NEW EVIDENCE OF VOLCANIC ASH IN THE LOCKATONG FORMATION OF THE NEWARK BASIN

Early continental rifting normally includes a period of eruption of alkali magma, such as may be seen in the East African Rift Valley today, prior to the lithosphere thinning that eventually allows fissure eruptions of flood basalts. However, in the Newark basin, the earliest evidence for eruptive volcanism is tholeiitic Orange Mountain Basalt (OMB). It is assumed that there might have been alkali volcanism prior to OMB volcanism but no volcanoes have been found in the pre-OMB rock record. This study of sedimentary rocks from Fort Lee, NJ has found geochemical evidence for traces of ash in a layer of fine grained sedimentary rock from the Lockatong Formation, stratigraphically earlier that the OMB. Several samples of sedimentary rock have been analyzed, all of which have major element concentrations typical of rocks of quartzose provenance, as would be expected in the continental interior. Rare earth element (REE) concentrations in most samples are similar to typical rocks of this provenance. However, one sample has much higher overall concentrations of REEs, and the REEs are also more fractionated. The concentration and fractionation are more akin to felsic or alkali igneous rocks. It is likely that the bulk of the minerals in the sample are detrital in origin, but that the REEs are concentrated in small amounts of volcanic ash. This would be the first evidence of volcanic activity in the Newark Basin, prior to the Orange Mountain Basalt. The samples interpreted as ash were collected about one meter beneath the base of the Palisades sill but are stratigraphically separated from the onset of OMB volcanism by the upper Lockatong and the entire thickness of the Passaic Formation. This proposed volcanic activity, therefore, must have preceded OMB volcanism by millions of years and if confirmed by ongoing testing may force important reinterpretations of the petrology and tectonic development of the Newark Basin.