports the field observations, with the clastic dike sample displaying the same rare earth element signature as the samples from the channel.