A SUSPECTED VOLCANIC ASH UNCOVERED BY TRACE ELEMENT CHEMISTRY OF SEDIMENTS IN THE NEWARK BASIN

An outcrop of the Stockton Formation beneath the Palisades Sill in the Newark Basin was exposed during a construction project in North Bergen, New Jersey. The outcrop is mostly stacked channel sandstones, typical of this part of the rift basin. There is evidence of low-grade metamorphism due to the intrusion of the sill, but no remobilization of major or trace elements is associated with this. The geochemistry of various sandstones and siltstones was determined by ICP-AES and ICP MS, and we examined thin sections to identify minerals present. Samples of channel sandstones and floodplain deposits mostly had geochemical signatures related to their provenance, except for two samples. Both of these samples had high concentrations of thorium (148ppm and 32.7ppm) and other trace elements, which could indicate traces of volcanic ash.

Further examination of these samples showed that they contained layers that were dark in color, due to the presence of smoky quartz and some small, dark opaque minerals. Smoky quartz is dark gray in color due to radiation interacting with aluminum impurities in the mineral structure. This radiation came from the rock, due to the high concentration of thorium. We then separated the light and dark part of each sample to see which has higher concentrations of thorium and other trace elements for comparison. In the sample with the higher thorium concentration, there is a clear distinction between the dark part (218 ppm Th) and the light part (40.5 ppm Th). The dark part has elevated concentrations of other trace elements, including U, Pb, Zr, and REE, that are less pronounced. The sample with the lower concentrations did not show the same strong signature when separate parts were re analyzed, suggesting that the source of thorium is less consistent here. Comparison with the recent ash flow from the East African Rift Valley suggests that these trace elements may come from volcanic eruptions from a fractionated magma chamber. The importance of this project is that we will be able to better understand the types of volcanoes in the Newark Basin nearly 200 million years ago, expanding the very limited data set of volcanic ash from this area.