Transmission of Flood Basalts through a Shallow Crustal Sill and the Correlation of Sill Layers with Extrusive Flows: The Palisades Intrusive System and the Basalts of the Newark Basin, New Jersey, U.S.A.
The Palisades Intrusive System consists of a 350-m-thick early Jurassic sill together with thinner comagmatic sills and dikes exposed within the Newark Basin of New Jersey and New York. The Palisades System is overlain by flood basalt that is interbedded with early Jurassic redbed formations. New and recently published data indicate that some of the basalt flows correlate with geochemically defined layers within a central well-exposed sill portion of the Palisades System at Fort Lee, New Jersey. Our interpretation views the sill as a progressively inflated conduit through which huge volumes of flood basalt flowed. The geochemical data are consistent with a Palisades sill fed by three compositionally distinct intrusion events. The first magma flowed through the sill and broke out near the northern end as three Orange Mountain basalt flows. Each of the three extrusive pulses is identified within the lower 150 m of the sill on the basis of distinct geochemical reversals. The end stage of each pulse was characterized by pyroxene phenocryst accumulation within the sill. Magma from a second source inflated the sill by an additional 170 m after approximately 260 k.yr. of minor intermittent igneous activity interbedded with sediments deposited above the Orange Mountain basalt. The second magma extruded as a highly fractionated 150-m-thick Preakness basalt flow and crystallized as a central layer of Palisades diabase of similar composition. Subsequent extrusions of relatively thin Preakness flows (magma 3) correlate with upper layers of the Palisades sill. We interpret the distinct layering of the Palisades sill as injections of magmas that were largely prefractionated at deeper levels and then modified to varying degrees by in situ processes.