NEW DATA FOR THE 201-MA JACKSONWALD LAVA FLOW, NEWARK BASIN, PENNSYLVANIA: FLOW CHARACTERISTICS, PETROGRAPHY, AND MINERAL CHEMISTRY

The Jacksonwald Lava Flow (JLF) is part of the Central Atlantic Magmatic Province, a large igneous province associated with the rifting of Pangaea. The JLF is located SE of Reading, PA, in the western end of the Newark Basin, and is one of three remaining basalt flows in PA. It was previously dated as 201 Ma (Marzoli et al., 2011, Lithos 122, 175-188). It is estimated to be about 150 m thick. The JLF, underlying diabase intrusions, and Tr-J sediments were folded into a NW-plunging syncline. The only published petrologic description of the JLF is by Edgar Wherry in 1910 (Transactions of the Wagner Free Inst. of Science, available online). We examined four sets of outcrop from near the fold hinge and along the south limb of the fold and found features not previously described. Wherry (1910) stated that there was no evidence that the basalt was erupted into water or wet sediments, but we found decimeter- to meter-scale pillows consistent with extrusion of basalt into Passaic Fm lacustrine sediments. The pillows are tilted and crosscut by small mineralized fractures that make them more difficult to identify. We also found columnar joints that have been tilted about 47° from vertical. The outcrop near the fold hinge and close to the flow top contains many half-moon vesicles. Brecciated lava is found on the ridge top and may be closest to the original flow top. These characteristics are all similar to those found in the Orange Mtn basalt in NJ described by Puffer and Student (1992, GSA Spec. Pap. 268, 261-277). Three samples were analyzed by Dr. Pollock using XRF and found to be high-Ti quartz tholeiite. The compositions correlate with published analyses of the JLF and the Orange Mtn basalt. We analyzed mineral compositions using a FEI scanning electron microscope with Oxford Inca energy-dispersive x-ray spectrometry. All samples of the JLF contain phenocrysts of plagioclase (An₇₁ core), clustered augite (En₅₁Fs₁₂Wo₃₇), and some contain altered olivine. The same minerals and compositions are found in the chilled margins of the nearby Birdsboro Dike, which may have been the feeder for the JLF. The sample from near the flow bottom contains the most augite phenocrysts and has the highest Cr concentration. The sample from near the flow top is finer-grained and plagioclase is more skeletal and altered.